"jim rozen" wrote in message
...
In article , Harold & Susan Vordos says...

Exactly as I do, too. I've found that the tops of parting tools
respond
well to the radius of the wheel, so I grind what surely must be no more
than
three degrees of positive rake on the tool as an almost last operation.
One has to start well back of the edge and extend the grind back to the
solid portion of the shank. Tools seem to have superior chip flow and
cutting ability with this minor variation. The only negative is as you
push the tool back from sharpening, the cutting edge drops slightly, so
they
must be shimmed up. I keep a huge collection of shims on the headstock
of
my lathe, so that's no problem at all. For those that use the insert
type
tool holders (KDK, Aloris, etc.), it's a simple screw adjustment.

Interesting. I keep my home-grown grooving/cutoff tools with
purely zero top rake - I don't grind the top face at all. This
seems to work pretty well, given that these are most often
used as grooving or chamfering tools (with the indexable
toolpost kicked over a notch or two) and never get buried
more than about 1/4 inch deep in a part. Beyond that and I
reach for a real cutoff tool.

The Aloris holders do indeed put that three or so degrees of
back rake on their cutoff tools, but again I tend to run mine
at zero back rake, in a home-made holder. If they didn't
work tolerably well, I would consider making one with
rake (angled bottom to the holder) but so far it just hasn't
been needed.

Jim

It's been so long since I started grinding my parting tools that way that I
can't honestly say what prompted me to do it, but I have a hunch it was a
result of poor chip flow in a deep cut. The minor amount of positive rake
is quite helpful in parting, especially in materials like stainless. I'm
not sure anything helps you when parting mild steel! :-)

You've likely noticed that stainless responds quite well to positive rake.
It becomes a balancing act, too much and you experience tip failure from
chipping or over heating, , too little and you get tip failure from chip
welding and abrading.

Long ago (still an apprentice at Sperry) I was turning some stainless plates
round, from what I recall to be about 1/8" thick material. . A fairly
young journeyman, new on the job, walked up to me and suggested I'd have a
lot better luck if I'd use HSS and plenty of positive rake, so much rake
that he sounded like he was nuts. The plates being machined were being
held between thick plates by tailstock pressure so you could machine the
periphery fully and deburr them. the holding plates being smaller in size.
At that point I felt I had nothing to lose, so I ground a HSS tool with
considerable positive rake and a rather broad and gentle chip breaker. I
then ran the machine slower, and fed the tool directly on the full width of
the plate, using a fine feed, maybe .002" and ran coolant on the cut.
Couldn't believe my eyes. The chip came off beautifully, the tool held up
for the job to be completed. The parts had an excellent finish when done.
Overall, it was much faster than using carbide. Along with a few other
cherished memories, that one has stood out in my mind. A definite
variation from suggested machining practice that has outstanding results.
I've used that concept many times since.

Maybe that helps explain why I grind the top rake on my parting tools? Hard
to say. I just know I do. :-) They definitely cut better than they do
without the angle.


Harold