Ball screws, backlash is BAD! rebuild? run it? nut 101

[bob1112]

We bought an older 96 Haas VF1 to put in my smaller shop. We were just leveling things and testing accuracies when we found that Z and Y have a bunch of slop from something. I was getting about .0015 in the Y and .0035 on the Z just with an indicator. This is with the factory preset comps in the control! I really wish I had a better way to check backlash.

We have really kind of narrowed down to ball screws and nuts. I can get very little axial movement (.0002) out of the lead screw and you can bring the table all the way back and try to move the table in Y with no movement. If I put it in the center, you can hear it move a bit. The indicator showed .0025 or so. We are is a time and financial pinch right now so wondering if there are any ways to cheat to get some lash out??? I was hoping if the nut balls are softer than the screw, I might be able to reload the nuts for now. I have one pulled back but cannot figure out how it would even come apart or be loaded.

I would like to just pull the end off the screw, unthread the the nut and reload. I know, sounds good in theory. I would also like to know if we can even make a decent part on this machine as is? We do not need to hold .001 but if finish and repeatability will be a big problem, we may need to do it.

Where would I go for a rebuild if needed? How much $? Never had to deal with this. Thanks guys.

[schbrownie]

Haas is known in the industry for their great customer service and their high quality factory technicians. You can call them and they will give you a quote for replacing the ball screw in your machine.

Remember... moving components are wear components and eventually need to be replaced. If you have an older machine, it just might be time to replace the screws... especially if you purchased the machine used--who knows if the previous owner did any of the preventative maintenance!!! (A few drops of oil each day really can save you thousands of dollars in repairs) It's possible that you'll be able to live with the runout that's in your machine--especially if you don't have to hold +/-0.002" tolerances. You might end up with a slight start/stop line wherever your start/stop cutting (except in the case of a sharp corner), but you can polish this out with some sand paper or emory cloth.

I think there might also be some settings deep inside the controller to adjust for backlash, and that might be all you need to do... but I'm not sure exactly how to do it. Check the machine's manual (or download one from Haascnc.com)

Haas Customer Service Website:
http://www.haascnc.com/about_service.asp#service
Service Dept. Toll-free Number: (888) 817-HAAS or (805) 278-8500


Good Luck

~Jon

[drwc]

I am no expert, but i wonder if the preload on your ballnuts can be adjusted? That is the means that backlash is eliminated on the ballscrews that i am familiar with.

Good luck
Wayne

[bob1112]

I used to think you could tune the nuts but that simply is not the case with most designs. I was just thinking about how to make a nut adjustable. Hell, that would be king SH&* on the Mori and Makino stuff to keep things tight.

The fact is, it would be like tightening the race of a bearing up. VERY precise. We have the nuts out and already did some ball swapping and reinstalled one nut to test it will good results. Went from .0035 backlash to .0018. Looking better.

Just gotta find a ball company that specializes in odd sizes that that.