ARGH! backlash killing parts! Anyone have some info?


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Thread: ARGH! backlash killing parts! Anyone have some info?

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    Angry ARGH! backlash killing parts! Anyone have some info?

    Hi all-

    I bought a COA mill in 2012 and have used it for light stuff here and there, and recently have started working with a partner making (a little) money with it. The problem is, i seem to have a backlash issue. The X axis is showing about 15 thou BL and the Y is at about 4, and for some reason, seems to increase and decrease with no apparent reason why. I tightened the machine's gibs until I thought it would bog the servos, I stoned the gibs flat, etc and it keeps coming and going. So, i decided to try and do a little research, and found that some folks use a spring washer between 2 captured ball nuts in order to take up whatever slack is in the screws. I dis some measurements (OD of lead to OD of lead is 0.783in, measured 5 leads per inch) and ordered myself up a couple of ballnuts I thought would match and a few bevelle washers from eBay, tore the mill down to the screws and tried to put the nuts on.

    No dice.

    The ball nuts go on as far as the non-ball barring leads cut into them, then bind to a dead stop where the BB start. I tried both sides of the ballscrew, put the ball nut on both ways and the same problem repeats with both barrings in both orientations and on both ends of both ballscrews.

    So, I figured I would contact COA, and as can be seen in another thread on the company, nobody's home to answer questions.

    Anybody here know what the hell the specs of the screws are? I was under the impression it was 3/4 RH thread 5 leads/inch. Am I wrong? Does anyone have any information or advice outside replacing the screws/ballnuts? I'm stuck, and need to get this resolved or I can't make accurate or repeatable parts...

    Thanks in advance-

    Farasien

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    Gold Member Jim Dawson's Avatar
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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    Are you sure it's not a 20x5mm, rather than a 3/4 x 0.200?

    Another cause of backlash is the thrust bearings, are they tight?

    Jim Dawson
    Sandy, Oregon, USA


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    Quote Originally Posted by Jim Dawson View Post
    Are you sure it's not a 20x5mm, rather than a 3/4 x 0.200?

    Another cause of backlash is the thrust bearings, are they tight?
    Also....
    what about the union between drive motor and leadscrew ?



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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    If all else checks good, look at the balls, themselves. It is possible the balls are bad. They can be replaced and even made larger to take up wear. It is possible the screw is bad. Look for more wear in the middle. Screws can be rebuilt, but it may be cheaper to replace them. As others have said, look hard at the couplers and thrust bearings.



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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    Quote Originally Posted by Farasien View Post
    Hi all-

    I bought a COA mill in 2012 and have used it for light stuff here and there, and recently have started working with a partner making (a little) money with it. The problem is, i seem to have a backlash issue. The X axis is showing about 15 thou BL and the Y is at about 4, and for some reason, seems to increase and decrease with no apparent reason why. I tightened the machine's gibs until I thought it would bog the servos, I stoned the gibs flat, etc and it keeps coming and going. So, i decided to try and do a little research, and found that some folks use a spring washer between 2 captured ball nuts in order to take up whatever slack is in the screws. I dis some measurements (OD of lead to OD of lead is 0.783in, measured 5 leads per inch) and ordered myself up a couple of ballnuts I thought would match and a few bevelle washers from eBay, tore the mill down to the screws and tried to put the nuts on.

    No dice.

    The ball nuts go on as far as the non-ball barring leads cut into them, then bind to a dead stop where the BB start. I tried both sides of the ballscrew, put the ball nut on both ways and the same problem repeats with both barrings in both orientations and on both ends of both ballscrews.

    So, I figured I would contact COA, and as can be seen in another thread on the company, nobody's home to answer questions.

    Anybody here know what the hell the specs of the screws are? I was under the impression it was 3/4 RH thread 5 leads/inch. Am I wrong? Does anyone have any information or advice outside replacing the screws/ballnuts? I'm stuck, and need to get this resolved or I can't make accurate or repeatable parts...

    Thanks in advance-

    Farasien
    After Paul left COA its not been as good for service and follow through, but if you can get ahold of the owner, Tony Page, he mostly knows his stuff. His salesman (Jim?) hasn't had good follow through in my opinion.

    My COA TK mill didn't have much backlash in X&Y, but Z had some using the weight of the head to take it up. Mine is gone now, but random backlash sounds like bad screws or maybe a few balls all together in the nut that are undersize. Seems odd though. I thought X & Y had a double nut screw. That should make up for a lot. Even a cheap Chinese single nut usually only has about .003 and some much less so it does sound like you have a problem somewhere.

    Its not the gibbs. Over tightening the gibbs will kill motors and drivers over time. Even if they seem to be working fine at the moment. I know first hand. On my bigger mill (Hurco KMB1) I set the gibbs by putting an amp meter on the driver power. When current goes up I back it off just a little and call it a day. I run full range of travel and look for tight spots too. They show up really nicely on an analog meter.

    Bob La Londe
    http://www.YumaBassMan.com


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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    Quote Originally Posted by Bob La Londe View Post
    After Paul left COA its not been as good for service and follow through, but if you can get ahold of the owner, Tony Page, he mostly knows his stuff. His salesman (Jim?) hasn't had good follow through in my opinion.

    My COA TK mill didn't have much backlash in X&Y, but Z had some using the weight of the head to take it up. Mine is gone now, but random backlash sounds like bad screws or maybe a few balls all together in the nut that are undersize. Seems odd though. I thought X & Y had a double nut screw. That should make up for a lot. Even a cheap Chinese single nut usually only has about .003 and some much less so it does sound like you have a problem somewhere.

    Its not the gibbs. Over tightening the gibbs will kill motors and drivers over time. Even if they seem to be working fine at the moment. I know first hand. On my bigger mill (Hurco KMB1) I set the gibbs by putting an amp meter on the driver power. When current goes up I back it off just a little and call it a day. I run full range of travel and look for tight spots too. They show up really nicely on an analog meter.
    Well, I ordered some new ballnuts, and its confirmed its 5x20mm, not the original imperial size I thought. Looking at the screw, it looks in good shape. I bought the mill in 2012 and its seen light use until recently (I was almost exclusively cutting out wood parts). Now, I've been playing more with aluminum and a little steel, so I wouldn't think that worn screws would be a problem. Its always stayed lubed, my feeds were fairly slow and cuts were fairly shallow, so wear is probably not the issue, I would think.

    The motor/screw interface is tight, and the nuts at the end of the screw were tight as I could get them without galling the screws (holding so I can tighten).

    Any idea where I might get my hands on larger balls for the nuts?



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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    VBX sells bearings in a number of sizes. You also may be able to find the size you need on ebay. I'd search and see if someone has repacked your specific nuts and see what size is needed and works best.



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    Gold Member Bob La Londe's Avatar
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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    Azvideo here on CNCZone (I can never remember his name) repacks single nut screws with larger bearings as needed and he claims to get under .001 backlash that way. I seem to recall that getting just the right size bearing from nut to nut is a bit fiddley.

    I also think I remember some guys who repack ball nuts will install every other ball slightly smaller than the optimum for some reason. I don't recall exactly. Might have had something to do with speed.

    I'd have to go back and check things out, but around 2012 give or take I think is when ownership of the COA changed. You may have gotten an old owner COA machine.

    Bob La Londe
    http://www.YumaBassMan.com


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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    Quote Originally Posted by Farasien View Post
    .........

    Any idea where I might get my hands on larger balls for the nuts?
    Maybe here? https://bearingballstore.com/index.p...ndex&cPath=391

    Jim Dawson
    Sandy, Oregon, USA


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    Gold Member Bob La Londe's Avatar
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    Default Re: ARGH! backlash killing parts! Anyone have some info?

    It just occurred to me that you may be experiencing head nod. I seem to recall seeing the head actually visibley fall back into position when finishing a modest cut in steel or a heavy cut in aluminum. Always had to cut pockets undersize and take a light finish cut. Always had to cut outside profiles over size and take a light finish cut. Sometimes a second finish cut or (spring pass) Some things like interrupted cuts were almost impossible to keep head nod from being an issue.

    Have you actually checked the table for backlash with a dial indicator.

    Bob La Londe
    http://www.YumaBassMan.com


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ARGH! backlash killing parts! Anyone have some info?

ARGH! backlash killing parts! Anyone have some info?