Thread: Y axis stalling

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  1. #31
    Quote Originally Posted by richie00boy View Post
    OK, before I take things apart a bit more and get lost, here is what I have found.

    1. The motor knob becomes stiff when the leadnut under the gantry can be at almost any point along the length of the leadscrew, i.e. not just at the ends.
    Ok you can't feel for stiffening properly with the motor connected because of the Detent of the motors to do it properly you MUST remove motor.

    Quote Originally Posted by richie00boy View Post
    I'm thinking that there could be two ways for bending the leadscrew. One if the leadnut was not mounted centrally, and the other if the chassis is out of square or the leadscrew not exactly in the middle of each end piece. How can I check for all of these? Because there is also the possibility that a bent leadscrew has been fitted at the factory - which I'm hoping is the case and my machine is fundamentally sound. Also I would not be that bothered if the leadnut was not mounted centrally because if I convert to ballscrews I would be fitting a new mount.
    Without seeing the exactly how the ends are fixed and the leadnut mounts to gantry then it's hard to say. But if it's how I think then to get a rough idea you'll need to remove the bearing at one end and the run the gantry to that end with leadnut attached and see if the screw is central in the hole.? This should give you some indication if it's misaligned.
    The most common reason for bending is because the leadnut is either lower or higher than the ends in middle of it's travel where it's the most bendy it then lifts or pulls down on screw when approaching ends bending screw in process.

    To check if screw ends are equal is a simple case of measuring from one reference edge to screw centre(or edge) for both vertical and horizontal. IE: Machine side to screw for horizontal and End plate edge to screw for vertical.

    Then if ok loosen the leadnut mount from the gantry so there's no pressure on it and run the gantry to one end. Now check for any gap between Mount and gantry.? Now Run the gantry to other end and see if the gap is the same. Do the same at several points along the screw.
    If all is ok the gap should be same or very very close to none, ideally you want light interference fit.! Now it's just a case of shimming the gap so when tightened there's no pressure or pulling on the screw.
    If there's no gap and there's tight fit between leadnut mount and gantry then you'll need to remove some material from leadnut mount.
    If your leadnut mount doesn't bolt onto flat surface and instead has drop bracket that which fastens to gantry cross brace then you'll need to loosen the bracket to slip fit and put witness marks on bracket and brace. Then run gantry up and down it's length and watch if marks move.? If they do then try loosening lead nut from the bracket and find balance using both bracket and leadnut mounts, failing that you'll have adjust the end fixings.!!

    If the machine is out of square then you'll feel it going tight when pushing gantry up and down it's length with leadnut disconnected. Being honest thou with the sloppy linear bearings on these machines I doubt you'll feel it. That said for it to be bad enough to bend leadscrew you'd most certainly be able to visually see it.!!

    Obviously I'm giving adjustment suggestions blind has I can't see your machine and how things are fixed but hopefully should give you an idea of what i mean.!!! . . .. . . OR I could have just blown your mind. .
    Last edited by JAZZCNC; 01-08-2013 at 08:05 PM.

  2. #32
    The stiffening is a lot worse than other axis though and I can see the leadscrew is bent so I think I'm satisfied with that as the fault diagnosis. The gantry runs nice and free up and down the rails with the leadnut disconnected, so I'm happy that things are square. I can't feel any play.

    I don't think I can do these other tests being mentioned because the leadscrew is bent, therefore it's going to make the leadnut not align with the bearing hole when looking through, etc. If it was the leadnut alignment that bent it, well I'm ditching that anyhow.

    So what I need now is to spec and purchase a suitable replacement ballscrew system. Then once I have that I can do these trueness checks. I'm going to have to do them anyway to work out the mounting for the ballnut aren't I?

    I'll start a new thread for the ballscrew.

  3. #33
    Just as a last check, here is a video of the leadscrew showing the runout. The leadnut housing has been unbolted from the gantry to show how it rocks due to the bent leadscrew. Hopefully you will agree that this is the cause of the problem.

    http://www.readresearch.co.uk/person.../leadscrew.mp4
    Last edited by richie00boy; 02-08-2013 at 02:45 PM.

  4. #34
    Quote Originally Posted by richie00boy View Post
    Just as a last check, here is a video of the leadscrew showing the runout. The leadnut housing has been unbolted from the gantry to show how it rocks due to the bent leadscrew. Hopefully you will agree that this is the cause of the problem.

    http://www.readresearch.co.uk/person.../leadscrew.mp4
    Take it you have the mounting bracket loose.? . . . But No not at all I've seen worse than that an still work fine.!!. . . . . For this to cause stalling it should only possibly be at very high speeds and even then with that amount of wobble over such short distance I doubt it would.!!

    If this is causing your stalling then I think you may have the motors tuned too high.? What do you have set for Acceleration & Velocity in motor tuning for each axis.?

  5. #35
    Tuning settings are further back in this thread and the comments then said they were fine.

    I've now unplugged the motor and been turning the knob to move the Y axis up and down, it's not that stiff really. This is with the leadnut mounting loose, so it's just pushing the gantry on the lip and is free to rock about a bit.

    I also tried putting the Y axis drive on the X axis motor and running that and there is no stalling. I tried by hand to give some resistance to the head moving and it just powered through it.

    Could I have a dodgy motor?

  6. #36
    Quote Originally Posted by richie00boy View Post
    Tuning settings are further back in this thread and the comments then said they were fine.
    Ok I've been back to start to familiarise my self with original issue.!! . . . . . Did you change the wires.?

    Quote Originally Posted by richie00boy View Post
    I've now unplugged the motor and been turning the knob to move the Y axis up and down, it's not that stiff really. This is with the leadnut mounting loose, so it's just pushing the gantry on the lip and is free to rock about a bit.
    Does it go tight when you tighten the mounting.? If so then you may need to shim the space between mounting and gantry surface. Some times just a piece of paper can make difference between binding and not.!!

    Quote Originally Posted by richie00boy View Post
    I also tried putting the Y axis drive on the X axis motor and running that and there is no stalling. I tried by hand to give some resistance to the head moving and it just powered through it.
    No wouldn't expect it to stall either.!! The Y axis is tuned for half the speed of X axis and X axis is half the weight of Y axis. So in simple terms you have roughly twice the torque pushing half the weight.!!


    Quote Originally Posted by richie00boy View Post
    Could I have a dodgy motor?
    Very unlikely Steppers are very robust things they either work or they don't.

    Basicly if you haven't got bad mechanical binding or some things badly bent or misaligned like it seems to be turning out you haven't, then it's either incorrect motor tuning or you have electrical/electronic problems, either from wires or failing drive or possible PC.(thou don't think so here)

    knowing there's no mechanical binding then my next place to turn, if you haven't already, is to change the wires and connectors.
    I've seen them cause this sort of thing on these TB based drives before, strange but it's happened.

    In mean time It may be worth playing with motor tuning to find the point were it doesn't stall. Can be tedious but some times the difference between stalling and not stalling can be less than 50 on either vel or Accl.
    Get pen & paper and write down settings for each axis before starting and be methodical, only change 1 setting at time and write it down then try.
    Start with Y axis and with Velocity first then Accel straight after, do one then the other until you find the balance.
    My approach for getting into ballpark is to half the current setting then try, if Ok, then increase by half the difference. . . . IE 1000 original try 500=Ok so try 750=Not Ok try 625=Ok try 560 etc
    This work for both Vel & Accl but best if only change one setting then do the other, All you have to remember is you can't have high Velocity and high Acceleration. When you are near usually raising one means lowering the other.

    One thing to try is lowering the other Axis settings slightly and see if it affects the Y axis.? Don't think so because it's not happening while moving other axis.? Or is it.??

    All that said your tuning for the Y axis is painfully slow to me so going lower to stop stalling would drive me crazy.!!

    After changing wires and lowering tuning if it's still doing it then I suggest you dump the Electrics and buy some real drives.!!

  7. #37
    Bit of an update. I think it's now sorted. There were a couple of issues, first was that it turned out the steps setting was 65kHz in Mach3, set that down to 25kHz. This became apparent trying a friends PC which was a little lower spec than mine.

    The other issue was/is some binding on the leadscrew which I have improved by loosening the motor mount screws and running the axis up to the motor end and tightening the screws back up. I had to do this twice, the second time trying to do it on the tightest position, as the first time there was still some binding up at the top end.

    Will see how it goes, maybe the binding will get worse again once I put the heavier 800W spindle on. But for now it's running the best it's ever run. I was able to speed the axis up to match the x axis.

  8. #38
    richie00boy I didn't know you were interested in CNC as well as diyaudio!

  9. #39
    Well it's a means to an end rather than interest. All I need now is a big CNC for speaker cabinets lol.

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