DoN. Nichols wrote:
On 2012-02-20, Pete C. wrote:

Larry Jaques wrote:

On 20 Feb 2012 04:20:40 GMT, "DoN. Nichols"
wrote:


[ ... ]


One problem -- I need the hub out of the failed one to complete
it. Unless I make myself a double-D broach to make them without the old
hub.

Don, how are double-D holes made? I just spent 5 minutes looking at
broachmaker sites and they don't have many pics. One listed it as a
round shape. Do they drill a hole and fit the broach in, moving it
up and down across an axis to cut the slot, or what?


I seem to have missed the original of this, so I'll reply to the
reply.

In the case of the original Tektronix gear, the hole was made by
casting the hub to form it. The hub was made of Zamac (pot metal), and
it had a taper on the outside to make it easier to get out of the mould.
Broaches are expensive and wear, while Zamac moulds are relatively cheap
and last a *long* time. Especially since the Zamac hub was used to cast
a plastic gear around, instead of the brass one which I made.

However, (below is the description of how to use them. To
*make* one, I would have to start with some drill rod, turn it (between
centers) to a little over the final OD, mill a pair of flats at 180
degree separation to get the double-D shape, then turn a taper on the
whole thing from full dimension at one end to a round pilot at the other
end which just fits the starting hole (a little larger than the
across-flats dimension). Then plunge an angled cutter into the shank at
regular intervals to provide a sawtooth wave profile. Then take it out,
go to a heat treating oven, quench to harden, heat again to draw the
temper to something strong enough to hold an edge, but not so brittle
that it will break when you look at it. Then back to the lathe, cover
the ways with oil soaked newspaper to protect them, and mount a toolpost
grinder, and use that to take each step down to the proper diameter and
clearance angle, and you finally have a broach which can be used as
below to broach the D-shaped hole.


That's basically what I did to make a broach of this form to adapt a
Pegler tap stem to a different knob a few year ago. The minor OD was
5.6mm and the major one 7mm and did that in 8 stages over 40mm. As I
have a DRO on the lathe I didn't cut the taper you mention but rather
went straight to cutting the stages and the back clearance angle, I also
didn't leave it over size for final grinding but that would depend on
the accuracy required . After hardening and tempering I touched up the
sides slightly with a stone and it worked very well in brass for the few
I needed to do. All in all it wasn't a very time consuming thing to make
and was the first multi stage broach I've done, I had previously made a
few to cut internal serrations in blind holes, again for tap adapters.

I have a collection of commercially made broaches, but they are
all either square or hex -- no Double-D ones. And I've never *made*
one, just studied what I have to tell me how to make one should I really
need to do so.


Like most broaching, it will be drilling a pilot hole and then pushing
or pulling the broach through to finish the full hole profile. Each
cutter step of the broach will progressively start from the pilot hole
round diameter and expand out a few thousandths towards the final hole
shape. This is why specialty broaches are long and expensive, every
cutter stage is a different profile.


Yes -- and you need a long travel arbor press (or something
else) to drive the broach. And care to make sure that the driving force
is on axis so you don't bend and snap the broach. (Better are the draw
type, but they require a different style of driver -- ideally hydraulic,
I believe.)

Enjoy,
DoN.