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  1. #1
    Join Date
    May 2010
    Posts
    98

    Machining arcs

    I am using VCarve Pro to machine 4 simple arcs to create one cycle of a perfect sine wave. I save the tool path using the Arcs post processor for WinCNC and then machine the path with my ShopSabre servo controlled router. In one of the arcs I get an imperfect bump that is consistent from part to part. I have tried fitting a spline to the arcs and machining that path but the bumps are then more pronounced. The only thing I have found to help is to export the g-code using the standard GCode post processor instead of the arcs version. It still isn't perfect however. I know this is within the capability of my machine to produce so can only suspect it is within the G code itself. If anyone can suggest how to fix it I would appreciate your expertise.

  2. #2
    Join Date
    Jul 2016
    Posts
    660

    Re: Machining arcs

    I imported your DXF on a 3D environment, it had double lines in it, can you tell me what`s the exact measurement on your drawing? let me try to do a custom CAM on it and post back the G-code here for you to try on your machine



  3. #3
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    Mar 2003
    Posts
    35538

    Re: Machining arcs

    There are duplicate arcs, but I don't see anything wrong with the .dxf file that would cause "bumps".
    Gerry

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  4. #4
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    Sorry about the double lines. I did clean the geometry in VCarve Pro before exporting the g-code though. I contacted ShopSabre and they told me not to use the WinCNC post-processor but the ShopSabre post-processor instead. I had no idea they had their own! I did that last night and the part was definitely improved. I need to make several more tonight to confirm.

  5. #5
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    Well it doesn't appear that the shopsabre post processor fixed the problem. The exact measurements for the arc series are:
    1. Peak to trough is 127mm
    2. Arc radius = 199.71mm
    3. distance between top and bottom arc = 584.2mm
    4. There are four arcs

  6. #6
    Join Date
    Jul 2016
    Posts
    660

    Re: Machining arcs

    Quote Originally Posted by HomerSimpson View Post
    Well it doesn't appear that the shopsabre post processor fixed the problem. The exact measurements for the arc series are:
    1. Peak to trough is 127mm
    2. Arc radius = 199.71mm
    3. distance between top and bottom arc = 584.2mm
    4. There are four arcs
    What kind of material you want to cut sir? also how big is your tool bit used and how thick is your material



    This is now your arcs, are you refering to cut only the arcs? or including the square lines?

  7. #7
    Join Date
    Mar 2003
    Posts
    35538

    Re: Machining arcs

    The radius of the arcs in the dxf is 399.42mm.

    I really don't know what the problem is. The .dxf doesn't tell anyone anything, as it really has nothing to do with your issue. The problem is either in the g-code, or your machine.
    Can you post a pic of the issue, and the g-code?
    Gerry

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  8. #8
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    The end mill is 3/8" diameter. The material is 1/2" MDF. If you connect a straight line to the top and bottom set of arcs to complete the profile, that is what I am trying to cut out. Thanks for your help!

  9. #9
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    Gerry,
    I'll take some pics tonight and attach them but for now, here is the latest gcode.
    Stan

  10. #10
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    Mar 2003
    Posts
    35538

    Re: Machining arcs

    Two things I would do.
    It looks like you are cutting 4 panels at a time. There is no space between the panels (except the 3/8" for the bit). I'd separate them by about 1/8", so 1/2" between them.
    I would ramp in, rather than plunging.
    I think the plunge move is probable putting the marks on your panels, but I'm not sure.
    Gerry

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  11. #11
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    Thanks Gerry. I have been doing a ramp lead in rather than plunging. I had just forgotten to do that when I created this file. The plunge is on the flat side though and the 'bumps' are on the arcs, right at the transition from one arc to the next. The imperfection isn't on every arc transition however, which makes it more difficult to diagnose.

  12. #12
    Join Date
    Jul 2016
    Posts
    660

    Re: Machining arcs

    You can try this code sir, Im not really sure of the dimensions though I just set it to 584.2mm, I made it into two passes to try it out on your machine and test it out. I also just copied the speed and feed of the G-code you posted, the plunge rate is 300 then the cut speed is 600

    Code:
    ( T3 : 9.5 )G21 G90 G91.1 G64 G40
    G0 Z3.0
    ( T3 : 3.0 )
    T3 M6
    ( First Pass )
    G17
    M3 S1000
    G0 X0.0 Y0.0
    G0 Z1.0
    G1 F300.0 Z-6.35
    G1 F600.0 Y-593.8789
    G1 X18.6615 Y-593.2409
    G1 X32.5577 Y-591.8112
    G1 X46.3239 Y-589.4358
    G1 X59.8956 Y-586.1258
    G1 X73.2096 Y-581.8966
    G1 X86.2036 Y-576.7679
    G1 X98.8169 Y-570.7639
    G1 X110.9907 Y-563.9124
    G1 X122.668 Y-556.2454
    G1 X133.7943 Y-547.7989
    G1 X144.3177 Y-538.6122
    G1 X154.0188 Y-528.8988
    G1 X173.6506 Y-510.5416
    G1 X195.4041 Y-495.1702
    G1 X219.0497 Y-482.9067
    G1 X244.1461 Y-473.9798
    G1 X270.2251 Y-468.5561
    G1 X296.8 Y-466.7369
    G1 X323.3749 Y-468.5561
    G1 X349.4539 Y-473.9798
    G1 X374.5503 Y-482.9067
    G1 X398.1959 Y-495.1702
    G1 X419.9494 Y-510.5416
    G1 X439.5812 Y-528.8988
    G1 X449.2823 Y-538.6122
    G1 X459.8057 Y-547.7989
    G1 X470.932 Y-556.2454
    G1 X482.6093 Y-563.9124
    G1 X494.7831 Y-570.7639
    G1 X507.3964 Y-576.7679
    G1 X520.3904 Y-581.8966
    G1 X533.7044 Y-586.1258
    G1 X547.2761 Y-589.4358
    G1 X561.0423 Y-591.8112
    G1 X574.9385 Y-593.2409
    G1 X593.6 Y-593.8789
    G1 Y0.0
    G1 X574.9385 Y-0.638
    G1 X561.0423 Y-2.0677
    G1 X547.2761 Y-4.4431
    G1 X533.7044 Y-7.7532
    G1 X520.3904 Y-11.9824
    G1 X507.3964 Y-17.111
    G1 X494.7831 Y-23.1151
    G1 X482.6093 Y-29.9666
    G1 X470.932 Y-37.6335
    G1 X459.8057 Y-46.08
    G1 X449.2823 Y-55.2667
    G1 X439.5812 Y-64.9801
    G1 X419.9494 Y-83.3373
    G1 X398.1959 Y-98.7087
    G1 X374.5503 Y-110.9723
    G1 X349.4539 Y-119.8992
    G1 X323.3749 Y-125.3228
    G1 X296.8 Y-127.142
    G1 X270.2251 Y-125.3228
    G1 X244.1461 Y-119.8992
    G1 X219.0497 Y-110.9723
    G1 X195.4041 Y-98.7087
    G1 X173.6506 Y-83.3373
    G1 X154.0188 Y-64.9801
    G1 X144.3177 Y-55.2667
    G1 X133.7943 Y-46.08
    G1 X122.668 Y-37.6335
    G1 X110.9907 Y-29.9666
    G1 X98.8169 Y-23.1151
    G1 X86.2036 Y-17.111
    G1 X73.2096 Y-11.9824
    G1 X59.8956 Y-7.7532
    G1 X46.3239 Y-4.4431
    G1 X32.5577 Y-2.0677
    G1 X18.6615 Y-0.638
    G1 X0.0 Y0.0
    ( Second Pass )
    S1000
    G1 F300.0 Z-12.7
    G1 F600.0 Y-593.8789
    G1 X18.6615 Y-593.2409
    G1 X32.5577 Y-591.8112
    G1 X46.3239 Y-589.4358
    G1 X59.8956 Y-586.1258
    G1 X73.2096 Y-581.8966
    G1 X86.2036 Y-576.7679
    G1 X98.8169 Y-570.7639
    G1 X110.9907 Y-563.9124
    G1 X122.668 Y-556.2454
    G1 X133.7943 Y-547.7989
    G1 X144.3177 Y-538.6122
    G1 X154.0188 Y-528.8988
    G1 X173.6506 Y-510.5416
    G1 X195.4041 Y-495.1702
    G1 X219.0497 Y-482.9067
    G1 X244.1461 Y-473.9798
    G1 X270.2251 Y-468.5561
    G1 X296.8 Y-466.7369
    G1 X323.3749 Y-468.5561
    G1 X349.4539 Y-473.9798
    G1 X374.5503 Y-482.9067
    G1 X398.1959 Y-495.1702
    G1 X419.9494 Y-510.5416
    G1 X439.5812 Y-528.8988
    G1 X449.2823 Y-538.6122
    G1 X459.8057 Y-547.7989
    G1 X470.932 Y-556.2454
    G1 X482.6093 Y-563.9124
    G1 X494.7831 Y-570.7639
    G1 X507.3964 Y-576.7679
    G1 X520.3904 Y-581.8966
    G1 X533.7044 Y-586.1258
    G1 X547.2761 Y-589.4358
    G1 X561.0423 Y-591.8112
    G1 X574.9385 Y-593.2409
    G1 X593.6 Y-593.8789
    G1 Y0.0
    G1 X574.9385 Y-0.638
    G1 X561.0423 Y-2.0677
    G1 X547.2761 Y-4.4431
    G1 X533.7044 Y-7.7532
    G1 X520.3904 Y-11.9824
    G1 X507.3964 Y-17.111
    G1 X494.7831 Y-23.1151
    G1 X482.6093 Y-29.9666
    G1 X470.932 Y-37.6335
    G1 X459.8057 Y-46.08
    G1 X449.2823 Y-55.2667
    G1 X439.5812 Y-64.9801
    G1 X419.9494 Y-83.3373
    G1 X398.1959 Y-98.7087
    G1 X374.5503 Y-110.9723
    G1 X349.4539 Y-119.8992
    G1 X323.3749 Y-125.3228
    G1 X296.8 Y-127.142
    G1 X270.2251 Y-125.3228
    G1 X244.1461 Y-119.8992
    G1 X219.0497 Y-110.9723
    G1 X195.4041 Y-98.7087
    G1 X173.6506 Y-83.3373
    G1 X154.0188 Y-64.9801
    G1 X144.3177 Y-55.2667
    G1 X133.7943 Y-46.08
    G1 X122.668 Y-37.6335
    G1 X110.9907 Y-29.9666
    G1 X98.8169 Y-23.1151
    G1 X86.2036 Y-17.111
    G1 X73.2096 Y-11.9824
    G1 X59.8956 Y-7.7532
    G1 X46.3239 Y-4.4431
    G1 X32.5577 Y-2.0677
    G1 X18.6615 Y-0.638
    G1 X0.0 Y0.0
    G0 Z3.0
    G0 X0 Y0
    G0 Z0
    M5
    M30

  13. #13
    Join Date
    Mar 2003
    Posts
    35538

    Re: Machining arcs

    Is the machine stuttering, or pausing, where you get the bump? My guess is that it has to do with the code, and WinCNC is pausing when it shouldn't be?
    I see some instances where your code has I8.0499, and I8.05. These should probably be the same. I'm thinking that some minor math issues are causing WINCNC to see tangent arcs as being not tangent?
    Gerry

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  14. #14
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    Gerry,
    Yes the machine appears to have a slight stutter at the location of the defects. I haven't checked the code line-by-line for the file which I attached earlier, but I had checked it for the one I am attaching below, which was generated using the WINCNC post-processor, and everything appears to be perfectly symmetrical, yet I still see the defect. Last night I tried some settings as suggested by ShopSabre in the wincnc.ini file, having to do with speeds and accelerations around the arcs. They didn't help. This is a puzzler!
    Stan

    N75 G00 X0.000 Y0.000
    N80 G00 Z0.800
    N85 G00 X0.100 Y-0.087 Z0.200
    N90 G01 Z-0.600
    N95 G01 X23.100
    N100 G03 X23.287 Y0.100 I0.000 J0.187
    N105 G03 X20.728 Y5.987 I-8.050 J0.000
    N110 G02 X18.287 Y11.600 I5.235 J5.613
    N115 G02 X20.728 Y17.213 I7.675 J0.000
    N120 G03 X23.287 Y23.100 I-5.490 J5.887
    N125 G03 X23.100 Y23.287 I-0.187 J0.000
    N130 G01 X0.100
    N135 G03 X-0.087 Y23.100 I0.000 J-0.187
    N140 G03 X2.472 Y17.213 I8.050 J0.000
    N145 G02 X4.913 Y11.600 I-5.235 J-5.613
    N150 G02 X2.472 Y5.987 I-7.675 J0.000
    N155 G03 X-0.087 Y0.100 I5.490 J-5.887
    N160 G03 X0.100 Y-0.087 I0.187 J0.000
    N165 G00 Z0.200
    N170 M5

  15. #15
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    I am just finding time to coming back to this problem. See attached pix which show the bumps at the tangency point between the two arcs. I have tried lowering the accelerations, and velocities. I have tried fitting a bezier spline. I have tried importing the dxf geometry from solidworks into vcarve pro and have also created the geometry natively in vcarve. I have put a 1mm line between the two arcs. Nothing fixes the problem.Attachment 373622Click image for larger version. 

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ID:	373624Click image for larger version. 

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ID:	373626

  16. #16
    Join Date
    Jun 2013
    Posts
    1041

    Re: Machining arcs

    What happens when you cut a circle. Do you have issues with them being round or defects at the quadrants?

    Ben

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  17. #17
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    I machined a circle tonight which was created using the same arcs from the problem file. It was perfect--no bumps at the intersections. Of course, the difference is that with a circle, the arcs are all facing the same direction, whereas the arcs are going in the opposite direction at the tangency points, like a sine wave.

  18. #18
    Join Date
    Dec 2013
    Posts
    5717

    Re: Machining arcs

    Do you have a constant velocity mode available? Or can you post the file as ''convert arcs to lines'' and run the part as all G1 moves?

  19. #19
    Join Date
    Mar 2003
    Posts
    35538

    Re: Machining arcs

    Is it maybe a backlash or backlash compensation issue?
    Gerry

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  20. #20
    Join Date
    May 2010
    Posts
    98

    Re: Machining arcs

    Jim, I have fitted a bezier spline to the arcs and machined those. Is that what you mean? This yielded worse results.

    Gerry, good question about backlash. I hadn't thought of that, even though I should have. I am currently trying to get the factory to machine the same profile and if they see the problem too, this could be a reason.

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