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Thread: .018 backlash??? What's wrong?

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    Default .018 backlash??? What's wrong?

    Ok, IH masters. I've been running my IH mill converted to CNC with their kit for the past 2 years or so. I haven't checked backlash in a LONG time. I knew it was off, but none of my parts were all that critical, so it wasn't a problem. My programs are usually VERY large (taking 8-12 hours to make one part).

    Yesterday I decided to check the backlash and the X and Y are way off. X is .012 and Y is .018. So I started ripping Y apart to verify what's going on.

    The movement is in the double ball nut (I'm almost positive). I can rotate the gear on the end of the ballscrew probably 1/4" by hand before the Y actually begins to move. So there is a great deal of "play" when changing directions.

    I will pull the ballscrew/ballnut out tonight to see whats going on, but anyone have any idea how this could be? Everything seems tight. The ballnut mount appears to be tight and not moving. I don't see any movement in the outer bearing, although I haven't taken the cover off to verify - I guess it could be the spacers on the outer bearing that are somehow worn?

    Any ideas what else to look for? If it is the ballnut, anyone know where to get replacement parts for it? (balls, or ballnut)

    Thanks for any suggestions!

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    There are just two areas where it can be, first would be the bearings in the block which holds the screw, if loose there would be lost motion.
    then the next would be the bolts that hold the screw and ball nuts mounting block to the table. If they got knocked loose or stretched then = lost motion.
    You will find it in there somewhere.

    Don
    IH v-3 early model owner


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    if you didn't loctite the bolts in that hold the angular contact bearings almost garunteed that its there. i had to cut down an allen key to get in there and tighten mine up,



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    Yep, I'd look first at the bearings. I had a problem with mine after a few months of operation. Ended up adding a shim between them to take up the backlash. Believe the shim went between the outer races. Worked like a champ.

    Bob



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    Guys,

    The more I think about it, the more it makes sense that it's the bearings. I just couldn't see how it could be the ballnut. I'll definitely start there. Can I get everything apart without taking the gear off the end of the ball screw? I know I loctited it, so it might be a challenge to get off. But I'm guessing I'll have to add a shim anyways, so I better find a way.

    Thanks for the help!!!!!!!!!!!!



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    SDF,
    It all depends on what locktite was used to lock it on the threads and how you intend to hold the ball screw so it can not rotate.
    The ball screws are so hard from the rolling process that they are near impossible to hold onto. I have been trying to find an effective way for a while now. I used some old locktite that is extremely tenacious and won't let go. Locktite will however go liquid at 312 deg far BUT getting it that hot without overheating everything else is quite another task and would take a torch better than a propane to do the job effectively, it would have to raise the temp quickly enough to get the gear loose before the heat convects away, not easily done.
    I have been tempted to pick a spot on the screw and grind some feature to allow me to grab the screw firmly, but, I really don't want to do that and weaken it in the driving end of the screw, and it would have to be in a location where the ball nut does not get to.
    What I am facing is that I used the wrong form of locktite and now regret it.
    I would prefer to not have the sprockets mounted by thread in the first place, but to have them splined or keyed.
    In short, it would be best to have the sprocket off to reset the bearings. And, be very careful to keep all chemicals away from the encoder and don't touch or whip the disk. The ink is berry juice bug spit and will easily get taken off.

    Don
    IH v-3 early model owner


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    I doubt you can fix it if it's the bearings without removing the pulley. The threads most likely will be buggered up, but don't take much to fix.

    I put mine back together without locktite to prove to myself that I had it fixed, then removed the pulley and locktited it back on. Use a good strap wrench on the pulley and a couple of vise grips on the end of the ball screw to get it tight as you can.

    Bob



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    I had the same issue- I needed to add a shim in the bearings- don't remember if was on the bearing OD or ID. If you are having difficulty removing the setscrews from the pulley it may be worth buying a LH drillbit. Probably $10 well spent. If the problem is due to loctite on pulley threads perhaps heatgun + wet rag on ballscrew part may soften the loctite a bit. Rather than grinding a flat which is not great at transmitting torque, maybe drill a hole 1/4" or so deep and use a spanner wrench. When putting the pulley back on, I would not recommend tightening it as much as you can, as you can easily exceed the recommended preload on the AC bearings. Ideally, you would look up the spec, but IIRC the instructions are something like finger tight plus a tweak.
    Put dial indicator on end of ball screw, near the motor. Grab ballscrew with vicegrips in a noncritical area, and then try to push/pull the screw back in forth. If indicator doesn't move then you have gotten rid of any backlash in the AC bearing area. Go ahead and tighten setscrews medium firm and then put carriage back on. Then put indicator on carriage, vicegrips on non critical area of turned ballscrew end, make sure gibbs aren't too tight, and rotate clockwise/counterclockwise to measure the backlash in the nut. May want to hold up a piece with marks/put stops in to make sure rotation angle is consistent. Probably good to write everything down. Then start to tighten the gibbs and see if the backlash values change. Also, make sure that mill is bolted down-don't want to have the entire mill moving on you when you're doing this.
    Probably best to wait for an evening when you have plenty of time and no distractions.

    Hope this helps.
    -Matt



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    If the source of the .018" backlash is indeed the angular contact bearings, using a shim would be a very temporary solution for worn out set of bearings. Also, .018" movement is a tremendous amount of wear and not likely coming from one source.

    If I could suggest, before you take anything apart, place an indicator to measure the movement of the table, and place a second indicator at the end of the ballscrew. Push and pull on the table to see if both the ballscrew and the table move the same amount, which they likely won't. If the table moves more than the ballscrew, then your problem lies also in the ballnut.

    With the amount of axial movement that you are describing, it could be possible that the angular contact bearings were never installed correctly to begin with. Are they arranged "back-to-back" or "face-to-face"? This is important to know. It sounds like they may be arranged in "tandem" which would allow such movement.

    I'm suprised that you didn't experience some pretty dicey machining under these conditions.

    Scott



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    Thanks again for all the suggestions, guys!

    Pulley actually came off without too much trouble (just working on the Y axis right now). Here's what I found. I'm guessing everyone is right and it is in this area that I'm having the issues.

    The bolts holding the bearing cap were not tight at all. More than finger tight, but probably not much more. Pulling the bearing cap off, everything looked good with the bearings. There was one shim that was between the bearings that was pretty "worn". I THOUGHT there were supposed to be 3 shims between those two bearings. I'm not sure if the others were disingrated or not. It didn't look bad, so it's hard to say. I tried to measure the shim that was left so I can replace it and I'm guessing it's something like:

    15mm inside x 22mm outside x .25mm thick

    So, the question is, what do I replace that shim with? Any suggestions?

    Scott, sorry, but I already ripped everything apart before I read your post. How do I know how those bearings should be installed? What is the "face" and the "back"

    I attached some pics - yes, I know the outer bearing is "backwards" - just took a pic so you could see what it looked like.

    Bryan

    Attached Thumbnails Attached Thumbnails -bearing-cap-jpg   -innerbearing-jpg  


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    Bryan,
    Attached is an illustration that shows angular contact bearings in the "back-to-back" arrangement. When looking at the face of the outer race, one face will be more narrow the the other face. The thicker side is the "back". Looking at your pictures and description, it appears that the bearings were in a "face-to-face" arrangement which would be wrong for an outer race shim assembly.

    Preload of bearings in a "back-to-back" arrangement is achieved by tightening the bearing locknut and not by using a thicker shim. If the bearings are indeed angular contact, you should see two vee's marked on the outer race similar to this <> (that is back-to-back) and they should be aligned with each other during assembly.



    Last edited by scotta99fan; 08-02-2011 at 01:29 AM. Reason: image didn't appear


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    I am going to chime in and say it is the bearings too. although you may have to add a shim between them, you might be able to get away with tightening the cap bolts down even further. I seriously doubt you have created .018 of wear in the bearing or between, so something else is going on. You may have just vibrated the bearing caps loose enough to create that much play so just try tightening them down (I use vise grips since an Allen wont fit) while turning the hand wheel to make sure you dont bind the bearings and see if it improves. If you do have them mounted Face to Face instead of Back to Back as Bryan described, then that would be your first problem, but you will still need the shim between them to take up a small gap that exists between the center races because of slop in the bearings. The shim is what gives you a better pre load and was a fix for an earlier problem with excess lash in the bearing assembly.

    I had actually cracked a race in one of my bearings and found the press fit on the cap to be fairly loose. enough so that both bearings can slide in and out in the actual mount rather than having slop between them if you don't tighten the cap screws sufficiently, the pre load on that is very tricky.
    Also I know that IHCNC has since abandoned the preloaded double ball nuts for a different arrangement, they work great for a while (years that is), but if you pile a couple of hundred pounds of set up equipment and stock (100 lb rotary table+tail stock+40lb billet), you start to get a little stretching in the pre load springs and then they wear out a good bit faster. Mine still work, but I get about .002-.004 of slop if I have 150+ lbs on the table.

    also take extra care not to damage the encoders on these things when taking them apart, they cost 60 bucks a pop and its not too hard to mess them up enough to where you are loosing steps and don't realize it because the counter is missing lines.

    I love my IHCNC, but they are a kit machine which means they are a work in progress.



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