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So everywhere I read I hear that carbide inserts for metal machining are extremely brittle and they will break very easily if you say drop them. However, you put them in your tool holder and spin up your lathe or mill and you can easily use them to cut hard materials. Why does the insert not have a problem cutting the material but would shatter if you dropped it? If you use one on a lathe and all of a sudden a piece of metal spinning at quite the speed smacks into the end of it is that force not going to impart similar forces on the material?

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Tungsten carbide tooling is not typically so weak that it will outright break merely from dropping.

However, WC tools are brittle, by which it is meant that when a stress is applied above some limit, instead of undergoing plastic deformation they will shatter. So the term brittle refers to the way the material responds to stress, and it does not mean the material cannot withstand any degree of stress. Ductile materials respond to high stress through plastic deformation (and eventually failure).

Another thing to bear in mind is that during machining operations, cutting forces are applied gradually, are typically in the range of hundreds of newtons, and are usually not concentrated at a particular spot. Whereas if a tool unintentionally moves into material rapidly without being able to cut, the forces can exceed tens of thousands of newtons, and tools will easily shatter. Also, when dropped, impact stresses can exceed the strain limit of the material, if for example a heavy tool with a thin cutting edge lands directly on the thin edge. In that case the edge might chip.

I have dropped WC tools many times and they have never broken. In fact, just now, to test this, I dropped a large heavy 16 mm end mill (I have access to a lot of machine tools) from head level on to a steel plate several times and there was no fracture. Tools with certain shapes may chip however.

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  • $\begingroup$ I'm a little confused as to how moving into a material rapidly has a force that are any different than if you take it slow. If a point 1mm from the outside of a cylinder has some amount of torque if you come up slowly on it or come up quickly on it it's going to exert the same amount of force on the carbide insert no? So if its enough to shatter it in one case why isn't it enough to shatter in the other? $\endgroup$ – csteifel Oct 4 '18 at 4:26
  • $\begingroup$ I mean in the case where it's not cutting, e.g. if the spindle is turned off. $\endgroup$ – Al Nejati Oct 4 '18 at 5:06
  • $\begingroup$ Additionally though, cutting faster requires more force (and more spindle power) than cutting more slowly. The reason is because when you cut faster, you are creating thicker and/or more chips, which require more force to remove. $\endgroup$ – Al Nejati Oct 4 '18 at 6:00

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