Chasing Backlash

I'm resurrecting this thread because backlash has reared it's ugly head, again. While .0008" of backlash was acceptable after I chased my tail a couple of years ago it has come back to haunt me. As some of you know I recently moved and as of a couple of weeks ago had my machines up and running, except for the CNC mill. Finally got it operational this past week. Last time I used it was in late February and at that time the quality of my finished parts needed some attention. The first order of business was to tune it up before making chips.

First I adjusted the X, Y and Z gibs by snugging them up until the motors missed steps then I backed off the gib adjustment screw until each axis ran throughout it's travel smoothly and without missing steps. I found that backing off the adjuster 1/2 to 1 turn was about right. Next step was to check backlash. Whoa! .0065" of backlash on the X axis and .008" on the Y. I had a dial indicator mounted on the table with the tip contacting the spindle. Jogged the table to remove backlash, set the DI to "0", set Mach3 jog increment to .100" and jogged the table. The Mach3 DRO read +0.1000" and the dial indicator made one full revolution to "0". I then jogged the table in the "-" direction. The Mach3 DRO read 0.0000" and the DI moved .0935". Checked it several times and it was repeatable. So where is the backlash coming from? Tight gibs, incorrect AC bearing preload, loose drive coupling, bearing/motor mounts flexing?

I decided to check the AC bearing preload so I disassembled the X and Y bearings housings. The bearings looked good and were smooth rotating by hand. My goal was to get .004" to .006" of preload. I installed the bearings and inner race spacers in the bearing housing with a .002" to .005" shim between the outer races and tightened the retaining nut. Measuring with a dial indicator I noted the lateral movement of the ball screw and then added a shim the thickness of the lateral movement plus .004" to .006" shim for preload.

Well, this kind of worked. I assembled the the X axis and still got .0065" difference in readings between my DI and the Mach DRO. Next step was to check my bearing housing/motor mount for flexing. All was good there; no detectable movement with a .0005" DTI. Now it's gib adjusting time. I backed the front gib adjusting screw out 1/2 turn and snugged up the rear screw. Did my backlash check and got .005". This is looking up! I backed the gib adjuster out another 1/2 turn. Checked backlash and it got better, .0038". Another 1/2 turn out, ran the test and backlash stayed at .0038". Ran the adjuster in 1/2 turn and checked backlash at .0038". Scratching my head trying to figure out where the slop could be when it hit me that I didn't do a final ball screw lateral check after the last AC bearing shim pack adjustment. I mounted my DI to the table and set the needle on the end of the ball screw. Jogged it in one direction, set the DI to zero, and jogged it the opposite direction. Voila! .003" of ball screw movement. I'll add a shim tomorrow and re-check before moving on to the Y axis.

I'm writing this long description to explain how I approached this problem so others can see what I did and to get feedback from the Hobby-Machinist brain trust on how I could have done it differently and/or better.

Thanks for reading.
 
Count me interested in this, too. I never got my mill free of backlash last year when I built it, but got sick of working on it.

A few weeks ago, something happened that made me check backlash and it had gotten worse on all axes. My guess is that it's caused by parts wearing. I corrected the backlash settings in Mach3 and continue to use it. I also guess I do things that are less critical than you and others do.

I have a list of projects that I'd like to do to the mill, but I don't see perfecting the mill as what it's there for. It's a side project that takes away from whatever I'm working on, so I mostly want to just get 'er done.
 
Today was semi-successful. Not bad but not as good as I had hoped. As I said in my original post I had .003" of X axis ballscrew lateral movement so I added .008" (.003" of clearance + .005" of preload) to the shim pack between the AC bearing outer races. Assembled the bearing housing, coupling and stepper motor. Checked ballscrew lateral movement and had zero movement. Looking good! I then checked backlash using the same procedure described above. My DI showed .001" to .0015". Not so good but better than what I started with. Checked the Y axis screw and adjusted it the same as the X axis. About the same backlash as the X axis. At this point I don't have a clue where the backlash is coming from. Maybe I need to increase preload. It could be clearance in the X and Y axis gibs. My setup for checking backlash could be less than optimal. Don't know but will look into each of these.

I wanted to get back to a job I was running before the backlash issue came up if nothing else but to see if the external radius surface finish was any better. Well, I was pleasantly surprised. The finish was not perfect but significantly better. If I can get the backlash closer to zero the finish should get better still.

If I can find the source of the remaining backlash I will post it.
 
Is it possible to get some drawings or pictures of what you're describing? You're talking about stacking shim stock washers between two sets of ball bearings on the X-axis ballscrew mount?

My thinking is that I built all of the parts for my CNC conversion with my Sherline CNC stuff and it's possible I got stuff wrong that I just didn't know better about.

The Sherline, btw, uses some fancy Kerk Teflon coated leadscrews with spring loaded antibacklash nuts that are supposed to eliminate lash, bit it still has some. Right around .004", very much like the Grizzly I converted.
 
TomS said:
Been chasing .003" of X axis backlash on my PM-932 CNC for four days now and haven't made any headway. Haven't checked the Y axis yet. It's time to tap into the Hobby-Machinist brain trust.

I've measured the backlash two ways. One method I used is to mount a .0005" DTI in the spindle and touch off on the vise. My other method is mounting a mag base with a 1" travel DI on the saddle, again touching off on the vise. I preload the dial indicator and set them to zero and also zero the Mach3 X axis DRO. I then jog the table in the opposite direction in .001" increments until the DI needle just begins to move and note the X axis DRO reading which is .003"-.004". For a comparison and to eliminate stick slip I tried the second method using a 1" travel indicator and jogged the table 1/2" and got the same backlash readings.

I've adjusted the gib several times and run the tests and no change in backlash except when I adjusted it too tight. I also adjusted the preload on the ball screw AC bearings. Having .003" to .004" of backlash I added .006" to the shim pack. Backlash is still .003" to .004". If I take shims out backlash increases.

Checked the ball screw for lateral movement with a DI and it's zero.

I installed Linear Motion Bearing double ball nuts a few months ago and was able to cut a pocket that was within .0005" of being round. Haven't tried this again so maybe it's time to retest.

What is considered acceptable backlash? Zero would be nice but I don't think it's a realistic expectation.

Am I checking backlash correctly? From what I've read on the internet I am.

Could it be bad AC bearings? These are the original Chinese bearings that came with the bearing blocks.

Could it be the coupling? I'm using split clamp style double diaphragm couplings with two set screws on each end. Although I don't like to run with rigid couplings, as a test I'm go to either make or buy one to eliminate that possibility.

Thanks in advance for your advice.

Tom S.
Click to expand...
Odd's the screw is worn too
I have found on lot of machines the screw is worn in center but ends are not worn .
I did a new mill with chrome plating on the screw. I had for 20 years and had no wear

Dave

Sent from my SAMSUNG-SM-J320A using Tapatalk
 
MontanaAardvark said:
Is it possible to get some drawings or pictures of what you're describing? You're talking about stacking shim stock washers between two sets of ball bearings on the X-axis ballscrew mount?

My thinking is that I built all of the parts for my CNC conversion with my Sherline CNC stuff and it's possible I got stuff wrong that I just didn't know better about.

The Sherline, btw, uses some fancy Kerk Teflon coated leadscrews with spring loaded antibacklash nuts that are supposed to eliminate lash, bit it still has some. Right around .004", very much like the Grizzly I converted.
Click to expand...

Look at Post#6. It shows a couple of illustrations that are very similar to my setup. Post's #8 and #22 also have illustrations. My AC bearings are mounted back-to-back.
 
Smithdoor said:
Odd's the screw is worn too
I have found on lot of machines the screw is worn in center but ends are not worn .
I did a new mill with chrome plating on the screw. I had for 20 years and had no wear

Dave

Sent from my SAMSUNG-SM-J320A using Tapatalk
Click to expand...

I wouldn't expect the screw or nuts to be worn as the mill doesn't have that much run time on it. But you do make a good point. I'll check backlash at both ends of travel and see if there is a difference.
 
TomS said:
Look at Post#6. It shows a couple of illustrations that are very similar to my setup. Post's #8 and #22 also have illustrations. My AC bearings are mounted back-to-back.
Click to expand...

I did a Homer style "D'oh!" here. I should have remembered that.

I had some backlash on my rotary table, too. No ballscrews, but there are gears inside of course. It measured to be two steps of the motor, IIRC.
 
I'm reviving this thread because I haven't really resolved my backlash problem. While my measured backlash is still about .001" - .0015" in both X and Y my gibs are set so loose that the table tilts downward about .005" at the extents of the X axis travel. Not good if you are trying to machine a flat surface.

So today I decided to dig further into the cause of my backlash issue so I can tighten the gibs to where they should be. This what I found.

Here's a picture of the new ball nut.
20170207_113953_resized.jpg


Here's a picture of the X axis ball nut with the anti-rotation keys removed and the two nuts tightened together with hand pressure only. Notice the mis-alignment of the anti-rotation key slots. If I put a .002" feeler gauge between the spacer and nut and hand tighten the slots line up.
IMG_0444.JPG

I haven't checked the Y axis nut yet but I suspect I will find a similar situation with it. Not having done this before I'm not certain if hand tightening then using a feeler gauge is an accurate enough method to determine ball nut preload. Should I go with a .002" shim or more? How much more, .0005", .001", ???
 
Good question ! I do it by feel, when I'm happy with the way is feels then I quit adding pressure (shims). There is probably a specification but I like to do it by hand.
 
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