What size is the lead screw So I am fairly new to 3d Printing and have a question that is probabley stupid. I built a 3D printer kit a while back but now I want a large printer to play with. I would like to build a 300X300X400 clone of a 3D printer. But I am not sure what size lead screws to by they come in 300mm length but once you connect the coupler and the to the screw that takes 20mm's that the axis can not travel too so isn't it more like 280mm length? Or do most people round up to the next size or ???  

I would really like to thanks anyone that can help me in advance Thanks alot
You'll need to do some calculation to figure out how long of a lead screw you need. The best solution would be to mock up the entire printer in CAD so you can visualize how everything fits together. Not only is the coupler going to take up some space, but the nut also takes up some space, and perhaps (due to design constraints) you won't be able to have the nut go up right against the coupler so you'll need some more space. Unfortunately, there isn't a general "just take the length of your Z-axis and add X millimeters"-type formula.