Need Help! X axis spring back on vice squaring ?


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    Default X axis spring back on vice squaring ?

    Hi there,

    I have just done a CNC conversion on a new Optimum MH28V mill and am having a strange thing happen when i went to indicate my vice in for the first time on the new machine.

    I indicated the vice in with my DTI in the quill but when i move in the X axis to the other side of the vice jaw and stop the reading on the DTI will keep moving for another 0.02mm ? Then settle back to 0. I move the table in the other direction along the X axis and same sort of thing where the DTI will overshoot a bit and then come back to 0 ?

    I thought it might be the way oil from my one shot oilier backing up and the pressure was pushing on the table and then when the table is not moving the oil pressure dissipates and lets the table fall back into place but thas probably a long shot ???

    I just cant figure out what might be causing it. Has anyone else had this happen ? Its hard to indicate the vice in as when the table is moving its out of square. But when the table is stopped it settles back into square ?

    Things i have done to problem solve:

    Tried a different DTI = same result
    Tried DTI on a mag base instead of in the quill = same result
    Checked and adjusted gib tighter/looser = same result
    Loosened fixed and floating bearing blocks and realigned = same result

    Here is a video of what is happening



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    Gold Member handlewanker's Avatar
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    Default Re: X axis spring back on vice squaring ?

    Hi, as it's only the X axis that is moving I would "guestimate" that you have a way oil cushion build up that is pushing one side of the X axis dovetail away.....as soon as the motion stops the oil build up drains away.......you could try a thinner way oil........perhaps the seals at the end of the X axis dovetails are allowing this to happen......remove the seals at one end and see what happens.

    A moving body with a fluid between it will "aquaplane" when the body is in motion.....it's called dynamic something or other.......can't remember the exact term.

    If the forward motion is slow the pressure dissipates but with fast motion you get a denser layer between the 2 surfaces that will create a pressure.....having slack gibs will exacerbate the problem......thjs will create a void for the pressure to move the slides back and forth.

    Do a test........with a dial indicator against one end of the table.....grip the table on the ends and attempt to rock it sideways against the dovetails......the dial indicator could move a few thou but if it moves a lot the gibs are too slack.

    Slack dovetail jibs are one solution people use on a manual mill retrofit to overcome dovetail friction and stepper motor stalling......one reason why linear rails are so good in that concept as they work practically metal to metal without jamming up.

    If this pans out to be the cause, you might need to have a more powerful stepper motor or servos to move your axes.....slack gibs are NOT the way to go......maybe thinner way oil.
    Ian.

    Last edited by handlewanker; 10-01-2017 at 10:08 PM.


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    Default Re: X axis spring back on vice squaring ?

    Hi Ian,

    Sounds like what it might be possibly. I am using Moble vactra #2 way oil so not sure if they do a thinner one ?

    As a test i did try to really tighten up the X gib over tight and the motor still drove it ok but it didnt change the behavior of things. Its a 640 Oz-in NEMA 34 from Homann Designs.

    I dont have any seals on the ends of my dovetails.

    I guess i could try wiping some of the oil off the x axis and see what that might do as a test ? Or disconnect the oil lines to the X axis to relieve the back pressure ?



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    Default Re: X axis spring back on vice squaring ?

    Hi, be careful on wiping the oil off as the X axis does do a bit of speed and that means you do need to be riding on a film of oil at all times.....cast iron does not like running on a dry surface.

    It's also possible, but not very likely, that the dovetails are not all that well mated.....hardly likely on a new mill but it could happen.........my last course of action would be to strip the table off and blue up the slides.....that takes some skill and is time consuming.

    The truth is, the dovetails must guide the table in 2 planes accurately.....any variation from the truth is a strip down to look and see situation.
    Ian.



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    Default Re: X axis spring back on vice squaring ?

    Interesting! I wonder if you might shed more light on the problem if you put something on each end of the table and trammed them to see if the problem was still there when the table was at the extremes. If you have a second DTI or even a indicator, maybe setting it up to read the top of your vise jaw as you did your test may narrow your search.

    What I find interesting is the slo-mo speed at which the dial comes back to zero. It sure looks like parts floating on a oil film that slowly ebbs away...but a typical oil film wouldn't be .002 thick would it?

    Make sure the saddle is tight and not wiggling. A ballnut that's not 100% aligned with the ends could possibly be putting the table in tension.

    Good luck and keep us posted, I would love to know the outcome of this bizarre problem.

    Stuart

    "THE GRIZZ" photo album - https://goo.gl/photos/yLLp61jooprtYzFK7
    Youtube - https://www.youtube.com/channel/UCT2lq9obzEnlEu-M56ZzT_A


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    Default Re: X axis spring back on vice squaring ?

    Quote Originally Posted by atomarc View Post
    ...
    What I find interesting is the slo-mo speed at which the dial comes back to zero. It sure looks like parts floating on a oil film that slowly ebbs away...but a typical oil film wouldn't be .002 thick would it?...

    Stuart
    Note that he is using a metric DTI so that 0.02mm is equal to 0.0007" or less than a thou. Could the oil film be half a thou? Absolutely. The question is why it builds up and bleeds away in one direction. Ideally it should be equal font and back and wouldn't introduce this error.
    I'd try taking the gib out and checking it on a flat surface for any bend.
    Or try this - its a long shot but easy enough - disconnect the line from the one shot oiler. Bending a hose can reduce its volume, pumping oil into the ways, which then bleeds out slowly when you stop.

    One detail question: Do you have double ball nuts with preload by chance? Torque from the ballscrew on a ballnut with high preload could be a reason for the movement being positive one way and negative the other way.

    Interested to know the answer to this mystery.
    Mike

    Last edited by ninefinger; 10-01-2017 at 08:24 PM. Reason: asked for more info


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    Default Re: X axis spring back on vice squaring ?

    Thank for the helpful tips.

    I tried a few things and i think am making some progress ( at the expense of introducing other problems).

    So i put my other DTI on the top of the vice jaw as well as the one on the face of the same fixed jaw. I see no spring back on the top of the jaw. Only the DTI touching the face.

    I also tried a DTI out on the far end of the X table and see a twisting motion where the DTI on the vice sees a movement in the +direction and the DTI at the far right end of the table sees a movement in the -direction. So that got me thinking maybe there is indeed a loose gib situation. I pushed and pulled on the table and saw about 0.05mm deflection. So i went and tightened up the Y gib a half turn and the X gib a half turn then pushed and pulled on the table and saw 0.04mm deflection. Keept adjusting the X and Y gibs till i got the deflection down to 0.02mm. Gibs are really really tight at this stage ( super tight actually). Went about 2 full turns from where they where on both x and y.

    Ran the DTI along the face of the vice jaw again and see only the tiniest little bit of spring back. Something like 0.0005mm. I call that good.

    Only now with the gibs that tight it gave double the backlash. Backlash setting in Mach3 was 0.018mm for X and 0.02mm for Y . Now they are 0.5mm for X and 0.04mm for Y. I think the ballscrews are now twisting under the extra strain (2005 screws).

    Next thing i will try is disconnecting the oil lines going to the X axis and see if there is back pressure there.

    Last edited by ti5m; 10-01-2017 at 10:13 PM.


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    Default Re: X axis spring back on vice squaring ?

    Just an update.

    Doing some more investigation work and it seems to be coming from the Y axis. I loosed off the gib on the X axis as it was really very tight at this stage and i didnt like to run it quite THAT tight so i backed it off a 1/4 turn and ran another test and it didnt make anything worse. So i backed it out another 1/2 turn and surprise it didn't change the reading at all. So i backed it out to where it was before i tightened it and the DTI reading is still good.

    So that was an interesting find and made me wonder about the Y axis gib and what would happen if i backed it off too. So i loosened it 1/4 turn and the problem started to come back. Loosened it some more and it got worse.

    So it was in the Y axis after all which is a bit strange as the Y is not moving when i am tramming the vice in. Only the X axis should be moving so that's why i thought it must be something in the X axis that's not right.

    I did try disconnecting the oil pipes to the X and Y axis but didn't see anything come back under pressure while moving the X and Y around.



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    Default Re: X axis spring back on vice squaring ?

    Hi, the too tight gibs are a no no as that will give you more problems,,,,,they should be adjusted to "ideal" run tightness.......... I think the run tightness will give you the miniscule variation at all times as there is a running clearance between the metal faces and oil is elastic......probably most people haven't noticed it on their machines.
    Ian.



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    Default Re: X axis spring back on vice squaring ?

    Quote Originally Posted by ti5m View Post
    Just an update.

    Doing some more investigation work and it seems to be coming from the Y axis. I loosed off the gib on the X axis as it was really very tight at this stage and i didnt like to run it quite THAT tight so i backed it off a 1/4 turn and ran another test and it didnt make anything worse. So i backed it out another 1/2 turn and surprise it didn't change the reading at all. So i backed it out to where it was before i tightened it and the DTI reading is still good.

    So that was an interesting find and made me wonder about the Y axis gib and what would happen if i backed it off too. So i loosened it 1/4 turn and the problem started to come back. Loosened it some more and it got worse.

    So it was in the Y axis after all which is a bit strange as the Y is not moving when i am tramming the vice in. Only the X axis should be moving so that's why i thought it must be something in the X axis that's not right.

    I did try disconnecting the oil pipes to the X and Y axis but didn't see anything come back under pressure while moving the X and Y around.
    OK......that is a good result.....I think you might have a gib on the Y xis that is not all that flat.........easy to pull out and test etc.

    If the Gib is OK the problem might lie in the actual machining of the dovetails on the Y......it's a big job to strip out the table and Y saddle just to blue up and see.

    The last thing is.......if the Y axis saddle is constructed on the light side it will flex if the gibs are over tightened........this will affect the X axis too as the table runs in the saddle top dovetails.

    IF......the Y axis dovetails are suspect in their machining and fit.....that is a manufacturing error.....bad workmanship etc.....it's a bit late to claim on the warranty once you've retrofitted it etc......your only recourse is to refit the Y axis dovetails, but that takes quite a bit of skill to achieve.
    Ian.



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    Default Re: X axis spring back on vice squaring ?

    Quote Originally Posted by ti5m View Post
    Thank for the helpful tips.

    I tried a few things and i think am making some progress ( at the expense of introducing other problems).

    So i put my other DTI on the top of the vice jaw as well as the one on the face of the same fixed jaw. I see no spring back on the top of the jaw. Only the DTI touching the face.

    I also tried a DTI out on the far end of the X table and see a twisting motion where the DTI on the vice sees a movement in the +direction and the DTI at the far right end of the table sees a movement in the -direction. So that got me thinking maybe there is indeed a loose gib situation. I pushed and pulled on the table and saw about 0.05mm deflection. So i went and tightened up the Y gib a half turn and the X gib a half turn then pushed and pulled on the table and saw 0.04mm deflection. Keept adjusting the X and Y gibs till i got the deflection down to 0.02mm. Gibs are really really tight at this stage ( super tight actually). Went about 2 full turns from where they where on both x and y.

    Ran the DTI along the face of the vice jaw again and see only the tiniest little bit of spring back. Something like 0.0005mm. I call that good.

    Only now with the gibs that tight it gave double the backlash. Backlash setting in Mach3 was 0.018mm for X and 0.02mm for Y . Now they are 0.5mm for X and 0.04mm for Y. I think the ballscrews are now twisting under the extra strain (2005 screws).

    Next thing i will try is disconnecting the oil lines going to the X axis and see if there is back pressure there.
    It's nothing to do with the oiling system, the Gibb is not flat, as others have suggested, and may be the table ways have not been scraped correct, The Ballscrew mounting also if not parallel with the table travel, will cause the same problem, this is not an unusual problem with these Chinese Mills, a lot have had to fix the gibbs and the ways on these machines, you are using the right oil, you can never have too much, although to much will just spill everywhere and be wasted, but it will not cause this problem

    Mactec54


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    Default Re: X axis spring back on vice squaring ?

    I just took the Y gib strip out and put a machinists square blade across it and it dose appear to have a very slight bend in it. I could only just fit a 0.0015" feeler gauge between the gib and square at the worst spot. I dont suppose they can be straightened can they ?



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X axis spring back on vice squaring ?
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