Let’s Shave a Yak Together

Have you ever heard the expression “yak shaving” before? Allegedly invented at the MIT AI lab a decade or so ago, it’s what you call all the stupid, annoying work you have to do first in order to be able to do what you’re trying to get done. When you find yourself looking for a tool to make a tool that you can use to make the tool you need (and which you could buy from Enco for $25), you’re shaving a yak.

Anyway, I spent a good six hours today trying to figure out the best way to cut a hole in a piece of plastic. To be specific, to cut out two holes in the face of the enclosure for the keypad and the LCD display. In order to do this, I need a way to clamp the enclosure securely. Neither a vise or double-sided tape would work, and there’s not enough room to get step clamps snug enough. So, it was going to require a fixture.

The enclosure front panel luckily has a bunch of holes with threaded inserts (to hold the two halves together), so I figured I could just use these to screw it down to a piece of tooling plate. So, all I had to do was precisely drill 12 holes (4 for dowel pins to locate the fixture, 4 1/4″ holes for screws that hold the fixture to the mating plate on the mill, and 4 for the screws that would hold the enclosure to the fixture). As this was one of those cases where every .001″ counted, it was slow work.

Nonetheless, I happily test-fitted the hold-down screws for the fixture (spot on), the hold-down screws for the enclosure (3 perfect, one hole needed to be drilled .010″ oversize), and the dowel pins (one came out oversize due to cheap drill bit, but 3 pins should be plenty). The screws that hold the fixture to the mill come down from the top. The screws that hold the enclosure come up from the bottom. So, you need to screw the enclosure to the fixture, then screw the fixture down onto the mill. Easy enough, if time-consuming.

Can you see what’s coming? When I tried to actually put it all together, I realized that once I screwed in the workpiece, it covered the holes where the hold-down screws for the fixture went. If I screwed the fixture down to the table first, there’d be no way to attach the workpiece. So, this would be me:

So, long story short, I’m going to have to make some clamps to go around the edges that can be secured from the top. The earlier efforts weren’t entirely in vain as they’ll make it easier to attach straightedges to locate the workpiece properly, and the end result will be a fixture that’s easier to use as I’ll be able to swap workpieces without having to unmount the fixture. Still, I’ve probably got another 4-5 hours of work ahead before I can even *try* the CAM program I wrote to mill out the two holes. Anybody need some extra yak hair?

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