Richard J Kinch wrote in
:

D Murphy writes:

http://www.sommatool.com/manuscripts/broaching.asp

Do I follow that action correctly: the cutting tool is tracing out an
internal hex path by spinning axially and wobbling at the same time?

Actually you are broaching with a hex shaped broach in the case we are
taliking about. Almost any shape can be broached, the broach is always
the same shape as the shape you need to put into the work. So the broach
isn't tracing out the shape in the sense of profiling if that's what you
are saying. The axis that the broach spins on is kicked off at a one
degree angle. The workpiece drives the rotation of the broach. Since the
broach's axis of rotation is kicked off on an angle there is a small
point of contact at any given time. This gratly reduces the amount of
pressure it takes to push the broach into the work. The broach is still
forming the hole by being the same shape that you need to broach into the
work. The easy way to visualize what is going on is to hold a small cup
or drinking glass up against your hand so that the rim is flush against
your palm. Now tilt the glass a couple of degrees. That is the contact
that the broach has against the part. Now if you rotate the glass and
your hand together, keeping the glass tilted, you can see that in one
rotation that the entire rim of the glass will eventually make contact
with your hand, one small spot at a time. Now if you think about the
broach pusing into the hole you can see that by having less contact at
any given time, it will take less pressure to push the broach into the
hole. The main limitation is your inch per revolution feedrate. The
broach is in a sense cutting a helix the same way that a turning tool or
threading tool does. A coarse pitch thread has a grater helix angle than
a fine pitch one does. You always have to have a greater side clearance
on a threading tool than the helix angle of the thread. Otherwise the
tool will rub. The helix angle created by the broach and the rotation
can't be greater than the amount the broach is kicked off at (usually 1-
1.5 degrees) otherwise you would no longer have that single point of
contact, you would have full contact and a higer broaching pressure. I
think that Somma has a formula on their site for calculating the max feed
rate. Usually it ends up being a small value .002"-.003" per rev. If your
confused after reading all this, try the glass thing it usually turns the
light on.

--

Dan