The New Hotness

The original Tektronix Circuit Computer cursor was probably die-cut from a larger sheet carrying pre-printed hairlines:

Tek CC - genuine - detail — Tek CC – genuine – detail

Machining a punch-and-die setup lies well beyond my capabilities, particularly given the ahem anticipated volume, so milling seems the only practical way to produce a few cursors.

Attaching a cursor blank to a fixture with sticky tape showed that the general idea worked reasonably well:

Tek CC - Cursor blank on fixture — Tek CC – Cursor blank on fixture

However, the tape didn’t have quite enough griptivity to hold the edges completely flat against milling forces (a downcut bit might have worked better) and I found myself chasing the cutter with a screwdriver to hold the cursor in place. Worse, the tape’s powerful attraction to swarf made it a single-use item.

Some tinkering showed a single screw in the (pre-drilled) pivot hole, without adhesive underneath, lacked enough oomph to keep the far end of the cursor in place, which meant I had to think about how to hold it down with real clamps.

Which, of course, meant conjuring a fixture from the vasty digital deep. The solid model includes the baseplate, two cutting templates, and a clamping fixture for engraving the cursor hairline:

Cursor Fixture - build layout — Cursor Fixture – build layout

The perimeter of the Clamp template on the far left is 0.5 mm inside the cursor perimeter. Needing only one Clamp, I could trace it on a piece of acrylic, bandsaw it pretty close, introduce it to Mr Belt Sander for final shaping, and finally drill the hole:

Tek CC Cursor Fixture - clamp drilling — Tek CC Cursor Fixture – clamp drilling

The Rough template is 1.0 mm outside the cursor perimeter, so I can trace those outlines on a PET sheet:

Tek CC Cursor Fixture - Rough template layout — Tek CC Cursor Fixture – Rough template layout

Then cut the patterns with a scissors, stack ’em up, and tape the edges to keep them aligned:

TekCC Cursor Fixture - Rough template — TekCC Cursor Fixture – Rough template

Align the stack by feel, apply the Clamp to hold them in place, and secure the stack with a Sherline clamp:

Tek CC Cursor Fixture - outline rear clamp — Tek CC Cursor Fixture – outline rear clamp

The alert reader will note it’s no longer possible to machine the entire perimeter in one pass; more on that in a while.

The baseplate pretty much fills the entire Sherline tooling plate. It sports several alignment pips at known offsets from the origin at the center of the pivot hole:

Tek CC Cursor Fixture - touch-off point — Tek CC Cursor Fixture – touch-off point

Dropping the laser alignment dot into a convenient pip, then touching off X and Y to the known offset sets the origin without measuring anything. Four screws in the corners align the plate well enough to not worry about angular tweakage.

The OpenSCAD source code as a GitHub Gist:

	// Machining fixtures for Tek Circuit Computer cursor
	// Ed Nisley KE4ZNU Jan 2021

	Layout = "Show"; // [Show, Build, Cursor, Clamp, Rough, Engrave]

	/* [Hidden] */

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	HoleWindage = 0.2;

	Protrusion = 0.1; // make holes end cleanly

	inch = 25.4;

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

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
	FixDia = Dia / cos(180/Sides);
	cylinder(d=(FixDia + HoleWindage),h=Height,$fn=Sides);
	}

	//———————-
	// Dimensions

	CursorHubOD = 1.0*inch; // original Tek CC was hard inch!
	CursorTipWidth = (9.0/16.0)*inch;
	CursorTipRadius = (1.0/16.0)*inch;

	CursorThick = 0.5; // plastic sheet thickness

	CutterOD = 3.175; // milling cutter dia
	CutterDepth = 2.0; // … depth of cut
	CutterLip = 0.5; // … clearance under edge

	ScribeOD = 3.0; // diamond scribe shank

	StudOC = [1.16inch,1.16inch]; // Sherline tooling plate grid
	StudClear = 5.0; // … screw clearance
	StudWasher = 11.0; // … washer OD

	CursorOffset = [-2*StudOC.x,0,0]; // hub center relative to fixture center

	// must have even multiples of stud spacing to put studs along centerlines
	BasePlateStuds = [6StudOC.x,2StudOC.y]; // fixture screws
	echo(str("Stud spacing: ",StudOC));

	CornerRad = 10.0; // corner radius

	BasePlate = [2StudWasher + BasePlateStuds.x,2StudWasher + BasePlateStuds.y,5.0];
	echo(str("Base Plate: ",BasePlate));

	EngravePlate = [5StudOC.x,1.5StudOC.y,BasePlate.z];
	echo(str("Engrave Plate: ",EngravePlate));

	TemplateThick = 6*ThreadThick;
	LegendThick = 2*ThreadThick;

	Gap = 3.0;

	//———————-
	// Import SVG of cursor outline
	// Requires our hub OD to match reality
	// Hub center at origin

	module CursorSVG(t=CursorThick,od=0) {

	hr = CursorHubOD/2;

	translate([-hr,-hr,0])
	linear_extrude(height=t,convexity=3)
	offset(r=od/2)
	import(file="/mnt/bulkdata/Project Files/Tektronix Circuit Computer/Firmware/TekCC-Cursor-Mark.svg",center=false);

	}

	//———————-
	// Milling fixture for cursor blanks

	module Fixture() {

	difference() {
	hull() // basic plate shape
	for (i=[-1,1], j=[-1,1])
	translate([i(BasePlate.x/2 – CornerRad),j(BasePlate.y/2 – CornerRad),0])
	cylinder(r=CornerRad,h=BasePlate.z,$fn=24);

	translate(CursorOffset + [0,0,BasePlate.z – CutterDepth])
	difference() {
	CursorSVG(CutterDepth + Protrusion,1.5*CutterOD);
	CursorSVG(CutterDepth + Protrusion,-CutterLip);
	}

	translate(CursorOffset + [0,0,BasePlate.z – 2*ThreadThick]) { // alignment pips
	for (x=[-20.0,130.0], y=[-30.0,0.0,30.0])
	translate([x,y,0])
	cylinder(d=4*ThreadWidth,h=1,$fn=6);

	for (x=[-30.0,130.0,150.0])
	translate([x,0,0])
	cylinder(d=4*ThreadWidth,h=1,$fn=6);
	}

	for (i=[-1,1], j=[-1,1]) // mounting stud holes
	translate([iBasePlateStuds.x/2,jBasePlateStuds.y/2,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6);

	translate(CursorOffset + [0,0,-Protrusion]) // hub clamp hole
	rotate(180/6)
	PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6);

	translate([2*StudOC.x,0,-Protrusion]) // tip clamp hole
	rotate(180/6)
	PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6);

	for (i=[-2:2], j=[-1,1]) // side clamp holes
	translate([iStudOC.x,jStudOC.y,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6);

	}

	}

	//———————-
	// Show-n-Tell cursor

	module Cursor() {

	difference() {
	CursorSVG(CursorThick,0.0);
	translate([0,0,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,TemplateThick + 2*Protrusion,6);
	}
	}


	//———————-
	// Template for rough-cutting blanks

	module Rough() {

	bb = [40,12,LegendThick];

	difference() {
	CursorSVG(TemplateThick,1.0);
	translate([0,0,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,TemplateThick + 2*Protrusion,6);

	difference() {
	translate([bb.x/2 + CursorHubOD/2,0,TemplateThick – bb.z/2 + Protrusion])
	cube(bb + [0,0,Protrusion],center=true);
	translate([bb.x/2 + CursorHubOD/2,0,TemplateThick – bb.z])
	linear_extrude(height=bb.z,convexity=10)
	text(text="Rough",size=7,spacing=1.00,font="DejaVu Sans:style:Bold",halign="center",valign="center");
	}
	}
	}

	//———————-
	// Template for aluminium clamping plate

	module Clamp() {

	bb = [40,12,LegendThick];

	difference() {
	CursorSVG(TemplateThick,-1.0);
	translate([0,0,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,TemplateThick + 2*Protrusion,6);

	difference() {
	translate([bb.x/2 + CursorHubOD/2,0,TemplateThick – bb.z/2 + Protrusion])
	cube(bb + [0,0,Protrusion],center=true);
	translate([bb.x/2 + CursorHubOD/2,0,TemplateThick – bb.z])
	linear_extrude(height=bb.z,convexity=10)
	text(text="Clamp",size=7,spacing=1.00,font="DejaVu Sans:style:Bold",halign="center",valign="center");
	}
	}
	}

	//———————-
	// Engraving clamp

	module Engrave() {

	difference() {

	hull() // clamp outline
	for (i=[-1,1], j=[-1,1])
	translate([i(EngravePlate.x/2 – CornerRad),j(EngravePlate.y/2 – CornerRad),0])
	cylinder(r=CornerRad,h=EngravePlate.z,$fn=24);

	translate(CursorOffset + [0,0,-Protrusion])
	CursorSVG(CursorThick + Protrusion,0.5); // pocket for blank cursor

	translate(CursorOffset + [0,0,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,EngravePlate.z + 2*Protrusion,6);

	translate([2*StudOC.x,0,-Protrusion])
	rotate(180/6)
	PolyCyl(StudClear,EngravePlate.z + 2*Protrusion,6);

	hull() {
	for (i=[-1,1])
	translate([i1.5StudOC.x,0,-Protrusion])
	PolyCyl(2ScribeOD,EngravePlate.z + 2Protrusion,8);
	}
	}

	}


	//———————-
	// Build it

	if (Layout == "Cursor") {
	Cursor();
	}

	if (Layout == "Clamp") {
	Clamp();
	}

	if (Layout == "Rough") {
	Rough();
	}

	if (Layout == "Engrave") {
	Engrave();
	}

	if (Layout == "Show") {
	Fixture();
	color("Green",0.3)
	translate(CursorOffset + [0,0,BasePlate.z + Protrusion])
	Cursor();

	color("Orange")
	translate(CursorOffset + [0,0,BasePlate.z + 10])
	Rough();


	color("Brown")
	translate(CursorOffset + [0,0,BasePlate.z + 20])
	Clamp();

	color("Gold")
	translate(0*CursorOffset + [0,0,BasePlate.z + 40])
	Engrave();

	}

	if (Layout == "Build"){
	rotate(90) {
	Fixture();
	translate([0,-((BasePlate.y + EngravePlate.y)/2 + Gap),EngravePlate.z])
	rotate([180,0,0])
	Engrave();
	translate(CursorOffset + [0,(BasePlate.y + CursorHubOD)/2 + Gap,0])
	Rough();
	translate(CursorOffset + [0,(BasePlate.y + 3CursorHubOD)/2 + 2Gap,0])
	Clamp();
	}
	}

view raw Cursor Fixture.scad hosted with ❤ by GitHub

The original doodle with some notions and dimensions that didn’t survive contact with reality:

I have no idea why the Sherline tooling plate has a 10-32 screw grid on 1.16 inch = 29.46 mm centers, but there they are.

The New Hotness

Tek Circuit Computer: 3D Printed Cursor Milling Fixture