# Chasing Backlash



## TomS

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.


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## JimDawson

0 is acceptable backlash and with double ball nuts should be achievable.  It sounds like you are setting up correctly.  My preference is to use the spindle at the indicator base point

I don't particularly trust the Mach3 DRO for a test like this.  With the motor removed, I would put a ''degree wheel'' on the lead screw, preload in one direction then rotate 1 turn in the preload direction, then 1 turn back .  The difference on the indicator is the backlash.  The degree wheel could be a stiff piece of paper with a single mark on it, just taped to the end of the lead screw or coupling.  The larger the diameter the better to get maximum resolution.

Are the AC bearings installed in the correct orientation?


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## TomS

JimDawson said:


> 0 is acceptable backlash and with double ball nuts should be achievable.  It sounds like you are setting up correctly.  My preference is to use the spindle at the indicator base point
> 
> I don't particularly trust the Mach3 DRO for a test like this.  With the motor removed, I would put a ''degree wheel'' on the lead screw, preload in one direction then rotate 1 turn in the preload direction, then 1 turn back .  The difference on the indicator is the backlash.  The degree wheel could be a stiff piece of paper with a single mark on it, just taped to the end of the lead screw or coupling.  The larger the diameter the better to get maximum resolution.
> 
> Are the AC bearings installed in the correct orientation?



I'll give your recommendation a shot in the morning.  I've had the drive end of the lead screw apart so many times in the last four days I can do it with one hand and blindfolded.

For info I cut a pocket and the bore matched my measured backlash.  I enabled backlash compensation, or so I thought, and recut the pocket.  The bore was .005" out of round!  Double checked my inputs ; .003" in X and .0018" in Y.  Even tried negative numbers and it locked up the motors.

Double and triple checked the AC bearing orientation and confirmed they are installed back to back.  Whats interesting is I measured bearing bore depth, cover spigot height and bearing widths to calculate the preload shim pack.  Put everything together with the calculated shim pack (.005" bearing preload) and had .0125" of backlash.  Hmmm!  I think the inner races were contacting each other.  Added a .015" shim and ended up with .003" backlash.  Added another .005" shim to the shim pack and still had .003" backlash.   

I adjusted the gib until it stalled the motor then backed it off until the motor ran the full length of travel without stalling.  Didn't want the table too loose and giving me false readings.

Tom S.


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## JimDawson

Do you have enough clearance between the outer races? They should have a spacer between them.


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## TomS

JimDawson said:


> Do you have enough clearance between the outer races? They should have a spacer between them.



The bearings drop into the housing.  Haven't measured the clearance but my guess is about .002" loose.  No spacers but I'm adjusting preload with shims between the outer races.  Is that what you are referring to?

Tom S.


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## JimDawson

TomS said:


> I'm adjusting preload with shims between the outer races. Is that what you are referring to?



OK, yes.  I guess I would have to see how the assembly goes together to really understand.  Normally I would expect the outer races to have a fixed spacer between them, then set the preload with a nut on the lead screw to tighten the inner races against each other.  I would think the outer races would be tight axially in the housing, maybe set with shims, but tight.  0.002 radial play seems a bit loose, but shouldn't really affect the axial play of the lead screw.  Normally the support bearing on the other end of the lead screw is pretty loose in the bore, but it is supposed to float axially.




And a bit better picture of the bearings


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## TomS

JimDawson said:


> OK, yes.  I guess I would have to see how the assembly goes together to really understand.  Normally I would expect the outer races to have a fixed spacer between them, then set the preload with a nut on the lead screw to tighten the inner races against each other.  I would think the outer races would be tight axially in the housing, maybe set with shims, but tight.  0.002 radial play seems a bit loose, but shouldn't really affect the axial play of the lead screw.  Normally the support bearing on the other end of the lead screw is pretty loose in the bore, but it is supposed to float axially.
> 
> View attachment 225211
> 
> 
> And a bit better picture of the bearings
> 
> View attachment 225212



Your second picture closely resembles my setup.  The only difference is the bearings are recessed in the bore about 1/16" and there is a spigot on the cap that fits the bore and locates the cap.  The shim pack serves the same purpose as the spacer.  Yes, the support bearing floats in the housing..

Thanks,


Tom S.


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## TomS

Well your advice about not trusting the Mach3 DRO's was right on.  I set up a disk on the ball screw, installed and aligned a pointer, preloaded the ball screw in one direction, zeroed the DI, then reversed screw rotation exactly one turn.   I have .007" of backlash, not .003" to .004"!  For reference the DRO read .0043".

Before running the test above I adjusted the X axis gib using the Bridgeport method.  I ended up with .0015" of indicator movement and called it good.  Before adjusting the gib my backlash was .005".  I'll back the gib off a bit.  

I tried measuring ball screw rotational torque using my in/lb torque wrench.  My lowest setting is 50 in/lb and the screw turned easily.  I even backed off the torque wrench setting one full turn and still didn't work.  Suffice to say rotational force of the ball screw in less than 25 in/lb.

Although they are Chinese origin I don't think the AC bearings are the problem.  Also rechecked ball screw longitudinal movement.  Still zero.  Don't remember off hand what the preload is but it's probably around .010".  I know it's way too much but I am chasing an unknown and with preload set at about .005" I had backlash.  I wanted to see if increasing preload would change anything and it didn't.  If I go below .005" preload backlash increases to about .012".  Maybe more using the degree wheel test.

I'm kind of at a loss as to how to approach this.  I've measured it a dozen different ways using three different dial indicators and adjusted the AC bearing shims pack thickness more times than I can remember.  I thought it might be the coupling but setting the jog at .0001" and carefully watching rotation in both directions I can see the coupling and ball screw rotating in unison.  If I was chasing a couple of tenths then I might dig deeper but .007" of rotational lag would be quite noticeable.

Here are a couple pictures of the driven side bearing housing components.  The two cylindrical items at the top are seal/compression sleeves.  The grease seals run on the OD and the length is such that when the bearing lock nut is tightened the sleeves and AC bearing inner races are compressed against the shoulder on the ball screw.  



This shows the register on the cap better than the picture above.


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## JimDawson

Given that the backlash changes with the shim packs, and the leadscrew/table/ballnut seems to have 0 backlash, then it would seem that the problem is in the AC bearing area.  Now the question is how to fix it.

I would be threading the lead screw for a preload nut even if it requires a redesign, but I have very little patience for this kind of adjusting system.  If that doesn't fix the problem then I would be tweaking the ball nuts or mounting system.


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## jbolt

Maybe I missed something but I do not see how the ball nut was eliminated as a possible source?


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## JimDawson

TomS said:


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





jbolt said:


> Maybe I missed something but I do not see how the ball nut was eliminated as a possible source?



I was going by Tom's quote above.  But you are correct, the ball nuts have not really been eliminated.


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## TomS

I attached a mag base to the table and set the indicator tip on the end of the ball screw.  Traversing the table in both directions the DI needle did not move.  If the backlash was in the ball nuts wouldn't I see that on the DI?


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## TomS

I decided to go back to square one and remeasure the bearing housing and other related parts.  Hopefully I can lay it out so it's understandable.  Here goes.

The bearing housing bore is .701" deep.  The bearing cap spigot is .078" tall.  Each bearing is .315" wide.  Doing the math, with the cap assembled, the effective bore length is .623".  Stack two bearings together (.315" wide) and my calculations result in .007" of preload.  I assemble all these parts onto the ball screw, including the front and rear spacer sleeves, and tighten the retainer nut.  Set my mag base on the table and positioned the DI on the end of the ball screw.  Moving the screw by hand I get .016" of backlash.  What am I missing?  After five days of no progress I had to walk away.      

Tom S.


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## JimDawson

Tom, I think you're right, time for a break.   

Not sure where the problem is, but I'm afraid you are going to find something simple.

Post a picture of the bearing area on the ball screw when you get a chance.


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## jbolt

Two bearings at .315 ea (assuming this is the measurement of the outer race) is .630 so stacked they are .007 taller that the pocket and cap. In this case the outer races are being compressed by the cap. This does not show the play in the AC bearings. To get the pre-load you need to measure the offset of the inner and outer race with the inner race suspended by the outer race and bearings. Once you know the offset and take into account if the inner and outer races have different widths then you can determine what the minimum shim thickness is to have the balls in full contact with the inner and outer races when stacked. Any shimming above this sets the pre-load.


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## TomS

JimDawson said:


> Tom, I think you're right, time for a break.
> 
> Not sure where the problem is, but I'm afraid you are going to find something simple.
> 
> Post a picture of the bearing area on the ball screw when you get a chance.



I'll try and take one in the morning before I head out of town.

Tom S.


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## cs900

so I had a fairly similar issue when I did my PM45 conversion. Had .003-.005 of backlash i just couldn't get out using a double ballnut set-up just like you. Well turns out I had too much preload on the double nut set-up. Went back and took all preload off the second ballnut and had .007 backlash. I then progressively increased the preload until I got to around .0005. I tired getting it closer, but seems to be that if I increase preload any more my backlash increases again. So I left it at that and I currently attribute it to something other than the leadscrew. Using the backlash comp and a good leadscrew mapping I hold +/-.001 fairly well.

Hopefully it's something simple like that, casing backlash can drive you nuts! I can understand why people just go buy tormachs now, lol.


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## TomS

TomS said:


> I'll try and take one in the morning before I head out of town.
> 
> Tom S.



Jim - here's the picture.  From the shoulder to the end of the bearing fit area is 15/16".  Length from the shoulder to the end of the threads is 1-1/2".

Tom S.


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## TomS

cs900 said:


> so I had a fairly similar issue when I did my PM45 conversion. Had .003-.005 of backlash i just couldn't get out using a double ballnut set-up just like you. Well turns out I had too much preload on the double nut set-up. Went back and took all preload off the second ballnut and had .007 backlash. I then progressively increased the preload until I got to around .0005. I tired getting it closer, but seems to be that if I increase preload any more my backlash increases again. So I left it at that and I currently attribute it to something other than the leadscrew. Using the backlash comp and a good leadscrew mapping I hold +/-.001 fairly well.
> 
> Hopefully it's something simple like that, casing backlash can drive you nuts! I can understand why people just go buy tormachs now, lol.



Thanks for the suggestion.  I can do that.  It's just a matter of pulling the table off.  Not sure how a Linear Motion double ball nut is put together.  I know there is a key that keeps them from rotating.

Tom S.


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## JimDawson

That's what I suspected.  What the heck backstops the inner race of the inner AC bearing?


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## TomS

jbolt said:


> Maybe I missed something but I do not see how the ball nut was eliminated as a possible source?



Jay - other than the dial indicator check I've already done what other checks can I do to either eliminate the ball nut or identify it as the culprit?

Tom S.


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## jbolt

JimDawson said:


> That's what I suspected.  What the heck backstops the inner race of the inner AC bearing?



There is a spacer between the inner race and a shoulder on the screw. 

This is a close representation.


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## jbolt

TomS said:


> I attached a mag base to the table and set the indicator tip on the end of the ball screw.  Traversing the table in both directions the DI needle did not move.  If the backlash was in the ball nuts wouldn't I see that on the DI?


The ball nut is attached to the saddle. The screw is attached to the table via the AC bearings and block. They both move together so if the AC bearings are good I would not expect to see movement there.  You need to measure with the TDI mounted off the table. I would lock the Y axis, mount a TDI in the spindle and bolt a 123 bock to the table or reference off the side of a vise and then apply force to the table in both directions to see if you are getting any deflection.


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## JimDawson

That is kind of a basakwards system.    I would want the outer races anchored solid in the housing with a spacer between them, and set the preload with a double locknut on the lead screw.  If it was mine, I would be changing that in a heartbeat at the first sign of trouble.


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## jbolt

JimDawson said:


> That is kind of a basakwards system.    I would want the outer races anchored solid in the housing with a spacer between them, and set the preload with a double locknut on the lead screw.  If it was mine, I would be changing that in a heartbeat at the first sign of trouble.



I'm not speaking for Tom but that is essentially how it is  as you described except instead of guessing what the preload is with a thick shim and just tightening a nut, thin shims are stacked between the outer races to set a known distance. Adding or removing shims changes the preload amount. 

The Chinese AC bearings and blocks have a large tolerance variation so to get them to work properly it is important to measure the fit of the bearings in the block as well as the play in the AC bearings. Once I know what the minimum shim thickness to get the bearings in full contact (no play)  is after measuring all the components I then add .001" shims at a time to add preload.


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## JimDawson

I understand what you are saying.  I have always set preload by feel rather than shimming, but an absolute dimension on the preload makes some sense.


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## TomS

jbolt said:


> The ball nut is attached to the saddle. The screw is attached to the table via the AC bearings and block. They both move together so if the AC bearings are good I would not expect to see movement there.  You need to measure with the TDI mounted off the table. I would lock the Y axis, mount a TDI in the spindle and bolt a 123 bock to the table or reference off the side of a vise and then apply force to the table in both directions to see if you are getting any deflection.



I'm learning something new every day.  I'm out of town until Tuesday.  Will check it when I get back.

Tom S.


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## jbolt

JimDawson said:


> I understand what you are saying.  I have always set preload by feel rather than shimming, but an absolute dimension on the preload makes some sense.



Having had a Chinese AC bearing fall apart in my hands I don't trust doing anything by "feel". I was talking to a bearing tech at one of the distributors I use about bearing preload and he said they order matched sets ($$$) with the internal races ground to a specific width so that when installed and the nut is tight the inner races are in full contact with each other and a preset preload is established. 

That's how I came up with using shims to set the preload. I may not know the value of the preload but at least I have a reference value if it turns out to be too loose or the bearings wear prematurely.


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## jbolt

cs900 said:


> so I had a fairly similar issue when I did my PM45 conversion. Had .003-.005 of backlash i just couldn't get out using a double ballnut set-up just like you. Well turns out I had too much preload on the double nut set-up. Went back and took all preload off the second ballnut and had .007 backlash. I then progressively increased the preload until I got to around .0005. I tired getting it closer, but seems to be that if I increase preload any more my backlash increases again. So I left it at that and I currently attribute it to something other than the leadscrew. Using the backlash comp and a good leadscrew mapping I hold +/-.001 fairly well.
> 
> Hopefully it's something simple like that, casing backlash can drive you nuts! I can understand why people just go buy tormachs now, lol.




cs900,

Are these the the typical Chinese double ball nuts? If so could you elaborate on how to adjust them?


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## cs900

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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## jbolt

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.


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.


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## MontanaAardvark

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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## TomS

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



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.


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## JimDawson

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.



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.


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## MontanaAardvark

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.



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?


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## ewkearns

I'm still trying to get my head wrapped around a .007 preload with a shaft that still turns!


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## JimDawson

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.


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## MontanaAardvark

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:


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.


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## JimDawson

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.



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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## TomS

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.


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## MontanaAardvark

How do you get a shim between the two ballnuts?  In mine that dark piece in the middle wobbled around a little but I was trying my best not to let them get apart.  


Bob


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## cs900

MontanaAardvark said:


> How do you get a shim between the two ballnuts?  In mine that dark piece in the middle wobbled around a little but I was trying my best not to let them get apart.
> 
> 
> Bob


well that's not right, the second ballnut should be tight up against the shim.

I also think a better option would be to put a belville washer stack in there so that you get preload that will compensate for wear on the ballscrew.


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## MontanaAardvark

cs900 said:


> well that's not right, the second ballnut should be tight up against the shim.
> 
> I also think a better option would be to put a belville washer stack in there so that you get preload that will compensate for wear on the ballscrew.



I should clarify that this is when I had the nut removed from the screw with the ballnut removal tool.  Still, how do you do that?  How do you take them apart to get anything in there and rebuilt it so that it's tight?


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## TomS

MontanaAardvark said:


> How do you get a shim between the two ballnuts?  In mine that dark piece in the middle wobbled around a little but I was trying my best not to let them get apart.
> 
> 
> Bob



I believe the shim is installed as part of the nut assembly procedure at the factory.  Thickness is calculated prior to assembly.  The shim on mine seems to be snug which is what I would expect.  I tried to measure screw axial movement with a dial indicator by pushing and pulling on the screw.  The readings were not conclusive but were significantly less than the .007" I measured with it assembled.  What's aggravating is I haven't been able to identify the source, or sources, of the backlash.  If I knew what was broke I could fix it.

Tom S.


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## TomS

Today was a good and enlightening day.  First, I will say my backlash is now about .0008".  Yahoo!!  I'll explain how I got there.  To address MontanaAardvark"s questions about the double ball nut and shim, my shim was snug but not tight.  I couldn't get a .002" feeler gauge between the nut and shim.  It looks like the shim is a two piece arrangement and is retained by the anti-rotation keys.  I tried taking mine apart but the key started to bend so I stopped before I broke it.  How the two nuts are held together I don't know.

After reinstalling the ball nut and screw and the AC bearings I checked nut backlash by mounting a DTI in the spindle with the needle against my vise.  Pushing and pulling the table in both directions I got a reading of about .002" +/-.001".  Not what I wanted to see but better than expected.  I adjusted the X axis gib using the Bridgeport method and settled on a reading of .002".  Ran a backlash test and got .007".  What a disappointment after having .002" of ball nut clearance.  I backed off the gib and the best backlash I could get was .0035".  Better but not good enough. 

I wrote a warm up gcode program several months ago and decided to run it to see if it made any difference.  Ran it for about twenty minutes and tested backlash.  I got .0004" repeatable readings.  Things are looking up!  I also found that the factory gib adjustment screws were pushing the gib to the side.  I cut 1/4" off the right end of the gib making it flat and square and made a bushing that slips over a stud instead of using a bolt, with an OD that just fits into the gib screw counterbore.  This works much better.  I was able further tighten the gib and maintain .0004" backlash.  After a bit more gib adjusting I settled on .0008" of backlash.

So after about a week of testing, measuring, cussing, retesting and remeasuring the source of my excessive backlash appears to be bad gib adjustment screws and not warming up the machine prior to taking measurements.  Lesson learned.  I've still got some fine tuning to do.

Thanks to everyone that contributed their experiences.  Without this forum I'd still be chasing backlash. 

Tom S.


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## JimDawson




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## MontanaAardvark

What Jim said. 

Before I took my mill apart, I don't recall actually measuring backlash, but I didn't feel any slop in the handwheels.  I figured taking it apart would destroy that anyway.  On the other hand, I think the gibs were farther in than they are now.  I need to take a look at them in the morning.  I think they may be loose.  

I tested my mill on rapids and I can get 280 IPM on all three axes.  Coming from a system where Z was 15 ipm and the fastest axis was about 50, it's amazing.   If I gave up a little speed and improved backlash, that would be an acceptable trade.  


Bob


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## TomS

MontanaAardvark said:


> What Jim said.
> 
> Before I took my mill apart, I don't recall actually measuring backlash, but I didn't feel any slop in the handwheels.  I figured taking it apart would destroy that anyway.  On the other hand, I think the gibs were farther in than they are now.  I need to take a look at them in the morning.  I think they may be loose.
> 
> I tested my mill on rapids and I can get 280 IPM on all three axes.  Coming from a system where Z was 15 ipm and the fastest axis was about 50, it's amazing.   If I gave up a little speed and improved backlash, that would be an acceptable trade.
> 
> 
> Bob



I don't have handwheels on my mill so wasn't able to detect backlash that way.  It showed up when I did a circular pocket.  The hole was out-of-round about .006".  I learned a valuable lesson that proper gib adjustment and warm up routine go hand-in-hand.  You can't do just one and expect to get good results, at least on my machine.

I have my rapids set at 100 IPM.  Tried it at 200 IPM and it looked like the machine was beating itself to death on short moves.  It's what works for me.

Tom S.


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## MontanaAardvark

TomS said:


> I don't have handwheels on my mill so wasn't able to detect backlash that way.  It showed up when I did a circular pocket.  The hole was out-of-round about .006".  I learned a valuable lesson that proper gib adjustment and warm up routine go hand-in-hand.  You can't do just one and expect to get good results, at least on my machine.
> 
> I have my rapids set at 100 IPM.  Tried it at 200 IPM and it looked like the machine was beating itself to death on short moves.  It's what works for me.
> 
> Tom S.



I agree that rapids are pretty meaningless because you can never cut at those speeds.   They really only matter if you have to do something with a lot of movements above the work and your time is money.  I did an array of holes on the front and read panels of my motor controller box.  Drill, retract bit, move to next hole, repeat, a large number of holes, followed by a "carriage return, line feed" to the next row of holes.  Rapids sped that job up nicely.   (Rapids on my Sherline were 15 IPM, a far cry from these speeds).

Pardon if this is like a rant, but I'm an engineer and I always see things this way.  I just don't see why cutting a circular cutout should be something backlash compensation can't fix.  In fact, it should be a really easy case.  Backlash only shows up when direction is changed, and in a circle, each axis changes direction twice at the most.  The CNC controller should know everything it needs to know.  It doesn't show up when the axis stops and resumes the same motion, or slows down and resumes.  Torque in a stepper is maximum as lowest speed so if the motor just slows down or stops without reversing direction, backlash doesn't apply.  It's not going to slip.  If it does, that's a different problem. 

I guess I just don't get it.


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## JimDawson

Computer generated backlash compensation is only partly effective and normally only works well on operations like hole patterns.  Cutting  a circle is where it doesn't work very well at all.  The only true backlash comp is when you have a closed loop servo system, and the encoders are on the load (table).  Even that is not 100% effective, because the table can float as the comp is applied.  The only real path to no backlash is to have a tight mechanical system that is heavy enough not to have any measurable elasticity in normal operation.

Take a look at this thread on the Machsupport forum.
https://www.machsupport.com/forum/index.php?topic=12143.0


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## MontanaAardvark

Well, I was a hardware engineer (EE, not mechanical) so hardware guys thinking software guys "just don't get it" is about as big a cliche as you can imagine.  

It seems like it ought to be easy.  Computer (do your best impression of Disney's Goofy, the idiot dog): "Oh, I got to change direction, time to crank out those special backlash steps".  If it needs to crank out something like 12-1/8 steps, that's different.  That's a screw that's too steep, and a system design problem. 

I could go on, but I don't want to bore everyone.


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## Metal

silly question since we are allready wandering off topic:

Where did you get your import ballscrews? I've been poking around at them for my bridgeport and prices are elusive while import screws seem to come with lots of horror stories.


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## MontanaAardvark

Metal said:


> silly question since we are allready wandering off topic:
> 
> Where did you get your import ballscrews? I've been poking around at them for my bridgeport and prices are elusive while import screws seem to come with lots of horror stories.



I got mine from a place called Linear Motion Bearings.  They are Chinese imports, so if that disqualifies them you should know.  The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.  

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.  


Hope that helps,
Bob


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## TomS

MontanaAardvark said:


> I agree that rapids are pretty meaningless because you can never cut at those speeds.   They really only matter if you have to do something with a lot of movements above the work and your time is money.  I did an array of holes on the front and read panels of my motor controller box.  Drill, retract bit, move to next hole, repeat, a large number of holes, followed by a "carriage return, line feed" to the next row of holes.  Rapids sped that job up nicely.   (Rapids on my Sherline were 15 IPM, a far cry from these speeds).
> 
> Pardon if this is like a rant, but I'm an engineer and I always see things this way.  I just don't see why cutting a circular cutout should be something backlash compensation can't fix.  In fact, it should be a really easy case.  Backlash only shows up when direction is changed, and in a circle, each axis changes direction twice at the most.  The CNC controller should know everything it needs to know.  It doesn't show up when the axis stops and resumes the same motion, or slows down and resumes.  Torque in a stepper is maximum as lowest speed so if the motor just slows down or stops without reversing direction, backlash doesn't apply.  It's not going to slip.  If it does, that's a different problem.
> 
> I guess I just don't get it.



I cut a 2-1/2" through pocket in 3/8" thick aluminum at .100" DOC, .220"stepover, 8000 rpm,


MontanaAardvark said:


> I got mine from a place called Linear Motion Bearings.  They are Chinese imports, so if that disqualifies them you should know.  The main way to buy from them appears to be through eBay,
> http://stores.ebay.com/linearmotionbearings
> 
> In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.
> 
> I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.
> 
> 
> Hope that helps,
> Bob





MontanaAardvark said:


> I got mine from a place called Linear Motion Bearings.  They are Chinese imports, so if that disqualifies them you should know.  The main way to buy from them appears to be through eBay,
> http://stores.ebay.com/linearmotionbearings
> 
> In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.
> 
> I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.
> 
> 
> Hope that helps,
> Bob



That's the same place I got mine.  Price and service was excellent as was delivery.  Keep in mind these are not top of the line screws and nuts.  If you want to machine within a few tenths you should look elsewhere.  

Tom S.


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## MontanaAardvark

I've spent a bit more time on mine and reduced the Y axis backlash to .001.  I made a tool to hold the AC bearings in place and under compression while I tightened the jam nuts on the ballscrew.  Just a little piece of 1/8" thick aluminum scrap I had.  It looks like the motor mounts, but that's just by coincidence.  I cut some long edges off it and was left with the more or less triangle shape. 



I tried to do the same thing on the X-axis but had a fit problem.  Somewhere along the way, I tapped the X-axis end cap for 10-32 screws instead of 8-32, so these 8-32x 3/8 screws didn't work.   I'll get some 3/8" 10-32 screws and try again.


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## TomS

MontanaAardvark said:


> I've spent a bit more time on mine and reduced the Y axis backlash to .001.  I made a tool to hold the AC bearings in place and under compression while I tightened the jam nuts on the ballscrew.  Just a little piece of 1/8" thick aluminum scrap I had.  It looks like the motor mounts, but that's just by coincidence.  I cut some long edges off it and was left with the more or less triangle shape.
> View attachment 226288
> 
> 
> I tried to do the same thing on the X-axis but had a fit problem.  Somewhere along the way, I tapped the X-axis end cap for 10-32 screws instead of 8-32, so these 8-32x 3/8 screws didn't work.   I'll get some 3/8" 10-32 screws and try again.



I understand what you did but not sure why you did it.  Your bearing housing has a cap, right?  Does the cap not compress the AC bearings in the housing?

Tom S.


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## MontanaAardvark

TomS said:


> I understand what you did but not sure why you did it.  Your bearing housing has a cap, right?  Does the cap not compress the AC bearings in the housing?
> 
> Tom S.



The cap is part of the motor mount, which sticks out far enough so that when it's in place, I can't get a handwheel onto the ballscrew shaft.


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## Metal

TomS said:


> I cut a 2-1/2" through pocket in 3/8" thick aluminum at .100" DOC, .220"stepover, 8000 rpm,
> That's the same place I got mine.  Price and service was excellent as was delivery.  Keep in mind these are not top of the line screws and nuts.  If you want to machine within a few tenths you should look elsewhere.
> Tom S.



Absolutely not, I'm trying to resolve on the BP:
1: my screws are worn out in the middle so if I adjust the nearly .1 backlash out I lose like half my table
2. constant adjustment due to 1: means chasing a moving target of backlash comp depending on where I'm cutting which is a huge pain in the butt.

If I could get accuracy down to a few thou reliably all across the table I would be very happy.

What size screws did you get (or the code if you are feeling nice)?  from what i can tell 25mm is about as big as he gets which (to me) looks like a far cry from the size of these leadscrews


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## jbolt

I'm running 2005 (20mm) ball screws on my PM-932 conversion on all 3 axis. The lead screws were 25mm. The size of the ball nut restricts what will fit on the X & Y axis on my machine. I would email Chai and see if he sells larger ball screws if that is what you have. He does not list all of what he sells on his ebay page. cgxfred@aliyun.com

I recently ran across this company that also does ground ball screws. I have not ordered from them but have been thinking about getting a quote. https://www.aliexpress.com/store/437307

A lot of people use the Nook ball screws in lieu of the imports. I have not priced them to see what the cost is vs import but it might be worth looking into if you are concerned about the imports.


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## TomS

Metal said:


> Absolutely not, I'm trying to resolve on the BP:
> 1: my screws are worn out in the middle so if I adjust the nearly .1 backlash out I lose like half my table
> 2. constant adjustment due to 1: means chasing a moving target of backlash comp depending on where I'm cutting which is a huge pain in the butt.
> 
> If I could get accuracy down to a few thou reliably all across the table I would be very happy.
> 
> What size screws did you get (or the code if you are feeling nice)?  from what i can tell 25mm is about as big as he gets which (to me) looks like a far cry from the size of these leadscrews



I'm using 1605 ball screws on the X and Y and a 2005 on the Z.  I had Chai machine the ends for my particular application so they would not directly transfer over.

Tom S.


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## TomS

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.


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## MontanaAardvark

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.


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## TomS

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.


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## MontanaAardvark

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.


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## Smithdoor

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.


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


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## TomS

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.



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.


----------



## TomS

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



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.


----------



## MontanaAardvark

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.



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.


----------



## TomS

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. 




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.



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",  ???


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## JimDawson

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.


----------



## TomS

JimDawson said:


> 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.



Thanks Jim.  I tried a few different feeler gauge thicknesses and found out that once the backlash/clearance is taken up it doesn't take much more to lock up the ball screw.  So in my situation it looks like .002 - .0025" should do it.


----------



## TomS

Took a month for the shims to arrive but it looks like shimming the two nuts solved the backlash problem, I hope.  I tried a .0025" shim between the nuts but it locked up the ballscrew.  Put in a .002" shim and you could feel a slight drag when turning the screw by hand.  I installed the nuts and screws and adjusted the gibs.  The X axis gib was easy but the Y was problematic.  The adjusting screw that moves the gib inward was twisting the gib.  I was getting line contact on the top and had a large gap on the bottom.  I've had problems with these adjusters before so I made a plate that uses a set screw to push the gib in.  Works great!

Here's a picture - 




For the backlash check I clamped a piece of 1/4" thick aluminum in the vise and pocketed a 1-1/2" diameter hole.  The hole was round within .0005".  Tried the pocketing routine three more times with the same results.  Maybe, just maybe, my backlash problem is solved.  Only time will tell.


----------

