Analyses of File Hardness by the aid of Sensory Test

File hardness test is popularly used in workshops because of its simplicity and quickness in practical operation, and of its unnecessity of any hardness testing instruments. It is popularly believed that the differential limen at the test is about ±4 HRC or so, but no systematic experiment has been reported. By the aid of sensory tests file hardness test is analysed to obtain the relation between the sensory quantity and HRC hardness scale and the interval scale of file hardness. And the following results are obtained :(1) Significant correlations are observed between the file hardness and HRC hardness.(2) The main factor affecting the file hardness test is cutting force of the file to be felt by the hand of the observer.(3) Applying paired comparisons method, it is confirmed that no significant difference among individuals of the observers is recognized in the test, and the difference limen is about ±1 HRC or so.