Well this conversation has spiraled down a rabbit hole more times than I could count while reading through the 7 pages
Quinn, you are correct about the DMM drive limitation, I faced the same issue when setting up my servo spindle although I didn't run into a need for ultra fine positioning like you do. Totally agree with your concerns.Unfortunately, your only solution would be to select a drive/servo with a high encoder resolution AND a higher input frequency. I am using Allen Bradley Ultra 3000 drives with AB bulletin MPL motors. Something like 2 million counts per rev with a 4MHz input frequency.
You are also correct about belt stretch. I would be willing to bet that your machining on a part held still by the belt and servo would have pretty poor rigidity even with the wide belt. The problem is that DMM servos (and most servos) don't respond well to high frequency loads like would be encountered during machining (tuning issues) and a belt will never be perfectly rigid. If you were to tension the belt high enough to get acceptable stiffness, you would likely run into problems with side loading of the shafts and issues at higher speeds. You might want to consider a disc brake for clamping the workpiece when indexing.
Last comment on the belt (found through painful experience) is at higher speeds, timing belts suck air into the teeth of the pulley and make a loud pop when exiting. When this happens a few hundred times a second, the belt noise can get pretty crazy. I talked with engineers at SDP-SI and their recommendations were to use a belt with smaller teeth (I changed from a L series to a GT3 series pulley), select a curvilinear tooth profile (GT2, GT3, etc), select a fabric faced belt, and if wider belt widths are required for stiffness or load, use multiple thinner belts in tandem (allowing air to escape between the belts). This coupled with a low belt tension took my spindle from a painful 86dBA to a comfortable 68dBA. This was at 5000rpm.
At the end of the day, tuning the servo to hold acceptable stiffness will most likely contribute 90% of the issue you have with deflection under load compared to the belt.
Don't claim to be an expert on anything here, but my comments come from knowledge gained as an engineer at a major industrial automation company doing servo motor applications (and tuning
).
Lets try to keep everything civil here, it is better for everyone who would want to learn from this conversation in the future.