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IndustryArena Forum > OpenSource CNC Design Center > Arduino > Bad cut on angled lines and arc
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  1. #1

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    Bad cut on angled lines and arc

    Hello guys,
    I am new here, as the hobbist in CNC I use grbl few years now.
    I have some problems and think I am not able to solve it by myself.
    I have feeling that it must be somehow arduino or grbl related.

    I can cut very nice smooth surface finish on X and dual Y.
    Problem is when machine goes into something not straight, like 45° or Arc, nice surface
    finish is gone and edgy. If I run program with spindle off, just cutting air,
    I can feel by hand on non-straight lines these vibration shaking.
    I have to mention that I checked everything mechanical, machine
    is squared, I use preloaded C5 ballscrew, there is no backlash. As motors
    I use Clearpath from Teknic with proper tuning. There is no tension on
    machine, everything is super tight and free to move. If I push hard on gantries,
    there is no measurable bending. As controller I use Arduino Uno with Grbl shield (no stepper drivers),
    I do not need that shield, but it is easier wiring with it for clonning Y to A.
    I have to mention that also changing feed rates does not help and does
    not affect surface finish on these non straight lines, always edgy.
    I tried to generate g-code in different programs - same result, also different g-code senders - same result.
    I modified several times grbl setting, also tried different steps/mm, junction deviation... still same.

    Does anybody know what causes it, I suspect there is something with grbl that cannot synchronize somehow XY movement ?

    I add photo:
    First two are cutting on X and on Y, its nice smooth cut. Other two are edgy, 45° and Arc on circle.
    On 45° is visible repetitive pattern in waves.



    I would be glad for any help.

    thanks

  2. #2
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    Re: Bad cut on angled lines and arc

    The X and YA finish isn't that bad.

    The 45° line is probably just 1 line of gcode. That rules out the gcode sender and the CAM package.
    The driver and controller are absolutely capable of keeping 2 steppers synchronized in motion.

    The stepper resolution could cause such a problem. At what resolution (Steps / mm) are you running ($100, $101, $102 grbl setting)?

  3. #3

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    Re: Bad cut on angled lines and arc

    Hi, I use 640 steps/mm, also tried 320, 160, 80. Still same. Yes, X and YA is ok, It can be much smoother with slower speed. Could it cause when ballnut is not attached to plate precisely 90°, is it so sensitive for it ?
    I am not sure how I adjusted nut at X, so I have to check it.

  4. #4
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    Re: Bad cut on angled lines and arc

    Quote Originally Posted by Arche View Post
    Hi, I use 640 steps/mm, also tried 320, 160, 80. Still same.
    Did you change the settings on the grbl controller and the stepper driver?

    Quote Originally Posted by Arche View Post
    Could it cause when ballnut is not attached to plate precisely 90°, is it so sensitive for it ?.
    I don't think this will influence the finish.

    You could try climb milling versus conventional milling. climb milling should give a better finish but is sensitive for play and flex. There is not only play in the ball nut, but also in the shaft coupler, guides, shaft bearings, etc.

    You could do a finish cut at a shallow depth of cut to get a better finish. This will also reduce the cutting forces and this helps if the mill / router isn't rigid enough.

    Positioning the part can make a difference. Routers are less rigid in the mid of the table.

    Can you post the gcode used to mill the 45° part

  5. #5

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    Re: Bad cut on angled lines and arc

    Yes, I change setting on both, grbl and stepper driver. Also tried climb vs conventional, climb has nicer finish, but result is same wavy pattern. I milled cube 60x60mm, after measure its 60x60, so it is very precise.
    Shallow has same result. Machine is very solid, no bending, so it does not matter if I cut on left side center... I tried it.

    Could it be something that I use GRBL Shield attached to Arduino Uno and somehow pulses from XYA can influences themself ? (I have no steppers on them, I use only side pins for step/dir, cloning Y on Arduino Uno is not cable friendly). I do not understand electronics, so maybe...


    Here is codefor 45°:


    G0 Z2.
    G0 X100.621 Y199.616
    G1 Z-15. F400.0
    G1 X199.616 Y100.621 Z-15. F1800.0
    G2 X199.616 Y97.369 Z-15. I-1.626 J-1.626
    G1 X100.621 Y-1.626 Z-15.
    G2 X99.011 Y-2.3 Z-15. I-1.626 J1.625
    G2 X97.369 Y-1.626 Z-15. I-0.017 J2.299
    G1 X-1.626 Y97.369 Z-15.
    G2 X-1.626 Y100.621 Z-15. I1.626 J1.626
    G1 X97.369 Y199.616 Z-15.
    G2 X100.621 Y199.616 Z-15. I1.626 J-1.626
    G0 Z2.

  6. #6
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    Re: Bad cut on angled lines and arc

    According to the Gcode, you run at 640 steps/mm and 1800 mm feed rate. That are 19200 steps/sec about 19kHz. The max step rate of grbl running on an Uno is about 30 kHz. I prefer not running faster than 10 kHz. You could reduce the steps/mm because most routers don't benefit for more than 200 steps/mm . However, I don't think this is the cause.

    According to the Gcode, only one move is used for milling a 45° side so that is OK.

    Could it be something that I use GRBL Shield attached to Arduino Uno and somehow pulses from XYA can influences themself ? (I have no steppers on them, I use only side pins for step/dir, cloning Y on Arduino Uno is not cable friendly). I do not understand electronics, so maybe...
    If that is the case, your problems would be not so repeatable. I don't suspect the controller or wiring to be the cause of this problem

    I use Clearpath from Teknic with proper tuning.
    I use only side pins for step/dir, cloning Y
    I suspect that there is "resonance" in the feedback loop of one or both of the axis. As long as only one axis is moving, you won't notice this. You could run at 1/10 of the speed, that should change the pattern on the 45° side.

    Tuning is only optimal at one rate/load combination.
    If you drive one axis by 2 motors (cloning Y) you have to tune the motors for this situation. I have no experience in running 2 closed loop motors on 1 axis. I found this thread, maybe it helps.
    https://www.cnczone.com/forums/diy-c...wer-one-2.html

    The tuning software can probably show the position error during milling, in a graph. If so, you can locate the motor(s) that causes the problem.

    I am going to post your problem on the dutch CNC website. Maybe some one there recognizes the problem.

  7. #7
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    Re: Bad cut on angled lines and arc

    It could also be something mechanical that has a relation to the spindle.
    What is the pitch of the spindles you are using?
    What is the "pitch" of the pattern in the 45° cut?
    How is de motor connected to the spindle?

  8. #8

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    Re: Bad cut on angled lines and arc

    Hi, I will try look for this resonance errors, I have some diagnostic tool for it. I can try cut 1/10 speed if it helps.
    Also I ordered new screw for X because the source of it is uknown, after that I will have C5 preloaded all of them. I will recieve it in one month I think.
    On current screw I have zero backlash, I measured it. On dual Y are both C5 preloaded. As spindle I use regular 2,2 Kw water cooled spindle, chinese ones. I do not understand what spindle pitch means ?

  9. #9
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    Re: Bad cut on angled lines and arc

    Quote Originally Posted by Arche View Post
    I can try cut 1/10 speed if it helps.
    It could help in finding the cause of the problems. If the pattern doesn't change, the cause is probably mechanical and related to the rotation of the spindle. If the distance between the pattern lines decreases, the problem is likely "time related" (tuning).
    Quote Originally Posted by Arche View Post
    I do not understand what spindle pitch means ?
    By spindle I mean the ball screw. The movement of 1 turn of this screw is the pitch.

    How are the motors attached to the ball screw?
    Is the Y-axis driven by 2 ball screws and 2 motors?

  10. #10

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    Re: Bad cut on angled lines and arc

    10mm pitch, all of them, It is 1610. Motors are attached by couplers to screws and then through thick plates with FK and FF.

  11. #11
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    Re: Bad cut on angled lines and arc

    10mm pitch, all of them, It is 1610. Motors are attached by couplers to screws and then through thick plates with FK and FF.
    At a feed of 1800 mm/min the screws will run 180 RPM. Long screws could resonate!!
    What is the size of your screws?
    What type of couplers are you using?

  12. #12

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    Re: Bad cut on angled lines and arc

    All of them 1200mm, I think it is not resonate, still it is 3 turns per second, that is not many.
    Coupler type is similar to the picture.

    I used diagnostic for position errors and there is wave pattern. I contacted support, they will look at it.
    So I will know more next week.

  13. #13
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    Re: Bad cut on angled lines and arc

    I got this document from the dutch CNC site for "auto tuning" a dual motor axis. It is from Teknic.
    Attachment 480958
    I used diagnostic for position errors and there is wave pattern. I contacted support, they will look at it.
    How large is the position deviation in that wave pattern?
    At witch axis did you found this pattern?

  14. #14

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    Re: Bad cut on angled lines and arc

    I know this tuning, I did it several times but it seems does not work properly.
    Lets wait to next week, I will know more. The pattern is on all axis

  15. #15

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    Re: Bad cut on angled lines and arc

    Hello,
    so I had servise assistance from engineer, they tuned
    both axis profiles and position error curve is now flattened. They
    told me that maybe I will not recognize difference in cut, because waveness of previous
    curve was not so extreme. They were right.
    I did test of 45° cut and it is little bit better, but still its not how
    I would image it should be. I would really imagine something close to straight cut
    of just X or Y, they are so smooth. I will make another test with new ballscrew when it comes
    for X. I add photo of slight improvement, but still it is not like solo X or Y.
    Attached Thumbnails Attached Thumbnails cut.jpg  

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