Tag: Slipstick

Machine Shop

Laser Cutter: Sheet Holder

Applying a laser cutter to paper-like materials requires balancing two contradictory imperatives:

  • Hold the sheet flat to avoid distortions
  • Have nothing below to avoid schmutz on the bottom

This seemed like a good compromise:

Sheet Holder - Tek CC bottom deck
Sheet Holder – Tek CC bottom deck

The orange 3D printed blocks hold aluminum miniblind blades:

Sheet Holder - steel sheet magnet pads
Sheet Holder – steel sheet magnet pads

The curved slots hold the blades flush with the upper surface and align their top sides parallel to the laser beam, giving the beam very little blade to chew on near the focus point and allowing plenty of room below the sheet to dissipate cutting fumes.

The gold-ish squares are thin steel sheets covered with Kapton tape, painstakingly filed en masse from small snippets:

Sheet Holder - filed steel pads
Sheet Holder – filed steel pads

The first iteration used precisely laser-cut refrigerator magnet pieces, in the expectation a crappy rubber magnet would provide just enough attraction to let a neodymium magnets hold the paper flat, without risk of blood blisters between fingers and steel:

Sheet Holder - ferrite magnet pads
Sheet Holder – ferrite magnet pads

As expected, contact with the neo magnet completely wiped away the alternating pole magnetism in the rubber sheet, leaving a weakly attractive non-metallic surface. Alas, the rubber had too little attraction through a laminated sheet of paper, so I switched to real steel and risked the blisters.

Most of the blocks are narrow:

Sheet Holder Bracket - solid model
Sheet Holder Bracket – solid model

The four corners are wider:

Sheet Holder Bracket - wide - solid model
Sheet Holder Bracket – wide – solid model

They’re symmetric for simplicity, with recesses for the magnets / steel sheets on the top. The through-holes have recesses for M3 SHCS holding them to T-nuts in Makerbeam rails, with a slightly overhanging alignment ledge keeping them perpendicular to the rail.

The magnets come from an array of worn-out Philips Sonicare toothbrush heads:

Sheet Holder - magnet holders curing
Sheet Holder – magnet holders curing

They’re epoxied inside a two-piece mount, with the lower part laser-machined from 3 mm acrylic to put the two magnets in each assembly flush with the lower surface; the green area gets engraved 1 mm below the surface for the steel backing plate. The 1.5 mm upper frame fits around the plate and protrudes over the ends just enough for a fingernail grip:

Magnet Holder Cuts
Magnet Holder Cuts

The epoxy got a few drops of fuschia dye, because why not:

Sheet Holder - trimmed magnet holders
Sheet Holder – trimmed magnet holders

The garish trimmings came from slicing the meniscus around the lower part of the holder off while the epoxy was still flexy.

The holders must be flat for clearance under the focus pen:

Sheet Holder - focus probe clearance
Sheet Holder – focus probe clearance

Some experimentation suggests I can raise the pen by maybe 2 mm (with a corresponding increase in the Home Offset distance) , but the switch travel requires nearly all of the protruding brass-colored tip and there’s not much clearance under the nozzle at the trip point.

With all that in hand, it works fairly well:

Sheet Holder - Tek CC cutout
Sheet Holder – Tek CC cutout

The lower deck has very little margin for gripping, which is why the four corner blocks must be a bit wider than the others.

The lamInator tends to curl the sheets around their width, so most of the clamping force should be along the upper and lower edges to remove the curl at the ends. This requires turning the whole affair sideways and deploying more magnets, which is possible for the smaller middle and upper decks:

Sheet Holder - Tek CC middle deck
Sheet Holder – Tek CC middle deck

Protruding SHCS heads on the four corners snug up against the edge of the knife-edge bed opening for Good Enough™ angular alignment.

Plain paper (anything non-laminated) seems generally flat enough to require no more than the corner magnets.

It’s definitely better than the honeycomb surface for fume control!

The OpenSCAD source code as a GitHub Gist:

// Bracket for sheet holder
// Ed Nisley KE4ZNU 2022-09-09
Layout = "Show"; // [Show, Build, Blade]
/* [Hidden] */
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
Protrusion = 0.1; // make holes end cleanly
ID = 0;
OD = 1;
LENGTH = 2;
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
h=Height,
$fn=Sides);
}
// Sizes
Magnet = [10,30,0.5]; // magnetic sheet size
//Magnet = [10,14,0.5];
MagnetRim = 1.0;
Screw = [3.0,5.5,3.0]; // SHCS OD=head LEN=head
MakerBeam = 10.0; // beam size, screw = half height
BeamRecess = 0.5; // slight overhang for alignment
BladeSlot = 0.15 * 4; // slot with plenty of clearance
BladeSocket = 5.0; // recess to hold miniblind
BladeWidth = 24.6; // miniblind width
BladeM = 1.6; // height of miniblind curve
BladeSides = 12*8;
BladeRadius = (pow(BladeM,2) + pow(BladeWidth,2)/4)/(2*BladeM);
BladeAngle = 2*asin(BladeWidth/(2*BladeRadius));
echo(BladeRadius = BladeRadius);
echo(BladeAngle = BladeAngle);
Block = [Magnet.x + 2*MagnetRim + ceil(BladeRadius*(1 - cos(BladeAngle)) + 2.0),
Magnet.y + 2*MagnetRim,
BladeRadius*sin(BladeAngle)];
echo(Block = Block);
// Cutter for spline recess
// approximately correct and good enough
module BladeRing() {
rotate([90,0,0])
translate([0,0,-BladeSocket])
linear_extrude(height=2*BladeSocket,convexity=2)
difference() {
circle(r=BladeRadius,$fn=BladeSides);
circle(r=BladeRadius - BladeSlot,$fn=BladeSides);
}
}
// Overall bracket
module Bracket() {
difference() {
translate([0,-Block.y/2,0])
cube(Block,center=false);
translate([Magnet.x/2 + MagnetRim,0,Block.z - Magnet.z/2 + Protrusion/2])
cube(Magnet + [0,0,Protrusion],center=true);
for (j=[-1,1])
translate([0,j*Block.y/2,MakerBeam/2 - Protrusion/2])
cube([3*Block.x,2*BeamRecess,MakerBeam + Protrusion],center=true);
for (j=[-1,1])
translate([Magnet.x + 2*MagnetRim + BladeRadius,j*Block.y/2,Block.z])
BladeRing();
for (j=[-1,1])
translate([Block.x - 2.0 - BladeSlot,j*Block.y/2,5*ThreadThick/2 - Protrusion/2])
cube([2*BladeSlot,2*BladeSocket,5*ThreadThick + Protrusion],center=true);
translate([MakerBeam/2,Block.y,MakerBeam/2])
rotate([90,0,0])
PolyCyl(Screw[ID],2*Block.y,6);
for (j=[-1,1])
translate([MakerBeam/2,j*(Block.y/2 - Screw[LENGTH] - 1.0),MakerBeam/2])
rotate([-j*90,0,0])
PolyCyl(Screw[OD] + HoleWindage,2*Block.y,6);
}
}
//----------
// Build it
if (Layout == "Blade")
BladeRing();
if (Layout == "Show")
Bracket();
if (Layout == "Build")
Bracket();
view raw Sheet Holder Bracket.scad hosted with ❤ by GitHub
Machine Shop

Homage Tektronix Circuit Computer: Laser-Engraved Painted Hairline

Having established that laser-engraving the hairline fuses the protective film to the PETG cursor, I wondered if the bond would allow filling the hairline by painting it with good old Testors enamel:

Tek CC - Painted Hairline - finished
Tek CC – Painted Hairline – finished

Which is, straight up, the best hairline I’ve ever made!

I use a fine-tipped paintbrush, but there’s no need for finesse:

Tek CC - Painted Hairline - mask detail
Tek CC – Painted Hairline – mask detail

After the paint has cured a bit, peeling the film removes everything that isn’t inside the trench:

Tek CC - Painted Hairline - mask removal
Tek CC – Painted Hairline – mask removal

The clouds on the clear side come from PETG vapor condensing on the transparent film protecting the bottom of the cursor.

A closer look through the cursor shows bubbles in the bottom of the trench, presumably from boiling PETG:

Tek CC - Painted Hairline - detail
Tek CC – Painted Hairline – detail

The hairline measures 0.3 mm across and, unlike previous attempts, is perfectly consistent from end to end.

Now I can go into mass production:

Tek CC - Painted Hairline - production
Tek CC – Painted Hairline – production

I love it when a plan comes together …

Machine Shop

Laser Printer vs. Laser Cutter: Alignment & Scale

The setup for cutting the Tektronix Circuit Computer desks looks like this:

Tek CC - Bottom Deck cutting setup
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
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.

More study is indicated …

Machine Shop

Homage Tektronix Circuit Computer: Laser-Engraved Hairline Improvement

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
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
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
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
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
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:

https://softsolder.com/2020/05/06/tek-circuit-computer-cursor-hairline-scraping/

https://softsolder.com/2021/01/26/tek-circuit-computer-cursor-hairline/

https://softsolder.com/2021/02/16/tek-circuit-computer-sawed-hairline-fixture/

https://softsolder.com/2021/04/13/tek-cc-milled-cursor-vs-speed-vs-coolant/

https://softsolder.com/2021/04/15/tek-cc-milled-cursor-mvp/

Plenty of Quality Shop Time™ along the way, though …

Machine Shop

Homage Tektronix Circuit Computer: Laser-Engraved Hairline Tests

This worked out surprisingly well:

Tek CC - laser-etched cursor hairline
Tek CC – laser-etched cursor hairline

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
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:

Tek CC Cursor - LightBurn layout
Tek CC Cursor – LightBurn layout

For a more systematic test I aligned a cursor engraving fixture I built for the Sherline atop the laser’s honeycomb platform and wedged it into place with eccentric stops, then dropped a cursor milled on the Sherline in place:

Tek CC Cursor - laser fixture alignment
Tek CC Cursor – laser fixture alignment

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
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
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
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
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.

[Update: dot mode can produce a continuous trench that looks even better! ]

Machine Shop

Tek CC Milled Cursor: MVP

What a difference 100 µm can make:

Hairline V tool tests - 0.3 mm 10 kRPM 24 ipm
Hairline V tool tests – 0.3 mm 10 kRPM 24 ipm

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
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
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
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
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.

Machine Shop

Tek CC Milled Cursor vs. Speed vs. Coolant

After getting the Sherline running with the Mesa 5I25, I could return to milling cursor hairlines for the Tek Circuit Computer:

Hairline V tool - fixture
Hairline V tool – fixture

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:

Hairline V tool - 0.2 0.3 0.4 DOC 10K RPM - Kapton fixture
Hairline V tool – 0.2 0.3 0.4 DOC 10K RPM – Kapton fixture

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:

Hairline V tool - 0.2 0.3 0.4 DOC - Kapton Sharpie
Hairline V tool – 0.2 0.3 0.4 DOC – Kapton Sharpie

Peeling the tape and applying a cloth moistened with denatured alcohol showed three gnarly hairlines:

Hairline V tool - 0.2 0.3 0.4 DOC 10K RPM - Kapton start
Hairline V tool – 0.2 0.3 0.4 DOC 10K RPM – Kapton start

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:

Hairline V tool - 0.2 0.3 0.4 DOC 10K RPM - Kapton mid
Hairline V tool – 0.2 0.3 0.4 DOC 10K RPM – Kapton mid

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
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
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
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
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.