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  1. #1
    Join Date
    Apr 2007
    Posts
    67

    Slipping Axis

    G"day,
    I haven't been on here in a long time but I have a question someone might be able to answer.
    The problem I am having is with a cnc converted knee mill that I did myself.
    What is happening is when I drive X or Y axis in either direction with the drive belts connected (timing belts & pulleys) it goes fine for a bit the it has a moment where it "slips" a little and then continues.
    Of course this is no good for cnc jobs.
    I have so far eliminated the belts and pulleys as culprits, which only leaves, the ballnuts or motors.
    If I disconnect the belts it is free as a bird, it just doesn't like being loaded up.
    Funnily, the Z axis doesn't "slip" and that is under a heap of load with an un counterbalanced table that has to weigh well over 50 kg, maybe more.

    So any hints, as I'm stumped.
    Thanks.
    Deane.

  2. #2
    Join Date
    Jun 2007
    Posts
    3757
    Find max speed of an axis while jogging and not losing steps.
    Back of, say 20%.
    Now increase acceleration until rapid direction changes lose steps.
    Back off acceleration, say 30%-40%.
    Do this for each axis, using jog keys.
    Write down the settings, so the next time you need them they are easy to find. LOL
    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.

  3. #3
    Join Date
    Apr 2007
    Posts
    67
    I have slowed it right down but it still happens.
    What's more it happens when I manually crank the axis with the feed wheel, but like I said, take the drive belt off and it is smooth as silk.
    I thought it might have been the balls in side the ball nut not running smoothly or binding or ??
    Thanks for the help though.

  4. #4
    Join Date
    Jun 2007
    Posts
    3757
    With the belt off, it must be smooth to turn, and possibly a little stiff, but no tight spots.
    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.

  5. #5
    Join Date
    Apr 2007
    Posts
    67
    Yes it is nice and smooth and that is the confusing bit.
    Could it be the stepper motor (Nema34 1200oz (I think from memory)

  6. #6
    Join Date
    Nov 2007
    Posts
    42
    Deane,

    Could you describe the entire machine.

    What brand machine did you start with.

    What brand and models of motors/drives/break out boards.

    Computer make & model.

    What screws/timing pulleys/couplings were used and how they were assembled (loctite or set screw or welded).

    Having the details will make it easier to help find the problem.

    BR,

    Mark

    The problem I am having is with a cnc converted knee mill that I did myself.
    What is happening is when I drive X or Y axis in either direction with the drive belts connected (timing belts & pulleys) it goes fine for a bit the it has a moment where it "slips" a little and then continues.
    Of course this is no good for cnc jobs.
    I have so far eliminated the belts and pulleys as culprits, which only leaves, the ballnuts or motors.
    If I disconnect the belts it is free as a bird, it just doesn't like being loaded up.
    Funnily, the Z axis doesn't "slip" and that is under a heap of load with an un counterbalanced table that has to weigh well over 50 kg, maybe more.

    So any hints, as I'm stumped.
    Thanks.
    Deane.[/QUOTE]

  7. #7
    Join Date
    Apr 2007
    Posts
    67
    I can't give in depth detail at the moment,it's 2.00 am, but it's a generic free standing knee mill, see pic.
    Electronics are typical Chinese rubbish, Nema 34 stepper motors 1200 oz. I think. Ballscrew & nuts on all axis, T5 timing pulleys and belts, pinned to ballscrew shafts.
    So just a basic, cheap conversion, running Mach3.
    As I have previously detailed, belts, pulleys have been renewed, secured to the shafts with grub screws and pinned for good measure.

    Click image for larger version. 

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    Click image for larger version. 

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  8. #8
    Join Date
    Dec 2005
    Posts
    238
    Hi Dingo,

    Nice looking machine.
    You have mentioned a couple of thing I just wanted to clarify.
    You mention that you have renewed the belts and pulleys. From this I take it that this was converted years ago and has been working fine up until recently. The reason I ask is to establish if this is a new conversion that has never worked or an old one that has. A large stepper of 1200 ozin would usually have poor speed characteristics and needs to be run by an expensive, matching driver but you used the expression Cheap Chinese which is a concern. It makes me think of resonance as your issue which would show up as the motor starting to turn then the axis stops and the motor whines. Then as the move comes to an end the whining would stop and the axis starts to move again. Is this what you are seeing?
    This video shows resonance issues and the difference adding a damper to the motor can make.

    [ame=http://www.youtube.com/watch?v=8Am69_F93yA&list=UL]65 ipm - YouTube[/ame]

    Another comment you have made is that the issue happens when you manually crank the axis with the feed wheel. If this is the case it has to be a mechanical issue as the steppers play no part in manual turning of the feed wheel. The only thing it could possibly be is a faulty leadscrew but 2 faulty ball screws doesn't sound right no matter how you slice it. Especially since you say they turn nice and smooth. I have misunderstood something but not sure what I am missing.

    Hope some of this helps.

    Cheers
    Peter
    The ingenuity of idiots is unlimited.
    http://www.youtube.com/user/cncnutz

  9. #9
    Join Date
    Apr 2007
    Posts
    67
    Thanks for the reply.
    This mill was converted by me about a year ago, but has basically not been used very much at all as I was pretty crook for a while and have only just decided to do something about this problem.
    Before I got sick I had a sign to route and it kept on messing up the sign when I ran the file.
    I finally slowed the axis speed to a snails pace just to get the job to finish without screwing up.
    As I have previously built a cnc router before I am quite familiar with most aspects of the mechanicals, and I know about resonance but not so much the electrical side.
    Cheap chinese is what I could afford at the time, and it still applies.
    I purposely chose the largest package deal available in my price range, seeing that it had to move a lot of weight especially raising the whole table assembly.
    The machine weighs 500 kg, so a big part of that weight is the table.
    funnily enough that works without fault, it si the X & Y axis that are the problem.
    Unloaded the ballscrews move very freely, but when the belt id attached to the steppers, there is significant drag/resistance, which I assumed a large stepper motor must have, previously I have only played with 400 oz size motors.
    Anyhow it's got me buggered !
    BTW I replaced the timing belts as I thought they had probably stretched, plus one had a few mangled teeth.
    So I ordered some new ones and put them on, plus I made some idler/tensioner pulleys to take out any chance of the belts slipping, but all to no avail.
    I would like to get it working properly but don't want to rush out and buy stuff I probably don't need as I don't use it that much now as I have changed directions and now have a cnc laser which is what I use the most, the mill is only for wooden signs that need to be routered.

  10. #10
    Join Date
    Dec 2005
    Posts
    238
    Do you have the specs of the steppers and the driver you are using?
    Also what voltage power supply came with the kit?

    Cheers
    Peter
    The ingenuity of idiots is unlimited.
    http://www.youtube.com/user/cncnutz

  11. #11
    Join Date
    Apr 2007
    Posts
    67
    This may not be the place, 3Axis Nema 34 Stepper Motor Dual Shaft1600oz CNC kit or MILL FREE SHIPPING | eBay but it looks exactly like the hardware I got.
    Motors were 1600 not 1200 oz.

  12. #12
    Join Date
    Nov 2007
    Posts
    42

    motor speed

    Deane,

    Sorry, can't look at ebay thru my current internet access.

    Check the speed you are running the X & Y axis motors. If different than the z axis. Change it to the same as the z axis and see if the problem goes away.

    On steppers usually the bigger the motor the slowerr the top speed before missing steps.

    Mark

  13. #13
    Join Date
    Dec 2005
    Posts
    238
    Hi Deane,

    Had a look at the driver kit on ebay. The driver is capable of decent current and also a good working voltage to power the motors. It's a pity the rest of the kit is not matched to take advantage of it.
    I looked at a couple of 1200 ozin motor specs on a website and if the motors you have are the same as the ones I looked at then it might not be too bad a match.
    Are the motors you have 4 or 8 wire.
    If they are 8 wire are they connected series / parallel or half coil?
    A stepper wired in series have poor speed compared to the same motor wired in parallel or half coil and driven by the same voltage.
    Does the motor have a part number on it? We might be able to find the spec's for it.

    Cheers
    Peter
    The ingenuity of idiots is unlimited.
    http://www.youtube.com/user/cncnutz

  14. #14
    Join Date
    Mar 2013
    Posts
    0
    I know this is an older thread, but hopefully this helps someone. I had a similar slipping problem on my current project. I even took the motor apart thinking that something was horribly wrong, but everything was solid and moved freely. I finally figured out that the slipping had to do with pulsing the motor too fast. Think of it this way... if you send pulses faster than the motor can respond, it won't rotate fully to the next step detent before the polarity changes and it gets pulled back to the previous detent. If this happens at full speed, the motor will appear to be slipping. The solution on my system was to reduce the distance/sec the axis was allowed to travel, which in turn reduces the number of pulses/sec required, and eliminated the "slip".

    Steve

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