I need to make a new knob for an old altimeter (1920s vintage) but I don’t know how to make a square hole.
The altimeter has a 1/8th inch square post (coming out of the case) that is taped for a small screw to hold the knob on. I can make the round knob but I don’t know how to cut a square hole into the knob about 1/8th inch deep. I’m assuming I should be working in brass as the post is brass (I think). All I have is a lathe (1940’s vintage) and a small drill press. Any suggestions?
A few ideas. One, you might try to "borrow" a square hole of the right size, from some other part, that you could insert into a larger drilled hole in the knob. You may be able to buy a "square hole sleeve" of the correct size. Check www.reidtool.com to see if they have any. I know they have square hole sleeves, but I'm not sure they go down to 1/8".
As far as actually making a square hole. If it's a through hole, you could try filing out the corners of a round hole with appropriate needle files.
Or, make a square hole broach out of drill rod. Machine (or file) a 1/8" square on the end of a piece of maybe 3/16" drill rod, with the square section being maybe 1/4" long. Put it in the lathe and countersink the end just a bit so it forms sort-of cutting edges on th end. Heat up with a torch and quench to harden, then align with a pre-drilled 1/8" dia. hole in the knob and press in with your drill press quill to plane out the corners.
Getting the broach aligned with the hole may be a little tricky on a drill press. Unless it's aligned really well it will want to go off to one side or the other.
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Drill a 1/8" dia hole deeper than you need the square hole. (For chip clearance) Then press a piece of 1/8 square keyway stock into the hole to broach the hole square. (Beleive it or not there are special drills that drill square holes but I do not know if they come that small and it would not be worthwile to buy one for one hole.) Gary P. Hansen
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Don't rule out 5 minute epoxy. Many a $1000 custom knife is held together using this stuff. You can mix some lamp black with it for a black color. It has the benefit of not getting brittle.
another option. hammer form a piece of round brass tubing onto a square mandrel. Then, drill a hole in your know, insert the squre tube and solder in place. Should hold till hell won't have it.
edited to add:
if you need to remove it just heat knob lightly with propane torch and the epoxy releases it's grip.
[This message has been edited by Your Old Dog (edited 03-03-2006).]
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Another possibility is to drill a hole that just clears the square and use a set screw to retain the knob on one of the flats of the shaft.
Okay, a 1/8-inch square hole 1/8-inch deep, right? This is how I would do it with the tools you have. Actually all you need is the lathe.
Chuck up a brass knob blank in your lathe and drill it all the way thru with a 1/8- inch drill bit. Then turn the material around in the chuck and drill or bore another hole about .250 (or whatever) to within 1/8-inch of coming thru. File, or broach, the remaining material square. Then make a brass plug for a press fit into the .250 hole. Machine outside to desired shape.
[This message has been edited by Mike Burdick (edited 03-03-2006).]
All excellent ideas. I recall seeing some small brass square tubing at the Hobby store may try that and some epoxy in a round hole. I think I will try all of the suggestions just to see how well they work.
I had to make a winding key for a table clock years ago. It needed a square hole about that size. Keep in mind the hole doesn't have to be entirely square, just enough corners to make it hold. I drilled a hole a bit under the diagonal dimension and ground the shank of an appropriate drill bit square. Tapped it in and voila, square cornered hole.
Oh yeah, the drill is a bit difficult to get back out but I will leave that as an exercise for the readers.
[This message has been edited by Evan (edited 03-03-2006).]
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Use a square drill. Yes, they do make such things. They consist of a hollow square-cornered broach with a drill running through the center.
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[This message has been edited by Leigh (edited 03-03-2006).]
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