Split/edit part in FreeCAD? I'm trying to modify some parts on this thing. I'm kind of new to dealing with the modeling aspects of this.

I am attempting to widen the part essentially 3mm. This would involve modifying the chain segments, and the anchor points.

I have already attempted this with 3D Builder and the result was less than stellar. I split the part, cloned off about 3mm of the middle of the X-Chain (including the top &amp; bottom bits). The intent was to break the part, translate one of the halves off 3mm, add back the 3mm into the gap.

When I put all the parts back together and merged them in 3D Builder, the resultant model, while visually on screen appeared to be one piece, the slicing proved otherwise.

Is there a way that I can do the above in FreeCAD? I'm learning this so tutorial links would be more than sufficient. But if there is someone who can explain this process to me in FreeCAD that would also be appreciated.
When you want to change an STL file in FreeCAD then this is how I do it. I open the STL file in FreeCAD and select the option in the Part/Component sub-menu: Make a form from mesh (hope that I translated it well. I use the Dutch version). It can take quite some time but when it is finished you have an editable object. Just remove the STL object and do what you want.

When you finished then you can perform all actions like if you created it from scratch with FreeCAD.

I am a beginner in 3D design so there might be 'smarter' solutions. When I want 1 special part then I create as many cubes as needed and perform a MINUS action on the solid and a cube. After a while I end up with only the part I need.