Question about leadscrew

Mostly Printed CNC - MPCNC Advice
1

Hello everybody :wink:

I dont understand the mounting of the brass nut for the T8 leadscrew. (I am currtently building a MPCNC and have no printed everything, starting the assembly). Are really the M3 screws directly cut into the plastic the only thing holding down the brass nut?? What happens, when you move the mill down to cut into the material? There is some force to dig into the material, and this force tries to pull out these tiny M3 screws with the brass nut, doesn’t it?

(sorry for my english - I am from Austria)

 

Thanks a lot!

2

Gravity and the Z axis pushing down on the brass nut holds it in place in the Z axis (vertically). The M3 screws keep it from spinning in place. The middle assembly the brass nut attaches to only moves in X and Y, not Z. The Z axis lead screw pushes against the brass nut to raise or lower the Z axis. Unlike a typical i3 (Prusa) style 3D printer where the lead screw remains in place and pushes the brass nut up, on the MPCNC, the brass nut remains in place and the threaded rod pushes itself (taking the rest of the Z assembly with it) up. So the brass nut is never being pulled away from the middle assembly. It is always being pushed against it. So the M3 screws are not need to attach the brass nut to the middle assembly, just to keep it from spinning.

I hope I repeated that in enough ways to get past any language barrier!

 

3

I am assembling a new Prusa I3 MK3 at home. I was surprised to see that they still have those t8 nuts on top. It makes so much more sense to put the nut under the assembly and let gravity help hold it all together. Granted on a printer there is less weight so you still need to rely on the M3 screws and nuts so it makes little difference.

4

Thank you for your quick replies!

So you mean the weight of the vertical moving Z parts is always resting on the nut holding it down. Ok! It would only be forced upwards if the “digging in force” during the cut would be greater than the weight of axis and the mill - which is probably not gonna happen right?

 

I’ve seen pictures where the nut is on top of the XYZ part and some where it is on the bottom. The top is correct, right?

5

Top is correct. If it was on the bottom, and those little screws let go, the whole assembly would pretty much fall straight through whatever you were cutting, then start a fire.