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Slow cutting of curves?

Discussion in 'CNC Mills/Routers' started by Chillimonster, Mar 10, 2016.

  1. Chillimonster

    Chillimonster Well-Known

    May 6, 2015
    Likes Received:
    Been having a very minor issue when cutting anything with an arc in it such as my arcade bartop side panels.

    When cutting the straights its flying along, but the curved fronts seems to slow it right down?

    I was thinking it may be the post processor i'm using in Vectric Aspire - i'm using G-Code MM (.tap)

    Would switching Post Processor help matters to something such as G-Code mm ARCS (.tap), or maybe something else.

    I'm using a CNCXPro as the driver.

  2. David the swarfer

    David the swarfer OpenBuilds Team
    Staff Member Moderator Builder Resident Builder

    Aug 6, 2013
    Likes Received:
    this depends greatly on what the post proc is outputting.
    if it is converting arcs into a lot of small line segments, outputting only G1 moves for the arcs, then, yes, that can slow it down since the controller has to
    a) do a calculation for each line segment
    b) start and stop for each line segment.
    c) if they are tiny, then there are LOTS of them and it might then spend most of its time receiving and processing the many lines of Gcode.

    you can fiddle with the GRBL settings to help it transition faster through the curves, or you can use a post that outputs real arc segments (G2/G3 codes).
    the settings are
    $11=0.020 (junction deviation, mm)
    $12=0.002 (arc tolerance, mm)
    and you can read about them at
    Configuring Grbl v0.9 · grbl/grbl Wiki · GitHub
    (make sure you are running the latest GRBL!)

    you can cut this code to see what the effect is of using real arcs, this will cut a circle using 5 arc segments, so it might hesitate slightly at 4 points around the circle.
    (Generated by SketchUcam V1.4a-78373)
    (Bit diameter: 3.0mm)
    (Feed rate: 2000.0mm/min)
    (Plunge Feed rate: 1000.0mm/min)
    (Material Thickness: 6.0mm)
    (Material length: 2500.0mm X width: 1200.0mm)
    (Overhead Gantry: true)
    (Plunge Diam First)
    (Optimization is ON)
    G90 G21 G49 G61 G17
    M3 S30000
    G00 Z5.000
    G00 X7.320 Y12.154
    G00 Z0.500
    G01 Z-6.300 F1000
    G03 X11.6957 Y10.7326 R21.5 F2000
    G03 X31.7533 Y4.2155 R21.5
    G03 X46.8541 Y25.0 R21.5
    G03 X37.7974 Y37.4655 R21.5
    G03 X31.7533 Y45.7845 R21.5
    G03 X7.3197 Y37.8455 R21.5
    G03 X7.3197 Y12.1545 R21.5
    G00 Z5.000
    G00 X0 Y0 (home)
    Bryan Cope likes this.

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