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Getting oval circles

Discussion in 'CNC Mills/Routers' started by jajohnsen, Dec 17, 2016.

  1. jajohnsen

    jajohnsen Well-Known
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    So i finally got around to get my ooznest ox working, but i have a slight problem in getting it dialed in...

    I'm trying to cut a test circle of 49mm but it turns out slightly oval, to eliminate as much as possible i've made the circle in the mach3 wizard.

    I'm using ESS and a PMDX-126 Board. my steps should be ok at 26.69485777 step for both X and Y axis.

    when i measure the circle it is only 48.5mm on the Y and X axis. but when i meassure on 45 degress to x or y it is the correct Dimension.

    What am i missing or where should i look to correct this?

    Here is the g-code

    (Code by Newfangled Wizard, 17.12.2016)
    (Version 2.86)
    (Program Posted for Wood )
    G0 G49 G40.1 G17
    G80 G50 G90 G98
    G21 (mm)
    (***** Circular Groove/Cutout *****)
    M6 T7
    M03 S24000
    M9
    G00G43 H7 Z1
    G00 X20 Y-2.25
    G41 P1.5
    G01 X22.25 F1000
    G03 X24.5 Y0 R2.25 F1000
    G01 Z-0.5 F500
    G03 X-24.5 Y0 R24.5
    X24.5 Y0 R24.5
    G00 Z1
    G03 X22.25 Y2.25 R2.25
    G40
    G00 X20 Y2.25
    M5 M9
    M30


    any help appreciated
     

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  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
    Staff Member Moderator Builder

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    Generally this is caused by loose set screws in the pulleys so best guess would be to start there.
     
    Mark Carew likes this.
  3. jajohnsen

    jajohnsen Well-Known
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    yes grub screws for the pulleys are checked is checked, tight as can be.
     
  4. jajohnsen

    jajohnsen Well-Known
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    if i try the set steps per unit wizard and ask for 100mm i the lengtht is correct in both x and y.
    I tried to make a 50x50mm Pocket With the mach wizard and it turns out square but only 49.8mmx49.8mm
     

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  5. jajohnsen

    jajohnsen Well-Known
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    could it be that i need to enter some backlash settings in mach?
     
  6. jajohnsen

    jajohnsen Well-Known
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    played with the backlash settings and that helped, measured the backlash With a dial indicator and got to 0.45 for correction. but now
    i get a small ridge just where the compensation is happening. and i can se the router is jumping right when its making the adjustment.
    if i hold on to the gantry you can feel it's the steeperss making a quick change.
     
  7. jajohnsen

    jajohnsen Well-Known
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    i think there is a compination of backlash and deflection going on here. i just redid my my step settings measuring off the gantry with my dial indicator and it's dead on. but if i touch it slightly i can se neadle deflection and if i push some there is no problem getting the needle to move +- 0.3-4 mm

    How much deflection and backlash is normal to get?
     
  8. Giarc

    Giarc Master
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    Is there any chance that your endmill was slightly undersized? That may explain the square being slightly smaller. My 1/8 inch endmill was just slightly smaller than expected so when I cut plates with a 5mm hole in them, the holes were real tight on the bolts. The actual endmill dimensions can be entered into the cam software.
     
  9. jajohnsen

    jajohnsen Well-Known
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    I'm not in front of it right now but did measure it earlier and believe its right.

    I'm starting to think it is deflection mostly causing this. As with my dial gauge i can clearly see deflection when i give it a slight push. I'm going to go over the build and see if i can get i alittle better in the next days.
     
  10. Ryan Lock

    Ryan Lock Master
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    0.3-0.4mm does seem on the high side. See if you can get the belts tighter. Maybe try setting the steps/mm to what they should from calculation so 26.6666, and then adjust the backlash from there.
     
  11. jajohnsen

    jajohnsen Well-Known
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    Thank you for the suggestions, I'll play with it tonight when i get back home from work:)
     
  12. jajohnsen

    jajohnsen Well-Known
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    So played with the belt tension and whell tightnes, and it's getting better.

    tightened belt, is alot tighter now and that seems good.
    tried first with tight wheels and that made it worse, then losened them so i can turn them by hand alittle and that seems to be good.

    circle with out out backlash on in mach3 seems to make the circles a little oval, buth with backlash on they are good, except for ├ą small edge when it makes the backlash adjustment for the y axis
    a square is perfect with backlash on and about 0.2mm off without.

    I will play some more with backlash to see if i can find a compromise for the backlash settings.

    right now it is as follows:
    Y: 0.45
    X: 0.30
     
  13. Ryan Lock

    Ryan Lock Master
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    Sounds like your getting there. I haven't got much experience with Mach3, but maybe try reducing the Backlash speed.
     

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