The New Hotness

Having a tool length probe station on the Sherline, I had to build one for the MPCNC:

MPCNC Tool Length Probe - plotter pen — MPCNC Tool Length Probe – plotter pen

It’s little more than a flange atop a wide base:

MPCNC Tool Length Probe - Slic3r preview — MPCNC Tool Length Probe – Slic3r preview

The flange offset puts the switch actuator on the midline of the base, not that that matters, and the base features rounded corners and a suitable legend, because I can.

I clipped the PCB’s through-hold leads nearly flush and stuck it to the flange with 3M permanent foam tape, which seems to work much better than screws & inserts for simple things that need never come apart.

The Protoneer CNC Shield includes a Probe input on the GRBL-compliant A5, although it took me a while to find the legend on the SCL pin in the I2C header. I moved the endstop power jumper to another header, then conjured a quick-and-dirty connector:

Protoneer CNC Shield - Tool Probe Wiring — Protoneer CNC Shield – Tool Probe Wiring

When I embed the endstop switch PCB in epoxy, I’ll add a drop to the connector while engaging in Magical Thinking. The whole Arduino + CNC Shield must go into an enclosure after I finish measuring the motor currents.

To forestall discussions about switch repeatability and accuracy, suffice it to say the MPCNC doesn’t claim to be much more than a woodworking router, so those switches seem Good Enough.

The OpenSCAD source code as a GitHub Gist:

	// MPCNC Tool Length Probe Station
	// Ed Nisley KE4ZNU – 2017-12-08

	/* [Extrusion] */

	ThreadThick = 0.25; // [0.20, 0.25]
	ThreadWidth = 0.40; // [0.40]

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	/* [Hidden] */

	Protrusion = 0.1; // [0.01, 0.1]

	HoleWindage = 0.2;

	/* [Sizes] */

	EndstopPCB = [40.0,16.5,1.6]; // endstop PCB
	ComponentHeight = 6.0; // max component height above PCB

	SwitchOffset = [35.0,21.0,4.75]; // touch point center from lower left PCB corner
	SwitchTravel = 2.5; // first touch to switch actuation

	TapeThick = 1.0; // foam mounting tape

	WallThick = 4.0; // basic wall & floor thickness

	BaseRadius = 5.0;

	Base = [IntegerMultiple(EndstopPCB[0] + 2*BaseRadius,20),
	IntegerMultiple(EndstopPCB[2] + TapeThick + WallThick + 2BaseRadius,225),
	2.0];

	NumSides = 8*4;

	TextDepth = 2*ThreadThick;

	//- Build it

	union() {
	difference() {
	hull()
	for (i=[-1,1], j=[-1,1])
	translate([i(Base[0]/2 – BaseRadius),j(Base[1]/2 – BaseRadius),0])
	resize([0,0,Base[2]])
	intersection() {
	cylinder(r=BaseRadius,h=BaseRadius,$fn=NumSides);
	sphere(r=BaseRadius,$fn=NumSides);
	}
	translate([0,0,Base[2] – TextDepth])
	linear_extrude(height=TextDepth + Protrusion) {
	translate([0,-10,0])
	text(text="MPCNC",size=8,spacing=1.05,font="Arial:style=Bold",halign="center");
	translate([0,-18,0])
	text(text="Tool Probe",size=6,spacing=1.05,font="Arial:style=Regular",halign="center");
	}

	}
	translate([0,
	WallThick/2 + TapeThick + SwitchOffset[2],
	(EndstopPCB[1] – Protrusion)/2 + Base[2]])
	cube([EndstopPCB[0],WallThick,EndstopPCB[1] + Protrusion],center=true);

	}

view raw Tool Length Probe Station.scad hosted with ❤ by GitHub

The original doodles show a severely over-complexicated solution desperately searching for an actual problem:

MPCNC Tool Length Probe - doodles — MPCNC Tool Length Probe – doodles

Putting a large flat pan at the end of a relatively long lever arm, with the pivot arranged to put the pan level at the switch actuation point, made sense at the time. Give the relatively small tools I expect to use, directly ramming them into the switch lever should work just as well.

Putting all that complexity in harm’s way seemed like a Bad Idea when I sat down and looked at it in cold blood.

The New Hotness

MPCNC: Tool Length Probe Station