What's a good target for backlash?

[alanambrose]

Hi,

Can anybody give some good target numbers for backlash? This is for a Warco WM-14 mill converted a while ago (not by me). The numbers right now are X/Y/Z ~40/80/80 microns. I want to improve the 'lash as I'm seeing some milling artifacts sometimes on curves. Sometimes the newbie doesn't know whether it's the machine or themselves :). X&Y are direct stepper driver, Z is belt and pulleys. To measure the 'lash, I'm moving back and forth +-5mm and using a 1 micron resolution indicator.

TIA, Alan

[routercnc]

If they are ballscrews then the 1605 ones I have measured have about 50 microns of backlash. So this might be a guide depending on what is on your machine. Try pushing the table by hand with a DTI on the end of the ballscrew if you can reach it. This will show you if the play is in the support bearings so you may be able to tighten the nut.

[AndyCD]

Would you still compensate for that measurement in the control software?
Mach 3 has a "backlash compensation" option I've been trying to make work...

[dazp1976]

I got my DFU 1605's down to 20 micron, and that was not easy.
Shimming the DFU nuts with alu foil was a chore, and getting the AC bearings fitted correctly, amongst other little bits n bobs.

I do now have max limits on rapid velocity of <3000mm/min with the DFU nuts being a bit tight, so it's a trade off.

If you want rapids higher than 3000mm/min on 1605's, I think your minimum limit is 40 micron.

[Robin Hewitt]

Software control of backlash requires tight Gibb's.

I prefer to stretch my leadscrews and use two nuts sprung apart with Belleville washers.

Arranged correctly that gives zero backlash in one direction and no backlash, until the springs are overcome, in t'other.

[Muzzer]

Yes, that's a pragmatic solution. As long as the allowable preload is less than the max machining force, it's a neat and simple approach for ballscrew backlash.

It's easy to focus just on the ballscrew but I found on my machines that it's a stackup of several elements including the gibs (the table won't always move in the direction you want otherwise), spring and movement in the connecting elements (ballnut and ballscrew mounts etc) and of course the spindle itself. Unless your movements are friction free, there will also be some degree of springiness too. I managed to get the backlash in my HSK double ballscrews themselves down to under 10um but the total still ends up between 50-80um in some places.

You have to think about springiness as well as lost motion. Most Youtube Warriors who boast about "holding tenths" are measuring backlash with no forces acting, even if they are using a valid measurement technique. Once you apply machining loads, those figures will increase further.

The notion that you can simply dial out backlash in software is magical thinking. When you are climb machining manually, you will find there is a % engagement of the cutter where no significant force is required to feed the work into the cutter. Less engagement results in the work feeding itself, more requires you to push the work into the tool. So backlash will apply one way when you are using more engagement and the other way when using less engagement. The backlash model used in most controllers assumes conventional milling from what I've seen. The only solution is minimise backlash the very best you can. And only count on backlash compensation for conventional milling of shallow cuts (low % engagement) eg finishing passes. However, some cutters don't work well with light cuts, particularly when conventional cutting....