When I assembled the gantry and tested the motion for the first time, I was a bit dissapointed because it seemed to be somewhat jerky. Something wasn't quite right. After playing around with some things I found the cause. It seems the nylon nuts were not sitting quite square to the 1/4-20 pan head bolts used as the shafts of the idler pulley assemblies. This caused the shaft not to sit quite in a co-axial arrangement with the bearings which was causing the pulley to wobble. Further, the pulley bore doesn't quite match the OD of the threads of the bolt, so when the set screw is put in, it pushes the pulley off center. In other machines I have built, the shaft colars are about 0.001- 0.002" larger than the shaft and would fit very snugly (almost hard to get on sometimes). If they were at all loose, I would just touch the shaft with some sand paper to raise some burs. I once cut a shaft much to small that was very important and the machinist helped me fix it by carefully knurling the surface to make the part snug again. Well, not having that, I thought about what I had around the house to bring up the OD of the threads. What I found that worked very well was masking tape. I carefully cut a single layer of masking tape to fit around the shaft. It brought the OD up to the point where it took a bit of effort to push the bolt through the center fo the bearings. It stays on its own without any need for a retaining nut. It actually takes some doing to get it out. It does not seem to move when in operation even if the pulley is far away from the bearing. This also did the trick to keep the pulley concentric.
There is now almost no wobble in the idler pulley. The only way we are going to get better is by replacing the bolt with a ground shaft and locking collars, which I may do someday. I am concerned that the threads will end up cutting through the tape over time, thus loosing the shimming affect and introducing wobble again. It is important for the axis to move smoothly because these steppers are small and on fast rapids they could miss stepps if the wobble is particularly bad. Upgrading to a ground shaft with lock colars wouldn't be very expensive. I would be surprised if the whole thing would cost more than $10.
Here is a photo of the assembled arrangement.
- Tape Shimming idler pulley
Sorry, the photo is a bit out of focus, but you can see a slight line in the tape. That is actually where I cut the tape so that there isn't overlap. I will keep you posted how this solution works out over time. I have my doubts about the tape holdin it's own, but we'll see. Let me know if you have any other ideas.