With bearings and hardware in hand, MPCNC roller assembly was quick and tight.
Each of the four axis rails holds an MPCNC roller assembly. This device rolls along the rail using five bearings. It also holds the gantry structure, a stepper motor and pulleys for the drive belt.
The bearings used in the Mostly Printed CNC are type number 608. These have an 8 mm inner diameter and a 22 mm outer diameter. 608 bearings are commonly known as skateboard bearings, which is why they are easily available and quite inexpensive. Mine cost about thirty-five cents each, ordered from China. I bought the version of these that is sealed on both sides (2RS) which is a good idea for a dirty environment.
The cost effectiveness of the 608 bearings explains why the hardware is sized for 8 mm or 5/16 inch bolts, which is the inner diameter of the bearings. These bearings are most often used in 3D printers, as well.
The bearing bolts are 1.25 inches in length and mate onto nylon locking nuts. You can see in the pictures above the channel for the steel conduit. The bearings will roll along the conduit touching on three sides.
The two top bolts are 2.5 inches long. They hold the top bearings and also idler pulleys for the timing belt. I found that the holes were slightly undersized, but after some light widening with a round file, they were a perfect fit.
MPCNC Roller Assembly – Leave some play in the bearing bolts
According to the instructions for MPCNC roller assembly, it is important to leave a little bit of play in the three bolts holding the lower bearings. “These lock nuts can control tension on the conduit so do not snug them up until it is on the rail.”
If you fully tighten these nuts, there is no ability for the bearings to self-align on the rail. I made this mistake on the first assembly. When I slid in a piece of conduit to test for fit, I noticed that several of the bearings were not flat against the conduit. Rather, they were just touching on an edge. When I backed off the tightness of the nylon locking nut slightly (half turn) the bearings were able to fully and flatly align themselves on the rail.
By the way, I have had a small round file in my toolbox for forty years. Finally, it is getting some use!