"Christopher Tidy" wrote in message
...
Ed Huntress wrote:
"Christopher Tidy" wrote in message
...

Ed Huntress wrote:

snip

The channel was broached.


Probably after being hot-punched in a secondary forging operation. The
whole body could be forged and punched in two hits.

Thanks for the thoughts. That was my own best guess.

I'm a little surprised that it's possible to broach a deep channel with
such thin walls (only about 3/32" thick) without everything getting
mangled. Guess the key must be getting it hot enough.


Production broaching is not much like the kind we do in hobby work. The
broach itself looks more like a long, extremely coarse tapered file --
some as long as twenty feet or so -- with each successive tooth cutting a
thousanth or three more than the last one, typically cutting on all sides
at once (or not, depending on the job -- some cut on only one side at a
time). They come in two general types: pull-broaches and push-broaches.
They were made from a single piece of tool steel but recent ones have
replaceable inserts.

Another thing I've been wondering about: is the broach inserted into an
oversize hole and firmly held by guide rails, or is the broach
self-centring in a tight pilot hole? In other words, what determines the
precise location of the broached hole?

Push broaches are usually located with their own guides, but they can be
free-floating in jobs for which precise location is not important. In the
case of a an old wrench, it could be free-floating. The punched hole was
close enough.

The hole is undersize, not oversize. The leading end of the broach is
undersize, and then the teeth progressively cut the hole to size.

--
Ed Huntress