Do any R&P users have measurements on the positioning precision that they could provide? Direct and backlash-corrected information would be very helpful.
Mike Everman's ServoBelt literature quotes +-0.0024" repeatability; it would be interesting to see how this compares to the R&P devices that are becoming popular.
I am sure you are right; high accuracy always costs more, for any drive system. That is why it would be interesting to know if the R&P builders have characterized this, to compare the accuracy vs price point for these two high-speed drive systems.
From what I understand, helical r&p can achieve higher accuracy/precision due to the fact that there is more contact between the pinion and rack... I know that some of the larger gatry/portal style mills use helical r&p, with the higher end machines also incorporating linear encoders.
if u say to go to 12.003" position on the r&p but it goes to 12.004" and has 0 position error & sufficient resolution in the control system, that is accuracy (.001"). whether it is cummulative or not is a function of the particular units design - it can be either.
if u go to 12.004", no matter where you told it to go, go somewhere else, then command move back to 12.004 and it goes to 12.003 instead, that is repeatability.
repeatability is a function of the control system and feedback device (resolution & accuracy of say linear scale or encoder or resolver), not the r&p. only way r&p effects repeatability is if it has backlash that gets in the way.