Circles are not round


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Thread: Circles are not round

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    Default Circles are not round

    My machine is a Momus Ver.1 that I've been using for several years with Carve Pro and Mach3. I've been cutting 2 circle profiles about .100" apart. They come out not circles, but slight eclipses. The diameter is 8.00" and the ellipse minor diameter and major diameter will end up about .020" different. On the Carve pro preview, they are perfect circles, but on the machine, they are eclipses. I used a plotter pen and drew out the pattern to eliminate the cutting forces and got the same result - eclipses. The major and minor diameters are not orinented to the X & Y axis, but are tilted 45 degrees. And I have noticed that if I change from a climb cut to a conversional cut, (a clockwise vs, counter clockwise cutter path), the tilt angle of the eclipse will reverse as shown in the drawing below. The elipses are nice and smooth, with no breaks are sudden jumps. I have checked the drive sprockets, and motor mount bolts, the X & Y bearings, and anything else that might cause some looseness. Everything seems tight. The belts seem 'tight enough', but that is just my guess. I'm thinking that there might be some belt stretch happening since the error changed direction according to the path direction. I did a test by drawing a 8.00 X 8.00 box in VCarve Pro and it came out exactly 8 X 8 with the plotter pen, but only 1 stepper motor was running at a time. When doing the circles, both motors are running at the same time. That would indicate a problem with the controller or Mach3. I went throught the 'steps per' calibration on the Settings page of Mach3 and that made no difference.
    Does anyone have any ideas of what may be causing this? This is not a new problem, I've just lived with it since it has not effected what I was cutting. Now with the concentric circles, it does pose a problem.
    Thanks,
    RichardCircles are not round-elipse-jpg

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    I don't make mistakes ... I make 'unintended engineering design changes'.


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    Default Re: Circles are not round

    Does the problem exist no matter how large the hole is?



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    Default Re: Circles are not round

    Yes, but not as bad. The size of the error increases with the size of the circle.

    I don't make mistakes ... I make 'unintended engineering design changes'.


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    Talking Re: Circles are not round

    Belts are at play here!

    Alright... Probably just one belt on the X axis, it's loose.

    If it's not the belt you've got backlash.

    Easy fix!

    Website [url]http://whatisacnc.com[/url]


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    Default Re: Circles are not round

    That's kind of what I was thinking - belts. The X & Y are snug but not really tight. I was afraid of putting too much stress on the stepper motor bearings and the little 1/4" shaft if I tighened them up much. Since the X belt is longer, it would have more 'give' if it was not tighter than the Y. I can't find any backlash at all and the curves are smooth when the motors change direction - where the backlash would show up. I'll get after the belts tomorrow and post the results. Any idea on how to gage the proper tension on the belt?

    I don't make mistakes ... I make 'unintended engineering design changes'.


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    Default Re: Circles are not round

    Other possibilities:

    1) If I remember correctly, the belts and pinions used on the Momus are XL profile, maybe not the most accurate profile with it's more squarish teeth. And depending where you got them you could be getting backlash or error just from an inaccurate "mesh" of belt and pinion. Even with the relatively tight "wrap" of the belt on that pinion. Possibly a check of the belts and pinions, or switch to GT2 or GT3 profile could help.

    2) The bearings on the rails may be too tight, causing the belt to "load" in one direction and release in the other.



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    Default Re: Circles are not round

    Thanks for the suggestion. The belts are XL and came from McMaster. I don't remember where I got the pulleys. The error does not show up when moving all the way across the table in the X direction or the Y direction (over 10" the actual measured travel will vary only a few thou.) , but only when moving in the X and Y at the same time as in a circle, the error will vary +-.020 . With the error reversing direction on a 45 degree angle when changing from a CCW to a CW path as the drawing shows, seems to point toward the belt issue that you mention. I'll do some searches and change out the belts to the GT3 profile and see if that helps. Then I'll experiment with the motor settings if the belts & pulley changes do not help. I had not thought about the bearings, but it seems to move fairly freely. I'll mess with them too.

    I don't make mistakes ... I make 'unintended engineering design changes'.


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    Default Re: Circles are not round

    Quote Originally Posted by ref772 View Post
    Thanks for the suggestion. The belts are XL and came from McMaster. I don't remember where I got the pulleys. The error does not show up when moving all the way across the table in the X direction or the Y direction (over 10" the actual measured travel will vary only a few thou.) , but only when moving in the X and Y at the same time as in a circle, the error will vary +-.020 . With the error reversing direction on a 45 degree angle when changing from a CCW to a CW path as the drawing shows, seems to point toward the belt issue that you mention. I'll do some searches and change out the belts to the GT3 profile and see if that helps. Then I'll experiment with the motor settings if the belts & pulley changes do not help. I had not thought about the bearings, but it seems to move fairly freely. I'll mess with them too.
    Don't forget you need to change the pulleys too.



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    Default Re: Circles are not round

    Had too many projects the last few days to mess with it -- a car show to get ready for and an autocross. Yesterday I tugged on the belt right next to the pulley, simulating a move. I could see the pulley just start to turn before the carriage moved. The same movement was seen in the X and Y axis. The belts are tight and I did not think the bearings are too snug. I haven't loosened the bearings up yet. I assume that I should get the lightest load on the bearings with no free play. Thats kinda hard to do with the way the Momus bearings are adjusted. The strech in the belt might be caused by the bearings as suggested. I didn't expect that.

    I don't make mistakes ... I make 'unintended engineering design changes'.


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    Default Re: Circles are not round

    Quote Originally Posted by crob09 View Post
    Belts are at play here!

    Alright... Probably just one belt on the X axis, it's loose.

    If it's not the belt you've got backlash.

    Easy fix!
    what is back lash iam haveing same problem so i put on doulble belts and still doing the same ovel holes



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Circles are not round

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