Backlash - Steppers, Toothed Belts & Ballscrews

I converted my old Henry Milnes vertical mill last year. For those unfamiliar with it it is a very hefty lump. I used ballscrews and a two to one drive x and y from steppers and a bigger stepper with a three to one

to drive the existing acme thread of the knee and carriage on the basis that if I could get backlash lifting going on for a quarter of a ton of knee and carriage then I must be taking too big a cut!

The speed reduction obviously affects the feed and movement speeds but I find it more than adequate for me to keep up with. The ballscrews seem to have totally got rid of bachlash.

Dave Robinson.

David , I used to have a Henry Milnes so I know how heavy they are. Now have a Harrison,which is a bit lighter. The leadscrews are very worn now and I'm going to have to replace them and new nuts.

The X axis traverse is driven by a separate motor which,through gearing,turns the nut while the screw is locked. ... I 'm not sure if that can be replaced so thought of ballscrew and nut with a stepper motor to drive it ??? Sort of CNC in stages.

Everyone seems quite shy when it comes to motor sizes. Can you tell me what size you used ?

some pics would be nice, Thanks, John

Despite buying the parts in 2012 I only found the time to do the conversion this year. The toothed belt drives have worked well (1:1.67) and do not seem to contribute anything I can measure to the backlash.

The Y has an overall backlash of <0.01mm. X is giving a little trouble and backlash is 0.12mm. I think this is down to the preloaded bearing mount. The design of which was determined partly by the machines 1930's origin and partly due to a previous owner who appears to have used a large hammer to smash part of a casting to increase travel! I feel a redesign coming on.

So overall I need not have worried ballscrew+toothed belt+3Nm stepper was a good choice.

The Z is another matter I used a direct drive 11.5Nm stepper. Power is marginal on the ACME . While it runs at speed when the speed is reduced it bangs and bumps and goes nowhere. But the solution will be OT for this thread .

The mill is a CVA 79 and while I don't know the weight of the knee I suspect it is not far short of the Henry Milnes 1/4 ton mentioned above.

Trevor

I have an old Denford Ezymill and the steppers are connected to the lead screws by belt, seems to be fine and it was made in the 80's as far as I know. I'm not running it very fast though and it can climb mill no problem.

Steve

I am about to buy a Centec 2B. The backlash is rfeported as .004". I realise that some of this may be reduced by playing with the lead-screw nut, but can anyone tell me what you'd consider norm al or acceptable for this amount of backlash? A DRO takes care of a lot of the problem, but I think that unusually large backlash would be quite irritating and would limit climb milling options

Hugh

Correction chaps. The backlash isn't 0.004 but 0.04" Seems like a lot to me but I'ved always worked on more or less new Bridgeports, and my Emco FB2 (virtually unused since new) has about 0.002, so I've been spoiled.

I'd be ecstatic if mine was that low. I don't have any problems, but get involved with climb milling apart from just running back at the same setting to improve the finish, I think that is best left to big industrial machines

Your last post beat my reply, 0.04" is a lot. If you can't adjust it out then it's new leadscrew and/or nut time. I've done away with the fast feed pinion on mine, if I were replacing the nut I'd just have it solid, permanently engaged.

Probably sound advice Duncan, since in any case the fast feed capstan gets in the way of things and was deysigned for production, which isn't what I'll be doing. Many thanks. This forum is very helpful.