Charter Oak mill x-axis backlash help

**mattinaustin** · 03-27-2023, 02:15 PM

Hello,

I have had an old IH manual mill since 2007 and regretted never getting the CNC kit which led me to purchasing a used Charter Oak made around 2014 that seemed all ready to go. Once I got it set up and had a chance to test it it turns out the x axis was out at least .004 or more on a 2" pocket which I did a finish pass on. I found that I could turn the screw and it took a bit for the saddle to engage and move. Based on what I have read here it is either the preload got dropped on that screw or maybe there is a worn out ball nut. I believe this mill has only seen around 50 hours of actual machine time so I am guessing it is the preload. I do think the gibs could have been too tight, but not sure.

My challenge is how do I best access the nuts to actually check? I am a novice at this so not sure if I am going about it the right way. I removed the servo, the encoder (was harder to get off the shaft that I thought it would be), and now I think I need to remove the pulley which seems tightly pressed on. Before putting more effort into that I wanted to check here for any help. Attached are some pics that might help.

Here is a video of the slop in turning the pulley before it meets resistance. The screw is turning and tight in tandem with the pulley, but the saddle doesn't move until the slack is taken up. Also, I have tried this with the gib removed with no difference. https://youtube.com/shorts/_h0jAzcZsVM?feature=share

Any thoughts?

Thanks,
Matt

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**CNCMAN172** · 03-27-2023, 05:56 PM

Based on your photos and video you clearly have backlash. But this looks like a belt drive system not a ballscrew system?

**mattinaustin** · 03-27-2023, 06:22 PM

Definitely ball screw. Servo driven with encoder on the screw drive, not the motor so it shouldn't be negatively impacted by a loose belt.

I tightened that flange, but it didn't change anything. The lost motion seems like its either in ball nut engagement or maybe at the bearings. I can't get that pulley off though. I think it might have Loctite on it. There is hardly any resistance when the screw turns in the range I show in that video and then it hits and moves very smoothly.

**mattinaustin** · 03-29-2023, 10:07 AM

I think this sprocket might be threaded on. I will try to use a band wrench on it. It might also be the fasteners for the ball nut. I will update if I figure it out.

**mattinaustin** · 03-29-2023, 04:30 PM

Yep, the pulley is threaded on and has set screws that just go into the threads. I think it had a mild loctite on it, not sure what though. Tightened it just a little and the backlash went away. It got a little too tight though. There was a shim in between the bearings that was a bit chewed up. I ordered a new one and will re-tension and hopefully that will be that. I did not inspect the bearings, hopefully that is something I won't regret. I didn't want to lose their alignment so just left them be. I found some ring shim stock from McMaster that will save the hassle of making my own with a drill and snips.

Any recommendations for which type of loctite to use on this given it is moving a lot, but I also want to be able to adjust it without too much hassle in the future?

Thanks!