Originally Posted by cnczoner  I've measured 0.014" of backlash near the middle of the X-axis of my mill (converted to CNC), and I understand that eliminating that will give me a noticeably better finish in aluminum. I can also see that my circles are not exactly circular, btw.
I need no better than 0.004" error per every 4", (= 0.012" per foot) and from the specs I see on the different options, that is not a problem with any of the options. So eliminating the backlash (as much as reasonably possible) is the only real issue here. Also, I'm going to focus on only the X & Y axes for now.
Existing setup: 23mm (0.91") dia. acme screws. X-Axis total screw length is about 3ft (supported on both ends) and Y-axis is about 2ft. Travels are a bit shy of 20" x 7". Realistically, about 5-10 hrs of use per week, and slow (about 15-20ipm for cutting and 5ipm for finishing).
Possible options...
(1) Anti-backlash nut. I think I understand how these work  but I haven't found any for my thread size, and it seems like it would wear out fast. It would not be easy to get under the base of a 700lb machine regularly, so I don't want to have to replace or adjust it often (it was hell getting the mill up on the workbench with 3 guys and some jacks, etc). |
Have you seen the anti-backlash nuts from
DumpsterCNC? Those might give you some ideas. I think that before you do the ballscrew conversion, you should try to see if you can eliminate the backlash in your acme setup.
I've got an HF MicroMill converted to CNC and the X/Y axis' have little if any backlash on the stock acme screws. The backlash is taken care of using the stock split nut, which is basically a metal block with a slot cut in it and two set screws, and a thrust bearing arrangement at the motor end of the screw.
The thrust bearing was definitely the most critical part of getting rid of the backlash, and tightening it up worked very well. You might go over your setup again and double check if you actually have thrust bearings and that they are properly tightened. I used a small pry bar to snug them up real good...
As far as finding acme screws in that size, I would check McMaster Carr, they stock a lot of acme screws....
HTH,
Chris.