Machine Shop, Software

Drag Knife Calibration: Downforce and Speed

The drag knife faceplant suggested I must pay a bit more attention to fundamentals, so, with a 60° drag knife blade sticking out a reasonable amount, the next step is to see what effect the cutting “depth” (a.k.a. downforce) and speed have on the outcome.

A smidge of GCMC code later:

Drag Knife Cal - depth - overview - Camotics sim
Drag Knife Cal – depth – overview – Camotics sim

It’s not obvious, but each pattern steps downward by 0.5 mm from left to right. With the spring force equal to 375 g + 57 g/mm, the downforce ranges from 400 to 520 g over the five patterns.

Laminated scrap, meet drag knife:

Drag Knife Cal - Depth - as cut
Drag Knife Cal – Depth – as cut

Pulling up on the surrounding scrap left the patterns on the sticky mat:

Drag Knife Cal - Depth - extracted
Drag Knife Cal – Depth – extracted

Which suggested any cutting force would work just fine.

Flushed with success, I cut some speed variations at the minimum depth of Z=-0.5 mm = 400 g:

Drag Knife Cal - Speed - 0.5 mm - as cut
Drag Knife Cal – Speed – 0.5 mm – as cut

The blade cut through the top laminating film, the paper, and some sections of the bottom film, but mostly just scored the latter.

Repeating at Z=-1.5 mm = 460 g didn’t look much different:

Drag Knife Cal - Speed - 1.5 mm - as cut
Drag Knife Cal – Speed – 1.5 mm – as cut

However, the knife completely cut all the patterns:

Drag Knife Cal - Speed - 1.5 mm - extracted
Drag Knife Cal – Speed – 1.5 mm – extracted

As far as I can tell, the cutting speed doesn’t make much difference, although the test pattern is (deliberately) smooth & flowy like the Tek CC deck outlines. I’d been using 1000 mm/min and 2000 mm/min seems scary-fast, so 1500 mm/min may be a good compromise.

The GCMC source code as a GitHub Gist:

// Calibrate Drag Knife - speed & feed
// Ed Nisley - KE4ZNU
// 2020-03 values for MPCNC
//-----
// Dimensions
CutIncr = -0.5mm;
BottomCutZ = -2.5mm;
SpeedRatio = 2.0;
MaxSpeed = 2000mm;
MinSpeed = MaxSpeed / 8;
StripWidth = 10mm;
CornerRadius = StripWidth/2;
PatternSize = StripWidth * [3,3];
PatternSpace = 1.25;
SafeZ = 10.0mm; // above all obstructions
TravelZ = 2.0mm; // within engraving / milling area
FALSE = 0;
TRUE = !FALSE;
if (!isdefined("TestSelect")) {
TestSelect = "Depth";
}
comment("Test Selection: ",TestSelect);
//-----
// One complete pattern
// Centered at ctr, ctr.z=cut depth
function Pattern(ctr) {
local d1 = CornerRadius; // useful relative distances
local d2 = 2*d1;
local d3 = 3*d1;
local d4 = 4*d1;
goto([-,-,TravelZ]); // set up for entry move
goto(head(ctr,2) + [-d2,d3]);
move([ctr.x + d2,-,ctr.z]); // enter to cut depth
arc_cw_r([d1,-d1],d1);
move_r([0,-d4]);
arc_cw_r([-d1,-d1],d1);
move_r([-d4,0]);
arc_cw_r([0,d2],d1);
move_r([d2,0]);
arc_ccw_r([0,d2],d1);
move_r([-d2,0]);
arc_cw_r([0,d2],d1);
move_r([d4,0]); // re-cut entire entry path
goto([-,-,TravelZ]); // exit to surface
// goto(head(ctr,2));
}
//-----
// Start cutting!
goto([-,-,SafeZ]);
goto([0,0,-]);
goto([-,-,TravelZ]);
if (TestSelect == "Depth") {
comment("Depth variations");
s = MaxSpeed / 2;
feedrate(s);
c = [0,0,-]; // initial center at origin
for (c.z = CutIncr; c.z >= BottomCutZ; c.z += CutIncr) {
comment("At: ",c," speed:",s);
Pattern(c);
c.x += PatternSpace * PatternSize.x;
}
}
if (TestSelect == "Speed") {
comment("Speed variations");
c = [0,0,-2mm]; // initial center at origin
for (s = MinSpeed; s <= MaxSpeed; s *= SpeedRatio) {
comment("At: ",c," speed: ",s);
feedrate(s);
Pattern(c);
c.x += PatternSpace * PatternSize.x;
}
}
goto([-,-,SafeZ]);
goto([0,0,-]);
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