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Thread: Worn leadscrew?

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  1. I wonder what would cause this error on my parts. Till recently, all worked well, but on my last 5 parts i noticed error on x axis. It is the same error on all 5 parts. It is cut circle and there are also 40 x 7.1mm holes made circular around that circle. Error is 0.5mm only on x direction. See the illustration bollow what i mean:

    https://i.imgur.com/wB1vJnK.png

    The holes positions have same error, so i assume it is worn leadscrew / nut, but when i move the x axis by hand, i dont feel any movement, so i wonder, is this mechanical error or is there some electrical issue? i use chinese servo motors (bst motion, krs?? 400w) and cslab csmio ips controller.

  2. anyone?

  3. #3
    Doddy's Avatar
    Lives in Preston, United Kingdom. Last Activity: 11 Hours Ago Has been a member for 6-7 years. Has a total post count of 803. Received thanks 128 times, giving thanks to others 34 times. Referred 1 members to the community.
    I've a couple of questions - really out of curiosity (and boredom - 2 weeks away from my machines) than progression to a solution, and I have zero experience of servos. But you should be able to answer the following with only a short time in the workshop.

    Firstly, what, if anything, has changed since the machine worked well? I'm assuming that you'll reply will you have not deliberately changed anything.

    Presumably you can swap the drive/positional feedback between the X/Y servos?, have you tried this?, what's the result? Then repeat with swapping the X/Y servo drivers. You should be able to isolate to either servo or driver in error... if not, then it's likely mechanical.

    You don't mention: Is your part a progressive multi-depth cut?, If you was losing steps then you would expect any error to accumulate and might expect to witness this as you cut through the material. I'm inferring from your post that this (accumulated error) is not the case, and the error is repeatable.

    You don't mention: If you move the workpiece centre elsewhere on the X axis and repeat the cut does the problem remain? If it did this would tend not to support the leadscrew argument.

    Have you tried any measurement of x-axis positioning? get a sheet of material on your bed and engrave a rule (Y stroke every 10 mm across the X-axis, say) - from X=left though X = max. Cut one rule in the left-to-right direction, and a second in the right-to-left direction. In each case ensure that the X-axis velocity is expected to be zero when you're engraving the Y stoke (this is just to test the X-positioning). Are these the same? (If not I'd be checking the fasteners on the ball nuts, ball-screw supports, spindle and even machine bed).

    Repeat but with a hand-crafted sinusoid (y = scale * sin (x * some-fettle-factor)) (cutter permanenting engaged with material) so that you're now testing the positional accuracy with the x-axis under load/ and not coming to rest) - measure the positional accuracy of the Y=0 crossing point. I'm not sure what this would prove other than ensuring that there's no significant time-constant in the servo positioning (is this the servo controller gain?).

    Is this with your DIY spindle?, if so you should be very intimate with the build of the spindle - are you confident there's no play in the spindle under load? Again, check fasteners also.

  4. #4
    Quote Originally Posted by Husky1 View Post
    The holes positions have same error, so i assume it is worn leadscrew / nut, but when i move the x axis by hand, i dont feel any movement, so i wonder, is this mechanical error or is there some electrical issue? i use chinese servo motors (bst motion, krs?? 400w) and cslab csmio ips controller.
    If the drives were tuned correctly and you changed nothing and the error is repeatable then it's a most likely mechanical issue. However, I'd expect you would feel 0,5mm easy enough. That said it's the first place I'd go looking before started messing around with tuning. Set the drives to show current and see if they pull higher current than usual or in different parts of travel, they could be binding in one spot and causing the motors to fall behind slightly but then catch back up, this wouldn't fault the drives but the error would still show on the part.!

    After this, I'd look for loose electrical connections. Then move onto re-tuning the drives.

  5. #5
    Quote Originally Posted by Doddy View Post
    Presumably you can swap the drive/positional feedback between the X/Y servos?, have you tried this?, what's the result? Then repeat with swapping the X/Y servo drivers. You should be able to isolate to either servo or driver in error... if not, then it's likely mechanical.
    Doddy it's not that simple with Servo's Esp if running analog (And if Husky is who I think(Mat) then sure he is running analog) because each drive is tuned to the motor and it's load, so it's a lot of work not just a simple switch over like steppers.

    Quote Originally Posted by Doddy View Post
    You don't mention: Is your part a progressive multi-depth cut?, If you were losing steps then you would expect any error to accumulate and might expect to witness this as you cut through the material. I'm inferring from your post that this (accumulated error) is not the case, and the error is repeatable.
    He's using AC brushless Servos so there are no steps involved. The encoders deal with position and feed it back to the drive so corrections can be made. You get what's called following error so if drives not tuned correctly to the motor and it's load then the error becomes larger, up to a point then drive will fault.
    A correctly tuned setup will keep this following error to a minimum which doesn't affect part quality. But if anything changes like start putting much heavier than usual loads on table or binding or mechanical issues then the following error becomes greater and will show in the part. If the following error is on the edge of acceptable or binding etc is isolated just to one spot then because of how servos work the servo loop always allows the motor and position to catch back up so don't get a fault. The allowable following error can be adjusted in the drive so it could be set to a lower value which will help catch any trouble before the part is ruined, but it's a fine line between right and wrong.! I've also given a very simplified explanation of servo tuning , in practice, there's more to it.

  6. Thank you very much for the replies. I havent had time for the machine since i am away most of the time during the holidays. I will work on the problem next week and see if i could resolve the issue. I did have 5 minutes to check the voltage on the drives and this is what it shows (i could set only voltage info, very confusely translated servo manual) :

    https://i.imgur.com/OhkgXSr.jpg

    I see lots of voltage deviations, could this be an issue?

  7. I have engraved multiple rulers (10mm spacing) on a blank plate. First i engraved with x axis (from right to left and then vice versa) and the the same for y axis, overall 4 rulers. The result is the same as it was on my 5 parts with the error. X axis has 0.5mm error, y axis is ok. Engraving from left to right and vice versa doesnt show any error when comparing those two cuts. So i guess no mechanical error.

    On the pics bellow it shows where the error appears. It is in the middle area, approx 0.5mm error. I circled where the camera shows correctly because of its lens.

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    On following pics i fixed the ruler in the center of the engraved ruler and it shows the same error as on the part:

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    This must be something with the servos. Is this fixable with servo tuning considering i have no clue how to fine tune the servos?

  8. #8
    That just looks like your steps per setting is wrong. It's not moving the amount you think it is.? The first thing you need to do is check that it moves the amount you tell it with one move. Using a ruler to measure against isn't accurate enough you need to measure with digital calipers at least or ideally linear scale.

  9. I put the ruler there just for illustration of the error. Today i played with mach3 sets steps and the digital caliper and it shows exactly the same error. On pic1 i calibrated the x axis in mach so it moves exactly 460mm, then i set caliper to 200mm and placed it on 3 different locations, left, right and center. Pic2 shows what error machine makes. Have no idea how to fix that. I will inspect the connections next and swap motors. Maybe the encoder is faulty? Just guessing.
    Click image for larger version. 

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    Last edited by Husky1; 13-01-2020 at 02:42 PM.

  10. #10
    How are the motors connected to the ballscrews.?

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