Archive for July 30th, 2010

Pin Spanner for 3.5 mm Audio Jack Nut

The external antenna jack on the Totally Featureless Clock is, by necessity, recessed way down in a hole (because I can’t get to the inside of the now-finished half-inch-thick case to gnaw it out from there). Perforce, that puts the locking nut out of reach.

Solution: a pin spanner wrench. I’m sure they’re available commercially, but what’s the fun in that?

The male threaded part of the jack is 0.230 inch OD, the nut is 0.313 OD, and the notches are 0.030 wide and 0.020 deep. Raw material is about two inches of 5/16-inch air-hardening drill rod, not that I’m actually going to heat treat it for this application.

Face off the end and drill the guts out with a 15/64-inch drill.

Drilling central recess

Drilling central recess

Grab it in the 3-jaw chuck bolted firmly to the table, then mill off anything that isn’t a pin. Don’t grab it in the milling vise, which doesn’t have enough oomph to hold a slick steel cylinder in place; don’t ask how I know this.

Milling pins in 3-jaw chuck

Milling pins in 3-jaw chuck

Set Z=0 at the top surface of the spanner-to-be and XY = 0 on the axis of the cylinder, of course.

Manual CNC, feeding the commands into EMC2’s MDI slot and then mouse-clicking the stored commands to avoid reduce typing errors. For my setup, Y=±0.171 to produce the 30-mil pin and  X=±0.4 to clear on both sides.

After cutting the first side at 3 k RPM, feed 2 inches/min, and 10 mils per pass, I whacked the other side off in one giant 20-mil bite. I’m such a sissy…

A bit of heatshrink tubing improves the griptivity and it’s all good.

Finished spanner engaged in nut

Finished spanner engaged in nut

This is the sort of thing you do once, drop in the baggie with the rest of the connector nuts, and use for years thereafter. I should’a done it years ago, but I’ve been able to not quite butcher the nuts with a needle-nose pliers…

[Update: It turns out a commercial nut driver was available, at least in one special shop in one special place, but no longer. For my delicate uses, that shaft into the jack isn’t really needed.]