Running screws through a long plastic hole in the MPCNC Z Mount may not work if the holes are not clean.
As shown in the Mostly Printed CNC assembly instructions, the Z Mount is used on the vertical axis to hold together two vertical pipes and a threaded rod. The threaded rod is attached to a stepper motor and is used to move the tool up and down on the Z axis.
The original MPCNC Z Mount is too short vertically. It is 30 mm tall, but there is not enough space for the rigid coupler to clamp onto both the stepper motor shaft and the threaded rod. (I think this is because the currently used stepper motors have longer shafts than the originals.) This is shown above in the center. Ryan Zellars has designed a replacement MPCNC Z Mount that is now 40 mm tall, an additional 10 mm of vertical space. This is shown on the left. Originally I had printed to old shorter mount. When I discovered the spacing problem, I printed the revised taller mount.
The problem I ran into concerned using support material in the vertical holes used to attach the stepper motor. In the future, I will try printing vertical holes like this without support material. Try as I might, I could not clean out these holes properly and the screws kept sticking on the way through the taller mount.
So, what to do?
MPCNC Z Mount Compromise
As shown on the right, I have printed two extensions to raise the height of the smaller mount. These extensions were created by using Meshmixer to plane off about 6 mm height from the original model and then print them. Meshmixer is a great tool for modifying existing 3D models. It imports STL files, among other things.
These extensions are held in place by the through-hole motor attachment screws. I also applied some Superglue. By switching to 16 mm long M3 screws, I am now able to properly attach the motor to the extended MPCNC Z motor mount. With the extra 6mm of clearance, there is enough space to connect in the rigid coupler on the two shafts above the bearing.
Since both the extensions and the original part have three solid plastic layers on the top perimeter, I think this modification will hold up under stress.