I think accuracy and resolution are being confused here. Accuracy is solely dependent on the lead accuracy of a particular screw, and not on its pitch or how many revolutions the motor may make per turn of the screw. Speed reduction via gearing or belts, or the use of a fine pitch screw doesn't increase accuracy. It increases the apparent resolution, but this can be misleading in that you're showing resolution to .0001 yet the position may not be within .005 of the indicated position. General statements cannot be made about the lead accuracy of ball screws versus lead screws either. For example, I have a manual jig borer with ground Acme screws that will equal or exceed the lead accuracy of the ball screws in any CNC machine I own, and these are CNC machines that come with 6 figure pricetags. I've made a lot of crossfeed screws for large engine lathes one of my customers owns. Although these machines all have DRO's on them, we still use .003/ft acme rod to make the screws instead of the cheaper .009 material. That way, if something happens to the DRO, the operator can simply continue by reading the dials and know he's getting reliable readings. You can usually make an Acme lead nut with less backlash that the ones readily available for purchase. The typical commercial acme nut is tapped rather than having the thread chased. By grinding a threading tool to the proper Acme form but leaving it a bit narrow, you can chase the thread to depth, and then by advancing the start point, either via the compound or via tool offsets if you're using a turning center, and gradually widen the thread until the matching screw will thread thru the nut with firm hand force. Running the two back and forth a few times will burnish the thread in the nut and the resulting combination will have very little backlash. Works great for a good tight manual machine, but still not adequate for a cnc mill or lathe IMO. Regarding accuracy and resolution, always remember, in the vast majority of CNC setups, the position you read on the screen is, in actuality, simply the position the control commanded the motor to go to.  |