My shop assistant came home with a five-dollar tag sale find: either a genuine antique car horn or a reasonable facsimile lashed together by an underemployed Pakistani shipbreaker. The original rubber bulb had long since rotted away, but the brass reed worked fine and the horn gave off a mighty honk! when given sufficient wind.
She bought a replacement bulb with hardware definitely made by the shipbreakers, knowing full well that the internal thread on the end of the new bulb’s brass stem couldn’t possibly match up with the external thread on the old horn. We sketched out some possibilities and decided to make a bushing over the horn’s stem with an internal thread: easier than a very short, perilously thin, double-threaded adapter ring.
She measured various dimensions of both pieces and we consulted Machinery’s Handbook. The horn has a really crusty 32-tpi thread somewhere between 1/2 and 9/16 inch, which is not standard at all. Heck, it’s not even metric. (#include standard-metric-goodness-rant)
The fitting also has an internal pipe thread (!) for the brass reed assembly. We eventually filed a few bits off the reed’s mounting dingus in order to clear the final bushing ID.
Some poking forced the scrap pile to disgorge an aluminum cylinder of exactly the right size for the bushing, with a nice half-inch hole right down the middle. Using a half-inch bolt with a center-drilled end as a mandrel, we brass-hammered it to line up pretty true, and she cleaned off the OD while learning about the quick-change gearbox; a round-nose bit at 104 tpi puts a nice zeepy (her term) finish on aluminum.
We left it stout, rather than trying to turn it down to a thin and elegant shell, because that was the easiest way to get things done. She’ll epoxy it to the horn stem and apply some Loctite to the horn bushing.
A lot of rummaging in the tool cabinet’s recesses produced a taper-shank drill slightly larger than the bulb stem. She drilled out most of the cylinder’s guts, leaving just enough for the threads at the far end, counting 1/10-inch turns on the tailstock all the way.
That pile of razor-edged swarf is now prized possession…
She bored out the narrow end to what seemed like the right minor diameter, given that we really didn’t have anything more than a guesstimate of the thread dimensions. I figured we could just continue threading, eating away at the ID, until it fit.
I don’t do a lot of internal threading, but we found a suitable threading tool, lined things up, and she learned about single-point threading by cutting a thread to match that horn. No measurements worth mentioning; this wasn’t the sort of job requiring a Go-NoGo gage.
I stayed away while she completed the threading, apart from consoling her when she discovered why you shouldn’t hand-rotate the chuck with the quick-change gearbox disconnected. We picked up the thread again and she completed the mission.
Here’s the raw thread before beveling the entrance.
And then it fit! Verily, the horn itself was the Go-NoGo gage.
This was the second part she’s turned on the lathe; I’d say she’s doing just fine.
Now, that was some Quality Shop Time…