Posts Tagged Slipstick
Although the original Tektronix Circuit Computer had relentlessly monochrome scales, a dash of color added a festive holiday look:
Well, OK, that’s excessive.
The intent was to see how the pens behaved, with an eye toward accenting general-purpose circular slide rule scales with a few colored characters.
The green pen shows how I built the arrows by drawing a line through vertical arrow characters:
I like blue ink entirely too much, having used a blue pen as my daily writer for most of my adult life:
Red ink for “backwards” scales and suchlike would work well, even if it’s too vivid for the tick marks:
Those are all on unlaminated plain paper, with plenty of room for improvement.
Seeing as how I’d be doing all the “tool changes” manually, optimizing the plotting sequence would be mandatory: one pen change per color per deck!
Cheap 1 mm pens produce scratchy lines:
More expensive 0.5 mm Pilot Precise V5RT pens produce well-filled lines:
For reference, the cheap pens use a collet holder:
The Pilot V5RT pens use a custom holder:
A 3D printer really simplifies making things!
Although a sex bolt works as a central pivot, even the shortest one available in a cheap assortment is too long for three paper decks and an acrylic cursor:
An eyelet / grommet intended for leather crafting works better:
That’s the front side, with the stylin’ rounded head, in “gunmetal” gray. The shank is 5 mm ID (the advertised size), 5.5 mm (-ish) OD, 4 mm long beyond the 10 mm OD head. All dimensions vary unpredictably between sellers, so expect nothing in particular and you won’t be disappointed.
The back side gets the washer:
The entire stack is 1.7 mm tall: three 0.4 mm laminated decks and the 0.5 mm polypropylene cursor. The 4 mm shank length seems excessive, but works out well in practice, even if I need more practice at smoothly swaging shank over washer. It’s sufficiently good looking in person.
Note: the washer goes on convex side outward!
The set includes a hole punch suitable for leather work and slightly too small for paper, plus the swaging punch and die required for the washer.
The first pass at cutting laminated decks for the Homage Tektronix Circuit Computer left little uncut snippets at the starting point of the cut. The point of the drag knife blade trundles along behind the cutting edge and, when the ending point equals the starting point, leaves an un-cut sliver as it’s retracted vertically:
The knife blade isn’t aligned in any particular direction, so it can leave a nick on either side as it enters the deck vertically at the start of the cut.
Gradually entering the deck along the cut line gives the blade enough time to swivel around to the proper alignment before it gets down to serious cutting. Continuing the final cut past the starting point then allows the blade to recut anything remaining from the entry move.
The middle and top decks have windows exposing the scales:
The paths are basically two arcs connected by semicircular cuts, but with ramps on each end recutting the entry and exit paths:
The entry path in the upper left slants downward from the
TravelZ level of 1.5 (-ish) mm to Z=0, with the nose of the blade holder flush against the surface and the blade sunk to its full length. The vertical path to Z=-2 (-ish) increases the cutting pressure from roughly the preload value to preload + 2*(spring rate), so the blade won’t ride up under the cutting forces.
The path then goes completely around the window at Z=-2, then ramps up to the
TravelZ level again.
All of which produces a neat cutout that sticks to the Cricut mat when I peel the rest of the deck off:
That’s a middle deck before I started laminating them, but you get the general idea.
The GCMC code (extracted from the complete lump) looks like this:
local WindowArc = 54deg; local ac = -17 * ScaleArc + ScaleRT/2; // center of window arc local r0 = DeckRad - ScaleHeight; // outer local r1 = DeckRad - 2 * ScaleHeight; // inner local aw = WindowArc - to_deg(atan(ScaleHeight,(r0 + r1)/2)); // window arc minus endcaps p0 = r0 * [cos(ac + aw/2),sin(ac + aw/2),-]; p1 = r0 * [cos(ac - aw/2),sin(ac - aw/2),-]; local p2 = r1 * [cos(ac - aw/2),sin(ac - aw/2),-]; local p3 = r1 * [cos(ac + aw/2),sin(ac + aw/2),-]; goto(p3); arc_cw(p0 +| [-,-,0],ScaleHeight/2); // blade enters surface move([-,-,KnifeZ]); // apply pressure arc_cw(p1,r0); // smallest arc arc_cw(p2,ScaleHeight/2); // half a circle arc_ccw(p3,r1); arc_cw(p0,ScaleHeight/2); arc_cw(p1 +| [-,-,TravelZ],r0); // exit from cut goto([0,0,-]); goto([-,-,SafeZ]);
Having measured the angular position of the window and its size on the original Tek CC, I compute the coordinates of the four points where the semicircular “end caps” meet the longer arcs, then connect the dots with arc_xx() functions to generate the G-Code commands. As always, using the proper radius signs requires trial & error.
While I was at it, I added entry & exit moves for the deck’s central pivot hole and outer perimeter.
I’m pretty sure the right CAM package would take care of that, but GCMC operates well below the CAM level.
To judge from the dislodged pigment grains, the original Tektronix Circuit Computer probably used then-new laser printing on good-quality paper, laminated between plastic sheets:
A Pilot Precise V5RT cartridge on plain paper (20 lb 98 white), also laminated, looks pretty good:
But a black V5RT pen on HP Glossy Presentation Paper (44 lb, 160 g/m²), also laminated, is spectacular:
The glossy Presentation paper is hard enough to keep the pen ball from sinking in, producing much finer lines. In round numbers:
- 0.2 mm – Tek laser-printed (?) original
- 0.3 mm – green V5RT on plain paper
- 0.2 mm – black V5RT on glossy Presentation paper
The CNC 3018XL plotted / drew everything at 2400 mm/min = 40 mm/s, with minimal wobbulation in the lines and none worth mentioning in the characters.
The pen ball sometimes pulls a dot of ink off the glossy paper as it rises at the end of a stroke; perhaps matte paper would produce more traction on the ink.
You can see small blobs at the end of some strokes, but the fancy paper prevents most of the bleeding visible in the previous tests. Pilot V5 pens definitely dislike card stock.
The results looks great in person without magnification, so maybe none of that matters.
A reproduction circular slide rule from the mid-1960s may not be the cutting edge of consumer demand, but the pen version of a Tektronix Circuit Computer came out pretty well:
A Bash script compiles the GCMC code with eight different parameter combinations to produce pairs of G-Code files to draw (“engrave” being aspirational) and cut (“mill”, likewise) the three decks and the cursor.
Better paper definitely produces better results, so I must rummage through the Big Box o’ Paper to see what lies within. Laminating the decks improves their durability and matches the original Tek surface finish.
Setting the XY origin to dead center on each deck requires carefully calibrating the USB video camera, with the end result accurate to maybe ±0.1 mm around the entire perimeter. Both machines move equal linear distances along both axes, which was definitely comforting.
Having made half a dozen cursors from various bits of acrylic, none of which look particularly good, demonstrates my engraving hand is too weak for a complete slide rule:
With logarithmic scales in hand, however, adapting the GCMC source code to produce general-purpose circular slide rules with only two decks and smaller diameters may be the way to improve my engraving-fu, as a full-scale Tektronix Circuit Computer would chew up three square-foot plastic sheets.
A general-purpose slide rule would need multi-color (well, at least bi-color) labels and digits for red “inverse” scales to remind you (well, me) they read backwards. Some slipsticks use left-slanting italics, left-pointing markers (“<2”), or other weirdness, but they’re all different.
An early small-scale version engraved on ABS came out OK, modulo poor ink fill:
Engraving the decks on hard drive platters doesn’t count:
All in all, it’s been an interesting exercise and, as you may have guessed, will become a Digital Machinist column.
The GCMC and Bash source code as a GitHub Gist:
Which turns out to be the case:
That’s a stack of three “Homage” Tek CC bottom decks under a Genuine Tektronix Circuit Computer.
The black scale at the top of the picture (and the bottom of the stack) came from a 1 mm cheap pen in the collet holder, the two green scales come from a 0.5 mm Pilot V5RT cartridge in its new holder, and the Original is (most likely) laser-printed back when that was a New Thing.
As always, paper makes a big difference in the results. The brownish paper is 110 pound card stock with a relatively coarse surface finish. The white paper is ordinary 22 pound general-purpose laser / inkjet printer paper.
The 1.0 mm pen (top) doesn’t much care what it’s writing on, producing results on the low side of OK: some light sections, no blobs. Perfectly serviceable, but not pretty.
The Pilot V5RT really likes better paper, as it bleeds out on the card stock whenever the CNC 3018XL so much as pauses at the end of a stroke. Using white paper slows, but doesn’t completely stop, the bleeding, making the blobs survivable.
I’ve been using card stock to get stiffer, more durable, and more easily manipulated decks, but the improved line quality on the white paper says I should laminate the decks in plastic, just like the original Tektronix design.
No surprise there!