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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