1. Field of the Invention
This invention relates to an apparatus for determining size. More particularly, but not exclusively, the apparatus has been developed for automatically determining the area or dimensions of an indentation made by the indenter of a hardness tester such as a Vickers hardness tester, or the planar area, profile or silhouette area of an article.
Hardness testing is widely used in industry for the control of material quality and heat treatment. For information on a finer scale, for example on the depth of surface hardening or the distribution of subsurface tempering, micro hardness testing is used, in which the maximum dimension of the indentation may be 0.005 to 0.01 mm. Micro hardness studies are also important in investigations of failures due to cracks and fatique.
The principle of hardness and micro hardness testing is that an inverted pyramid, ball, or cone is pressed into the surface of a metal test piece and the size of the resulting impression is measured. This measurement is then converted to a numerical hardness value by comparing with tables.
In practice, the method is complex. All indenter movements must be carefully controlled for reproducability. The measurement procedure can be tedious and lead to eye strain, particularly when using a microscope with cursors for repetitive size measurements. These measurements frequently involve many separate measurements because many indentations have to be made in the test piece and the average size ascertained, because of the variability of the hardness of the material over the surface thereof.