garageFab.cc

Another attempt to kill several birds with one stone:  

• Test belt-driven mini-router design
• Test solutions for motor coupling backlash (Oldham coupling, pulley on motor shaft)
• Drill the cutting mat in a few hours

The mat drilling contraption is basically the new belt-driven design of the mini-router, with the exception of Z-axis which is also belt-driven (mini-router still uses leadscrew on Z-axis). Dremel flex shaft with the drill chuck and 1/32" bit is mounted to Z axis. The results are rather mixed. 

X and Y axis have no problem moving at up to 100 IPM, and perhaps faster, which is pretty good. On the other hand, there is about 1/32" of backlash in both axes. The backlash is coming from a very small belt slack. I suppose tensioning the belts really tight should reduce it, but the bigger culprit here is relatively high friction of the sliding elements, which makes it a problem to accurately move anything except for a very light stage. This seems to be the case even when the sliding element pads are completely relaxed, however sometimes misalignment can produce this extra friction (e.g. two parallel rails are a bit closer/farther to/from each other than their sliding elements).

The backlash appears to be more pronounced on Y axis where the motor drives the upper belt which transmits the motion through the threaded rod to the lower belt, essentially doubling up the belt slack. X axis is driven via the Oldham coulping and even though the load on it is heavier and the belts are longer, the backlash on it is the same if not less than on Y axis. The backlash is visible in the hole pattern on the mat - the rows of holes are in Y direction. Compare to the job done by the leadscrew driven mini-router.

Still, I would have drilled the mat if not the other problem on the belt driven Z stage. It is also related to friction. To shorten the drill time, the down stroke feedrate is 20 IPM and the up stroke feedrate is 100 IPM. To avoid stalls on a lifting motion at this feedrate, sliding element should have very little friction with the rail. Unfortunately, this comes at the expense of a large slop in the sliding element, to the point that the drill bit visibly wobbles and bends on the down stroke. Attempts to tighten up the sliding element resulted in Z axis stalling on up stroke, not pulling out the drill bit completely and eventually snapping it - see the streaks on the mat.

I think I'll resort to using the earlier 80/20 compatible bearing-on-rod design for the Z stage; also hopefully will be able to tighten the belts more and see if this reduces/elmininates the backlash.