MadSci Network: Engineering

Re: Is their a limit on how sharp a blade can be?

Date: Fri Jan 4 14:15:27 2008
Posted By: David and John Free, Post-doc/Fellow, MFA, MFA
Area of science: Engineering
ID: 1177962553.Eg

Hi Jim

The concept of "sharpness" is nether well defined nor understood.
How a blade cuts, and elementary questions like "Why and when do 
serrations help" remain unanswered.

Sharpness is NOT hardness. Thus a harder blade (or even a blunt stone) 
can damage a fine blade.

A blade sharpened to taper to "a single line of molecules" could not be 
any sharper. But GOOD blades work by being resistant also to SIDEWAYS 
forces - which our "line of molecules" certainly would not be!
Also the best blades are made of carefully tempered, heat-treated alloys -
 in other words combinationns of many elementary atoms, (Fe, Cr, Vd, C, 
S, P, etc) so simply could not exist as a "line-of-molecules" because 
that would change the molecular structure.

It seems that a cut is made in a softer material by the pressure of the 
blade. This deforms the material being cut, bending and stretching it 
until it fractures. This allows the blade to enter the crack and force 
the crack to propagate by the very high stress concentration around its 

To cut concrete you are better off with a saw. The teeth of the saw need 
cooling (water is fine). For the most difficult materials a cutting dust 
would be added (as a slurry) to the water. Diamond being the hardest 
material known is often used for this. So the say acts as a "conveyor 
belt" for the cutting dust!


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