The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

The New Hotness

  • OMTech 60 W Laser: Speed vs. Corner Radius Wobbulation

    Experimenting with little squares showed the Y axis has a definite wobble:

    Subpixel Zoo - Quattron RGBY Shifted - detail
    Subpixel Zoo – Quattron RGBY Shifted – detail

    Which suggested a simple test:

    Cornering - overview
    Cornering – overview

    I adjusted the laser power to compensate for the speed, with the result being a line burned into white cardboard. The lines are a bit under 0.2 mm wide, roughly the width of the focused spot.

    The controller settings for the X and Y axes:

    KT332N - X Y Axis Parameters - 2025-02-18
    KT332N – X Y Axis Parameters – 2025-02-18

    The acceleration values may be affected by the limits in this section:

    KT332N - Cut Engraving Parameters - 2025-02-18
    KT332N – Cut Engraving Parameters – 2025-02-18

    Assuming the Y axis acceleration is 3000 mm/s², the RepRap calculator shows the Y axis speeds within the 30 mm distance along the vertical sides:

    RepRap Accel Calculator - 3000mm-s2 30mm
    RepRap Accel Calculator – 3000mm-s2 30mm

    Extracting the useful bits and lining them up for comparison:

    Cornering - detail
    Cornering – detail

    The first column in the test results shows perfectly square corners have no problem at any speed, because the controller decelerates to nearly a stop before changing direction.

    Rounding the corner to 0.5 mm introduces a distinct wobble in the Y axis that doesn’t change much, probably because the controller still decelerates as it approaches the corner.

    The 1 mm radius corners show a distinct overshoot at all speeds. The peak overshoot doesn’t change much between 250 and 500 mm/s, because the RepRap calculator shows the machine barely reaches 250 mm/s by the middle of the side, so 500 mm/s isn’t any faster.

    The first overshoot is about 0.2 mm, the first undershoot is a little over 0.1 mm, and the rest are barely visible.

    The 2 and 4 mm radius corners have barely visible wobbles. Whether that is due to the head not flexing as much due to the lower acceleration around the larger radius I cannot say.

    The machine may not follow the simple RepRap acceleration profile when approaching a corner, let alone a rounded corner.

    I think attempting to reduce the overshoot by fiddling with the belt tension / hardware fasteners / whatever will be unavailing. The laser head runs on a linear rail along the gantry with plenty of unbalanced mass hanging off the bottom:

    OMTech 60W beam alignment - head X plane
    OMTech 60W beam alignment – head X plane

    Moving the beam 0.2 mm on the platform by pivoting around the rail 6 inch = 150 mm above amounts to only 0.08°, far less than anything I can measure while adjusting the mechanics.

    Slowing down doesn’t help nearly as much as I expected and rounding the corners makes it worse.

    Word has it that much spendier machines behave better, which is both comforting and unhelpful.