Options to eliminate backlash?

[Zumba]

A dial indicator placed at the end of the leadscrew will tell you whether it's flopping about (when the direction changes). There should be no movement.

[ckm]

Originally Posted by cnczoner Isn't this something I can test for? I expect that if I do my regular backlash test (bring the spindle to zero position from both directions and measure the difference with a dial indicator), then I could measure the play/slop in the thrust bearing with a dial indicator on the end of the leadscrew....right? Because if this really is the main problem, yet it would be a no-brainer to try this first.

As far as I know (and that's not much...), you can test for backlash overall, but there is no way to seperate backlash from the nut vs the thrust bearing. You can try tightening down the bearing nut until it's almost unmovable and see if that eliminates most of the backlash, I guess.

All I know is that in my mill, the thrust bearing (or lack there of) was a major source of backlash.

Chris.

[NC Cams]

It may interest you to know that even preloaded ball screw support bearings fitted to GROUND Hiwin's will result in pretty sloppy backlash depending on the condition and tune of associated hardware.

Do a search for "EzTrak" on the Zone. In my earlier postings, I explain in chapter and verse how I went about methodically eliminating the slop from my mill. Yes, this is a bit different than yours but the methodology remains the same - get rid of sloppy, motion losing and/or stifling members.

First, you need to establish a budget - then you need to select the best parts that are within your budget and proceed methodically.

For ball screw thrust bearings, you want THE HIGHEST CONTACT ANGLE, HIGHEST PRELOAD PAIR OF BEARINGS YOU CAN AFFORD. These will pretty much eliminate the bearings as slop generators - period paragraph.

After that, you'll find that ball nut preloading adds a bit of backlash and/strangely enough, gibb tighness and/or looseness adds a bunch more - even with horribly expensive ball screw bearings and/or ground screws.

Getting to a thou or less is not that difficult. Getting to 0.0005" takes a lot of detailed work and good parts. Oversized balls may improve backlash potential on a rolled screw but it won't do anything to fix lead error - only a ground screw will do that.

Check with Hiwin as they also sell ground screw retrofits for many mills. Once you run/operate a mill with deadly smooth and backlash free lead/ball screws, you'll never want to run something sloppy again.

And don't forget to adjust the gibbs - you'll be amazed at what gibb tuning does for backlash elimination.

[cnczoner]

Still on this, though I've been away for a bit recently. I still have not checked the slop etc, and I need to re-build the power supply as the one in the controller got fried by a power outage/surge last week during a thunderstorm. But it was a cheezy 24V switcher and I had already ordered the parts to build a linear 70V, 10A supply.

I did setup an automated ebay search for ball screws and so far nothing in my range that I like. But it does help me identify brands that I can then research. So far, double nuts cost far far more than single nuts.

Zumba, you mentioned "about $12/foot for the screws, $120 per nut". What brand or source is this, if the nut really is preloaded?

Cheers,
-Neil.

[cnczoner]

BTW, is there a simple way to make a preloaded double-nut using 2 single nuts? Easiest design I can think of would be to have one with the RH flange threads and the other with LH flange threads, then have a F-F coupler with LH and RH threads which would be used to adjust the preload. But flange threads only seem to come in one direction. Other thought would be to have flanges on both nuts, then run two screws through them on either side (with the screw threaded on one, and free to slide on the other), and a spring between them on the screw. Any better/simpler/proven ideas?

[Zumba]

Neil, the last time I checked, Rockford sold their double preloaded nuts for around those prices, give or take. Nook might be marginally more expensive, but not much.

Yes you can make your own preloaded assembly using two single nuts, but you need two flanges. I make my own flanges so they cost me about 3 bucks in materials using A36 steel. If you buy them, they're like $30 and up. Ouch.

You don't need to mess with RH/LH. They should both be RH. The flanges are locked to the nut with a setscrew. In order to preload, one nut remains fixed (flange securely bolted) while the other is preloaded using disc springs of some sort. How much preload you need is up to trial and error. More preload = less chance of backlash from heavy loads, at the expensive of added friction. Calculate your loads to figure out just how much preload you need. If the combined load of acceleration + friction + cutting force is say 50 lbs at max speed, no need to add 200 lbs of preload.

Your thrust bearings should be preloaded the same amount.

[tpworks]

plan to make your own preloaded ball nuts
http://www.homecnc.info/ballnuts.htm

Tom