text:
Are these the right types of electrical connectors for hooking my Switching Power Supply up to a wall outlet?
From what I understand, when you hook up the Switching Power Supply 12v Dc 30a 360w to the wall outlet, you have to be very careful; careful not to get the wires mixed up; careful not to have anyone or anything touch the leads (in fact the first proper project I intend to print out will be a casing to fit around the switching power supply), or just order one from someone.
Now there are three wires that go into the US wall of particular concern, and these wires come out of a standard PC cable with the female end cut off, and they hook the power supply.  Like the external casing, these three wires are also insulated, and when you take the insulation off the bare wires and connect it to the power supply, you have to use Electrical connectors of some sort to connect them to the power supply's screw leads.
I bought some electrical connectors just for this purpose, but I'm not entirely certain they will be good for this purpose, so I thought I'd check here first.

There are specifications on the back:
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬                  ╬        AWG           ╬  Wire Size  ╬ Stud Size ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬ Spade Terminals                                                   ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬  YF1.25-35 (red) ╬        22-16         ╬   0.5-1.5   ╬    3.7    ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬ Ring Terminals                                                    ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬   YF1.25-4 (red) ╬        22-16         ╬    .5-1.5   ╬    4.3    ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬ Butt Splice                                                       ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬
╬   BF-1.2SS (red) ╬        22-16         ╬    .5-1.5   ╬   n/a     ╬
╬──────────────────╬──────────────────────╬─────────────╬───────────╬

Not sure if I should use ring or spade terminals, and I don't know what wire size to use; and I don't know what wire grade is inside a standard PC power cord or even if these are safe connectors to use for this.

It is okay to just use bare wires in the type of screw connector found on your power supply. They're designed for it; they have a little plate under the screw that prevents the wires from being frayed by the screw.
If you want neater wire termination, you should use one of the spade type ones. Pick the smallest size that fits your wires.
PC power cords are generally fitted with an IEC C13 plug, and those are rated for 10A (meaning also the wires themselves will be able to carry at least that much current). This is fine for any home 3D printer which usually draws a fraction of that.

Specifically, a power supply rated 30A at 12V, is therefore taking in a bit over 3A at 120V. Wire gauge 18 to 24 or so is fine (depending on air circulation, bundling, etc.). See [http://www.engineeringtoolbox.com/wire-gauges-d_419.html]. Wires on the 12V side should be heavier.

@TextGeek thank you for the link!  Why don't you make it the answer and I'll select it: http://www.engineeringtoolbox.com/wire-gauges-d_419.html

The block on the supply will accept the bare wire

you could use the yellow in the middle on the right, but the screw on the block essentially does its own crimp.


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What are the other materials which can be used in DLP Printing, other than polymers?
I know that melted polymers are used for DLP (digital light processing) printing, by displaying the model onto the liquid material, which is the melted polymer.
But, can I use materials like steel, carbon fiber, etc?
If yes, then what precautions and arrangement should I make to my apparatus for coping with their high melting point?

Note that "digital light printing" doesn't exist. There's one article where the writer incorrectly uses the term, but DLP stands for "digital light processing" - it refers to the technology that is used in some beamers and subsequently used in some 3D printers.

@TomvanderZanden thanks for clarifying.  I'd edit the question. But still, my question remains the same. Can it be done with high-melting-point materials? If yes, how?

DLP doesn't use "melted polymer". It uses a photosenstive resin which is at room temperature and polymerizes when exposed to (UV) light. At no point is any material melted.
As such, steel or carbon fiber can not be used as these materials are not photosensitive.

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How is 3D printing done in space?
This article states that 3D printing has been accomplished in outer space, on the International Space Station.
I'm curious as to how this works differently from 3D printing on Earth. Are there any extra measures that needed to be taken to ensure that the filament would be correctly extruded onto the print bed, or during other steps?

Also, if anyone can come up with better tags, that would be helpful.

Most likely, the 3D-printers used on ISS does not incorporate some fundamental difference that allow them to print in zero gravity.
Some people over at 3Dprint.com raised a very similar question, and figured that when turning their 3D-printer upside down and on it's side:

there’s not really much difference at all. It’s quite interesting to see how the orientation has little effect on the quality.

One of the early 3D-printer models - the Bukito printer - demonstrated that their printer was so portable it even could print on the move, and upside down.
In other words, some consumer 3D printers already print upside down, and so they would probably print in zero gravity as well!
(That's the short story anyway. Have a look at Ryan's post, who gives a great description of the more intricate parts of space printing!)

Also check out: https://m.youtube.com/watch?v=jUPG5fatJQc

To answer your question, you have to consider how the melted filament sticks to the print bed and to other layers, and if gravity has any affect on how it sticks. The answer is that gravity does not have any real affect on the stick-to-itivity of the filament. Instead, the plastic bonds to the print bed surface, and then subsequent layers fuse with the previous layer. Nor does gravity have any affect on how the filament is fed or how the belts and gears move. Certain filament roll holders may not be able to be used if they do not clamp the roll down, and the printer also needs to be clamped down. But, perhaps surprisingly, there isn't really anything else that needs to be done differently to make a printer work in space.

The first big space-specific issue is actually air quality. You can't just open a window to air out the molten-ABS smell from the ISS!
FFF printers put out fumes and nanoparticles. In a space station, the same air gets recycled over and over, and the air purification systems have a specific set of contaminants that they are optimized for, as well as a design capacity for air turnover and chemical removal rates that won't be adjusted just because somebody's printing a space-ratchet today. Protecting cabin air quality is a huge design factor for any experiment that goes into space.
The Made in Space printing experiments on the ISS to date were performed in one of the vacuum experiment chambers, so any unfiltered fumes (or fire flare-ups) could be vented directly to space if required. In the long run, this isn't going to work -- other experiments may need the vacuum chamber, or "production" printers may be too large to fit. So the printer needs to have its own internal air purification system.
Another MAJOR design constraint is launch survival. Rocket payloads must be designed for extreme g-forces without 1) damage, or 2) significant internal shifting of mass which would affect the payload center of gravity.
Total payload weight is also quite important here: lifting mass to low Earth orbit is EXPENSIVE.
Surprisingly, the microgravity environment itself isn't that big of a deal. Molten plastic is highly viscous and pretty much stays where you put it long enough to solidify, as long as it's sticking to something. But two impacts do come to mind.

First, an unsecured filament spool will try to unwind itself. Gravity won't provide the contact friction we usually rely on to keep spools from bird's-nesting. (Think about it: a tightly-wound spool is literally a giant coil-spring.)
Second, heat flows are different in microgravity -- you can't rely on passive convection to cool the print or the motors. Accommodations must be made for sufficient forced airflow and heat-sinking on anything that requires cooling. And that includes the enclosure itself, since, as mentioned above, the print chamber must be sealed up tight for air quality control.

Finally, reliability is critical. Amazon doesn't deliver to the ISS (yet). Even a single stripped screw may take the printer out of commission for months until a replacement part can be fit into an upcoming supply launch. Having the printer catch on fire because something shorted would be catastrophic.
So, really, it's all about making a printer robust enough to make it up there, operate safely, and never break. Printing upside-down is trivial in comparison.

You last statement summarizes it very well. Great post! I think this should be marked as the correct answer for now.

Great answer! I've just asked What kinds of activities, experiment and, procedures done on the ISS must be done in chambers vented to space? and also How are particulates monitored on the ISS? Are they distinguished at all by size and type?

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My endstops have 4 female plugs, but the examples on the RepRap Prusa i3 Site have 3; what are each of them for?

The Sainsmart Endstops I picked up are different from the ones described in the RepRap Prusa i3 Rework electronics assembly wiki;  they have 4 female plugs that go into the RAMPS 1.4 board instead of 3:

Since these endstops are different, how do I hook them up,  and what do the markings on them mean?

The website shows exactly what each wire is for.  Both middle wires are ground, the wire on the same side as the lever is the signal wire and the last wire is the power wire.

Okay I didn't buy it off if their website and so I was unaware of the documentation.  Thank you!


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Do you provide ventillation for your 3D printer?
The university of Austin published a study saying that 3D printers emit toxic chemical vapors into the air when printing.  They specifically point out that PLA vapors are shown as possible links to cancer.
http://pubs.acs.org/doi/abs/10.1021/acs.est.5b04983
Do you ventillate your printer so you don't breathe the vapors?  I'd be interested to hear what others in the community are doing.

I think you are misquoting the study here. "We are not aware of any relevant information regarding the inhalation toxicity of lactide, the primary individual VOC emitted from PLA filaments."

I am voting to close this question as off-topic, as it is not a question about the tools and applications of 3D printing. Rather, it is a poll regarding whether people ventilate their printers.

A molecular biologist would argue that literally everything is "linked" to cancer, even breathing (oxidation causes free radicals which can form chemical bonds with DNA molecules thereby modifying and damaging them). So yeah, I'd suggest to not assume something is dangerous until sufficiently proven otherwise, otherwise you'll become paanoid.


text:
Chocolate printing
I have seen lots of printers that print chocolate using a syringe with molten chocolate. But, even cooler, would it be possible to print chocolate using some kind of feed system for continuous chocolate printing, so large objects and for a prolonged time, not only lasting the content of one syringe with molten chocolate ?
Things to consider are IMO:

How to keep the chocolate long enough in a molten, viscous state
enough to print ?
Chocolate needs a tempering temperature, which
means it needs to be around 32-37 degrees celsius, else it doesn't
shine but gets a dull look (or turns white after a while).
Chocolate
is food, so you need foodsave equipment in the whole chain that is in
contact with the chocolate.

Maybe a peristaltic pump that keeps pumping the molten chocolate to the extruder, which might be a valve that can be open/closed from G-code ?

For some cases you are better of printing a two part negative (mold) from food safe PLA and pouring molten chocolate from a hole you designed in your editor. I think there are scripts/tools for generating molds. They are usually used for making candle or soap molds and such but could be adopted for choloclate. That mold will not be too reusable as tiny pieces can get trapped between the layers and rot. But you could use it to make a batch of cholocate figures in a day or so.  On the contrary, if your print takes 10+ hours a syringe printed might not work because it might spoil during printing.

continued: the walls of the mold might need to be covered with some food  safe material to prevent the choloclate sticking to it

Hello @Dimitri Modderman, I noticed your question has been up for a while now. Have any of the answers below been able to solve your question? If so, would you mind accepting the appropriate answer. If not, what is missing so that we may help you further? Also, if you have figured it out on your own, you can always answer and accept your own solution. Thank you.

I'm voting to close this question as off-topic because it is a recommendation question.

Update: I found a nice article about chocolate printing: https://all3dp.com/2/chocolate-3d-printer-all-you-need-to-know/

You are searching for chocolate extruder. I did not find one, which would fulfill all your requirements. You have to adapt each solution.
Zmorph3d Liquid paste extruder

https://zmorph3d.com/cake-and-chocolate-extruder/

According video on the page you insert chocolate in liquid form. That could be solved with heated chocolate container.
Syringe based extruders

http://www.open-electronics.org/3drag-is-now-printing-with-chocolate/
http://richrap.blogspot.de/2012/04/universal-paste-extruder-ceramic-food.html
http://www.instructables.com/id/Chocolate-Extruder-for-Ultimaker/

You can use a 2 liters syringe. And if this is not enough then you can refill during print.
Convert pellet extruder
Printing from chocolate pellets is simpler then printing from plastic pellets. Therefore if you use foodsave parts to build such a extruder then this is useable for you.

https://www.youmagine.com/designs/universal-pellet-extruder-reprap-3d-printing

Cooling

3DRAG CHOCO (Chocolate 3d printer) Cooling system explained

Shop
by Open-Electronics

Extrude for chocolate
Syringe Heater for 3Drag chocolate printer

Great answer. One comment I would make this a bit less Link dependent. So that when in the future and a large portion of these links no longer work, the answer still works.  IE posting the names along with the links.

I would look into what Hershey did for this chocolate printer in junction with 3ds. Behold state of the art Chocolate 3d printing.. The
CocoJett
That said there is little said about their tank system.
Far as the actual question it self. There is nothing to facilitate a large pool of continuous chocolate. It would be easy enough to build something that stirred the chocolate and a pump that feeds it in. At that point we are talking about a fair amount of new engineering. One could figure out how to map the E extrusion to a pump that would refill what was spent.
This section is just a bit of speculation..
I wonder. It seems to me that the way to go for XL coco printing would be powder. Bind it with an edible adhesive (hot sugar water maybe?). Have the printer refrigerated. Or just have a simple system that can change out the syringes of coco. Last you could just pause it while you change syringes.

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Does wood filament damage the printer nozzle?
I recently found out carbon fiber and glow in the dark PLA can damage the printer nozzle, now I'm suspicious of all the "exotic" filaments.
So, does wood filament cause damage to the nozzle? (under normal use, or at least what someone who only used PLA/ABS before would consider normal use)
Let's assume a normal quality brass nozzle - not some cheap stuff that didn't even came in the correct size to begin with and not some premium reinforeced nozzle - and reasonable quality filament.

related: https://3dprinting.stackexchange.com/questions/14402/how-do-i-know-if-a-filament-is-abrasive

EDIT: It seems many haven't shared my same experience and have their own input and proof, please view others answers below! For the record I've been using Hatchbox Wood PLA
Original Answer:
Wood filament has the same effect on the nozzle as PLA, so, no, it shouldn't. It's just some wood shavings mixed with PLA, after all.
Materials like Carbon Fiber and Stainless Steel are abrasive and will wear down a brass nozzle. You would want a hardened steel nozzle in that case.

This answer is wrong, and also based on hypothesis, not experience.

Wood filled filament often is regarded as abrasive for brass nozzles. This answer is wrong.

@Routhinator and FarO Thanks for your comments. Just fyi, it's been 5 years since I wrote that comment. So 5 years of continuing to use wood filament. My same stock brass nozzle on my i3 mk2 is still doing great, even with long wood prints. So...it is based on experience, but not your same experience :)  I'm sure different filaments and different nozzles cause much variation though, so it's definitely worth being mindful in all cases.

@Alex Maybe you could add which wood filament you're using, in case other people want to try what you've found to be a non-abrasive variety.

this is highly dependant on which filament you use - a very low wood filler is much less abrasive than high wood content.

If you haven't been to their site before, you should check out the forums on 3DHubs. There's a lot of how-to's. A quick Google search yields this link to a similar question.
The key thing to note is that in all technicalities, any material you run through the nozzle is going to cause some sort of wear on your nozzle. How quickly depends on the material or composition.
The answer to the question linked above relates it spot on to sandpaper. If you have sandpaper made out of metal (ie stainless pla), it will scratch your skin fairly easily. If you have sandpaper made out of tree bark (ie laywood pla), it probably won't scratch your skin as bad, but it'll still scratch. And just for poops and giggles, lets say you have sandpaper made out of pla; it'll take a while, but you could eventually make your skin raw if you rub the plastic against your arm long enough.
It is typically recommended to use one nozzle for each material type as to avoid cross-contamination of materials in your printing. With this idea in mind, if you are using many types of materials, you can also minimize failed prints due to clogging and other "damaged nozzle" type troubles.

"Sandpaper made out of PLA isn't a reasonable analogy here because the PLA melts when passing through the nozzle. The embedded wood or metal particles do not; they retain their rigid solid state.

Wood PLA is too abrasive for a brass nozzle and will wear it out until it becomes a straight pipe after about 12 hours of printing with it.
This answer is based on first hand experimentation. I'm attaching photographic evidence of what to expect if you do not use hardened steel at minimum for Wood PLA.
This image shows that the nozzle after a 48 hour print is a complete straight pipe. It was mostly a straight pipe after about 12 hours. I let the print complete to take stock of the entire result.

Here I am measuring a piece of filament that's been extruded after running the Wood PLA through the nozzle for 48 hours.

And here is how the layers degrade. When the issue started I thought maybe I had a clog, however my extruder was exhibiting perfect extrusion behaviour. The result looks like inconsistent flow, and it is. The extruder is moving at a rate appropriate for a 0.4 mm output, but the nozzle is now 1.7 mm wide.

I haven't used such fiber myself (yet), so this is an educated guess rather than experience. Assuming you mean the kind that has real tiny wood bits embedded, it shouldn't wear a brass nozzle badly, because the nozzle is considerably harder than the wood. Carbon fiber or steel embedded in filament are much harder, so would be expected to wear the nozzle much more.
That said, wood-based filament will still wear the nozzle slowly -- even a hardened steel drill bit gets dull with use, even if you only cut wood with it.

Then again, so will normal plastic by that logic.

-1 for guesswork instead of either personal experience or reference to a reliable source. If you can find some references to support your answer (other than those provided by other answers), I'll gladly switch it over to a +1.


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Whats the difference between a basic rapid prototyping machine and a 3D printer?
In general 3D printers are compact and smaller than RP machines. That's ok. But, what's the difference? 3D printers can be used as RP machine too.

Just curious- where did you hear that a 3DP is smaller than a RP machine? The context in which you heard/read it can clarify the question a bit.

All rapid prototyping means is automatically producing a physical part from a cad model. 3D printing is a way to achieve rapid prototyping. There are 2 main methods of rapid prototyping: additive, and subtractive.
A 3D printer is additive- you add materials to an object layer by layer.
Usually, when people talk about a subtractive machine, they are talking about a CNC mill (or lathe), which tend to be extremely large (most are over one ton). You start with all the material there, and you subtract the material that you don't want. This might be what you are thinking of.

To add to the above answer, a very simple way to say it is that FMD 3d printer is a rapid prototyping machine which has a component called an extruder which pulls and melts plastic which the machine then lays down.

A sintered metal printer is a version of a 3D printer that is rapid, but expensive. Seen 1 for 800,000$aud
Uses laser to melt metal particles like titanium.

Welcome to Stach Exchange 3D Printing. I am wondering: how is your answer relevant to the question asked? Does it say anything about the difference between a rapid prototyping machine and a 3D printer? If it does, I don't think that comes clear from your answer. Remember that the Stack Exchange sites are not forums, but rather Q&A sites.

@TormodHaugene it looks like it was an addition to Daniel's answer as LeoErvin did with his answer.

@tbm0115, I guess you could be right. Perhaps it would have been better as a comment. :)

@TormodHaugene both probably would :)


text:
What are some good FOSS or free tools for editing STL files for 3D Printing?
I'd like to customize and modify some parts on Thingiverse, beyond just simple scaling for 3D printing.
I've been looking for some tools that convert the STL files into something that is easily edited, but so far all that I've found are really buggy and crash frequently as soon as one loads a reasonably complex model.
Are there any free open source software tools that people can recommend that handle STL importing and editing? If not FOSS, what about just "free or nearly free for hobbyists, ed, non-commercial?

They exist; Blender for one. An exhaustive list is off topic here.

Please note that STL, as a file type, is not designed to be edited and is susceptible to corruption if brought between multiple CAD programs. For this reason, it is often difficult to convert an STL back into a file type that is "CAD friendly". It's like crumpling a piece of paper or opening a JPEG a million times.

I'm not sure about that. Sounds more like there being different revisions of the format and/or some 3d editors not creating the files properly which other 3d editors with more strict parsing will not load, than a limitation of a format. Personally I've created as well as edited STL files in Blender and used them in Repetier Host and I've never had an issue loading a file in both.

@LeoErvin I was trying to convey the issues of 3D editors not importing/exporting properly as you said. Look into the following Wikipedia page as one of my sources https://en.wikipedia.org/wiki/STL_(file_format) . For a program like Blender, which can heavily rely on proper orientation of the triangulated solid, it is understandable that you haven't encountered an issue. However, there are other (not as well developed) applications that may choose to ignore or not use certain parameters within the file as they are not within scope of the application. So, just be careful which software you use.

You can use OpenSCAD. It's a script based powerful CAD open source software under GPL.
On Thingiverse, a lot of things are made with this CAD software (the Customizer flagged ones) and you can just download their source code (.scad) and directly edit them !
You can also import existing STL and edit them like they were a primitive shape like a cube. You can then interact with them by doing binary operations, adding parts, etc.
It is hard to start with if you have never coded, but that's worth it :

OpenSCAD is a software for creating solid 3D CAD models. It is free software and available for Linux/UNIX, Windows and Mac OS X. Unlike most free software for creating 3D models (such as Blender) it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects. Thus it might be the application you are looking for when you are planning to create 3D models of machine parts but pretty sure is not what you are looking for when you are more interested in creating computer-animated movies.

Good point mentioning OpenSCAD! When downloading from Thingiverse editing the .scad files directly is nice solution - if you know how to use it.

OpenSCAD isn't a bad solution - however tweaking parts is a bit challenging as there isn't a way, AFAIK to measure an existing object to offset and tweak accordingly.

If you have programming background, OpenSCAD is easy (but read up on the unusual scope rules); but if the original author was drawing in a GUI instead of coding, it may be hard to get any useful (human-readable) code out of their models and into OpenSCAD. Models of people, terrains, etc. are likely to be done with a GUI like Blender; engineered parts are likely to be done with OpenSCAD.

If you want to do basic edits to an STL, a program such as MeshMixer might be right for your. It offers the ability to combine and subtract models, add custom supports, and similar, in a fairly straight forward manner.
An alternative that allows your to do complex editing of STLs would be Autodesk Fusion 360. Here you can import STLs into meshes, which you then may convert into solids for further editing. It is more work to use, but a very flexible solution; you can i.e choose to only convert certain faces to solid, or use the mesh as a reference point for other designs.
As far as I know, neither of these are FOSS (free and open source),  but both are free for non-commercial use.

+1 for Meshmixer, love doing small fixes with it. It can't rescue everything, but it is certainly unique in its approach to modifying files (much more akin to literal sculpting than anything else I've used). Fusion360 often has issues with complex STL files (since it doesn't natively work in the STL format, so files have to be converted to the Fusion 'language').

There is no argument that the most successful FOSS 3D editor is Blender. It can both import and export STL files and export to STL or many other formats (DAE, OBJ, 3DS, FBX, DXF, WRL) so you can both use it to edit STL models as well as just use it as a STL converter.
https://www.blender.org/

Hi @Leo. I have downvoted your question, and here is why: stating that "There is no argument that..." strongly indicates that you cover up your own personal opinion as a fact - something that is frowned upon in the SE community. I suggest you either back up this statement, or find a somewhat more humble reformulation.

I don't care if it strongly indicates that it is my own personal opinion in your opinion. Statistically Blender is successful in all areas, which are feature completeness, stability, siz eof community (help), popularity (download count, number of users), development phase. These are statistics/facts. Whether my own opinion is also involved or not is even irrelevant at this point. So if you're going to downvote an answer which might be the best option for the person asking the question (the whole purpose of Q&A) for the sake of the "spirit of SE" then go ahead. I simply find that illogical.

please understand me correctly: I am not saying Blender isn't a great software, and perhaps the best alternative in this case. However, there is no argument that there are other 3D editors that can edit STL files as well (see what I did there?). In other words, I'm not disagreeing with you, but simply asking if you would consider rephrasing it. Whether you do or not is entirely up to you, but I believe your answer will be better if worded differently.

Blender is successful - and I'm not debating that.  My question wasn't meant to instill a pissing match.  That said - Blender is really more oriented at animation; and editing models for 3D printing is kind of a "hack", being that it is geometry is "unit-less". Hence one can model proportionally, but then you have to scale to the real world outside of Blender.

Tormod, please understand that I'm not talking like this because of my emotions, nor am I a fan of the software Blender, I'm just stating what I believe to be a fact. I see no reason to reword what I said simply because I believe what I said to be true. I don't think this is the situation where one needs to not seem biased or sound humble. As I mentioned, there are facts to back up what I said. Please understand that what you said about what SE community likes is also simply your opinion which I can disagree with. No pissing match here.

Jim, Blender is a 3d modeller which has taken a generalist approach like many other 3d modellers, meaning that it can do many things, it is not design specifically for one task. I can tell you with 100% certainty that Blender is not more oriented at animation making. Like I said, it's a general 3d modeller.  And geometry is not unitless, you can choose Metric or Imperial mode in this menu: http://i.imgur.com/D1mdhDy.png  By default it is "none" or "Blender Units" which is more useful for other things when you're better off thinking in relative dimensions rather than real world units.

Continued: there is also an addon for people who use it for 3d printing which is included with Blender now. http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Modeling/PrintToolbox  Not that it is necessary, but there's even that.

@LeoErvin just edited your answer to help maintain integrity to the "political correctness" of your response.

Sorry I could not approve that as in the link there was no statistical data, just mention that it has 3d printing tools built-in...

@LeoErvin, I'm sorry for my own negative response to your question. For some reason, the formulation provoked me, which it probably shouldn't have.

@Tormod Haugene. No problem, I believe political correctness is overused these days, for a Q&A site I just don't see a place for it, as I think it will both make the questions and the answers longer and therefore less readable, that's why I write like this.  Now if you asked me to mention some sources to my claims that would be reasonable to me.

@LeoErvin, I guess I'm more of the scientific writer type where you never make a claim without backing it up without at least 5 references. :-)

As someone who's in the 3d field I know almost everyone here knows what the major 3d modellers are capable of (Max, Maya, Blender, C4D, etc.) but we don't keep bookmarks to the things which led us to learn that. I'll try to dig up the facts.

@LeoErvin The link that I tried to add to your response was a SE link, which is preferred over another website link. Did you have a look at the link in the posted question? It leads to an article providing sound statistical evidence that Blender is most used in the 3D printing community.

Only a link that Blender has some tools for 3d printer users. Not a link that Blender is the most successful FOSS 3d editor which was what some people didn't like (FOSS means "free and open source", not about 3d printing).

I have used Wings 3D (http://www.wings3d.com) for just that purpose. It is very easy to use, and has many other useful functions built in. Not only can you reform the mesh, but you also you have the ability to import and export many other meshes such as .obj, and etc. When you have completed your modifications you can export them in one of several formats (I find it easier to just keep it in .stl) and then use netfab (also has a free version with some limitations as compared to the paid version) to fine tune the mesh if it is not printable in its current format. Wings will export your modifications in .stl format, but you may have to fine tune to get rid of holes blemishes and etc., and you can view your changes in several views such as Gaussian and other forms.
If you decide to try Wings 3D you need to go through the tutorials, since there is a multitude of possibilities, including making your own meshes, beginning with several basic forms such as squares, cubes, cylinders and several other basic geometrical figures.
Using these two programs I have been able to modify many meshes and arrive at a very printable object. One of the features of Wings which has been quite valuable is the smoothing app which reduces the sharp division lines on some models.
Since both programs are free you can try them and uninstall them if they do not fill your needs. Blender is also an excellent free program, but I find Wings to be much simpler to use.
Hope this helps

Wow, this is actually pretty well maintained surprisingly.  I will need to try this out.  As a software dev myself, I'm intrigued just because it's written in Erlang.  For those looking for the site: http://www.wings3d.com

If you want a WYSIWYG-style editor and are not content with OpenSCAD, I found 123D Design (Autodesk, definitively not FOSS but free for hobbyists) to have (on a Mac) a compromise between power and ease of use that most appealed to me.
If you insist on FOSS probably Blender comes closest, but I found it to be too idiosyncratic in interface and tools to be usable.


text:
Broken Print Bed
I have a Robo 3D. However A while ago, the print bed was fractured, and now it has a long crack cutting it in half. The bed still works because it is held together, by the screws holding the bed to the tracks. So I want to continue using it, because it still is fairly good, the heating element works fine, and a replacement bed is 80$, and I am unsure if the one sold on the RoBo 3D website will be compatible with my printer as I don't have the R1, but a version before that.

So my question is:
How can I best align the two glass fragments, to provide as flat a print surface as possible, and two how to best hold these two pieces in place, or if it would be best to invest in a new print bed?
EDIT:
Here is a image of the heating element as well to explain the situation with how it is attached
EDIT:
The RoBo 3D team have said that I just need to upgrade my y-axis with a object on thingiverse and then buy their new build plate. So I am going to experiment with a glass replacement, and if that fails to succeed then I will go along with their suggestion, and buy the upgrade. Thanks everyone for their help.

Hi, Arden. Does the glass stick to the heated bed somehow, or could you potentially replace the broken glass without also replacing the heated bed?

It appears that on this model, the heating element is part of some adhesive that is attached straight to the glass printing bed. There is a possibility that I might be able to remove it, but not a chance that I want to take

A little chip is fine, but I wouldn't print with that big of a crack. However, that doesn't mean you need to spend $80 on a new sheet of glass.
Option #1- If you want borosilicate glass, you can get a sheet from either McMaster (about $40+shipping for a 10"x10" piece, less for smaller) or from a local glass maker (the price varies a lot, so you should check that also if you want to go that route). However, you won't be able to drill holes (tempered glass will break of you drill a hole). Borosilicate glass has the advantage of being extremely heat-resistant, so it won't break from thermal expansion. If you go this route, you should pick up a few binder clips also to attach the sheet to the heated bed.
Option #2- You might be able to get picture frame glass from a hardware store. It's super cheap (mine was $3 or something from lowes) and they'll often cut it to exact size. You might be able to drill this, but you have to be extremely careful. Sometimes, the hardware store can drill holes in the sheet for you if needed. If you don't want to drill, a few binder clips will work just as well.
Just a note for if you are using binder clips: you might need to (depending on how your hbp is set up) get a piece of glass that is slightly smaller to allow for any bed leveling screws to have room. If you're going with the picture frame glass, you should make sure that the glass will be entirely within the heated part so that there isn't any heat gradient (that's how glass cracks/shatters).

Replacing your bed is the safest option, especially if you have a heated bed. Cracks and chips in the glass create an uneven surface for your prints, and can become a safety hazard; additionally, while borosilicate glass is very resistant to thermal shock, it can continue to crack or shatter when subjected to rapid/uneven temperature variations after damage. Borosilicate glass tends to break in large pieces rather than shattering (snap instead of splintering), but it's still a safety hazard.
It sounds like you have the Kickstarter version of the Robo3D R1; the current glass bed on the Robo3D uses magnets to hold the bed in place, so you'd probably need to replace your Y axis rails along with the bed. I would contact Robo3D support to see what options you have available for upgrading the bed and rails through them.
Aside from Robo3D, you have a few options:
 - If you plan to continue using the bed, Kapton tape is an effective surface application for general printing. It's very thin, but provides an effective surface for ABS & PLA adherence, works with heated beds, and is compatible with secondary adherence items (blue masking tape, PVA glue, ABS slurry, etc). Kapton tape can be damaged after several prints, so plan on having several sheets on hand.
 - You can replace your current borosilicate glass with a new sheet of borosilicate glass from a secondary source
 - Aluminum plates are an effective replacement
 - Lexan sheets also make an effective replacement, but I wouldn't recommend applying heat to them.

Thanks for the suggestion I am waiting for their reply on the request for replacement part. And if that doesn't work, then I will look into the Kapton tape.

NP. You should also think about adding a piece of cork over that heating element, it'll help insulate the coil and improve the heating of the bed.

IMHO your best bet is to use some kind of a tape. I do not have any experience with robo3dprinter but, I have been using glass with PET tape on in for past 4 years.
I have seen guys using Kapton tape as well. Never tried.
The main disadvantage on this solution is increasing the bed thickness, I am not sure if you are able to tinker with the Z level endstop, in case not this migh be an issue.
The other solution you can try is replace your bed with piece of glass and then apply PET tape or any other kind, I am also not sure if you are able to replace the bed print surface of not.
When using glass and PET tape, you need to get the surface of PET tape rough using sand paper, and you might have to apply so called Acetone Juice to make sure that your ABS prints will stick to the bed.
Acetone juice is basically Aceton with disolved piece of Natural ABS plastic this created sticky layer on your bed that helps to hold your prints on the bed while printing.

That was the solution that I have been using in the past, but I wanted to eliminate the necessary price for purchasing new tape. If I am unable to find a solution for the glass bed, then that is what I will return to using. And I am able to adjust the Z level end stop. Thank you for the suggestions of different kinds of tape. I have been using blue painters tape, and that has a fairly hefty price tag sometimes.

@ArdenRasmussen I see. Honestly I am not really su that you can fix the broken glass anyhow. You better prepare for replacing it either for original one, or just one cutted from nearby glassmaker.
Does your maschine at least allow you to position your print where ever you want? Or do you have to print from middle?
Have you tried to apply warranty in matter of this issue?

If your heatbed itself is aluminum/copper/other metal: Get a glass cut and replace it. The current one is probably attached to the heatbed with adhesive glue. If it the adhesive is cyanocrylate based, you can remove by rubbing with acetone. If silicone type, it will come off by some force. Don't worry, you won't damage aluminum. THe heating element is either inside the aluminum or under it.
If it's not alumunum it's likely a PCB heatbed. Then I wouldn't suggest to use acetone or some "antiglue" and would just try to apply a little force and see if the glass moves a bit. If no, don't bother, PCB heatbeds can break too. Then I'd just suggest to apply some epoxy to cover the holes surrounding the cracks and keeping the glasses uniform.
Also, about replacement glasses: picture frame glasses are very cheap. McMaster sells some high quality bearings and rods, but for glass there's no point to waste money. $5 is typical for that size borosilicate glass, if you're in USA (I'm not) I think your local Home Depot will cut it for free for you as well.

I agree with several others that your best bet is to replace the glass entirely. But short of that, you might try something like Loctite "Glass glue", which is essentially crazy glue for glass. Be sure to level and clamp well while drying, or you'll end up with it permanently not straight of course. You'd want it clamped to a very flat surface, and clamped pushing the broken edges together. But I think I'd just replace it.

If it's cyanocrylate based, you can always remove the glue with acetone if it ends up uneven.


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How to configure Cura to run the Z probe before heating
I'm using Cura as my slicing/printing software and I just started using the BuildTak printing surface.
The BuildTak is damaged by pushing a hot nozzle into it and my printer's (Robo3D R1+) autoleveling feature works by pushing the nozzle into the build surface.
Is there a way to configure Cura so that it runs the Z probe first, then heat up the nozzle?
My first sheet of BuildTak already has 10 small holes in it (at the homing position and at the 9 leveling touch points)

In Cura (and Slic3r), you can 100% customize what the printer does before printing your actual model through custom start/end g-code.
If you navigate to the Start/End-GCode tab in Cura, then select start.gcode, you can see what operations are run before each print begins. Lines prefixed with ; are comments, and does not affect the printing in any way.
Basically, we want to manually tell the printer to do the auto leveling before heating up the nozzle by editing the g-code in start.gcode.
G-Code generated with the default start.gcode:
If you try to slice some model with the default code found in start.gcode, you will get something like the following (depending on your printer):
; CURA AUTOMATICALLY INSERTS THESE TEMPERATURE CODES

M190 S70.000000 ; Set bed temperature to 70 degrees
M109 S210.000000 ; Set nozzle temperature to 210 degrees

; THESE ARE THE CODES FROM START.GCODE (for a ROBO 3D R1)

G28          ;move printer to endstops (the home position)
G92 E0       ;zero the extruded filament length
M565 Z-1     ;set z-probe offset
G1 Z5 F5000  ;move the printer 5mm above the bed
G29          ;run auto-leveling

; THE ACTUAL MODEL BEGINS HERE

;Layer count: 168
;LAYER:0
.
.

Analyzing the g-code output
At the top of this code snippet, we can see that Cura automatically inserts g-code for heating up the bed and nozzle to their respective temperatures with the M190 and M109 g-codes. This means the printer always will heat up the nozzle before reading the start.gcodes that we set. However, if we manually override M109 code in start.gcode, the M109 at the top will automagically disappear from the generated g-code output! (Thanks, @TomvanderZanden!)
We could therefore use the auto-leveling command G29 before manually setting the nozzle temperature with M109; specifically, we want to add M109 S{print_temperature}, which reads the Basic -> Print Temperature-setting in Cura, and replace {print_temperature} with it automatically.
Manipulating start.gcode:
In order to postpone heating the hotend till after probing, start.gcode could be something like:
G28          ;move printer to endstops (the home position)
G92 E0       ;zero the extruded filament length
M565 Z-1     ;set z-probe offset     <-----   ( YOU HAVE TO ADJUST THIS, READ BELOW)
G1 Z5 F5000  ;move the printer 5mm above the bed
G29          ;run auto-leveling
M109 S{print_temperature}    ;set nozzle temperature, and wait for it heat up

And that's about it! You can then use these codes in your start.gcode. However, you probably will have to recalibrate your z-prove offset.
Adjust z-probe offset:
Normally, auto-leveling is done with the nozzle heated for a reason: when the nozzle is warm, it expands slightly, moving closer to the bed. You might therefore have to adjust your Z-probe offset with the M565 command (as demonstrated in the snippet) to account for the increase in nozzle length when warm.
Remember:
Remember that when editing g-code in this manner, you will take full control of how the printer operates. You could therefore very well do something unintended, so keep the power switch close!

This answer misses a very important point: in Cura (and something similar works for Slic3r and Simplify3D) you can insert "M109 S{print_temperature}" into your G-code. It will replace {print_temperature} with the set printing temperature and not add its own M109. This way you don't need to do the ugly hack of setting the temp to 0.

@TomvanderZanden, you are entirely right! I didn't think that Cura would be smart enough to remove the first M109 if you set it manually in start.gcode, but it seems like it is! I will update the answer.

{print_temperature} didn't work for me. {material_print_temperature} did. Look here for more.

As pointed out in Markus's comment to Tormod's answer, {print_temperature} needs to be replaced by {material_print_temperature}, so the code now becomes:
G28          ;move printer to endstops (the home position)
G92 E0       ;zero the extruded filament length
M565 Z-1     ;set z-probe offset     <-----   ( YOU HAVE TO ADJUST THIS, READ BELOW)
G1 Z5 F5000  ;move the printer 5mm above the bed
G29          ;run auto-leveling
M109 S{material_print_temperature}    ;set nozzle temperature, and wait for it heat up


text:
How do people load filament, prime the printhead, flush out the old filament when changing colors when using a standalone 3D printer?
I am wondering how people that use standalone 3D printers (printers that have the ability to print autonomously from SD Card) feed in filament, prime the printhead and/or change filaments without a laptop ?
Do the printers have a menu to arrange all these tasks ?
I often only see the options to preheat the head to a certain temperature, but not to load/unload filament, extrude a small amount etc.
I understand this differs from printer to printer, but still am wondering about this.

I think you should reword your question to something like "How do printers print offline or from an SD card?"

That's not what the OP is asking though - printing offline is all well and good - stick the file on an SD card. All the other bits like leveling the bed or changing the filament are trickier if your printer doesn't have options for that built in (as mine doesn't). Doing this without a PC is possible, but non-obvious

Thanks MalphasWats, that is exactly what it is! Non-Obvious.

Also what printer do you have/are you looking at? If it uses Marlin or you can load your own firmware, the LCD should have functions for that. If it's proprietary or closed-source, then you might have to write the custom gcode.

My printer (IdeaWerk 150) is very basic and doesn't have any options for this from the screen.
I wrote a really simple GCODE file that brings the nozzle up to temperature, then runs the extruder for a few seconds, then waits, then extrudes for a bit again. I think it does this 3 or 4 times then stops. I can put the file (when converted to .x3g!) onto an SD card and run it whenever I need it.
I have a similar file that allows me to level the bed without a computer by just moving the nozzle around to a few key points on the bed and pausing for a few seconds.
I used a GCODE file generated by my slicer in verbose mode to get started, along with a list of codes
I don't have these files to hand at the moment, but this is my start.gcode:
(**** beginning of start.txt ****)
(This file is for a WeisTek IdeaWerk 150)
(**** begin initilization commands ****)
G21 (Metric FTW)
G90 (Absolute Positioning)
M18 (This disables the stepper motors.)

G92 X0 Y0 Z0 A0 B0 (Declare the current position to be 0,0,0,0,0)
(**** end initilization commands ****)

(**** begin homing ****)
G161 Y X F2500
G92 X0 Y0 Z0 A0 B0
G1 X5.0 Y5.0 Z-5.0 F450
G162 Z F450
G161 Y X F2500 (Home X axis maximum, go until reaching the end stop.)

G92 Z142.4 ( ** Set Bed Height ** )

G92 X-75 Y-75 (set zero for X and Y)
(**** end homing ****)

M108 R8.0 (Extruder speed = max)
M6 T0 (wait for toolhead parts, nozzle, HBP, etc., to reach temperature)

G1 Z10 F500 (Bring bed up)

M101 (Turn on Extruder)
G04 P8000 (Wait for 8 seconds for flow)
(**** end of start.txt ****)

If I feed just this file into my printer, it will heat up the nozzle, bring the bed up to about 10cm below printing height and once the nozzle is at temperature, it turns on the extruder for 8 seconds.
Your printer will likely be different to mine - there are a few different flavours of GCODE and you will likely have different XYZ positions, so take a look at some GCODE generated by your own slicer and identify the different parts. The principle is the same.

RepRap based printers use LCD modules with control button and SD card. You can trigger operations like

nozzle heating (to change filament)
extrude filament
home axis to caliber bed level
...

Most used LCD modules are:

RepRapDiscount Full Graphic Smart Controller

RepRapDiscount Smart Controller, see video

RepRap firmwares (Marlin, Repetier) are supporting it. To get better view on what is supported, take a look on Marlin language file.

There are options for tablets. They are running software (for example) on some device that has internal storage, wifi, USB connection etc.
You can buy a new tablet, or reuse your old one just to be a controller.
Another great example is this app.
Apps have menus that can arrange everything for you, now it depends on what app do you use and what filament you use. It's very simple thing.

I have two different printers that can print un-tethered. The first is a Prusa inspired machine with an LCD and the second is a printrbot without an LCD.
The reprap machine uses a ramps 1.4 board programmed with Marlin that gives me the ability to control loading and unloading of the filament with simple menus.
Not all printers that have the ability to print from SD come with an LCD screen. My printrbot simple metal does not come with an LCD, it is an additional $100.
I am too cheap to buy one so I came up with an extremely low tech way of changing filament. I turn my printer on with an SD card that has an auto0.g file (this tells the board to load and run g-code on start-up), once the printer starts to move I kill the power remove the filament and put new filament into the extruder. I then press the lever to disengage the extruder motor and force the new filament into the hot end until the old color has been purged. Please note this method only works when you are replacing materials that have the same printing temperatures.

I know that my RoBo 3D has the ability to run untethered once the gcode file is saved to the SD card attached to the ramps board. It does have to be attached to the computer to start the print at first, but can then be unplugged from your computer. Since it is just a Marlin based Ramps printer this should work for similar 3D printers.
The gcode files contain the temperature of both the nozzle and the heated bed if you have one. The Marlin firmware will prime the temperatures for you.
Changing filament you would still need to set your hot end temp to swap in the new filament with a computer over USB though.
Now I personally use something called OctoPi which is a raspberry pi distribution preconfigured with Octoprint. OctoPrint allows you to control the printer, monitor or start prints over a web interface and even stream video to watch your prints if you want. While technically you printer is still tied to a computer(raspberry pi), it does not tie up the use of your laptop/desktop while printing. Plus this would allow you to put your 3D printer in more locations in your home as well.
OctoPrint Website http://octoprint.org/
For the OctoPi distribution http://octoprint.org/download/

Hi, if you name the gcode file auto0.g the printer will start printing as soon as it is turned on, letting you start a print without a computer at all. also, I got a cheap screen from aliexpress for my Robo3D and it works great

By a pretty large margin, the most popular "primary control via LCD" printers such as the FlashForge Creator line and Makerbot Replicator 1/2/2x run Sailfish firmware, which has options for all necessary control functions directly from the LCD. Preheat, load, unload, level bed, etc are all done via the LCD and seamlessly built-in scripts. There is no need to connect host software to do anything but update firmware and change rarely-adjusted settings like acceleration parameters. RepRap-style USB control of Sailfish printers is perfectly functional, but not really necessary 99.9% of the time.
This is a major reason why Sailfish printers have historically been popular for printer farms. They're designed for stand-alone operation, which is highly favorable when the printers greatly outnumber the associated computers. It's extremely rare to see a Sailfish printer from the last 5 years without a 5-button LCD panel.
The Sailfish firmware UI/UX is simply built around the LCD buttons rather than host software. This is generally more reliable and higher-performance than, say, Marlin via host because it reduces processor time spent on command transmission and eliminates all the possible failure modes in the PC and USB link.
Assuming all Sailfish printers will have LCDs with SD card support and focusing on that one interface really allowed the creation of an overall better printer. More optimization, fewer things that need to be configured by the end-user. But in exchange, you're limited in what kind of hardware Sailfish will support. It's a tradeoff.
In comparison, LCD support was more strapped onto RepRaps as an option rather than the core interface element. This tends to produce a host-driven interface design, rather than an LCD-driven interface design. Which is why operating "headless" tends to be more difficult in Marlin, Repetier, etc.

You don't need a specific menu for this (although some printers are more able than others to do the following procedure), just play with the filament loading procedure.
Although better print qualities are given by sticking (if possible) to the same color and filament, when you have to "flush out" any trace:

get an ABS filament of the most neutral color possible, getting a spool dedicated to this task might be a good idea
load the neutral ABS at 250 degrees (or higher) and let it extrude freely and quickly for at least 1 meter.
if you need to clean more, extrude at least 3 meters

Note:

if you can't get ABS (the best option) you can use PLA, but results may vary...
if you can't get neutral color (absence or almost no color pigment), white is the best choice, otherwise black. Neutral color is best by far for obvious reasons.


text:
Is it necessary to have both hot ends on a dual extruder printer at the same height?
I'm thinking of another extruder on my printer, and I'm curious about this one....
Is it necessary to have both hot ends on same height? Why yes / why not? (if there is not)

I have a dual extruder Replicator 1 and having the nozzles at the same height is a must and albeit a bit of a struggle otherwise. At one point, I had to disassemble my extruder head and the nozzles didn't line up quite right. There after, printing with the lower one obviously didn't have any troubles, however, printing with the high extruder made it so the lower extruder would scrape the molten plastic layer. This made my surface finish horrible and almost impossible for support structures to be printed.
Instead of fighting with my stock nozzle assembly to get everything perfectly lined up, I just shimmed the one side with some stacks of paper cutouts. This brought my extruders very close to even.
Also, you'll want to make sure excess plastic is cleaned off of BOTH nozzles when printing with either nozzle. I found that some prints would fail because of a small discharge from a previous print on the other nozzle.

I don't have dual extruder printer myself, but to my understanding having both nozzles leveled at the same height is critical for getting successful prints.
For typical FDM printers, the lowest point of the end effector should always be the nozzle. If you, for instance, mount a fan lower than the tip of your nozzle, it will eventually collide with the printed object.
The effect of having unequally leveled nozzle tips for a dual extruder printer will be exactly the same: one of the nozzles will either drag against or collide with the model during print; or, one of the nozzles will be to far away from the model, giving poor layer adhesion. Either way, the result will be sub-optimal.
So, leveling both nozzles equally is probably a good idea. You might want to have a look at this question on some advice regarding how to do it.

If your printer has no way to move the heads up and down, or otherwise out of the way, and your slicer isn't able to detect collisions and account for head height differences in the produced G code, then they must be at exactly the same height from the bed.
If extruder A is hanging lower than extruder B, then when B has printed at a certain level, and A moves across that area, A will hit the material B has printed.
Unfortunately, this is further complicated for newer printers because some of them rely on digital bed leveling.  If your bed isn't parallel to your X and Y axis movement, then you have to work very hard to make sure the heads are parallel to the bed, and with some mechanisms this isn't possible without making the mechanism itself parallel to the bed, so just mounting the new head at the same height as the other head might not be sufficient if your printer isn't perfectly calibrated already.
I have not yet seen a printer or slicer that manages heads of different heights, but I suppose it's possible to manage the problem with additional intelligence.  For instance, if the printer can move the heads up and down independently, it might move one down for printing, and the other up and out of the way when it's not printing, and then the opposite when it switches.  I can't think of a good reason to do this, though.
Another option would be to develop a slicer to generate G-Code that will prevent such collisions.  The printer would necessarily be limited in what it could print depending on the location of the second head, as there are some situations where it would need to print in a certain area, but a collision prevents it from getting to that area.
At any rate, no one has developed such software or mechanisms yet, so if you had good reason to mount the heads at different heights, you'd have to account for all of these factors and develop the complete solution yourself.

I want to say that the slicer in Octopi (RaspberryPi Print Server) has options for extruder height offsets as well as the normal X-Axis offset. Don't take my word for it though...

@tbm0115 Octopi uses Cura for its slicing, perhaps you can find a reference to multiple head use at different heights in that software.

I may have mistaken the Y-Axis offset for Z. Still a useful setting and it may be different as I'm pretty sure Octopi uses a Cura framework (not full-blown Cura). I found the following screenshot:https://www.flickr.com/photos/ultimaker/9536542189/

To add to the above answers, besides the obvious point that one hotend might collide with something another hotend positioned lower than it has printed, you also want both hotends to be positioned X microns above the bed at minimum height so your filament will stick to the bed properly. Position one a bit higher than the other and what it prints won't stick to the bed very well, position one a bit lower and it will hit the bed and clog/be unable to print the first layer.
The only reason I can think of when you'd want one hotend to be higher is a situation when you're not using both hotends and don't want the second hotend to drag on and deform a layer the main hotend has printed and is still warm and deformable. If that's an issue (I doubt) then sure, move it a bit higher.
If there are other reasons for what you would like to have the hotends at different heights, please add that to your question. I can't think of one myself.


text:
Printing multiple materials with different properties on the same device
I have searched the internet and found various 3D printers with different advantages and materials which they can print - some even multi color.
However, I cannot seem to find a printer that can print multiple material with different properties; for instance, simultaneously printing PLA and metal. Is there currently such a printer available or in development?

What metal printing technology do you have in mind?

To extend @amra's question: with "different material properties", do you mean common desktop 3D printer filaments, or such as plastic alloys, flexible materials etc., or completely different filaments with different extruder mechanisms?

Yes and no.

for instance simultanious printing of plas plastic and lets say metal. Is such a printer available or in development ?

Practically speaking, no.  Metal printing requires significantly higher temperatures than plastic, and the two processes are so incompatible that there is currently no good solutions that would allow one printer to print both in the same print. Whether extruding filament, laser sintering, or curing resins, the materials involved have to be fairly similar in processing environment to print adjacent to each other without issue.
There are many printers that are intended to print multiple materials by changing the print head.  You might, for instance, use a ceramic paste extruder, then change the head for the next print using plastic.
There have been efforts in the past, and some efforts are ongoing, to resolve this.  For instance wood's metal, a low temperature alloy, can be poured at temperatures compatible with plastics, so it's possible to create a printer that prints plastic, leaving troughs or voids in the plastic, then the same printer during this print would pour molten woods metal into these areas, which then solidifies into an internal metal structure.  These are intended for circuitry and electrical use, however significant problems still exist because the thermal expansion differences in these materials lead to stress and result in poor reliability.
So while some of these processes are being developed, this is still just in the experimental stage and there are significant problems to overcome before printers can print widely different materials in a single printing session.
Of course you can find plastics with such a wide range of characteristics that they can be seen as printing different materials.  Plastics imbued with wood fibers, printing next to conductive plastics with graphite, printing next to flexible plastics, etc, etc are now possible, and depending on your requirements they may meet your needs.

thanks for you extensive comment ! you say : So while some of these processes are being developed, this is still just in the experimental stage and there are significant problems to overcome before printers can print widely different materials in a single printing session.
do you know which experimental printers this are ?

@pwghost here's one example: http://blog.reprap.org/2012/04/some-more-printed-circuitry.html?m=1

For the most part, you can achieve this with a dual extruding printer. However, dual extrusion is best for either multi-color printing or printing with support material. For example, printing the part with PLA and all support material with water soluble PVA.
In practice, printing two completely different materials is not sound engineering practice as they have the potential to not make a well enough bond to each other. So, the case of pure metal and pure plastic, the two materials will not bond well because they will not both be in the same physical state together at any point in time.
However, your best option would be a printer like the MarkForged which uses a composite approach by combining a common binder (ABS, PLA, Nylon, etc.) and a strong material such as Carbon Fiber, Fiberglass, or Kevlar.

thanks for your comment, hmm interesting so there is no printer at all, which can print multiple materials ? why isn't there any research in this area to make it happen ? no demand for something like that ?

Technically Dual extruding FDM printers can print different materials, but they're going to be plastic or composite. I don't really think there is going to be a need for a machine like that. The focus is more into Material Science and developing composite material anyways. Metals are heavy, but the properties are great. Plastic is light, but the properties generally suck. The focus now, is to just combine them. These sorts of composites can be used on FDM and FFF printers.

do you think there's a market for plastic,glass (si) and metal printing in one machine?

Maybe, it just depends on what your focus for manufacturing would be. I personally don't see a how/why you would try to print glass aside art/crafts or refurbishments. You can, however achieve some of the properties of glass in the plastic.

me and somebody else have thought out a method which makes it (we think) possible to print metal, plastic and silicon in one model this makes it possible to creat metal/plastic/glas models and circuitry ect 1 ony step in one machine . Personally i think theres a market for that because it would make it possible to create complete functioning products. if we can get the resolution to 5um we can make pic controllers etc

That sounds like a great tool, if you can get it to work. Please feel free to ask more questions on the site in regards to your project, it sounds like it'll be a pretty involved process.

yes i also think it is, but i need to define the market and ofcourse. does the principle really work :-) thx for your answers. If you like you can get involved in the process?


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Bed leveling method?
Is there a good method or tool to level the bed of 3D printers? I find myself making small adjustments a lot and it's mostly just trial-and-error. A normal bubble level is of limited help and trying to figure out if the head is the thickness of a sheet of paper from the bed in all corners is beyond the capability of my vision.

A bubble level is worse than useless for leveling the bed because you don't want the bed to be level with respect to gravity, you want it to be level with respect to the printer's axes.

@Micael, which kind of printer do you have? Do you have auto-leveling?

The answers provide good methods, I would add level the bad when it is at printing temperature.

There are several ways.
1) Simplest is the business card/paper method.
This video shows how simple it is: Leveling the platform
You should feel the same amount of resistance between the hotend and the bed on all sides. On some 3D printers there are 3 screws holding the bed (eg. Solidoodle) and on others there are 4 screws on each edge (e.g. Prusa i3).
On some 3D printers you need to rotate a screw with a screwdriver to adjust it (e.g. Solidoodle 3) and on some you have a nut (mostly wing nut) (e.g. Solidoodle 4).
I find most people move the extruder around when adjusting the bed by controlling the motors. I think the quicker way is stopping the motors (in Repetier Host it's the "Stop Motor" button) and moving the gantry by hand.
Tutorial: 3D printing guides - Bed leveling
2) Second one is just more advanced version of the above. You use a dial indicator/ micrometer attached to your extruder and make sure the number on the dial is as close as possible on all edges.
Level the Print Bed on a RepRap / RepStrap 3D Printer with a Mitutoyo Dial Test Indicator
Micrometer op 3D printer
There are also digital versions of this tool.
You might need to print or make some other way a piece to be able to attach a micrometer to the extruder (there might be one for your 3D printer on Thingiverse already).
Now, remember I mentioned that you adjust the height of the edges of the print bed with a screw/nut? Well, since there is a lot of vibration during print those tend to get out of place and that might explain why you end up readjusting it over and over again. What I have found to solve this is to use a single drop of Loctite (cyanocrylate glue) in the middle of the nut/screw. A single drop will secure it in place but also not make it impossible to unscrew in the future.
3) Setting up auto-bed leveling on your 3D printer if it's modable enough:
3D printing guides: Setting up auto bed tramming leveling tilt compensation!.
Finally, if you can't get the bed leveled no matter what, it might be uneven. That's usually caused by warping. Both PCB beds and aluminum beds can get warped. With the former just sandwich a glass bed on top of it and it should be fine. Warped aluminum is pretty hard to flatten. In this extreme situation you might need to get a new aluminum sheet cut and drilled, preferably a mm thicker one which is less likely to warp under the same conditions. I think this happens when you either set the bed temp too high and/or the thickness of the aluminum bed is too small (bad or cheap 3D printer design).

The easiest way I know of (unless your printer has a Z-probe and automatic leveling), is to bring the nozzle(s) down fairly close to the bed (maybe 1/4" or so), and then move it around while watching for anyplace that doesn't look even. Adjust the bed until it seems even. You can just eyeball it, or use a ruler or object to measure.
Then bring the nozzle down closer, and repeat. Each time you move it closer, you'll be able to eyeball more accurately.
Once you're quite close, pull out a 3x5 card, or business card, or similar, and move the nozzle up or down until the card just fits between the nozzle and the bed (with no great pressure, but no space either). Again, move the head all around the bed, and do any remaining (tiny) adjustments so it's the same everywhere.
Of course if the bed is at all warped this will be much harder, or even impossible. So before starting, put a good-quality straightedge along it to make sure it's actually flat.

See also @OlafM's point about doing this with the system hot. That way if anything changes with temperature, you'll eliminate that variable.

After the first leveling, I usually don't need to touch it for long time, even if my bed structure is of wood and could warp in time.
I guess you need to adjust it because you turn on the heating and in the meanwhile do the leveling. Do it when the bed is hot and stable for 5 minutes and you'll see it will stay correct between prints.
Keep in mind that, in my case, the temperature changes the position of the extruder as well: do the leveling when everything is hot, even if without filament inserted.
Edit: if you print both ABS and PLA, a single leveling done at an intermediate temperature will be enough.
Also, be sure that the screws don't move due to vibrations. I used a foam washer to keep them in position. It warps during the first hours of application and then provides good friction.

If you don't have an auto-leveling build plate or a depth gauge attachment for your X-Axis, you're pretty much stuck with the "piece of paper" method. To perform this easier, try shifting the paper as you adjust the bed. As soon as the paper becomes difficult to move (the nozzle is touching now), back off the build plate by about a quarter turn.
Please also keep in mind how your printer is constructed. If your build plate arms are plastic, they are easily susceptible to warping if you have a heated build plate. Even if the warping isn't permanent, the build plate can technically shift during a print as the arms get hotter. Some people go as far as replacing these arms with aluminum versions.

I printed a faceplate for a Reprap X400 to use an electronic drop gage to level the bed:

I use a mechanical one in a snap-on holder myself.


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What could be causing my y axis to slip?
Occasionally, while printing, my y axis will slip and the layer will, from that point forward, be shifted, ruining the print.
What might be the causes of an axis slipping? I have tried cooling the motor which seemed to have been getting warm, and the belts are not too tight.
This does not happen with every print, and seems to be an intermittent problem.
My printer is a MendelMax RepRap, and the y axis is my moving bed.

Hi! What kind of printer do you have? Is your y-axis a moving print bed?

Added to post. RepRap and yes, it is the bed.

The belts being not too tight can be the problem. If I don't tighten my belts, the belt slips off the motor. What sounds does it make when it skips? Also, how much force does the bed require to move?

I know it's an answer to a somewhat different question than you've asked, but one thing you can do to mitigate axis slippage until you can figure out what's causing it is to home the X and Y axes between each layer. This will guarantee that if you slip during one layer, only that layer is off, and the next layer will be lined up correctly again.

FYI, the way stepper motors work is they are always "on", so being pretty warm when even not rotating is normal. That said they can skip steps if too hot, so adding a 4cm heatsink+fan is worth it and isn't hard or expensive.

Hello @Matt Clark, I noticed your question has been up for a while now. Have any of the answers below been able to solve your question? If so, would you mind accepting the appropriate answer. If not, what is missing so that we may help you further? Also, if you have figured it out on your own, you can always answer and accept your own solution. Thank you.

@MattClark, when you post a question like this one, it is very helpful if you can include photographs of the result.  I was confused (nothing new) by the photo in one of the answers, and wrote an answer addressing the particular problems it demonstrates, only to realize during my review that it wasn't your problem!  Pictures help people help you.   If you have found the cause, please accept the best answer.  It we didn't identify it, you might write and accept your own answer.  Our goal is to have a record of great questions and spot-on answers to help the next person.

(source: all3dp.com)
Your printer is skipping steps in the y-direction. This can have several causes. Take a look into Shifted layer guide on RapRap.org which lists 29 possible problems that can cause this issue and how to fix them.
First items of the list:

Driver current is too low
Driver current is too high
Belt too Loose
Belt too Tight
Loose Set Screw/Grub Screw
Belt or Bearing is binding
Speeds are too high
Acceleration is too high
...

When I was dealing with this issue on my RepRap I had to increase current to the particular driver.

Something else that I have also run into that can cause skipping is the filament not spooling smoothly.

worn out/low quality linear bearings and rods they slide on can contribute to 4, 7 and 8.

I had the same problem on my Mendel, and it turned out that speed and acceleration were too high. But as @amra said, there are a lot of thiings that can cause this problem.

That's an awesum print lol

In my experience, the most common reason for positional offset during printing, is the motor skipping steps due to physical impact.
Your stepper motors do not give positional feedback to your printer. So, if you forcefully move your motor during print, then the printer will not notice, and simply pretend it never happened.
In particular, the motor could skip steps if:

Your nozzle collides with erroneous extrusions (e.g. blobs) during print.
Your speed settings (jerk and acceleration) are too high for the mass (inertia) of the parts moved by the y-axis motor.

Smaller collisions and nozzle drag at high speed (e.g. during travel) could also cause this problem, since the strength of stepper motors is reduced at high speeds.

+1 for the note about no positional feedback. This is so frustrating, you spend sometimes thousands of dollars on a machine that doesn't know where it is.

@tbm0115 that is very true indeed! I am hoping for that to change as the treaty printing technology matures!

I thought I heard somewhere that the NEMA motors have a way to know which step they're on, but the controllers only interact with the motors in increments. There might be a way to at least create some form of "endstop" that recognizes the stepper is out of sync and "pauses/stops" the machine.

I believe most stepper motors only have an open loop control, which means that they don't know their own position. However, a closed loop control (with positional feedback) is usually not needed, since they have excellent precision and torque as long as the motor is properly scaled for their application. Also, for some applications - unlike in 3D printing - skipping steps doesn't really matter. You could,  add an encoder to the shaft for providing positional feedback. And just like you say, some steppers also come with such encoders integrated.

@tbm0115 Some stepper drivers can detect skipped steps by sensing variations in the current going to the motor. However, NEMA motors don't have a way to "know" what step they're on. There are no electronics inside, just some coils and magnets.

The motors can also be skipping step simply from overheating

The current to your motor driver could be set either too high or too low. If it's set too low then the torque might not be sufficient and the motor will skip steps. If it's set too high then the driver might overheat and occasionally shut down to protect itself.
Another option is that the printing speeds (or jerk/acceleration settings) are too high. I would start by reducing the travel speed (which presumably is higher than your printing speed) and see if that makes a difference.
The motor getting warm is normal and will not cause these issues.

From what I've experienced, there could be three potential reasons.

Your belt(s) could be loose. Simply loosen your Y-Axis motor and pull the motor until the belt is slightly more than taught (it will relax into a taught position). Then, tighten the motor securely in its place.
One of your axis endstops could be triggered mid-print. If you have a larger print, you run the risk of hitting an endstop, which could cause the machine to lose its coordinate system.
I found on my machine, if you run your program via USB (on MakerWare specifically, possibly others) there might be some sort of lag in the serial connection that could cause the entire program or coordinate system to shift. I repeated this issue multiple time using a USB connection and fixed it (repeatedly) by either running off of an SD card, using a different slicer (in my case the Cura plugin for OctoPi), or trying an earlier version of your software (this was my long term solution).

The latter worked best for me. I tried running MakerBot Desktop on my Dual Replicator 1, but ran into the same exact issue as you. In fact, I encountered this issue around firmware 5.0 on the Replicator as well (7.? is the latest). Finally I switched back to using MakerWare 2.4.? and everything worked fine.

Had Y axis stepping issues
Solved the issue by correct pressure on the guide wheels on the Y-axis track.
Too much pressure caused binding and the Y stepper motor to skip steps
Hope this helps some people

Given the last few questions you have I am going to say that you have too much mass.
F = m * a. If you are trying to move a heavy plate, you will need to reduce the Jerk setting. As well as maximum acceleration.
Post your firmware settings for more advice.
Also just for completion, sometimes the Pololus overheat, that can cause it too. As well as a loose belt.

I had a repeatable problem where my prints were shifting to the side after about 5mm. This was down to a loose z-axis guide rail that would come out of its end support about 5mm into the print, but appeared secure when the bed was set to its initial position. (My print head moves down). There was a grub screw hidden below a panel at the base of my printer.
I'm not familiar with the build of the MendelMax so this may be different for you.

Make sure your controller board/electronics board etc is as cool as possible - if not the axis may jump - that's what happened with me - after adding additional fans over the Ramps/Adruino - I did not have the problem again (so far)

I had the same issue. the only thing that helped me was settings.. uneven surface of layers caused collisions with the nozzle. I adjusted the flow by calibrating my steppers.. also the type of infill pattern you use can cause nozzle collisions.

I have some suggestions that might solve your problem

Try to use belt tensioner which is suitable from your printer.(You'll probably find one on Thingiverse)
The belt has teeth but your bearing which slides your belt does not. So try a bearing cover that has teeth. That will prevent slipping of the belt.
Most importantly lower your acceleration constant. This has a lot to do with missing steps from the motor.
Decreasing the print speed can help as well.

I fixed this problem on our MakerBot replicator yesterday. The extruder got dislodge on a print some time ago and even after re-attaching it, subsequent prints often shifted sideways on the bed unpredictably. I was skeptical that a firmware update was responsible, so I took a look at the carriage.
The carriage that the extruder attaches to has a belt that moves it from side to side. On that belt is a plastic clip about 2” long that is supposed to be attached to the extruder (actually, not the extruder itself, but the device the extruder attaches to). It had become dislodged and so the sideways motion was entirely relying on friction from the belt which would inevitably slip. Once I got the belt clipped onto the carriage properly, I was back in business.

My Y axis runs on a channel and I believe there was some grit or metal flakes in the channel left over from manufacturing. The wheels in the channel got stuck on the debris and caused the belt to slip. It made a horrible grinding noise when this happened.
So I blew out the channel with pressurized air and tested all the wheels.
I'll update if necessary as I test my fix with longer (taller) prints.
Update
Actually, the print had messed up g-code. The gcode file was corrupted.

What is the purpose of the picture to your answer? Do you have the rights to use it?

One possibility is that after some time, your bed's sliding rods become sticky (where printer but not yourself notice).  Turn printer's power off, spray windex on rods and bearings, slide the bed forth and back until it becomes slippery, wipe any excess around, turn power back on.  reconnect printer and send it to home xyz coordinates since you moved bed, messing up its xyz memory.

Why, of all things, windex? Most people recommend machine oil or grease of some kind. But windex!? I wouldn't risk that.

Windex contains: 2-hexoxyethanol, isopropanolamine, sodium dodecylbenzene sulfonate, lauramine oxide, ammonium hydroxide, fragrance, and Liquitint sky blue dye. These seem pretty nasty chemicals... Would isopropyl alcohol suffice? I guess that you are using Windex for its cleaning/de-greasing properties. Regardless, isn't it a good idea to re-instate some lube, for the bearings, after having removed the stickiness?

authors: سمير سعدي حمتو


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How to properly use Polyisocyanurate (PIR) foam with the heated bed?
From what I understand, it takes a really long time for the heated bed to heat up using an MK2a heated bed.  I've heard some people suggest that using Polyisocyanurate (PIR) foam (insulation that takes quite a bit of heat to catch on fire) can be used under the headed bed to make it heat up faster.
Now of course there are other methods for doing this too, for instance using a larger power supply, but at this point I'd rather just use the parts that I have without re-soldering many of the components on the RAMPS board.
I was wondering what a proper way to attach this to the bed would be for instance, should the springs go on the bottom or on the top of the bed?  Do I need some extra parts?  Are there any other considerations for doing this?

An alternative solution that is used with some Kossel Mini models, is a simple cardboard covered with aluminum tape (docs).
This solution has the added benefit of actually improving heat-up time, since the reflective surface of the aluminum tape reflects the heat back into the bed, instead of getting absorbed.
It is also a very simple solution, without costly materials. I guess mounting it properly could be an issue depending on your printer, but this is a fine solution for the Kossel models, at least.

If you use a seperate powersupply for your bed (or if your controller has a built in voltage regulator so it doesn't damage at higher voltages) you can sometimes find an small potentiometer near the connection terminals of your powersupply, turning this potentiometer up can raise the outputvoltage of your powersupply by a few volts max. In my case the potentiometer was orange and allowed me to turn up the voltage by almost 2Volts!
If your printcontroller is running from the same powersupply and is critical about input DON'T do this since it will damage the electronics.
Better option then is to find a higher voltage powersupply and power the bed Seperately.
A buddy of mine runs his MARK2a bed on 19V and it heats up very fast. (he uses an obscure 15V powersupply that was able to be cranked up to 19V). And he doesn't use insulation at all!

Orange?  Do you mean glowing orange?

@leeand00  That would be orange plastic.


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Ideal print bed: Glass or Aluminium?
Glass is always level, easy to clean, easy to work with.
Aluminium allows for the addition of automatic bed leveling with an inductive sensor and distributes heat a little more evenly.
When printing mostly ABS and PLA, which one is better?

Just to note, you can get foil tape to use auto leveling with glass

Interesting...  Do you have a link handy to the exact kind of foil tape you are referencing?

How does the foil tape affect adhesion?

I can't recommend an exact one, but anything that's conductive should work. I got the recommendation from here. Some people just use aluminum foil. Most likely, it would go on the corners or under the bed so adhesion shouldn't matter. Alternatively, you can use a contact sensor, such as the BLTouch, which is material independent.

The guys who make the DeltaPrintr have found a pretty neat solution to auto calibration where they put pressure sensors at each foot of the bed. Although not as straightforward to use with "normal" printer firmware, it means that you no longer need any specific mechanism on the end effector itself.
http://deltaprintr.com/shop/3d-printers/deltaprintr-kit/

How to setup up auto bed tramming leveling tilt compensation using inductive sensor and aluminium bed: https://youtu.be/EcGFLwj0pnA?t=106

Al is a material commonly used in heatsinks, so it's NOT going to distribute heat more evenly, it will cool down rapidly as one moves away from the heat source. Glass is very hesitant to thermal changes, so while it takes longer to warm up, it takes longer to cool down as well, which means the extremities will be closer in temp to the heat source once a thermal equilibrium is achieved.

I believe printing directly on aluminium is unwise, simply because it will expand when heated, typically giving the bed a concave or convex shape. Glass, on the other hand, does not (at least not significantly).
As pointed out in the comments below, the heat expansion of aluminium could potentially be mitigated by increasing the thickness of the bed, as well as heating it evenly. Also, a common solution is to place a glass plate on top of an aluminium bed, at the cost of a slightly longer heat-up time.
In my experience, printing directly on heated glass can be very practical and give a nice surface finish for some PLA variants and other materials that support it. I don't know if printing directly on aluminium can give similar benefits.

It seems that this can be mostly mitigated by getting quality aluminium, 3+mm thickness, and ensuring that the entire plate is heated evenly.  Thoughts?

On the other hand, many 3d printers have an aluminum bed then a glass on top of the aluminum.

@LeoErvin,Chase. Very good points. I have updated the answer.

hairsrpay or abs slurry is the way to go with bare glass and ABS. Or PET tape or Kapton tape on it. The latters also work with bare aluminum but not sure about the former.

Don't forget about glass and painters tape for PLA.

Also some say with glass you have less uniform heat distribution. I don't think that's a problem myself and maybe it has to do with not having a ful size or uniform heating element in the first place.

I print directly on my heated aluminium bed with glue stick on it. It works well for PLA (not tried with ABS). The bed shape don't change (3 mm thick but it's a Delta with a small diameter). I've got a nice surface finish.

Basically having an homogeneous temperature on a aluminium bed is impossible, or possible only at ONE temperature: the sides are losing heat more than the center, it's inevitable. You can have a non-homogeneous heating below it, tuned to heat more on the sides, but it would be optimal only for a specific temperature, the one for which you designed the heating path below. One compromise would be to heat only the circumference, maybe a sort of ring 3mm wide, so that the center is always slightly colder and it is pulled apart/stretched by the sides. At least it will not bend up/down.

I've been printing for years on Aluminium on one of my ptinters, never had any problems with the expansion of Aluminium. Works like a charm! Printing on Aluminium is not very different from glass; I do use 3DLAC as adhesive, but I also use that on glass.

I have aluminium 3 mm thick and 110x110 mm big, and it bends when heated. Inevitably the center becomes higher than the sides. It is not a problem with PLA and nylon, but noticeable with ABS.
I use Kapton on top of it, and the print sticks easily with PLA. ABS requires hairspray on top of it, and nylon requires glue stick (but it fails anyway if the piece is big).
If you use glass with a top layer for proper adherence, you should be fine. Aluminium below could help spreading the heat, but... it will bend and push the glass, therefore you increase the risk of breaking: the glass does not bend much, but internally it is ALREADY withstanding a lot of stress when hot, and aluminium below could push it to the limit.
Edit: it appears however that aluminium beds are in general the best ones:

Aluminium gives low adherence. Probably because its top of surfaces gives temperature fast. But using conductive metals is the right direction. There are some benefits:

Electromagnetic induction allows to heat faster and reach higher temperatures with same current.
Non-transparency allows to use mirror under bed to make heating bonus.

But's:

It will expand when heated.

I tried titan. Here is prooflink. Using 0.5 mm it gave good results on printing with temperature 120 degrees Celsius. But as I had bit crooked plate, I had to stop using it. Needing to heat it up 20 degrees more than glass makes it pointless, unless using microwave. Try borosilicate glass.

I print on Aluminium and on glass on several printers, there is no difference, both work very well; the mirror shine on glass printed parts is a little more beautiful though.

@0scar, it is generally may be good for PLA, yes, but you may experience bends for ABS as it needs higher temperature. Is it 3mm?

Yes it is 3 mm, but it also works for PETG and high temperature co-polyester filaments, no problem.

I have only printed on an aluminum build plate but I agree with Craig Anderson.  I use Marlin RC6 (I really should update to RC7 but I am lazy and RC6 is working well for me) and the mesh bed leveling system.  WELL worth the time to get it setup.  I use blue painter's tape and I print in both PLA and ABS, but mostly ABS.  I have very few issues with the part(s) not sticking to the build plate, actually the problem I have tends to be the opposite LOL!  I started to get good, repeatable results when I switched to mesh bed leveling and then great results when I changed my first layer to be 200 % flow.  YMMV but I print with 110 °C bed and 235 °C print head, use blue painters tape and mesh bed leveling.

I've got a full frame i3 with a glass printing surface, and an Anet A8 with an aluminium surface.
According to my optical thermometer there is a 15 to 20 °C temperature loss between the bed sensor and the glass surface at 65 °C, which is how hot I have to run the i3 to get adhesion with PLA.
I have not tried printing ABS yet on either printer, but I cannot see getting the glass surface temperature high enough to give good adhesion.
I use dilute PVA adhesive as on both surfaces, but it does not adhere well to the aluminium.

If your glass has nothing on it than your stated method of taking the temperature is not accurate due to emissivity of materials.  Basically, the sensor is going to read the temperature of what is reflected off the glass and not the actual temperature of the glass.  You need to coat the glass in something non-reflective to get an accurate temperature or use something that contacts the glass physically. See here for more information - http://www.raytek.com/raytek/en-r0/ireducation/emissivity

Here is a picture that demonstrates the above - http://blog.brixandersen.dk/wp-content/uploads/IR003969.jpg  If you notice the axial resistor which according to the scale is ~300C but the block around it is showing a much lower temperature because it's brass and reflective.  You'll also notice the tip is showing ~200C, this is because it more than likely has plastic smeared on the outside.

@tjb1 The emmisivity of glass is much higher than that of aluminium or brass. The measurement should be fairly accurate. It doesn't have anything to do with reflections: the amount of infrared radiation emitted by an object depends on its material and its temperature. Smooth, metal surfaces happen to have low emmisivity (i.e. do not give off much radiation) and thus are not read accurately, on the other hand glass has rather high emmisivity and should work fine.

Have you tried to match the temperature in between the heatbed thermistor and your optical thermometer ? It might be the case that the thermistor does not yield the same temperature as the thermometer for the same temperature. That might also influence the difference you notice in temperatures.

I use a 200 mm x 200 mm aluminum bed over an aluminum heated plate.  It is attached by bolts in the corners.  I use a proximity probe to run a leveling program and I print on the aluminum.  I don't see where my bed is bowing or causing attachment problems.  I run the leveling program with the plate at temperature (110 °C) and any bowing is handled by the software.  It works very well with ABS.  I haven't used much PLA and am still experimenting.

Hi, welcome to the site! The question asks for a comparison between aluminium and glass. Your answer seems to only focus on aluminium. If you have any experience printing on glass you might include that in your answer as well, as in its current state it doesn't really address the question.

I have used glass from the beginning and it works perfect.
(From old picture frame and old scanner)
I have tried glass with tape,
glass with hairspray and have had very good luck on printing on both.
Prefer hairspray :)
I have tried printing on the heated aluminum, but experienced the aluminum plate tend to warp due to temperature changes.
With a glass plate on top of the aluminum, you have a flat surface that don't warp. :)
So I would give glass thumps up!
EDIT: Printed on Glass with glue-stick. Wow. Sticks really good.

you could always slightly slot the holes on your aluminum bed so its not corner constrained and forced to bow in the middle..
My advice is use both and hairspray
currently testing the effectiveness of insulation on the bottom of the plate

Could this be reworded? As it stands right now it's not answering the question being asked