A test bar of known BHN, approximating the hardness of the specimen to be tested, is selected. Consistent accuracy is maintained when the test bar BHN is within 15% of the speci-men BHN and is of the same general material. (Testing non-ferrous materials with carbon steel bars causes impact errors that must be compensated for by applying correction factors to the test results.) The test bar is inserted into the Telebrineller instrument and the instrument placed upon the specimen.
When the Telebrineller instrument is complete with test bar (1), the anvil (2), encased in a soft molded rubber head (3), rests on the test bar. The rubber head and a similar resting block (4), provide non-skid footing. Below the test bar, a steel impression ball (5), secured in the base of the head, is in con-tact with both the test bar and the specimen. The anvil is struck sharply with a three to five pound hammer. The impact, regardless of force, is transmitted equally to the test bar and, through the impression ball to the specimen metal (6), making impressions in both. The diameters of the resulting impres-sions are directly related to the respective hardnesses of the test bar and the specimen.
A spacing bar (7), operated by a spring catch and but-ton, adjusts the test bar to a clear area for each test. By turning the bar to utilize the full length of all four faces, a total of 80 tests can be made with one bar.
MEASURING DIAMETERS
The bar is removed from the instrument and the Telebrineller Microscope positioned over the appropriate im-pression. The microscope provides a scale of 1/10 millimeter divisions and is designed so that the image and scale are bright and clear to the observer.