Tek Circuit Computer: 3D Printed Cursor Milling Fixture

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,$fn=Sides);
}
//----------------------
// Dimensions
CursorHubOD = 1.0*inch; // original Tek CC was hard inch!
CursorTipWidth = (9.0/16.0)*inch;
CursorTipRadius = (1.0/16.0)*inch;
CursorThick = 0.5; // plastic sheet thickness
CutterOD = 3.175; // milling cutter dia
CutterDepth = 2.0; // … depth of cut
CutterLip = 0.5; // … clearance under edge
ScribeOD = 3.0; // diamond scribe shank
StudOC = [1.16*inch,1.16*inch]; // Sherline tooling plate grid
StudClear = 5.0; // … screw clearance
StudWasher = 11.0; // … washer OD
CursorOffset = [-2*StudOC.x,0,0]; // hub center relative to fixture center
// must have even multiples of stud spacing to put studs along centerlines
BasePlateStuds = [6*StudOC.x,2*StudOC.y]; // fixture screws
echo(str("Stud spacing: ",StudOC));
CornerRad = 10.0; // corner radius
BasePlate = [2*StudWasher + BasePlateStuds.x,2*StudWasher + BasePlateStuds.y,5.0];
echo(str("Base Plate: ",BasePlate));
EngravePlate = [5*StudOC.x,1.5*StudOC.y,BasePlate.z];
echo(str("Engrave Plate: ",EngravePlate));
TemplateThick = 6*ThreadThick;
LegendThick = 2*ThreadThick;
Gap = 3.0;
//----------------------
// Import SVG of cursor outline
// Requires our hub OD to match reality
// Hub center at origin
module CursorSVG(t=CursorThick,od=0) {
hr = CursorHubOD/2;
translate([-hr,-hr,0])
linear_extrude(height=t,convexity=3)
offset(r=od/2)
import(file="/mnt/bulkdata/Project Files/Tektronix Circuit Computer/Firmware/TekCC-Cursor-Mark.svg",center=false);
}
//----------------------
// Milling fixture for cursor blanks
module Fixture() {
difference() {
hull() // basic plate shape
for (i=[-1,1], j=[-1,1])
translate([i*(BasePlate.x/2 - CornerRad),j*(BasePlate.y/2 - CornerRad),0])
cylinder(r=CornerRad,h=BasePlate.z,$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([i*BasePlateStuds.x/2,j*BasePlateStuds.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([i*StudOC.x,j*StudOC.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([i*1.5*StudOC.x,0,-Protrusion])
PolyCyl(2*ScribeOD,EngravePlate.z + 2*Protrusion,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 + 3*CursorHubOD)/2 + 2*Gap,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:

Cursor Fixture doodle
Cursor Fixture doodle

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.

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