Up to this time, surface roughness as a factor of measuring error has been left out of consideration. But in accurate measurement, careful consideration must be given to the surface roughness of the specimen.
In order to make clear the relation between surface roughness and the accuracy of measurement, an apparatus has been prepared by which real measuring force of micrometers can be obtained when the diameter of cylindrical specimens are measured.
The experiments are carried out by measuring the diameters of ground specimens made of four kinds of materials under different measuring forces. And the results are summarized as follows :
The diameter decreases as the measuring force increases and the decreasing ratio of the diameter to measuring force relates to both surface roughness and Brinel hardness of the test piece, and further measured values present hysteresis as the measuring force increases and decreases.
These facts mean that the peaks of the surface is yielded by the feeler.
A method is suggested, by which a reasonable diameter of a test piece can be induced. For this purpose it is convenient to introduce "profile factor" which is a ratio of the maximum height of surface roughness compressed by length measurement to the maximum height of its virgin surface.
When the profile factor is obtained, reasonable length of the object can be known when measuring force and material of the test piece are given.
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