Archive for March 18th, 2010

Beveling Some Edges

Clock case test fit

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

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

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

Final corner bevel

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