From: mulligan@advinc.com
Newsgroups: rec.crafts.metalworking
Subject: Re: Diamond cutting tools
Date: Fri, 02 Feb 2001 05:33:34 GMT

In article <20010201191625.22762.00000269@ng-cq1.aol.com>,
  pumaracing@aol.com (Dave Baker) wrote:

>  Modern carbides and ceramics
> are so good though I can't really see the need for diamond tools
> anymore in general machining applications.

I have used the polycrystaline diamond inserts for turning
and milling (insert type end mill, ceratip) of abrasive composite
materials like phenolic (yes, that is pretty abrasive) and also
G-10 and quartz-epoxy composite.

These are TPG-221 type inserts made by valenite, with full edge
diamond tips.  They have been very, very good value for their
cost, and hold a sharp edge even when cutting really abrasive
stuff.

Composites like G10 with glass fiber fillers will rapidly
degrade cemented carbide tooling.  The mechanism (so I have
been told, and nothing I've seen disagrees with this explaination)
is this:

Cemented carbides are not 100% silicon carbide in composition.
They are sintered of carbide powder in a cobalt metal matrix.
Now the cobalt metal is nowhere near as hard as the silicon
carbide particles, so when you are machining a glass loaded
composite, the glass fibers will selectively pick out the
cobalt binder metal until the carbide grains are unsupported,
whereupon they will fall out of the cutting tool.  More binder
is exposed, and then removed, and then more edge is lost.

Granted the PCD tools also have cobalt binder in them, but
perhaps the additional hardness of the diamond particles
makes it harder to remove binder from the cutting edge, or
perhaps there is less binder percentage-wise.

Obviously this in not really 'general' machining, but in my
applications, these tools were a lifesaver, as I had to
turn that abrasive stuff, and leave as little magnetic residue
(cobalt being very magnetic) as possible behind.

Jim

From: "Ed Huntress" <mike234@bellatlantic.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Diamond cutting tools
Date: Fri, 02 Feb 2001 06:13:34 GMT

>>Cemented carbides are not 100% silicon carbide in composition. They are
sintered of carbide powder in a cobalt metal matrix.<<

I'm sorry to be such an argumentative s.o.b. tonight, but the hard crystals
in cemented carbides are tungsten carbide, or tungsten and tantalum carbide,
not silicon carbide. And the binder is usually a mixture of iron and nickel,
with more or less cobalt added for toughness and high temperature tolerance.

But you have the general idea exactly right. <g>

>>Granted the PCD tools also have cobalt binder in them<<

Here I go again. No, the PCD tips -- the diamond part -- have no binder.
They are monolithic diamond. The diamond can be polycrystalline (I forget
how they're made), or, on the new diamond-coated inserts, vapor-deposited.
But it's all diamond either way. No binder. The same is true for ceramic
inserts, including those made of silicon carbide and aluminum oxide, and CBN
(cubic boron nitride, which is used for ferrous metals where you'd want to
use diamond, but can't because diamond reacts with ferrous metals).

With metal-matrix "carbides", the common ones we all call "carbide", the
sintering process just sinters the metal. The carbide particles are locked
in, like fruit in a fruitcake. With ceramics -- which often are carbides,
too, just to confuse you -- there is no binder. It is the ceramic particles
themselves that are sintered together. This requires major temperatures. I
think PCD is also sintered, although I forget.

Ed Huntress

From: mulligan@advinc.com
Newsgroups: rec.crafts.metalworking
Subject: Re: Diamond cutting tools
Date: Fri, 02 Feb 2001 07:14:37 GMT

In article <iese6.756$Nx3.194436@typhoon2.ba-dsg.net>,
  "Ed Huntress" <mike234@bellatlantic.net> wrote:

> the hard crystals
> in cemented carbides are tungsten carbide, or tungsten and tantalum
>carbide, not silicon carbide. And the binder is usually a mixture of
>iron and nickel, with more or less cobalt added for toughness and high
>temperature tolerance.

Yep, brain fart.  But the binder is, I belive, often predominantly
cobalt.

> But you have the general idea exactly right. <g>

Right, the Ni or Fe will be even easier to pick out than
cobalt.

> But it's all diamond either way. No binder. [pcd inserts]

I think I got most of my info from Valenite, who made the inserts.
There is, I belive, some binder in between the diamond grains.  But
I would be happy to be proven wrong.

> The same is true for ceramic inserts, ...

Things like Silcon Nitride, Al2O3, or the cermets are not really
sintered.  I think you are correct, there's no binder there.
I've used the cermets (silver color) and those are tough
cookies.


> I think PCD is also sintered,

I think so too - but that the binder is needed for the sintering
process - and that they use cobalt for this.  Although my information
is a bit fuzzy, the last time I investigated this was a year or two
ago.

I've never had any success with the CVD coated diamond inserts.
Basically they try to grow a thin layer of single-xtal diamond
on a carbide insert, but they have a real adhesion problem at the
edge, so those inserts invariably have (or had, as of a few years
ago) a honed, or turned, edge.  They were nowhere *near* as sharp
as the PCD ones I got from the same manufacturere (valenite) and
did not do nearly as good a job for me as the PCD ones.  I was
running them on a smaller machine (Hardinge HLV) and the postive
rake and sharp edge on the PCD ones was really required to get
any kind of decent finish.

I bought about 8 of those inserts, figuring they would eventually
have to go out and get re-sharpened.  But each one is as razor
sharp as when I got it.

Jim

From: "Ed Huntress" <mike234@bellatlantic.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Diamond cutting tools
Date: Fri, 02 Feb 2001 18:41:17 GMT

>>There is, I belive, some binder in between the diamond grains.  But I
would be happy to be proven wrong.<<

Both. <g> There is a binder that's inserted to aid somehow in the sintering.
But it either volatizes or diffuses out. The finished product is all
diamond.

Ed Huntress