PETG Diamond Drag Engraving Tests

The hairline on the second machined cursor looks pretty good:

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

Based on manually scratching some acrylic, the GCMC code retraced the hairline four times to help the Sharpie stick to the groove. Maybe fewer passes would be better?

Affix a PETG scrap to the milling fixture for some manual CNC action:

PETG - engrave through film
PETG – engrave through film

Just to see what happened, I made the first scratch through the protective film and, because it’s hard to tell which side is up, the scratch went through the white film.

Repeat several times with variations in number of passes & downforce:

PETG - engraving test - overview
PETG – engraving test – overview

Manual jogging FTW:

  • 2 passes, 300 g, through film
  • 2 passes, 300 g, no film
  • 1 pass, 300 g
  • 3 passes, 300 g
  • 4 passes, 300 g
  • 1 pass, 260 g
  • 1 pass, 330 g

A closer look through the PETG sheet, as you’d see finished hairline, with the scratches in the same order as above:

PETG - engraving test - detail grid
PETG – engraving test – detail grid

They may be easier to see against a blank background:

PETG - engraving test - detail plain
PETG – engraving test – detail plain

Or in a hairline’s natural environment:

PETG - engraving test vs Testors sample
PETG – engraving test vs Testors sample

The absolute best-looking line is at the top, with the diamond point scribing through the (white) protective plastic film.

Multiple passes average out the waves / glitches / irregularities, at the cost of broadening the hairline.

The bottom hairline suggests a single pass with more downforce produces a broader groove and a finer line of Sharpie ink at the bottom; the top appears more rounded and the bottom more ragged.

Doing one pass with enough pressure to cut through the thinner (?) transparent(-ish) film may produce a better overall result. This will require me to get the orientation right.

The Real Hairline in my K&E Deci-Lon slipstick is a smoothly engraved, neatly half-cylindrical, channel with a smooth thread of red (!) ink / paint / pigment laid along the middle. Obviously, my engraving hand is weak …

The nightmare scenario: engraving a smooth hairline groove, completely backfilling it with paint, sanding (that side of) the cursor smooth to leave the groove’s paint flush with the surface, then polishing the plastic back to full transparency. Even I agree that’s crazy talk, at least for a circular slide rule made with laminated paper decks.

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.

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 …

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.

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.

bCNC Rounding vs. G-Code Arcs: GRBL Error 33

While cutting the top deck of the Pickett-flavored Tek Circuit Computer on the MPCNC, this happened:

Tek CC - top deck - failed arcs
Tek CC – top deck – failed arcs

I traced the off-center circle with a marker to make it more visible, as it’s the drag knife cut that should have been the exit move after completing the window.

Huh. It never did that before …

The bCNC plot looked fine, but the Terminal log showed three Error 33 reports:

Failed arc command - bCNC screen - terminal and plot
Failed arc command – bCNC screen – terminal and plot

The GRBL doc has this to say about Error 33:

The motion command has an invalid target. G2, G3, and G38.2 generates this error, if the arc is impossible to generate or if the probe target is the current position.

The error messages don’t occur immediately after the failing G2/G3 command, because bCNC sends enough commands to keep the GRBL serial input buffer topped off. After GRBL sends the error message, it continues chewing its way through the buffer and, when bCNC notices the first error, it stops sending more G-Code commands and shudders to a stop.

The great thing about Free Software is that when it breaks, you have all the pieces. Looking into the GRBL source code provides a definition of Error 33:

// [G2/3 Offset-Mode Errors]: No axis words and/or offsets in selected plane. The radius to the current
//   point and the radius to the target point differs more than 0.002mm (EMC def. 0.5mm OR 0.005mm and 0.1% radius).

Which doesn’t quite match the code, but it’s close enough:

// Compute difference between current location and target radii for final error-checks.
            float delta_r = fabs(target_r-gc_block.values.r);
            if (delta_r > 0.005) {
              if (delta_r > 0.5) { FAIL(STATUS_GCODE_INVALID_TARGET); } // [Arc definition error] > 0.5mm
              if (delta_r > (0.001*gc_block.values.r)) { FAIL(STATUS_GCODE_INVALID_TARGET); } // [Arc definition error] > 0.005mm AND 0.1% radius
            }

I’ve drag-knifed maybe a dozen top decks with no problem, so figuring out what broke took a while.

The key turned out to be in the Terminal log, where all coordinates in the G-Code commands had, at most, two decimal places. The GCMC program producing the G-Code emits three decimal places, so bCNC rounded off a digit before squirting commands to GRBL.

After more searching, it seems I’d told bCNC to do exactly that:

bCNC Config - Round 2 digits - highlighted
bCNC Config – Round 2 digits – highlighted

Perhaps I’d mistakenly set “Decimal digits” instead of “DRO Zero padding” when I reduced the DRO resolution from three decimals to two? It’s set to “2” in the CNC 3018XL configuration, so this seems like a typical one-off brain fade.

GRBL doesn’t execute invalid commands, so the tool position remains at the end of the window’s outer perimeter while the next two arc commands fail, because their center offsets produced completely invalid radii.

The three failed arc commands should have cut the right end of the window, the inner side, and the left end, but left the tool position unchanged. The final arc command should have withdrawn the blade along the outer side of the window, but became a complete circle, with the commanded end point equal to the leftover starting point at the same radius from the deck center.

The same G-Code file fails consistently with Decimal digits = 2 and runs perfectly with Decimal digits = 3, so at least I know a good fix.

Protip: Keep your hands away from moving machinery, because you never know what might happen!

This seems sufficiently obscure to merit becoming a Digital Machinist column. More analysis is in order …

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.