The setup for cutting the Tektronix Circuit Computer desks looks like this:
Tek CC – Bottom Deck cutting setup
Four neodymium bar magnets hold the corners flat against the honeycomb and the neo disk magnet pins the center down, thus ensuring the red alignment laser meets the cutting beam at its focal point on the surface.
The triangular shapes mark the OD of the perimeter (177.8 mm) plus twice the cut margin on each side (2×2 mm), with the tick mark in the upper right ensuring I slap every deck down in the proper orientation. Aligning the two right marks to the edge of the honeycomb frame (with a straightedge for some offset) aims the deck’s 0° index along the cutter’s X axis.
The cut pattern origin is, naturally enough, the center point of the deck, so aligning the red dot to the center cross should put the OD cut at the place all around the perimeter. For confirmation, I fire the laser (“A single ping, Comrade.”) and verify the hole is in the middle of the cross.
Before cutting the deck, the laser also marks the corner shapes, so this may come as some surprise:
Tek CC Middle Deck Corner Targets
The laser printer (a venerable HP LaserJet 1200) produced the dark triangles and the laser cutter (a new OMTech 60 W) burned the light brown marks. The picture is a composite of the four corners, with the blank center removed to concentrate on what’s important.
The scrawls give the edge-to-edge distances in both inches (because that was the scale at hand) and converted to millimeters (because that’s how it’s laid out), with the L suffix for the laser marks.
What’s of interest is that you can’t overlay the two sets of marks by a combination of scaling and rotation with the centers (not shown) of the two patterns pinned together.
The laser measurements differ from the ideal 181.8 mm by 0.1 mm vertically and 0.4 mm horizontally. This may require dinking with the scale factors in the firmware, which I recall having weird values.
The LaserJet is definitely not a precise instrument, off by 0.4 mm vertically and a millimeter horizontally, with considerable variation. I think this comes down to unrealistic expectations for toner stuck to a flexible sheet wrapped around rollers and heated enough to melt dust into the fibers.
Entirely by accident, I discovered that engraving a hairline with LightBurn’s Dot Mode using 1 ms burns and 0.1 mm spacing produces a continuous trench, rather than the series of dots at 0.25 mm:
Tek CC – Cursor Hairline – 30pct 100u – oblique view
The left is at 20% power (12-ish W) and the right is at 30% (18-ish W), both filled with Pro Sharpie red ink.
The V-shaped groove is even more obvious when seen end-on:
Tek CC – Cursor Hairline – 30pct 100u – end view
In both cases, the travel speed seems to be about 10 mm/s regardless of the speed set in the cut layer parameters. The higher power level produces a slightly wider cut that doesn’t seem deeper, which I cannot explain.
Filled with red lacquer crayon, the hairline looks absolutely gorgeous:
Tek CC – Cursor Hairline – 30pct 100u – in place
Engraving the PETG sheet with the protective film in place produces a neat cut with the film edges fused to the plastic.
Cutting the outline and pivot hole in the same operation ensures everything remains perfectly aligned:
Tek CC – Cursor Laser Cutting
Scribble red crayon over the film, make sure the trench is completely filled, peel the film off with some attention to not smearing the pigment, and it’s about as good a hairline as you (well, I) could ask for:
Tek CC – Cursor Hairline – 30pct 100u – Width
The pigment in the trench is about 0.2 mm wide, with slight heat distortion along each side, and I’ll call it Plenty Good Enough.
Totally did not expect this!
Getting a good-looking hairline on a good-looking cursor turns out to be a major challenge, because there’s nowhere to hide the blunders. A few of the many dead ends along the way shows what’s involved:
Not knowing what to expect, I peeled the protective plastic off the styrene PETG sheet before cutting the perimeter, thereby dooming myself to about five minutes of polishing with Novus 2 to remove the condensed vaopor and another five minutes restoring the shine with Novus 1. Next time, I’ll know better.
Eyeballometrically, the hairline is a lovely fine line, but it’s really a series of craters on 0.25 mm centers filled with red Pro Sharpie marker and wiped off with denatured alcohol:
Tek CC – laser-etched cursor hairline – detail
That’s dot mode: 2 ms pulses at 20% power (about 12 W) with a line speed of 100 mm/s and 0.25 mm dot spacing. The craters look to be 0.15 mm in diameter, with a 0.15 mm blast radius merging into a line along the sides. The view is looking through the undamaged side of the cursor, so you’re seeing the craters from their tips.
I cut the cursor and engraved / etched the hairline in one operation, by just laying a rectangle on the honeycomb and having my way with it:
The six pips (small printed holes with ugly black outlines) intended for the Sherline’s laser aligner make this feasible, although the accuracy of the OMTech’s laser pointer requires precisely setting the focal point atop the fixture.
The corners of LightBurn’s tooling layer (the enclosing rectangle) match the corner pip positions, so framing the pattern should light up those four holes. Putting the Job Origin (small green square) at the center-left point lets me tweak the machine’s origin to drop the alignment laser into that pip.
AFAICT, burning a cute puppy picture pretty close to the middle of a slate coaster makes everybody else deliriously happy.
Setting up the cut layer parameters:
Tek CC Cursor – laser dot mode tests
Burning through the protective film, peeling it off, filling with Sharpie, and wiping with alcohol produces interesting results against a 0.1 inch = 2.54 mm grid:
Tek CC Cursor – dot mode 1-2ms 10-20pct
The angled top and bottom lines are the edges of the cursor, positioned with the craters on the top surface.
The bottom three lines at 10% power consist of distinct 0.10 mm craters incapable of holding much ink:
Tek CC Cursor – dot mode 2ms 10pct
The top three lines at 20% power have 0.15 mm craters and look better:
Tek CC Cursor – dot mode 1ms 20pct
The top line was a complete surprise: it seems a 20% duty cycle does not turn off completely between 1 ms dots spaced at 0.15 mm. I expected a row of slightly overlapping dots, which is obviously not what happens.
Punching the dots through the protective film eliminated the polishing operation, although I have yet to cut the perimeter with the film in place.
More experimentation is in order, but it looks like I can finally engrave good-looking and perfectly aligned hairlines on nicely cut cursors without all those tedious manual machining operations.
All three hairlines have 0.3 mm depth of cut, with the spindle running at 10 kRPM and the cut proceeding at 24 inch/min = 600 mm/min. All three cuts went through a strip of water + detergent along their length, which seems to work perfectly.
The cuts start on the left side:
Hairline V tool tests – 0.3 mm 10 kRPM 24 ipm – start
I cut the red hairline through the PET cursor’s protective film to confirm doing it that way is a Bad Idea™; the gnarly appearance is sufficient proof.
The cuts end on the right:
Hairline V tool tests – 0.3 mm 10 kRPM 24 ipm – end
Eyeballometrically, the cuts are the same depth on both ends, with a slight texture difference at the start as the X axis ramps up to full speed.
They’d be a bit stout on an old-school engraved slide rule, but look just fine laid against a laser-printed Homage Tek Circuit Computer:
Hairline V tool tests – 0.3 mm 10 kRPM 24 ipm – Tek CC
Flushed with success, here’s a fresh-cut red hairline in action:
Tek CC cursor hairline – V tool red fill
The end of the cursor sticks out 1 mm over the rim of the bottom deck, because I wanted to find out whether that would make it easier to move. It turns out the good folks at Tek knew what they were doing; a too-long cursor buckles too easily.
The trick will be touching off the V tool accurately enough on the cursor surface to get the correct depth of cut. The classic machinist’s technique involves a pack of rolling papers, which might be coming back into fashion here in NY.
That’s the fixture intended for Gyros circular saw blades, repurposed for V tool engraving. The V tool in the Sherline tool holder collet is one of the ten-pack from the CNC 3018, unused until this adventure.
The actual setup had a scrap cursor secured with a strip of Kapton tape:
Those are three passes at (nominal) depths of 0.2, 0.3, and 0.4 mm (bottom to top) with a pre-existing hairline visible just above the second pass. The spindle ran at the Sherline’s top speed of just under 10 kRPM with no coolant on the workpiece.
I touched off the 0.2 mm cut by lowering the tool 0.1 mm at a time until it just left a mark on the Kapton tape, after a coarse touch-off atop a 0.5 mm plastic card, and calling it zero.
Scribbling over the cuts with a red Industrial Sharpie looked downright gory:
The top hairline shows distinct signs of melted PET plastic along the trench, with poor color fill due to the Sharpie not sticking to / wiping off the smooth-ish trench bottom. The next one is the existing saw-cut hairline with the lead-in cut over on the left.
The 0.3 and 0.2 mm hairlines look much better, with less debris and more complete fill. Unfortunately, the right side of the Sherline’s tooling plate seems to be a few tenths of a millimeter lower than the left, causing the 0.2 mm hairline to … disappear … where the cutter skipped up onto the Kapton tape:
Now, in practical terms, this is the first time I’ve actually needed platform alignment to within a hundred microns in subtractive machining. As some folks discover to their astonishment, however, 3D printing does require that level of accuracy:
Thinwall Box – platform height
Engraving through a layer of tape isn’t the right way to do it and some coolant will definitely improve the results, so I ignored the alignment issue, remounted the same scrap cursor with the red hairlines on the bottom, pulled a strip of water + detergent along the tool path, cut the same hairlines, and colored the trenches with blue Industrial Sharpie:
Hairline V tool – 0.2 0.3 0.4 DOC 10K RPM – water cool start
The 0.2 mm hairline on the bottom becomes a line as the V bit begins sliding along the surface at 10 kRPM without cutting:
Hairline V tool – 0.2 0.3 0.4 DOC 10K RPM – water cool mid
The 0.3 mm hairline looks pretty good and the 0.4 mm hairline remains too rugged by the end of the passes. I think the actual depth of cut is at least 0.05 mm less than at the start:
Hairline V tool – 0.2 0.3 0.4 DOC 10K RPM – water cool end
Obviously, neurotically precise touchoff carries a big reward, as will aligning the tooling plate to an absurd degree.
A real machinist simply flycuts the top of an offending part / fixture / tooling plate to align it with the machine’s spindle, but I have a sneaky suspicion the real problem is a speck (or ten) of swarf between the Sherline’s table and the tooling plate; better cleanliness and attention to detail may improve the situation.
The tip is pretty close to the stated 0.1 mm. The included V angle looks like 22.5°, but the descriptions use the half angle, so it’s either a generous 10° or a scant 15°, take your pick.
It’s turning at 4000 RPM in the Sherline spindle, which is much too slow for such a tiny cut. No coolant, nothing fancy.
The lower left group ran at increasing depths from 0.0 to about 0.6 mm, with the deepest one looking surprisingly good.
It’s all manual jogging at either 12 or 24 inch/min and, when you (well, I) count the swirls across those 100 mil grids, the spindle really is turning at 4 kRPM. Gotta love it when the numbers work out!
These are obviously the best-looking hairlines yet, so I must tweak the GCMC source to do the right thing with the existing fixture.
The Smell of Molten Projects in the Morning 