Tek Circuit Computer: Acrylic Cursor Hairline

A slide rule needs a cursor with a hairline to align numbers on its scales:

The GCMC code generating the hairline is basically a move scratching one line into the surface with the diamond bit:

  feedrate(ScaleSpeed);

  goto([-,-,TravelZ]);

  repeat(2) {
    goto([DeckTopOD/2 - 3*ScaleHeight,0,-]);
    move([-,-,EngraveZ]);
    move([DeckBottomOD/2 + ScaleHeight,0,-]);
    goto([-,-,TravelZ]);
  }

Two passes make the scratch deep enough to hold engraving crayon / lacquer / ink, without making it much wider. Laser engraving would surely work better.

In lieu of actually milling the cursor, this code scratches the perimeter:

  local dr = DeckBottomOD/2;
  local hr = CursorHubOD/2;
  local a = atan(hr - CursorTipWidth/2,dr);   // rough & ready approximation

  local p0 = hr * [sin(a),cos(a),-];          // upper tangent point on hub

  local c1 = [dr - CursorTipRadius,CursorTipWidth/2 - CursorTipRadius*cos(a),-];
  local p1 = c1 + [CursorTipRadius*sin(a),CursorTipRadius*cos(a),-];

  local p2 = c1 + [CursorTipRadius,0,-];      // around tip radius

  feedrate(KnifeSpeed);

  goto([-,-,TravelZ]);
  goto([-hr,0,-]);
  move([-,-,EngraveZ]);

  repeat(3) {
    arc_cw(p0,hr);
    move(p1);
    arc_cw(p2,CursorTipRadius);

    move([p2.x,-p2.y,-]);
    arc_cw([p1.x,-p1.y,-],CursorTipRadius);
    move([p0.x,-p0.y,-]);
    arc_cw([-hr,0,-],hr);
  }

Three passes makes it deep enough to snap along the line:

If you look closely, though, you’ll find a little divot over on the left along the bottom edge, so I really must machine the thing.

Were I to go into production, I’d have to figure out a fixture, but I think I can just clamp a rough-cut acrylic rectangle to the Sherline’s table, mill half the perimeter, re-clamp without moving anything, then mill the other half.

Subtractive machining is such a bother!

The pivot holding the cursor and decks together is a “Chicago screw“, a.k.a. a “sex bolt“. I am not making this up.