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Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Tag: Sherline

Sherline CNC mill

Beveling Some Edges

Clock case test fit

The case for the Totally Featureless Clock is exactly what you’d expect: a solid black acrylic block with a Lexan Graylite faceplate. All you see are digits… no buttons, no knobs. Just the time, all the time.

I’d hoped to just epoxy the faceplate on and be done with it, but the edges really didn’t look right. A bit of rummaging turned up a Dremel 125 “High Speed Cutter” that looked to be exactly the right hammer for the job.

Dremel conical cutter in drill chuck

The Sherline CNC mill just doesn’t have the reach for a foot-long cut, so I clamped the case to the manual mill and grabbed the cutter in an ordinary drill chuck. This is absurd, but ya gotta run with what ya got…

According to the Dremel specs, the cutter should run at 20-30 k RPM, but, trust me on this, the mill doesn’t go that fast. I set it for 2000 RPM, the fastest it’s run in years, and hoped for the best.

The runout was breathtaking.

I aligned the case against against a parallel in one of the T-slots, which got it surprisingly close. A trial cut showed it was off by a bit, but two slight realignments (loosen clamps, slide gently, reclamp) and trial cuts put it spot on. That worked fine for three sides.

The cutter is about 6.3 mm dia and just over 9 mm long, so the cutting edge is inclined at almost exactly 1:3. That means a horizontal misalignment of 10 mils causes a vertical misalignment of 30 mils. Conversely, you can measure the vertical error and then tweak the horizontal to make the answer come out right.

Tweaking the alignment

The fourth side was off enough to make the final joint to the first side pretty ugly. I measured the vertical offset at about 80 mils, set the front magnetic block as a pivot and stuck a 25-mil feeler gauge between the rear block and the case. Remove the feeler, loosen the clamps, rotate the case, reclamp, and the cut was just about perfect. Certainly within my tolerances for such a thing… you can’t see it unless you’re looking for it.

The bottom picture shows the final bevel, hot off the mill table, minus the protective plastic wrap and plus a bunch of dust and adhesive smudges from the wrap. The end plate gives the vertical line down the right side and a slight discontinuity where it’s a few mils shy of the side. There’s a hairline around the whole case where the faceplate joins the black acrylic; I used transparent epoxy and a light weight to clamp the faceplate down, so the joint is uniformly thin all around.

A few passes on a sanding block ought to get rid of the tool marks and spiff the bevel up just fine all around.

I love happy endings.

Final corner bevel

2010-03-18
Acrylic Sheet Thickness Variations

Milling plate thickness

So I measured the thickness of the black acrylic sheet I’m using for the Totally Featureless clock and machined the rabbets to match. Went to assemble everything and the rabbets are too shallow!

Come to find out that the sheet varies in thickness from about 0.437 to 0.475 across the four pieces I’d cut and, of course, I’d measured the thinnest end of the thinnest piece. Makes no sense to me, as I’d expect the thickness to be pretty well controlled over a few feet of sheet, but that’s not how things went down.

The simplest solution was to mill a flat on the inside of the case to match the rabbet, so all four panel ends were the same thickness. The sketch below has the straight dope.

Acrylic sheet thickness fix

Milling with a 3/8-inch end mill at 2500 rpm, 10 ipm, in one pass with no cooling was OK.

I’ll insert some brass shimstock into the rabbets to make the outside edges wind up flush.

2010-03-10
Sherline: Milling a Too-long Rabbet
This is pretty much the same general idea and setup as the one I described there, but with the panel flat against the tooling plate.

Milling rabbet on top panel

The cutter sits at the far right end of its maximum travel. I made the rabbet in three manual CNC passes.

To set up for the rest of the cut:
- G0 X-4.25 to clear the left end of the panel
- Loosen the three clamps, slide the panel leftward
- Push the panel against the brass tubes
- Tighten the clamps
And away we go…

Complete rabbet

One disadvantage: you can’t do a final finishing pass along the entire length of the cut. There are tool marks at the stopping point, but nothing really objectionable on the back of a clock where a panel will cover the rather ugly guts.

The brass tube “locating pins” work surprisingly well.

About 2000 rpm with 3/8 inch end mill. Cut 1/8 inch wide and 0.25 inch deep @ 10 ipm in three passes. Finish pass 15 ipm at 0.257 deep to make it pretty.
2010-03-09
Sherline: Milling the Ends of Too-long Acrylic Panels
Having flycut the acrylic panels to the proper width, I had to cut them to the proper length, too. This picture shows the lashup I used to hold them down during the operation…

Clock top panel fixture

The brown bar sticking out to the left is one of the bookshelf struts that held the toolmaker’s vises down during the flycutting; it’s now secured to the Sherline table with a T-nut. A vise clamped to the bar serves as an end stop for the panels.

Brass tubing locating posts

A pair of brass tubes around studs serve as locating pins. To get the things lined up:
- Loosely clamp a panel down atop a spacing plate
- Push it back against the loose tubes: crudely parallel to X axis
- Snug the clamps
- Align the panel to the X axis using the laser
- Push the tubes against the panel
- Tighten their nuts
Top panel end trimming detail

Crude, but good enough for this purpose.

Then a bit of manual CNC to shave off the end. Half-inch mill, 1500 rpm, 150 mm/min, more-or-less 0.5 mm cuts. The panels don’t have to be any exact length, as long as the clock circuit boards fit inside, but the ends must be perpendicular and smooth for good gluing.

The exact part will come when I rabbet the side panels…

The side panel setup was much simpler: same brass posts, same spacer, no need for the long bar hanging off to the left.

Side panel fixturing
2010-03-07
Sherline: Flycutting Too-Long Acrylic Panels

Flycutting acrylic top panel

The Totally Featureless Clock will have a black acrylic case with a Graylite Lexan faceplate. The top & bottom panels are 11.75 inches long, which is much too large for the Sherline’s 9-inch maximum X travel.

Fortunately, in this case I can cheat.

This setup cut the panels to the proper width. A pair of parallel blocks, made from some mysterious glass-like material and ground very nicely flat, support the panel just over the body of the four toolmaker’s vises lined up along the tooling plate. I drilled the brown bookshelf rails to match the tooling plate and secured them with 10-32 studs.

The front rail secures the vise bodies to the tooling plate; they’re aligned parallel to the X axis by the simple expedient of laying a parallel along the back edge and matching that to the tooling plate. No real precision is in order here; the flycut is across the whole top edge.

The rear rail holds the movable vise jaws down; they tend to rise up just slightly when tightened, but the difference amounts to barely enough to release pressure on the parallel blocks. Not enough to matter, as it turned out.

The general notion is to flycut about 2/3 of the length of the panel, then slide it far enough to cut the remainder. Flip it over and flycut the other side the same way.

About 1000 rpm and 150 mm /min, cutting 0.5 mm or so on each pass.

This worked surprisingly well. I expected to find a bow in the middle due to an uneven bandsaw cut on the initial downward side, but it was all good; evidently the blocks were wide enough to average things out.

The joint where the two cuts meet turns out to be visible, but barely detectable with a fingernail: entirely suitable for this application. I’ll hit the sides with sandpaper on a sheet of plate glass before bonding them to the faceplate.

Flycutting the end panels was much simpler: one pass clears their entire length. I moved the clamping rails to simplify the whole process; turned out that clamping the movable jaw didn’t really gain very much at all while complexicating the slide-the-stock process beyond belief.

Flycutting end panels

Overall, the width varies by about two mils along the length of the long panels and they’re perfectly straight as measured against a surface plate. Definitely close enough!

2010-03-06
Improved Sherline Way Bellows
What with all the milling going on lately, I decided to replace the crusty bellows on the Sherline mill. The previous design worked reasonably well, but I’ve had a few tweaks in mind for a while.

Herewith, a PDF file with some Sherline Bellows – Improved:
- Color coded lines so you know which way to fold them!
- Unlined side up for a neat look
- Fits on Letter and A4 sheets
- Taping cuts and hints
The PDF page size is about 8×10 inches; call it 204×280 mm. Print it without scaling and it should just barely squeak onto the sheet. If you don’t have a full-bleed printer, the tips of the sides may get cropped off, but you can extrapolate easily enough.

Some assembly required:
- Cut it out
- Fold the central valleys (red) first, flatten it out again
- Fold the central ridges (blue) next
- Pleat the whole thing into a half-inch tall stack
- Squash it into a neat package to harden the folds
- Fold the tips along one side
- Fold the tips along the other side
- Squash the folds again
- Make the saddle cuts & fold the tabs
- Apply double-stick tape as noted (some on back)
- Install on your cleaned-up mill
- Admire!
The tip folding is the trickiest part. Basically, flip the first tip from a ridge to a valley, then chase the little transition folds into place. Repeat for each tip along that side, then do the other side.

It gets easier after you fumble around for a while.

My nimble-fingered daughter has offered to fold ’em for you. Stick a few bucks in an envelope and mail it to me; we’ll mail back two folded sets (two each, front and rear bellows) for your amusement. Kid’s gotta earn her college money somehow…

Address? Go to the QRZ.com database and search for my amateur radio callsign: KE4ZNU. Cut, paste, that was easy.

For the do-it-yourselfers, start with the PDF file in the link above. That’s the easiest way to get the correct scaling. The tabs on the ends should be 4.0 inches across on the printed page.

Rear Bellows

Front Bellows

Here are some 300 dpi PNG files, but you’re on your own for scaling.

If you want the original Inkscape SVG files, drop me a note.
2010-02-26
Cutting Pin Header Strips

Slitting dual-row connector

I needed a few strips of single-row pin headers, but the parts bin was empty.

I hate it when that happens.

The heap disgorged a handful of double-row strips and, of course, I Have A Machine Shop.

So: no problem.

This is, I admit, not cost-effective, but it took about 15 minutes to slit the aforementioned handful of strips right down the middle and get back to soldering.

The trick is to use an ultra-thin slitting saw, rather than a regular saw. The one here is 4 mils thick and the better part of 7/8″ in diameter; call it 0.1 mm x 22 mm. I think it came with one of the Dremel tool kits a long while ago.

Cut about 1 mm deep on the first pass, then cut through on the return to avoid having the saw deflect too much. Run about 100 mm/min, 1000 rpm, and no coolant. Line it up by eye, type manual CNC commands into EMC2, and it’s all good.

The trick is finding a mandrel that doesn’t collide with the vise; my larger saws have a rather thick screw-and-washer arrangement that doesn’t fit. I think some padding (chopped-up credit cards?) between the longer pins, mounting the vise vertically, and grabbing the longer pins would fix that. The catch might be clearance between the top of the vise and the bottom of the spindle motor.

Better to just buy some single-row strips. Sheesh… but if all you have is a CNC mill, you have plenty of solutions.

Another slitting saw repair is there…

2010-02-13