Fusion 360 and MPCNC Calibration holes too small Help!

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This topic contains 6 replies, has 4 voices, and was last updated by  Klaus Vaethe 9 months, 3 weeks ago.

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    Klaus Vaethe

    Hi all.

    I’m using the excellent Fusion 360 cam and the post made by the good people of this forum.
    I have calibrated my MPCNC using the regular methods (made a model with two holes 500mm apart, ran the code, measure and calculate (new_e_steps = old_e_steps * (500 / distance_actually_moved).

    Re-ran the Gcode, and the distance is now correct (500mm between holes..)

    The problem is that when I now try to make a hole it comes out too small.
    Made a model with a hole with diameter 10,3mm, and the actual hole it cuts is 9,8mm.

    I am using a 6mm flat end router bit and the “bore” setting in fusion 360 cam. there are no “materiel left” setting used and the simulation shows the operation is good.

    Here is the gcode for the hole.. could someone test it and see if it comes out correctly on their machine? (its a 50x50mm square with the hole and a chamfer in the middle.. the hole is 22mm deep.)

    ;T1  D=6 CR=0 - ZMIN=-23 - flat end mill
    ;Units in mm
    G92 X0 Y0 Z0
    M84 S1800 ;Change Stepper disable timeout to 30 minutes
    G1 Z15 F400
    G1 X24.35 Y25.6 F2000
    G1 Z2 F400
    G1 Z0.6 F500
    G1 X24.343 Z0.506 F500
    G1 X24.321 Z0.415 F500
    G1 X24.285 Z0.328 F500
    G1 X24.235 Z0.247 F500
    G1 X23.285 Z-2.672 F500
    G1 X23.321 Z-2.585 F500
    G1 X23.343 Z-2.494 F500
    G1 X23.35 Z-2.4 F500
    G1 Z15 F400
    M84; Turn steppers off

    Any suggestions would be much appreciated šŸ™‚


    Martin DB

    It shouldn’t be necessary to calibrate your mpcnc…. calculated steps per mm should work ok… how is your new steps?



    I would put the steps back to what they were. Measure your “bit”, proper end mills tend to be better dimensions. Make sure you do a light finishing pass on anything you are going to measure critically.


    Klaus Vaethe

    Hi, Thanks for the replies.

    I have no idea why it needed calibration, but when I told it to drill a hole, move 100mm then drill another hole it was 127mm between the holes.
    after first calibrating using these numbers, I decided to do it again but with 500mm to get better accuracy. It then moved 498,5mm, so i recalibrated again.
    Now it moved 500mm when i tell it to in either x or y direction. but holes are too small..

    I am using a proper milling bit. A 6mm flat nose milling bit with 1 flute. I measured the bit and it is exactly 6mm.
    When using the “bore” function in Fusion 360 cam, wanting a hole around 10,3mm, I have to make the hole about 1,2mm too big to get the size I want.

    Today I also experienced a runaway about 2 hours into a job (making 150 holes in my table for threaded inserts). Without warning it suddenly went straight to the side while in the middle of a bore operation, making a gorge in the table.. I have no idea why, but it happened once more later just 20 mins. into a similar job..

    I am using the latest Marlin on your site (the RC7). Only thing I have changed are the esteps for calibration.

    I am really unsure what to do now, as the runaway problem is beyond what I am able to troubleshoot. Why would it do this? I can’t find anything in the gcode that suggest it should.



    Are you using 20T pulleys instead of 16T?


    David Walling

    The runaway could be caused by vibes in the table affecting the controller or static in the conduit traveling along the stepper cables.


    Klaus Vaethe

    Hm.. I was sure I used 16T/GT2 pulleys.. I am going to double check.. thanks for the tip, Ryan..
    David, it just might be vibrations.. as the failure occurred when the spindle was on the same side of the table where the electronics are mounted.. I will try to detach the electronics from the table and see if that will remedy the problem. Thanks alot.

    I will update this thread when I have checked these options.

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