Before teeth can be successfully cut on a file blank it must have a thin film of oil spread over its surface. This applies whether teeth are to be cut by hand or machine. Without such an oil film the chisel will blunt and the teeth will not be sharp, however, with it, sharp teeth will be made and the chisel will stay sharp for a very long time. The exact mechanics of how the teeth are formed is not known but there are two distinct possibilities which both rely on the oil film. One is hydraulic forming where the chisel blow forces the oil into the metal's grain structure and bursts it apart to raise a sharp edge whilst the other is explosive forming in which the rapid impact of the chisel edge rapidly raises the temperature of the oil film locally and causes it to explode and burst the metal apart.
No experimental work on these possibilities is known of, but some experiments were carried out to try to measure the direct pressure required on the chisel to form teeth. The results of this failed to produce workable teeth and demonstrated that more complex mechanisms were at work than just a clean blow.
In practice, black lead is usually also added to the oil to darken the blank and give a good contrast with the cut teeth.
This work is Copyright to Ian W. Wright 2003. You may use it for your own private purposes but reproduction by any means or its use for commercial gain is strictly forbidden without the written permission of the author.