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From: Dave Baker
Subject: Re: Cutting hex holes in drill rod
Date: 11 Feb 1999
Newsgroups: rec.crafts.metalworking

>From: (Gary Coffman)
>Okay, maybe I'd better explain a bit. I was playing with my new
>machine, just seeing what it would do (and what I could do with
>it). I turned, I milled, I knurled, and when I was done I had the
>cutest T handle you ever saw. Now what good is a T handle
>without something for it to turn? So I decided to make a set of
>interchangeable bits for it.  Most of the commercial interchangeable
>bit tools seem to use a female hex socket to accept the bits. I began
>to wonder what was the best way to do that at home.
>I've broached keyways before, but I've never tried to broach
>a hex hole. Seems there needs to be some way to adjust
>depth of cut (I don't think I can cut it in one pass) as well as
>to index the cutter. Shims and a guide sleeve are used for
>keyways, but they're the opposite sex of what I need here.
>(Note that the hole I need should be deeper than the hex in
>an allen screw, I'm shooting for something on the order of
>1/2 inch deep and 1/4 inch across the flats.) I'll drill deeper
>than that for a small cylindrical magnet which will be press
>fit in place at the back of the hex hole to hold the bits.
>Now I could just braze a 1/4 inch deep socket on the end
>of the tool and call it a day, but I don't want to do it that way.
>I want this thing to be machined all in one piece. (Why? Just

Make up a fixture to hold the T bar you want to broach the hole in. Ideally the
end should be vertical but horizontal will do. Take a good quality allen key
and cut the short end off it. Drill a small hole about 4mm diameter through the
key about 1/2 inch from one end - you'll see why later. Then heat the other end
to bright yellow and quench in boiling water to get a superhard tip. Grind the
end to get sharp cutting edges.

Drill the hole in the piece to be broached at the size across the flats of the
allen key. Make it a bit deeper than you need so there is somewhere for the
swarf to go although if you want a bar magnet in there you'll have a smaller
deep hole anyway which will be fine.

Now you need a guide for the allen key to run in. Take a piece of steel about
3/4 inch thick and drill a hole through it just smaller than the size across
the corners (not the flats) of the allen key. Then tap the allen key through it
and you have just broached your first hole. Mount this piece of steel so the
hole in it is central to the part to be broached. You now have a guide which
will keep the allen key bang on centre all the time.

Now just tap the allen key into the workpiece a bit at a time and pull back to
blow out swarf. If the allen key sticks then use a small tommy bar in the hole
you drilled through one end of it. If you want the broached hole to be as dead
to size as possible then do this. Heat the workpiece in a flame to 200 degrees
C or so until you have broached to full depth. This will expand the workpiece
and the broached hole will be a tad undersize when cool. Then let it cool and
contract again and take a final cut in one hit to the full depth.

It's a lot of messing about to make one hole but you'll get it dead true and
dead to size if you do it this way.

Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

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