In article ,
John Albers wrote:
I am a home shop machinist. I have a SB 9" lathe. I am interested in
using carbide inserts and making my own tool holders. I know that HSS
is easier to work with but I still want to experiment with carbide. I
have been doing my homework but have a few questions and need some
clarification on a few points.

1. Any insert with a "N" in the second position is a negative insert.
Any insert with any letter other than an N in the second position is
a positive insert.

Yep. At least for lathe tool inserts. Not too sure about those
for milling cutters and such. And, of course, there are things like the
threading inserts (There are two styles, one is the "laydown" which have
the triangular inserts mounted more or less flat (there are solid
carbide anvils with various angles to allow you to tune the angle of the
insert to match the lead of the thread, since you can't just grind more
relief on the typical insert.)

The other style of threading insert is also triangular, but is
mounted on edge. I know of no way to tune this for the needed clearance
angles.

2. I am assuming that the positive and negative are refering to back
rake.

Basically.

3. Negative inserts if kept horizontal, really have no back rake on
their own and also no end clearance or side rake. Positive rake
inserts if kept horizontal, really have no back rake on their own just
end and side rake.

End and side clearance -- not rake.

The positive ones have whatever back rake may be contributed by
any chipbreaker which they may have -- if it comes close enough to the
edge.

3.5 Negative inserts are more economical because both sides can be
used.

They also are stronger, because there is more meat behind the
edge. The edges on positive inserts are more fragile.

4. Tilting down a negative rake insert gives end clerance and
negative back rake. This is why they are called negative inserts when
really they have no back rake on their own.

Right.

5. Tilting up a positive rake insert *should* give end clerance and
some positive back rake would seem to be the ideal situation for a
small lathe. However this creates an interesting paradox, all of the
sources that I could find, seem to want to operate a positive rake
insert perfectly horizontal. This would make it a zero back rake tool
(neither positive or negative). Why does everyone want zero back rake?

They don't -- except for turning things like brass. But they
depend on the chipbreaker to generate the effect of rake.

And if you tilt a positive insert, you actually *reduce* the tip
and side clearance -- which might not be too bad, depending on the feed
rate.

The tip clearance on a positive insert is formed by the
combination of the clearance angles on the two sides which join to make
the tip -- and it is affected by the angle at which they join. There
are *lots* of shapes. On my 12x24" Clausing, I use the triangular form,
while on my little Compact-5/CNC, I use the 55 degree diamond. The
choice for the Compact-5/CNC was made for be by the tooling which came
with it.

The triangular ones on the Clausing are the TNMP, and usually
uncoated, which gives a sharper edge. These give a total of six corners
which can be used (assuming that the wear to a given corner is not
sufficient to destroy the use of the flip side one).

The diamond shaped ones on the Compact-5/CNC are positive rake
ones, and the diamond only allows two of the four corners (the sharper
two) so there is a 3:1 ratio in favor of the triangular ones on the
Clausing. Most of my inserts for the Compact-5/CNC are also uncoated,
though I have a certain percentage of TiN coated ones.

The diamond shaped ones have three formats (in the collection
which I have). In one (the first ones I had were all of this type), the
chipbreaker ran around the full diamond shape. The other two either
have the chipbreaker only on the left edges (for right-hand-side turning
tools), or on the right edges (for left-hand side turning tools and for
boring bars). The corners are a bit stronger on the ones which have
only the half chipbreaker. I presume that there are also diamond-shaped
inserts with *no* chipbreaker (good for brass), but I don't have any of
them. I have a large collection of the other styles, thanks to an offer
from someone else in this newsgroup a few years ago. He was willing to
send a few samples free to anyone in the list, and as I was the only one
(at the time) who could use them, he wound up offering to sell me the
rest (several thousand, I think) for what I considered a reasonable
price -- which I gladly accepted.

6. I found a source suggesting using a TNMP insert. The chip breaker
goes clear out to the edge of the insert. Tipping this insert down 5
degrees gives you 5 gegrees end relief and 5 degrees positive back
rake. I think that this insert has chip breakers on both sides so it
could be turned over and used again. DOes anyone know for sure? I
think that this is the best way to go. Has anyone tried this?

Yes -- the chipbreakers are on both sides in the TNMP inserts.
These are what I use in the Clausing -- both in normal holders held in
Aloris style quick-change holders, and (lately) in the special holders
made by Aloris which directly accept two inserts -- one on each end for
the turning and facing operations. This makes for a more rigid setup,
and reduces chatter. There are two forms of this -- one for the positive
inserts (16P), and one for the negative inserts (16N). I have the
latter only. I won a couple of lots on eBay which had 100 of the TNMP
inserts and a straight holder (I later got the left and right holders
via MSC to go with the inserts which I already had. All of these are
uncoated, and seem to work well for what I normally do -- though for
certain materials, the TiN coating could improve wear life and friction,
while reducing the sharpness of the available edge (which isn't that
sharp on a TNMP anyway. :-)

Note, however, that most of my threading inserts *are* TiN
coated.

7. What effect does the tip radius have. I beleive that the larger
the raduis the smoother the surface finish on turned work. However,
the larger the radius the more power required.

And -- the more likely you will get chatter, because of the
higher forces.

Any help would be greatly appreciated. Thank You

One consideration for true negative rake inserts (not the TNMP)
is that it takes a more rigid machine to hold them without chatter. And
chatter can destroy an insert where a HSS tool would not have problems,
because the carbide is more brittle. I'm not really sure whether the 9"
South Bend is rigid enough (and has enough power) for true negative rake
insets, but it is probably sufficient for the Positive/negative ones
which we are discussing. The true negative rake ones take *lots* of
power, and are better for cutting really hard materials, where the
clearance for the positive inserts weakens the edge too much, as does (to
a lesser extent) the chipbreaker groove that turns a negative insert
into a positive/negative one.

I hope that this helps. Now off to read your other responses,
to see how many agree or disagree with me -- or offer other things that
I didn't think of.

Good Luck,
DoN.
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