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.

Sherline: Diamond Drag Engraving Tool Holder

Although I shouldn’t have used a hardened shaft for the case, the rest of the diamond drag tool holder worked out well enough:

Sherline Diamond Drag Holder - assembled
Sherline Diamond Drag Holder – assembled

The dimension doodle shows what’s inside and gives some idea of the sizes:

Sherline Diamond Drag Holder - dimension doodles
Sherline Diamond Drag Holder – dimension doodles

From left to right, it’s an M6×1.0 setscrew to adjust the spring preload, a spring harvested from a cheap clicky ballpoint pen, a machined cap, a 3 mm rod (which should be a hardened & ground shaft, but isn’t) surrounded by a pair of LM3UU linear bearings, a machined coupler, and the stub of a diamond engraving tool’s shank.

Tapping 15 mm of M6×1.0 thread inside of the case took an unreasonable amount of grunt. Next time, brass.

The setscrew gets a little boss to hold the spring away from the adjacent threads in the case:

Sherline Diamond Drag Holder - setscrew spring boss
Sherline Diamond Drag Holder – setscrew spring boss

The little machined cap has a somewhat longer spring guide to prevent buckling:

Sherline Diamond Drag Holder - shank cap spring guide
Sherline Diamond Drag Holder – shank cap spring guide

The spring fits snugly on the slightly enlarged section inside the last few coils, with the rest being a loose fit around the guide. When the spring is fully compressed, it’s just slightly longer than the guide and can’t buckle to either side.

The cap gets epoxied onto the 3 mm rod with some attention to proper alignment:

Sherline Diamond Drag Holder - shank cap alignment
Sherline Diamond Drag Holder – shank cap alignment

The other end of the rod has a 3 mm thread, which would be a serious non-starter on a hardened rod.

The shortened diamond tool shank gets epoxied into the gizmo connecting it to the now-threaded rod, again with some attention paid to having it come out nicely coaxial:

Sherline Diamond Drag Holder - diamond tool alignment
Sherline Diamond Drag Holder – diamond tool alignment

The LM3UU bearings got epoxied into the case, because I don’t have a deep emotional attachment to them.

Unscrew diamond tool, push spring onto cap, drop rod through bearings, crank setscrew more-or-less flush with the end of the case, screw diamond in place with some weak threadlock, add oil to rod, work it a few times to settle the bearings, and it’s all good.

A quick spring rate measurement setup, with a brass tube holding the diamond point off the scale pan:

Sherline Diamond Drag Holder - installed
Sherline Diamond Drag Holder – installed

The spring rate works out to 230 g + 33 g/mm for deflections between 1.0 mm (263 g) and 3.5 mm (346 g), so it’s in the same ballpark as the diamond tools on the MPCNC and CNC 3018.

Note: WordPress just “improved” their post editor, which has totally wrecked the image alignment. They’re all set to “centered” and the editor says they are, but they’re not. It’s a free blog and I’m using one of their ancient / obsolete / unsupported themes, so I must update the theme. Bleh.

CNC Kitchen Sink Strainer

Our Young Engineer recently rented a house, now knows why our sinks have CNC-machined strainers, and asked for something better than the disgusting stainless mesh strainer in the kitchen sink.

Being a doting father, I turned out a pair to get a pretty one:

CNC Sink Strainer - overview
CNC Sink Strainer – overview

They’re made from the same scrap smoked acrylic as the ones in our sinks:

CNC Sink Strainer
CNC Sink Strainer

They’re definitely upscale from the (not watertight!) 3D printed version I built for a Digital Machinist column to explain OpenSCAD modeling:

Strainer plate fill
Strainer plate fill

This time around, though, I rewrote the subtractive design in GCMC, with helical milling for all the holes to eliminate the need to change tools:

Sink Strainer - tool path simulation - CAMotics
Sink Strainer – tool path simulation – CAMotics

They’re done on the Sherline, because it has real clamps:

CNC Sink Strainer - on Sherline
CNC Sink Strainer – on Sherline

Four tabs eliminated the need to reclamp the stock before cutting the perimeter, but I should have ramped, not plunged, through the final cut between the tabs:

CNC Sink Strainer - tab surface fracture
CNC Sink Strainer – tab surface fracture

The handles come from the same chunk of hex acrylic as before, eyeballed to length, tapped 8-32, and secured with acrylic adhesive.

The GCMC source code as a GitHub Gist:

// Drill & mill sink drain strainer
// Ed Nisley KE4ZNU -- Digital Machinist 15.2 Spring 2020
// polycarbonate or acrylic sheet
// External clamps at corners
// Origin at center of sheet
//-----
// Dimensions
DiskOD = 80.0mm; // usual kitchen drain = 3-1/4 inch
DiskRad = DiskOD/2;
PlateThick = 6.0mm; // stock thickness
MillOD = 3.170mm; // measured end mill OD
HoleDia = 4.75mm; // 3/16 inch drain holes
ScrewOD = 0.18in; // knob screw clearance
NumRings = 3; // rings of drain holes
RingSpace = 1.5 * HoleDia; // .. between rings
MaxZCut = 0.25 * MillOD; // max cut depth
MillSpeed = 1000mm; // horizontal feedrate
NumTabs = 4;
TabTilt = 45deg;
TabLength = 5.0mm;
TabThick = 0.5mm;
SafeZ = 10.0mm; // above all obstructions
TravelZ = 1.0mm; // within engraving / milling area
MillZ = -(PlateThick + 0.5mm); // through disk into spoil board
TwoPi = 2*pi();
//-----
// Mill one hole
function MillHole(ctr,radius,turns) {
goto([-,-,TravelZ]);
goto(head(ctr,2) + [radius,-,-]);
goto([-,-,0]); // kiss surface
circle_cw(ctr,turns); // helix downward
circle_cw(head(ctr,2)); // remove last ramp
goto(ctr); // get elbow room
goto([-,-,TravelZ]);
}
//-----
// Start cutting!
goto([-,-,SafeZ]);
goto([0,0,-]);
goto([-,-,TravelZ]);
feedrate(MillSpeed);
// Mill center screw hole
comment("-- Center hole");
ctr = [0,0,MillZ];
MillHole(ctr,(ScrewOD - MillOD) / 2,ceil(abs(ctr.z) / MaxZCut));
// Mill hole rings
comment("-- Drain hole rings");
repeat (NumRings; ri) {
comment("Ring: ",ri);
rr = DiskRad - ri*RingSpace; // ring radius
comment(" radius: ",rr);
nh = to_int(floor(TwoPi*rr / (2*HoleDia))); // number of holes
comment(" holes: ",nh);
repeat(nh; h) {
a = (h - 1) * TwoPi / nh; // angle of hole
ctr = [rr*cos(a),rr*sin(a),MillZ]; // center point at ending Z
MillHole(ctr,(HoleDia - MillOD)/2,ceil(abs(ctr.z) / MaxZCut));
}
}
// Mill perimeter
comment("-- Perimeter");
r = DiskRad + MillOD/2;
goto([r,0,-]);
goto([-,-,0]);
ctr = [0,0,-(PlateThick - TabThick)];
circle_ccw(ctr,ceil(abs(ctr.z) / MaxZCut)); // ramp downward
circle_ccw(ctr,1); // remove last ramp
goto([-,-,TravelZ]);
comment("-- Tabs");
ta = 360deg / NumTabs; // between tabs
tsa = to_rad(TwoPi*((TabLength + MillOD) / (TwoPi * r))); // subtended tab angle
repeat (NumTabs; i) {
comment(" # ",i);
a = TabTilt + (i - 1)*ta; // tab center angle
p0 = [r*cos(a + tsa/2),r*sin(a + tsa/2),MillZ]; // entry on ccw side
p1 = [r*cos(a + ta - tsa/2),r*sin(a + ta - tsa/2),MillZ]; // exit at next tab
if (0) {
comment(" angle: ",a);
comment(" entry: ",p0);
comment(" exit: ",p1);
}
goto(head(p0,2)); // to entry point
move(p0); // plunge through
arc_ccw(p1,r);
goto([-,-,TravelZ]);
}
goto([-,-,SafeZ]);
goto([0,0,-]);
view raw Sink Strainer.gcmc hosted with ❤ by GitHub

All in all, a pleasant diversion from contemporary events …

HON Lateral File Cabinet Foot Repair

We bought the best-looking (pronounced “least bashed”) pair of hulking five-drawer industrial-strength HON Brigade Lateral File Cabinets from the local ReStore outlet’s assortment for Mary’s quilting fabric stash. They came with a steep discount, barely fit inside the Forester, caused minor interior trim damage, and should organize her entire stash.

One cabinet lost a foot nut at some point in its 16 year history:

HON Lateral File - foot hole - weld nugget filed
HON Lateral File – foot hole – weld nugget filed

The surviving foot nuts sported two weld nuggets apiece:

HON Lateral File - OEM front foot
HON Lateral File – OEM front foot

The hole had the remains of one nugget at the top left and looks like a manufacturing defect to me. Of course, we’re (at least) the second owners and the usual lifetime warranty no longer applies.

I can fix that.

Bandsaw a 1×¾ inch rectangle from 3/8 inch aluminum plate to match the surviving foot nut (which is steel, but aluminum will suffice for our needs). Break the edges, clamp in the Sherline, and mill a square protrusion to match the square-ish hole:

HON Lateral File - square nut - rough cut
HON Lateral File – square nut – rough cut

Drill a 17/64 inch hole (looser than the nominal F drill, because I’m a sissy) for a flat-head bolt from the Drawer o’ 3/8-16 Bolts, tap, and clean up.

A trial fit showed the nugget had to go before the nut would come even close to fitting flat into the hole:

HON Lateral File - foot hole - grinding
HON Lateral File – foot hole – grinding

The sheet metal around the hole had absorbed at least one mighty blow pushing the entire surface inward behind the front edge. To compensate, recess the nut’s front edge and slope the sides with a Dremel wheel to let the bottom face sit level:

HON Lateral File - square nut - taper grinding
HON Lateral File – square nut – taper grinding

Another trial fit showed the need for more recess:

HON Lateral File - square nut - deeper cut
HON Lateral File – square nut – deeper cut

Another spate of grinding made it sit mostly level on the decidedly non-level surface around the hole:

HON Lateral File - square nut - ready to install
HON Lateral File – square nut – ready to install

The beveled corners fit inside the swaged hole corners.

Grind paint / crud off the sheet metal and roughen the surface for good epoxy griptivity:

HON Lateral File - foot hole - ready for install
HON Lateral File – foot hole – ready for install

Stand the cabinet top-side-down to make the bottom level. I wish the basement had one more course of block, but it’s not to be.

Butter the nut with JB Weld epoxy, plunk it in place, apply excess epoxy to make a fillet around the edges, apply duct tape to guy the top of the bolt level-ish, and let it cure:

HON Lateral File - square nut - epoxy curing
HON Lateral File – square nut – epoxy curing

After the epoxy stiffened enough to hold its position, remove the bolt, file a crude ¼ inch hex, and saw a screwdriver slot to make it match the other feet:

HON Lateral File - new foot hex head
HON Lateral File – new foot hex head

Not the fanciest job I’ve ever done, but it now behaves just like the other ones and it’s all good. The HON Storage Files FAQ points to a Troubleshooting Guide showing how to level the thing with a hex socket from inside the bottom drawer.

The flat heads on those bolts are basically 25 mm OD steel plates calling for fuzzy felt bumpers on the Sewing Room’s wood floors. When properly leveled, the front will be ⅛ inch higher than the rear. Although they suggest a pencil should roll toward the back, the top sheet metal on this one may be sufficiently warped to confuse the issue; I have a long level well suited to the task.

The original dimension doodle includes metric offsets for cutting with a ¼ inch end mill:

HON Foot nut - dimension doodles
HON Foot nut – dimension doodles

All in all, a satisfying day in the Basement Shop …

ACM Poughkeepsie Presentation: Algorithmic Art

In the unlikely event you’re in Poughkeepsie this evening, I’ll be doing a talk on my Algorithmic Art for the Poughkeepsie ACM chapter, with a look at the HPGL and G-Code transforming math into motion:

Superformula - triangle burst - detail
Superformula – triangle burst – detail

The PDF of the “slides” lacks my patter, but the embedded linkies will carry you to the blog posts & background information:

See you there! [grin]