Posts Tagged Slipstick

Tek Circuit Computer: Cursor Fixture Adhesion

After removing debris, flattening the top surface, and generally paying more attention to detail, the PETG sheet has much better adhesion to the fixture:

Tek CC - Milled cursor - cleaned fixture
Tek CC – Milled cursor – cleaned fixture

This time, I traced the inside of a drag-knife cut cursor to extract the blank from the stock and, yes, used new double-sided tape under the lower white protective film on the PETG.

Fewer air bubbles means better adhesion:

Tek CC - Milled cursor - fixture adhesion
Tek CC – Milled cursor – fixture adhesion

Spinning the 1/8 inch end mill at about 5000 RPM produced finer swarf at the Sherline’s maximum 609 mm/min = 24 inch/min pace, with less uplift. I suspect Moah RPMs! would be even better, constrained by melting the plastic into heartache & confusion.

Scribe the hairline with the diamond tool, ease the finished cursor off the fixture, scribble Sharpie into the scratch, and wipe

Tek CC - Milled cursor - second try
Tek CC – Milled cursor – second try

It’s Pretty Good™ when seen against an un-laminated bottom deck drawn with a Pilot V5RT pen:

Tek CC - Milled cursor - unlaminated bottom deck
Tek CC – Milled cursor – unlaminated bottom deck

The diamond point tears a slightly gritty path through the PETG, which then looks a bit more granular than a real hairline. I’ve been using four passes for emphasis; perhaps fewer would be better.

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Tek Circuit Computer: Cursor Milling

The white separating film on the double-sided tape makes the cursor milling fixture look presentable:

Tek CC - Cursor milling fixture - 2-side tape applied
Tek CC – Cursor milling fixture – 2-side tape applied

Some deft X-acto knife work exposed the trench around what will be the cursor’s perimeter, in the hope of keeping tape stickiness out of the milling cutter.

Peeling off the white film and sticking a PETG cursor blank to the tape reveals I didn’t do a particularly good job of cleaning the rubble from the trench edges:

Tek CC - Milled cursor - bad tape application
Tek CC – Milled cursor – bad tape application

These PETG sheets arrive with a transparent film on one side and a white film on the other. The picture shows the white film on the bottom of the PETG sheet, with the dark areas corresponding to places where the film sticks to the tape and the tape sticks to the fixture. The lighter areas show an air gap in (at least) one of those interfaces; given the amount of clutter, I think it’s mostly between the tape and the fixture.

I milled the cursor with a 1/8 inch = 3.175 mm cutter:

Tek CC - Milled cursor - outline
Tek CC – Milled cursor – outline

The ball of swarf around the cutter wasn’t as threatening as it appears, because it had very little adhesive holding it together. The rows of swarf surrounding the PETG show why putting the tape all over the fixture isn’t a particularly good idea. ‘Nuff said.

Engraving the hairline with the diamond drag bit was entirely uneventful:

Tek CC - Milled cursor - hairline scribe
Tek CC – Milled cursor – hairline scribe

Four passes at Z=-2 mm = 300 g downforce put a delicate scratch across the surface. Run a fat black Sharpie along the hairline, wipe off the excess with denatured alcohol, and peel the white film from the other side:

Tek CC - Milled cursor - first try
Tek CC – Milled cursor – first try

It’s sitting atop the doodle giving the dimensions, such as they are, for the milling fixture.

The hairline came out so fine it makes the Pilot V5RT ballpoint pen lines look downright chunky:

Tek CC - Yellow Cardstock - Pilot V5RT - Milled Cursor
Tek CC – Yellow Cardstock – Pilot V5RT – Milled Cursor

Seen over the engraving test piece with scraped Testors paint, however, things look just the way they should:

Tek CC - Engraved - Testors Paint - Milled Cursor
Tek CC – Engraved – Testors Paint – Milled Cursor

In a techie kind of way, of course, which is the only way that matters on Planet Slipstick …

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Tek Circuit Computer: Cursor Milling Fixture

The original Tektronix Circuit Computer cursor is a floppy sheet of plastic with a hairline printed on it. I’m making the homage version from 0.5 mm PETG sheet with an engraved hairline:

Tek CC - radial text example
Tek CC – radial text example

But I don’t foresee enough ahem production volume to justify making a punch-and-die to cut the thing out, so I need a milling fixture to hold the sheet in place while I have my way with it.

Start by squaring up a suitably sized scrap from the Box o’ Plastic Scrap:

Tek CC - Cursor milling fixture - squaring sides
Tek CC – Cursor milling fixture – squaring sides

It need not be particularly square, but getting rid of the ragged edges seemed like a Good Idea. I think it’s polycarbonate and, yes, it’s just about that green in real life.

Align it square-ish to the tooling plate and drill three #7 holes on 1.16 inch centers to line up with the plate and clear the Sherline’s 10-32 screws:

Tek CC - Cursor milling fixture - hole drilling
Tek CC – Cursor milling fixture – hole drilling

The two outer holes will clamp the fixture to the table. The third hole may be useful to clamp a stack of cursors to the fixture, should I need more than a few.

Screw it to the tooling plate, mill the outline of the cursor into the fixture, apply a layer of double sticky tape, then cut out the cursor outline so the milling bit won’t accrete a giant whirling ball of adhesive & swarf:

Tek CC - Cursor milling fixture - 2-side tape applied
Tek CC – Cursor milling fixture – 2-side tape applied

I milled the perimeter 2 mm deep, anticipating a 1 mm cut depth for the cursor, and milled a small step inside the perimeter by compiling the GCMC code with a 2.5 mm cutter diameter instead of the actual 3.175 mm. I tweaked the cursor code for proper offset milling, about which more later.

With the tape in place, it’s not entirely obvious this will work the way I expect, but it wasn’t too difficult.

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Drag Knife Blade Extension

The battered corner of my bench scale shows it’s been knocking around for quite a while, but the drag knife blade tip seems pretty close to the first 0.5 mm division:

Drag Knife Blade - 0.5 mm
Drag Knife Blade – 0.5 mm

The blade extends from the LM12UU holder for the MPCNC.

Scribbling the blade across a scrap of laminated yellow card stock (about 0.4 mm thick) showed it didn’t cut all the way through the bottom plastic layer, even with the spring mashed flat.

So I screwed it out to 0.7 mm:

Drag Knife Blade - 0.7 mm
Drag Knife Blade – 0.7 mm

The scale isn’t quite parallel to the blade axis and maybe it’s sticking out 0.8 mm; setting a drag knife’s blade extension obviously isn’t an exact science.

In any event, another scribble slashed all the way through the laminated deck without gashing the sacrificial cardboard atop my desk, which seems good enough.

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Homage Tek Circuit Computer: Yellow Variation

An on-sale pack of yellow Astrobrights card stock tempted me:

Homage Tek CC - Yellow Astrobrights paper
Homage Tek CC – Yellow Astrobrights paper

The somewhat wrecked cursor comes from my collection of discards, because I haven’t yet figured out how to mill the outline and engrave the hairline on raw stock.

The paper isn’t quite the same color as my Genuine Pickett Model 110-ES circular slide rule:

Homage Tek CC vs Pickett 110ES colors
Homage Tek CC vs Pickett 110ES colors

Nor, of course, are the ticks and legends nearly as fine as you get with real engraving, but it’s probably Close Enough™ for anybody other than a Real Collector™.

The Pilot V5RT ink bleeds less on Astrobrights card stock than on the previous, somewhat coarser, card stock:

Tek CC - Yellow Astrobrights paper - bare
Tek CC – Yellow Astrobrights paper – bare

An automagic color adjustment bleaches the yellow and makes the black ink much more visible.

Laminating the paper crisps the contrast a bit, although it’s more obvious in person:

Tek CC - Yellow Astrobrights paper - laminated
Tek CC – Yellow Astrobrights paper – laminated

You can see tiny air bubbles over the darkest part of the ticks and letters.

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Diamond-Drag Styrene Engraving: Scraped Enamel

For the first time in a loooong time, I applied Testors Gloss Enamel paint to styrene plastic:

Engraving Testpiece D - Testors Enamel - red
Engraving Testpiece D – Testors Enamel – red

Two coats of black paint produced the larger areas along the inner scales and completely filled those engraved lines:

Engraving Testpiece D - Testors Enamel - red black applied
Engraving Testpiece D – Testors Enamel – red black applied

With exactly the correct paint on exactly the correct material, it cured into a non-removable layer. Being enamel, however, the last coat requires two or three days for a full cure, so this isn’t a short-attention-span project.

It’s “non-removable” unless you’re willing to abrade the surface:

Engraving Testpiece D - Testors Enamel - scrape sand - overview
Engraving Testpiece D – Testors Enamel – scrape sand – overview

Sanding tends to remove too much plastic, particularly when confronted with raised walls & suchlike along the grooves. The darkest scale down the middle was engraved with 300 g downforce and is deep enough to retain all its paint:

Engraving Testpiece D - Testors Enamel - sanded - 250 300 g - detail
Engraving Testpiece D – Testors Enamel – sanded – 250 300 g – detail

As expected, paint scrapers produce better results:

Engraving Testpiece D - Testors Enamel - scrape - 250 300 g - detail
Engraving Testpiece D – Testors Enamel – scrape – 250 300 g – detail

There’s not much visible difference between the 250 g and 300 g scales.

All the scraped lines are over 0.1 mm wide, with the heavier downforce producing maybe 0.12 mm.

The double-coated lines are flush with the (scraped) surface and visibly matte. The single-coated regions have the usual glossy enamel finish remaining deep in the lines & numbers, with a thin matte outline flush with the surrounding surface. It’s basically impossible to photograph those features, at least for me.

The colors are crisp & vivid: enamel paint is the way to go!

The next testpiece should run downforce variations from 300 through 500 g and speeds from 1000 to 2400 mm/min. Scraping off the raised plastic before painting should deliver a better ahem painting experience without much surface damage; the trick will be clearing all the debris from the engraved lines.

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Diamond-Drag Styrene Engraving: Scraped Sharpie

Applying only two Sharpie colors to the third quadrant of the engraving testpiece produces a more restrained result:

Diamond on styrene C - scraped red-black Sharpie - start
Diamond on styrene C – scraped red-black Sharpie – start

Instead of sanding the surface, I used a paint scraper to remove everything down to the engraved grooves. The scraper in the upper right is a Rubbermaid 54807, which is apparently no longer available. If I ever buy a new scraper, I’ll spring for a carbide blade.

A dirt speck under the plastic sheet can still obliterate the markings, though:

Diamond on styrene C - scraped red-black Sharpie - first clearing
Diamond on styrene C – scraped red-black Sharpie – first clearing

Overall, the results look just like a real slipstick:

Diamond on styrene C - scraped red-black Sharpie - 225 300 g scale detail
Diamond on styrene C – scraped red-black Sharpie – 225 300 g scale detail

The upper scale was engraved at 225 g downforce, the lower at 300 g, with corresponding differences in width & depth.

Seen at higher magnification with omnidirectional light through the microscope, the tick marks have more detail:

Diamond on styrene C - scraped red-black Sharpie - 225 300 g line detail
Diamond on styrene C – scraped red-black Sharpie – 225 300 g line detail

The upper ticks are 0.1 mm wide and the lower ticks a scant 0.2 mm wide. Both ticks on the sanded Sharpie sample were close to 0.1 mm, which suggests:

  • Scraping removes less plastic
  • The grooves have a flat-ish bottom and side walls roughly matching the slightly worn 60° diamond tool

Sharpie ink is, of course, soluble in alcohol:

Diamond on styrene C - scraped red-black Sharpie - alcohol wipe
Diamond on styrene C – scraped red-black Sharpie – alcohol wipe

That’s not unexpected, as I’ve been removing Sharpie with alcohol forever, but it’s worth keeping in mind. I don’t know if spraying a clear topcoat (Krylon FTW!) would provide good sealing with enough wear resistance.

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