Thursday, June 16, 2011

A Rotary Switch Knob Repair

Pictured below is a fairly typical tri-lite light bulb socket on a rather nice table lamp of my wife's. It has a silly little problem -- one of those things that leaves you wondering what they were thinking in the manufacturing plant.

That knob threads onto the switch's actuating spindle, which is fine, but the threaded hole in the knob is so deep that the knob threads on until it jams against the side of the socket, rendering the whole thing inoperative. Brilliant.

Happily, though, there's an easy fix for this.

The switch's spindle is threaded 4-40. That happens to be the same thread used on those little jackposts that are on older serial data port (RS-232) connectors on PCs and printers. (There's also a metric version, but the vast majority of them that I encounter are inch 4-40.) One of those jackposts will be just what I need to rectify the knob's problem. Here's a shot of the knob and a jackpost.

I'll shorten the knob by about the length of the jackposts's head, and install the jackpost in the end of the knob with CA adhesive. Then the knob will have a proper thread with limited depth.

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And here's the knob made ready.

And here it is back in service.

End of problem.

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