Elliott Omnimill backlash

I have an Elliott Omnimill that I use often and have added a DRO which I thought would help with the backlash in the slides, it does but it still a bit of an issue. Some years ago on this site I think there was some info on modifying the nuts or perhaps making new ones. I don’t want to start dismantling the mill until I know a bit more and have decided what to do about the problem. I am very tempted to make acetal nuts by the “heat melting” method using my lead screws as a pattern but can’t see how I’d mount the newly made nut as it would be essentially a round tube shape.

Anybody got any thoughts on the best way forward or do you think the actual lead screws will be too worn.

The acetal nuts will be more forgiving of leadscrew wear, so they would be a good way to start the renovation.

I have recently done some work on a Tom Senior X axis leadscrew which is 3/4 X 5 ACME. Firstly, I extended the thread by about 4 inches at one end to allow the fitting of two nuts for backlash reduction. Then I had to rework the entire length of thread to match the wear in the centre part. There was no visible wear, but having adjustable backlash without attending to the wear variation was not an option. I was single pointing and using the travelling steady on the lathe to get the insert to just rub the flanks of the most worn parts, to cut the less worn areas to match. It is very easy to line up the tool when the thread pitch is so coarse, so the work was done in several sections, and on both sides of the thread. It has been quite successful, the leadscrew runs right through the nuts smoothly without any discernible backlash, although I have not tested it properly yet. The old nut had about 0.030" backlash and looked like a vee thread, there was no flat top of the thread left.

This will be a job for me soon as well. Too much backlash in a x axis. I`m going to look at fitting a ballscrew but it might be too much work so other choices are make a complete new nut or make a nut holder which will take a separate nut made from round bar. Let us know what route you take and how you get on.

How much backlash? It's difficult to make suggestions without the size of the problem being quantified.

Andrew

Backlash should not be an issue, particularly while using a dro, as long as you are not trying to climb-mill. That is unless the backlash is so much that failure of the nut is imminent.

As well as Andrew’s point, the difference between mid point travel and the extremes would be a useful piece of information. It would indicate whether the wear is just the feed screw nut or the screw as well. Little point, or short-term repair, of fitting a new nut to a badly worn screw.

It would be good to know that acetal can resist climb-milling forces when used for a mill feed screw nut. I have my doubts on this but would love to be proved wrong. Lathe feed screws are generally a different ball game in this respect.

Thanks for the Replies so far. I've had a measure. It seems that the amount of backlash is consistent over the whole movement of the table. I have measured it as about 18 thou (0.4mm) . I moved the hand wheel to the left then right watching the dro as soon as the display changed I noted the reading on the hand wheel it was 18 thou where ever I tried. The next question would be have I adjusted the lead screw thrust races and the answer is yes I have they are as tight as I would want to go. So it does look as if I'll have to investigate further, I cant see that wear on the lead screw would be consistent over the whole length so with out a bit more investigation I'm no further forward. I'm sure you all know how it goes you just put up with it until one rainy day when boredom has set in you decide to investigate further.

The mill has seen some use before I bought it, all I have done is add inverters to run the 3 phase motors after digging out the star points. It's a good machine but I have difficulty keeping the vertical head part vertical as it all pivots and slides on a circular ram, and speed changing by belts is not ideal.

Ashley

I just tried the same test on my Bridgeport plus DRO; got 0.8mm (26 thou) on X and 1.2mm (47 thou) on Y. It doesn't stop me working to a thou or so if needs be. So I'd stop chasing a non-problem and just make parts!

Andrew

Don't forget that backlash can also be due to end float in the leadscrew.