Chasing Backlash

cs900 said:
well this was before they started offering the double ball nuts they have now, but yes it's 2 chinese single nuts preloaded with belville washers.
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Okay thanks. That's the way I did mine.

I used the double nuts on my router but I do not consider that a precision machine. I'm a little suspect of the Chinese double nuts. When I installed mine two of the three you could tell had some preload, the third did not fell like it had any so I put that one on the Z. I'm not sure if they can be adjusted.

The Chinese double nuts we use on the router at the high school have worked great for the last two years and that was built as a precision machine.
 
MontanaAardvark said:
I'm a bit late here, Tom, but did you use VXB bearings like these?

(http://www.vxb.com/7000B-Bearing-Angular-Contact-10x26x8-p/kit8872.htm)

I apologize if this is way out in left field, but the AC bearings seem like they could be part of this. I just did the Hoss DVD conversion on my G0704 and if I could upgrade it by replacing bearings, that might be a quick thing to try.


Bob
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Bob - thanks for your input. I'm using the bearings that came with the bearing blocks. They are Chinese vintage 7001 AC bearings. I am not convinced my problem is bearing related. I say this because I have preloaded the bearings as much as .015" and I still have backlash, .004' to .006" preload should be more than enough. Maybe I should just spend the money and buy new bearings. They are not that expensive. I've got a couple more tests to run when I get back in the shop. My fingers are crossed that one of them will be the smoking gun.

Tom S.
 
Bob, I copied this over from the Grizzly G0704 Cnc Conversion thread

MontanaAardvark said:
I note one of the guys said "0 is acceptable backlash and with double ball nuts should be achievable". Zero? To how many decimal places? I wonder if that's realistic. If you took a production mill from someone like Tormach and measured it, would you really get 0.000? If you do get 0.000, do you have to tweak that periodically to keep the low backlash? Parts wear out, after all.
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That was me. With preload on the ball nuts there is an interference fit, so therefore no axial movement is possible between the ballscrew and the ballnut. This also assumes that the ball track is not somehow floating inside of the ballnut, not sure if that is even possible. This also applies to the preloaded end support bearing, if everything is tight no axial movement is possible. Now, having said that, the overall system may have flex, stretch, or looseness in it that would manifest as backlash.
 
JimDawson said:
Bob, I copied this over from the Grizzly G0704 Cnc Conversion thread



That was me. With preload on the ball nuts there is an interference fit, so therefore no axial movement is possible between the ballscrew and the ballnut. This also assumes that the ball track is not somehow floating inside of the ballnut, not sure if that is even possible. This also applies to the preloaded end support bearing, if everything is tight no axial movement is possible. Now, having said that, the overall system may have flex, stretch, or looseness in it that would manifest as backlash.
Click to expand...

I answered this over on that G0704 CNC Conversion thread so I'll copy it over here, too.


If that came across as harsh or critical, I didn't mean it that way. By background, I'm an engineer (electrical - so mechanical is mostly out of my "comfort zone") and I'm just trying to understand what's going on. Obviously, nothing is zero backlash to the millionth of an inch, and probably not to a tenth (.0001"). It seems like something is always going to show up that looks like backlash. If it's always there, when do we stop chasing things? When is it "good enough"? I guess we all make that call.

As I said, I have backlash issues on my newly converted G0704, too. Quite a bit worse than Tom's (.010"). Before I take apart my mill, I'd like to have a good idea of what I'm looking for.

I had to re-ball one of the ballnuts off of one of Chai's ballscrew/nut combinations and while I know there's not supposed to be axial motion, I didn't see anything in there that would guarantee that. I didn't see a shim of any kind. I didn't take the two halves apart, though, so it's possible one was in there.

One of those things I've read is that people put larger ball bearings into the ballnuts and take out backlash that way. The way I think of it, there are something like 8 races in the linearmotion ballnuts. If you have 5 mils of backlash, maybe that means you make the balls .005/8 bigger or .000625". The ones I just bought were .1244", this would say to use 0.1250 bearings. Turns out the ebay seller I bought from sells them. Is that .005 divided over the 8 races the right way to think of this?

The thing that gets me is that ballscrews (really all screws) have imperfections from turn to turn so that the exact distance between turns wanders around the ideal. When I was looking at my screws with a dial indicator, some spots seemed to be off about .001" from one tenth (0.100") to another but averaged out to be right. It worked out that going from tenth to tenth it could be 0.1.00, 0.099 or .101, but over the few .500" intervals I measured it was perfect. Regardless of lost motion in the machine, that .001" is going to show up on some cuts. There are different "classes" of screw that have tighter or loser tolerances on this, and as you'd expect, the higher the class of screw, the more expensive. Now, having a circle out of round by .001, or a hole location off by .001 isn't always going to matter, but it's always there.

What about the way they deform under load? Under load, the balls go out of round and the thread they're in changes shape, too. It probably doesn't contribute to backlash, but it contributes to overall accuracy.

Sorry if this is rambling. As I say, I'm trying to understand all this. Is there a general tutorial approach to tracking down where backlash is coming from? A "do this, check that"? kind of thing?
 
I'm still trying to get my head wrapped around a .007 preload with a shaft that still turns!
 
Bob, not harsh or critical at all, but rather a good question.

If you bolt 2 pieces of steel together you now have 0 clearance at that joint, therefore, the static ''backlash'' is 0. If you can put enough tension on that that joint you can create clearance by exceeding the elastic limit of the joint, thus creating ''backlash''.

Having said that, in a properly designed machine tool you should never notice the elasticity of the components in normal operation. Will the components stretch a bit in use? Yes, but it should not be measurable by any tools that would normally be found in the hobby machine shop or in most industrial environments for that matter. For all practical purposes the backlash should be adjustable to 0 or at least what we would normally perceive as 0.

A recent project of mine is Alloy's Shizouka mill that is sitting in my shop. We found about 0.005 backlash in the X axis ball nut. It is a double nut arrangement and is adjusted by a shim pack. The problem was that even adding a piece of heavy duty aluminum foil to the shim pack proved to be too much preload. So what we did was drill & tap the end cap for set screws and manually adjusted for the correct preload. The ''correct'' preload in this case was what I felt was the correct amount of rotational resistance when turning the lead screw by hand with the entire assembly sitting on the bench. The table now has 0 measurable backlash in the X axis when measured with a 1um (0.000039 inch) scale.
 
Thanks, Jim. I only have a partial mental image of what you're saying, and I'm not sure it's right. First, I assume these double nuts look like the ones that Tom and I are using. Second, when you say, "drill & tap the end cap for set screws and manually adjusted for the correct preload", I get lost. My double ballnut screw looks like this:
Both_Mounted.JPG
When you say end cap, is that the flange on this (right end of nut)? Parallel to the screw?

My only progress today is that I made a little adapter sleeve and adapted the old Grizzly handwheels to the 5/16" shaft on the ballscrew. What this does is allow me to turn the free end of the X-axis. I didn't have a reasonable way to do this until today. The motor is in place, but even now, I find I can turn the handwheel quite noticeably before the table moves. We had guests and I couldn't go off into the shop all day, but tomorrow, I'll tear down the X-axis motor end.

Chances are that whatever I did wrong to the X-axis, I probably did it wrong on the Y.
 
Yours is a different mounting system than on the Shizouka. The ball nut pair was housed in a steel block as I remember and had an end cap on it. So by tapping the end cap for set screws, you could preload the nuts.

Here is a picture, the new set screws are the black dots at 3,6,9,12 o'clock. You can just see the mounting block that is attached saddle.
upload_2017-2-5_21-5-1.png

It doesn't look like that would work in your case. The flange on the right is part of the nut, and is harder than He!!, not going to tap that. I'd have to think about your's for a bit to come up with a way of doing it.

Another common system is that flanged nuts enter a block from either end, but the screw holes are slotted, so by rotating one of the nuts a bit, you can preload the system.
 
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Got back in the shop today. I didn't do the test jbolt suggested because the site was down all day and I wasn't able to read his post. So I decided to strip the machine down to a point where I could remove the X axis ball nut and screw for inspection. The nut mounting bolts were tight. I'll check the ball nut mount bolts in the morning. Visually the nut and screw looked fine. I couldn't feel the .007" of backlash between the nut and screw that I measured when the mill was assembled.

The ball nut is assembled with a shim between the two nuts. My assumption is preload is adjusted by grinding the shim. Don't know for sure because I've never done it.

Not being able to identify the cause assembly will be slow and meticulous with lot's of measurements. I'm sure I will have questions along the way.

Tom S.

20170207_114005_resized.jpg
 
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