Does Carbide Cut or Tear Metal?

[deltaenterprizes]

I was talking to an instructor at the local community college and his opinion is that carbide tears metal off instead of cutting like HSS does.

What say ye?

[Pherdie]

While I'm certainly no expert, it would seem to me that all cutting is a defined measure of tearing, if nothing else, at a molecular level.

[Arthur.Marks]

Also no expert, but it would seem to me that anything which can create a burr is "tearing". I have certainly read in textbooks that all metal cutting is a form of shearing. That said, perhaps we (and your instructor) need to first define the terms you're using. A.R.Warner, for instance, makes the same claim in much of their advertising.

[Rustybolt]

While I'm certainly no expert, it would seem to me that all cutting is a defined measure of tearing, if nothing else, at a molecular level.

That's pretty much what I have been led to believe. I also think that it would depend on the material, the machine, all kinds of factors come into play here.

[gwilson]

I think it depends upon how the cutting edge is ground. There are carbide inserts that have highly polished,very sharp cutting edges.

Of course,most of them don't,and they don't leave as clean a surface as HSS does,so,they could be tearing.

[justanengineer]

Its known as shearing. I am not sure what you mean by tearing or cutting.

[Mcgyver]

Its known as shearing. I am not sure what you mean by tearing or cutting.

exactly, dispel this notion of cutting or tearing. The tool bit exerts pressure causing a shear plane to be established ahead of it. That's it. Doesn't change if its carbide, diamond, hss, tool steel, positive rake, negative rake, zero rake, grinding, drilling, turning, milling.

did no one read the first chapter of that grade 11 machine shop text....including the college instructor (who should know better)?

[Dr Stan]

exactly, dispel this notion of cutting or tearing. The tool bit exerts pressure causing a shear plane to be established ahead of it. That's it. Doesn't change if its carbide, diamond, hss, tool steel, positive rake, negative rake, zero rake, grinding, drilling, turning, milling.

did no one read the first chapter of that grade 11 machine shop text....including the college instructor (who should know better)?

You're right on point. All metal "cutting" results in some smeared metal at the molecular level. This is why the first step in electroplating, post degreasing, is etching in order to remove the smeared metal.

A former shop owner & boss of mine discovered this the hard way. He had some bores "bearingized" which produces a highly accurate and smooth surface that uses a smearing action. They were then sent to the electoplater and returned as rough as a corn cob.

[J Tiers]

exactly, dispel this notion of cutting or tearing. The tool bit exerts pressure causing a shear plane to be established ahead of it. That's it. Doesn't change if its carbide, diamond, hss, tool steel, positive rake, negative rake, zero rake, grinding, drilling, turning, milling.

did no one read the first chapter of that grade 11 machine shop text....including the college instructor (who should know better)?

I think there is a different "level" of discussion here.......

Naturally all is at some point a shearing....

But, looking at carbide vs HSS.... the USUAL* carbide, such as brazed carbide cutters, are used with a substantially flat top.... no special rake, unless it is supplied by the tool holder, which the Aloris generally does not (some lantern style have a tilted "carbide" holder).

On the other hand, it is fairly common to grind a good amount of rake into an HSS cutter.... I like 15 or more degrees for some cuts.. others may use less, or more.

obviously, the GROSS LEVEL action of the plain flat-topped cutter, carbide or HSS, is more a "scraping", sometimes called "plowing", action. One might actually say, with some justification, that it is "tearing" the material. The cut material doesn't always flow off nicely, you need chip breakers etc to handle it, often ground into the cutter to raise the effective rake, or force the chips to crumple and break.

Cutting force tends to be high, as is the reaction against the work. Remember what some people always preach when this comes up..."your little hobby lathe isn't rigid enough to use carbide"... That's what they mean.

A high rake cutter, typically (but not only) made of HSS, seems to 'slice" the material. Cut chips flow off nicely, often forming thin ribbons if allowed to. You have to look much more microscopically to see the "tearing" and "plowing" action.

Cutting forces are typically low, and reaction on the work is also low.

I am perfectly certain that this is what folks are referring to when they suggest that "carbide tears the metal". They are talking about the "visible level" action. And about the surface finish.

Their error is not seeing that the geometry is the culprit... It is just that carbide is much more often used in geometries that DO "plow", or "tear" the material at the "visible level"....

of course if you turn up the speed, and hit the feed / DOC hard, you can get the same flat-topped carbide to give a really nice finish that does not appear torn at all. IF you have the machine that can do that. McGyver does, I don't.


* please don't get us tangled up just because one time you saw a really high rake carbide cutter.... we are talking the general case....

[Mcgyver]

they don't plough or tear they set up a shear plane ahead of the tool. The material doesn't know whether the cutter is hss or carbide