[CNC] Another Backlash Problem

You're not hijacking the thread, this is all good information for others down the road. :)

Yup, just take out the tie bar and unscrew the nuts. Add a shim and screw it back together. That is the same configuration as the Shizuoka has, and we tried to add an aluminum foil (0.002) shim. That was not successful, it just crinkled up. (It was a Sunday afternoon and that's what we had) :) That's when I came up with the idea of the grub screws because of the way the nut mounted in the housing. A hardened SS shim or something a bit tougher than aluminum foil would have worked I think. Available from McMaster.

I have always liked the idea of a screw adjustment, that way you can set it to what you want, and is adjustable in the future to compensate for wear. In your case, that might require a modification to the housing. That's what I will do with mine if needed.
 
You could cut a shim in 1/2 ( ) and try sliding it in there. The Grub screw is brilliant :) Van Norman use to do that on OEM headstock spindle bearings for their Crank Grinders. Was real handy when you adjusted the angular contact thrust bearings. Great thread :) Rich
 
Well I MAY have found the problem. I did a bunch of comparative measurements this morning and found that it was also giving me oversize parts on linear profiles, but only on the X axis. And always 0.010 oversize no matter the size, so it's not proportional to length as would be with the scale factor incorrect. It was adding 0.005 to each side of the part, not just adding 0.010 on one side as you might expect from backlash. I checked the G code and compared the tool path to the drawing and the part, the G code and drawing agreed. Then I checked the table travel against an indicator and found I was getting some strange readings, the DRO and the indicator didn't agree. Hmmmmm..... Backlash is about 0.002 so taking that into account it still didn't explain the readings I was getting.

Keep in mind that this problem developed rather suddenly, it just reared its ugly head the other day.

So I pulled the mag scale read head, cleaned it and reinstalled with 0.005 clearance, I had been running it at 0.015 clearance. I checked the table travel with the indicator again and it seemed to agree with the DRO now. Then did a test cut on the bearing pocket I'm working on. The test cut measurements: X axis = 2.250, Y axis = 2.250 :encourage:

I need to do a bit more testing before I'm going to say it's fixed, but it looks promising.
 
Last edited:
I think it's fixed, at least as far as I can go for now. I made a number of 0.010 test cuts using a few different cutting scenarios and the pocket is running about 0.0004 out of round at the 4-10 o'clock position but otherwise on the programmed size and round. The out of roundness at the 4-10 position has always been there, it's better now than it was. This also eliminated software rounding error as a possibility, there is a lot of math going on internally to cut circles and rounding errors start adding up.

I can live with it for now. When I complete the current project I'm going to tear into the machine and fix the ball screws.
 
Last edited:
JimDawson said:
The next time you decide to tear into your machine take a picture of the ball nuts and post it. There are a few different types so the way you attack the problem depends on what you have. On Alloy's Shizuoka we worked a little magic on the X ball nuts to make them easily adjustable, they were the shim type also and nothing we did would adjust them correctly. But we removed the shims and added 4 grub screws to the securing flange, this allowed us to adjust to the correct preload.

You can just see 3 of the four grub screws a the 3, 6, and 9 o'clock positions. Sorry, this is the only picture that I have of the ball screw, we didn't get any when it was on the bench.
View attachment 274169
Click to expand...
Hi Jim! Probably a shot in the dark this thread is still active at but I'm literally at this stage digging into my Shizouka AN-S. I've got a bit of slop in the X-axis, and I seem to remember stumbling onto a thread with you and Alloy mentioning that you worked some magic to remove the backlash . So am I correct in assuming that there is some adjustment to remove the backlash? I've not pulled the table off yet, and thought if there was play in the ballscrew I'd just get one moving and on it's way. Then I found this post ! Could you explain any more about the ballscrew in this machine?
Thanks again and I appreciate any insight you can give. Looking forward to posting progress as I move thought this machine.

I've removed the old bandit controller from the machine and currently have 6 chickens using it as a coop outside the barn and they have lots of room to spare.
 
Welcome aboard Al.

Well, this thread has been around awhile, but it's still alive. As you can see from the picture, we added the 4 grub screws in the hub of the flange. I can't remember exactly what it all looks like in there, but the assembly does allow the nuts to be tensioned against (or away from) each other. The original design was that the nuts are shimmed to set the preload, but the grub screws take the shims place.

You have to move the table to the left far enough to access the nut housing and be able to remove the cap screws securing the housing. The ballscrew slides out to the right.

We never completely removed the table, and supported it with a floor jack with some cribbing on it.

I'll try to answer any questions you have.
 
Thanks Jim!

I appreciate you replying! I bought this machine and had it trucked a 1000 miles here. We lifted if with a crane and set it on a 1/2 piece of plate steel then pushed it into the shop. I don't think I've ever seen anything so caked with dirt and oil before. haha. I'll post pics of it and I was thinking about starting a thread on it here. It's got the same tool changer on it and I've been searching for any info on that as well. I'm going to go Mach 4 industrial for the controller with new servos. Your changes to the tool changer were excellent. Did it end up working the way you wanted? I've heard them called "tool chuckers" almost as many times as I've seen them discussed but I'm hoping I can make use of it even if it's just as a library that I can rotate the tools through if I have to do manual tool changes. Thanks for the advice on how to remove the ballscrew- I'll get the engine hoist out tomorrow and get the table pushed all the way over to the left.

Again, thank you for the reply!

Al
 
Al Wooley said:
Thanks Jim!

I appreciate you replying! I bought this machine and had it trucked a 1000 miles here. We lifted if with a crane and set it on a 1/2 piece of plate steel then pushed it into the shop. I don't think I've ever seen anything so caked with dirt and oil before. haha. I'll post pics of it and I was thinking about starting a thread on it here. It's got the same tool changer on it and I've been searching for any info on that as well. I'm going to go Mach 4 industrial for the controller with new servos. Your changes to the tool changer were excellent. Did it end up working the way you wanted? I've heard them called "tool chuckers" almost as many times as I've seen them discussed but I'm hoping I can make use of it even if it's just as a library that I can rotate the tools through if I have to do manual tool changes. Thanks for the advice on how to remove the ballscrew- I'll get the engine hoist out tomorrow and get the table pushed all the way over to the left.

Again, thank you for the reply!

Al
Click to expand...

My pleasure.

I think the last time that tool changer changed a tool was in the video of it running. It does work. That machine has only had about 20 minutes of spindle time on it since the retrofit was done. Just really have no use for the machine.

Yes, please start a thread to document your rebuild. We like pictures :)
 
You must log in or register to reply here.
Back
Top