George wrote in
:

Richard J Kinch wrote:

D Murphy writes:

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.

OK, then, does this 1 to 1.5 degrees of tilt and wobble result in a
slightly conical hole being broached, or is perhaps the tool tapered
so that the leading edge is wider than the body, or what?

The leading edge is wider than the body.

Since the tool is always at a 1 degree angle to the work, the sides of
the tool must have a 1 degree or greater draft.

Ideally the tool advances at the same rate that it cuts. So a 1/2"
diameter tool should advance at 0.009" per revolution. 1/2*sin(1). If
it advances any faster than that then the tool becomes choked, if it
advances any more slowly then you get an interrupted or zig-zag cut.
Since all work material is elastic, you would actually cut a little
less than the ideal rate just to release the load on the non-cutting
edge of the tool.

Exactly right. The sin of one degree is .017452. The formula for
broaching is to take the diameter of the broach and multiply by .016 to
get the proper feed rate. For an internal hex you should be using the
distance across the corners, and not the distance across the flats.


There is some spiraling of the tool as it cuts, so the bottom of the
hole may be rotated with respect to the top of the hole. Spiraling may
be undesirable because it binds the body of the tool and prevents it
from wobbling freely. One solution to this is to reverse the rotation
in mid cut causing the tool to spiral in the opposite direction.

Spiraling is nearly nil in a shallow hole or a short O.D. length. If the
brach is deep, you pretty much have to reverse the spindle. Another
trick that helps if it's allowed by the drawing, is to groove the
workpiece so the broach essentially is cutting a series of short
lengths. If you look at the valve stems for faucets in a hardware store,
you will often see the spline on the end of the valve stem where the
knob mounts, has a groove in the middle of the spline. This allows the
spline to be cut on a multi-spindle screw machine, which doesn't have
the ability to reverse the spindle. Coolant is usually not required. If
you are going to use coolant on an internal broach, then the hole in the
blank must be larger than the broach is across the flats. If that is not
possible, there are companies that make wobble broaches with a vent
hole. Coolant does not compress very well.


--

Dan