Ballscrew pitch accuracy question

[pstockley]

I have a chance to pick up a nice ground ballscrew that I can use on my Hardinge lathe Z axis. The only problem is that it is 2tpi which is a bit of a course pitch. The servos I will be installing will have 2500 count encoders and with a 4:1 reduction using timing pulleys, I get a resolution of 0.00005" which is more than adequate. However, I am worried about inaccuracies caused by backlash in either the ballscrew nut, bearings or timing pulleys. This will be exaggerated by the high pitch. I was originally going to go with a 5tpi pitch ballscrew but the price is right on this one. I am looking for an overall accuracy from this lathe of about 0.0005". Does this seem reasonable with this ballscrew? What kind of pitch do commercial lathes run?

[fkaCarel]

Don't do it. If you compare 5 and 2 tpi you need a 2.5 times higher torque at the ballscrew to achieve the same at the slide. The choice of pulley's are dictated by the edges of availability, so it will be around 18/72. The big pulley will be monstruous, a 72XL pulley is already more than 4.5 inch. On the small pulley you get a low toothcount, which also handicaps resolution. General advice is a minimum of 5-6 tooth in grip with the pulley, but this is about stress, not backlash under stress. A 5tpi screw will give lower torque at the ballscrew and a better design for the reduction. Be kind for your lathe and spend some more.

Carel