[Richard-TX]

I have a common Grizzly/HF 12x36 taiwan lathe. If do some fairly precise machining with it. I have to hold tolerances of +0 -.0005 on some parts and other parts are +.00025 -0

In considering a CNC conversion, I want to keep the ability to operate it as a manual lathe too.

I was wondering how I can achieve this level of accuracy with a CNC conversion. The thought occurred to me that a CNC controller reading from a glass scale DRO should get me the accuracy I need. Since the accuracy of most glass scale DROs are pretty good, it seems that an expensive ball screw would not be needed.

Am I thinking about this wrong?

[TonyW]

I have a common Grizzly/HF 12x36 taiwan lathe. If do some fairly precise machining with it. I have to hold tolerances of +0 -.0005 on some parts and other parts are +.00025 -0

In considering a CNC conversion, I want to keep the ability to operate it as a manual lathe too.

I was wondering how I can achieve this level of accuracy with a CNC conversion. The thought occurred to me that a CNC controller reading from a glass scale DRO should get me the accuracy I need. Since the accuracy of most glass scale DROs are pretty good, it seems that an expensive ball screw would not be needed.

Am I thinking about this wrong?

Generally speaking, using a lead-screw and nut for CNC is not a good idea. You will always end up with end play and backlash problems. For simple CNC operations that may work, but the tolerances you specified really do call for using ball-screws. Ball-screws will have zero backlash and a positive preload on the screw. You can still have manual controls with the ball-screws, (Such as Milltronics does on their machines). It has spring loaded handles that release when CNC moves them.

As for glass scales, it can be ok but not optimal. The feedback you get would have to be handed off to your servos, and with the backlash that you would end up with on a lead-screw, you would not be able to hold those tolerances. Just easier to install servos with encoders.

By the way, not trying to toot my own horn, but have built machines for almost 25 years. Have put together a few CNC/NC machines in that time. Hope this helped a little