Is this the once-infamous "Contrary Ground Finishing Tool" as described by
Frank Burns in the article "Grinding Tool Bits for a Smooth Cut" in
the July/August '97 HSM magazine?

Grant Erwin
Kirkland, Washington

Ed Huntress wrote:
wrote in message
...

On Mon, 14 Jun 2004 15:28:51 GMT, "Ed Huntress"
wrote:


Congratulations! You've just reinvented the angled knife-tool, a

finishing

cutter from the 1930s. g

I'm being only slightly facetious. This is nothing like a traditional

knife

tool. It doesn't really have a standard name, but you'll see it called
several things, including an "angled knife tool." It should be called a
slicing tool, or something like that.

An angled knife tool typically is ground from round stock. It has a lot

of

front clearance and a lot of top rake, which you get by grinding a groove
into the top of the bar, roughly half-way through. You must hone it very
sharp. The front of the tool is ground straight across, so that, if you
presented it square to the work, it would create an impossibly wide chip.

So you don't present it square to the work. You rotate the cylindrical

tool

on its axis perhaps 15 degrees counter-clockwise, as you're looking from
behind the tool, into the work. The edge is now angled to the work so it

can

slice, or shear, on an angle.

Then you start a very light cut. If you have the tool height set dead
on-center, you make only a fairly narrow, very thin, knife-slice chip,

right

in the center of the cutting edge. Make sure you don't set this tool even
slightly below-center. Err on the up side, if you must err at all.

It's a near-last resort for getting a good surface on soft, gummy steel.

The

last resort is a file. g

Watch out, it can grab if you cut too deep. This is a problem mostly on
long, thin workpieces that can flex and climb over the cutter. In that
regard it's like cutting brass with a lot of positive rake.

Ed Huntress

Sounds pretty similar to the sort of tool I've been playing with.

I'm a bit puzzled by your emphasis on the importance of tool
height because skive tool cutting action is not affected by this.

I normally set the tool height so that it cuts over a narrow
region roughly in the centre of its 45 deg edge i.e. halfway down from
the top of the tool. However any part of the whole cutting edge can be
brought into play, cutting with the same cutting geometry, by simple
adjustment of the tool height. This is quite useful because, as soon
as the initial cutting edge region starts to dull, a fresh region can
be brought into play by a making a small change in tool height.

Jim




I think I had the geometry of your tool clearly in mind yesterday, but I
lost it, and I have to run now so I can't go through the mental gymnastics
again.

But, to answer your question: a skiving tool usually is a form tool, and it
can be -- and often is -- cut with no top rake and no front or side
clearance. Form tools don't cut to the side, they plunge-cut. To get front
clearance, they're run below center height, which has the additional result
of making the cutting angle negative.

They also can be given front clearance all around the form, which allows
them to be set on-center or even above-center. My experience with them comes
from two places, the primary one being a turret-lathe form-cutting
operation, and we used our tools set above-center to get effectively
positive top rake, with hand-ground front clearance.

Your tool actually sounds like something else, if I understand it correctly.
My comment about the top rake refers to the fact that these shearing cutters
I'm describing, or whatever you want to call them, typically have a lot of
positive rake and a lot of front clearance, which is the combination that
can cause a tool to grab and dig into gummy or "grabby" material, like brass
or hot-rolled mild steel. But the positive rake is intended to make a clean,
sharp slicing cut, and it's considered to be part of the tool design.

I haven't experimented much with finishing tools. I just take old designs
off the shelf and try them. I consider myself fortunate when one actually
works. d8-)

The thing that made me think of those shearing cutters when I read your
description was the idea that it was shearing chips off like a knife, which
is what the cutters that I'm familiar with actually do.

This is a place where a picture is worth 10,000 words, I think.

Ed Huntress