pipemakersforum.com

W.Pastuch wrote:I recommend asking The Great Wise Wizard Google.

PremalChheda wrote:I hate to poop on your great PMTAD find, but there is something out there that is even better and will last much much longer...

socrates wrote:

PremalChheda wrote:I hate to poop on your great PMTAD find, but there is something out there that is even better and will last much much longer...

And that is???

RadDavis wrote:You know what I hate? I hate when someone posts about a fantastic new pipe making tool they have found, and then, like some smug smartass, they tell us to use Google to find the fucking thing.

Post a fucking link.

Ridiculous.

Rad

I use tungsten carbide cutters to shave a pedestrian (but pure) formulation of acrylic plastic in some cases, and hardened rubber (vulcanite) in others. They must be replaced periodically because they get dull. I don't get it. Why/how does the soft stuff ever remove so much as a molecule of the enormously harder one?

Materials Scientist here.

Your problem is not due to the sharpness of your tool, nor the hardness of the tungsten carbide (WC) particles. First let me show you what your tool looks like up close:

http://i.imgur.com/YWr1g.png

The block particles are the very hard WC, and the matrix is actually just cobalt (Co). Advantages of a metal matrix composite include the advantages of the metal and ceramic, while trying to get rid of the metal and ceramic shortcomings. The cobalt is ductile, has a high fracture toughness, is easily formed and has high conductivity for dissipating heat. The WC ceramic maintains strength well as the temperature rises, has a high elastic moduli and is corrosion resistant.

The wear behavior of metal matrix composites is different from a single metallic counterpart because the composite materials have multiple constituents in their structure that wear at different rates. Under the low wear stresses (such as you probably encounter when cutting your PMMA), there is some abrasive wear. However as your tool cuts, "pull-out" of the WC reinforcement occurs and this increases the wear on your tool which is the biggest deciding factor.

The pull-out simply means the WC particles are literally being pulled out of the Co matrix in your tool blade. The hard particles themselves are not being deformed, they're simply leaving the tool as they are being "gripped" by the PMMA you are cutting. When the tool encounters something called a "critical wear stress", that means the stress at the WC-Co interface is too great for the bonding between the two components. The critical wear stress is simply a ratio between this interfacial strength between the Co matrix and WC particles, and the WC friction coefficient between the WC and the Co.

So let's take a look at what happens, and in what order:

1. The tool cuts the plastic and a minimal amount of wear on both the WC and the exposed Co matrix develops
2. Over time, a specific WC particle will be gripped by the plastic and a force will be exerted on it. This force will be parallel to the WC-Co interface, which puts a stress between these two phases.
3. Every once in a while, the stress is large enough to surpass the critical interfacial strength because the bonding between the WC and the Co is not strong enough, i.e. the friction between the two pieces is too small for the stress applied to the WC.
4. The WC particle will pull-out of the cobalt matrix, and some extra Co is exposed.
5. The strong, but not strong enough, Co matrix is exposed as the cutting surface, and it quickly erodes until the next WC particle is exposed for cutting.
6. Process repeats itself.

There have been numerous studies which blame the WC de-bonding and pullout as the main reason as to why tungsten carbide tools wear so quickly.

The solution? Better manufacturing and process methods, which will differ from company to company.

RadDavis wrote:George, they get dull for the same reason that razor blades get dull cutting your beard, or pocket knives get dull whittling cedar. It don't seem right, but that's the way it is.

wdteipen wrote:Where can one purchase this magically imbued artifact?

baweaverpipes wrote:

RadDavis wrote:You know what I hate? I hate when someone posts about a fantastic new pipe making tool they have found, and then, like some smug smartass, they tell us to use Google to find the fucking thing.

Post a fucking link.

Ridiculous.

Rad

I concur.