  Tungsten carbide inserts. 
The Boeing aircraft company operates a wonderful surplus store in Kent, Washington (near Seattle).  These are solid tungsten carbide cutting inserts: They each fit a particular milling bit, where they are clamped or screwed into place.  When they wear out, they can be replaced, allowing the body of the bit to be used indefinitely. 
Source: Boeing Surplus Store 
Contributor: Theodore Gray 
Acquired: 30 September, 2007 
Text Updated: 30 September, 2007 
Price: $2 
Size: 0.75" 
Purity: <50%
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