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:
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
It’s Pretty Good™ when seen against an un-laminated bottom deck drawn with a Pilot V5RT pen:
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.
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:
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:
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.
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:
It’s sitting atop the doodle giving the dimensions, such as they are, for the 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:
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:
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:
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:
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.
The dimension doodle shows what’s inside and gives some idea of the sizes:
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:
The little machined cap has a somewhat longer spring guide to prevent buckling:
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:
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:
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.
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.
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:
The surviving foot nuts sported two weld nuggets apiece:
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:
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:
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:
Another trial fit showed the need for more recess:
Another spate of grinding made it sit mostly level on the decidedly non-level surface around the hole:
The beveled corners fit inside the swaged hole corners.
Grind paint / crud off the sheet metal and roughen the surface for good epoxy griptivity:
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:
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:
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:
All in all, a satisfying day in the Basement Shop …