This article first appeared in Practical Engineering 1940 Vol1 No3. The information herein is accurate as of 1940. The piece brings to the attention of the reader developments within Engineering at the time.
Ball and Diamond Penetrators
One of the best methods of determining hardness under workshop conditions is the direct reading test which is a standard covered by British and American specifications and which is in general international use.
Ball and diamond penetrators are used, and after the application of a small initial load to eliminate surface imperfections, a major load is applied and the hardness measured by the depth of penetration produced by the major load. The reading is automatically indicated on a dial micrometer. Production speeds up to 300 tests per hour can be achieved. The impression is slight and most components may be tested in their finished condition.
Penetrator and Loading Device
Two standard forms of penetrator are used, one being a steel ball of 1/16in. diameter 3 and the other a diamond cone of 120°, terminating in a spherical tip. Both major and minor loads are imposed by means of weights acting through a sensitive and accurate weighing lever fitted with hardened steel knife edges.
Scales involving different loads and penetrators are in use, but the most popular are those designated "B" and "C." In each case, a minor load of 10 kg is applied when the specimen is brought into contact with the penetrator. For the "B" scale, a total load of 100 kg is employed in conjunction with a ball penetrator of 1/16 in. diameter.
In the case of the "C" scale, the total load is 150 kg, and is used in conjunction with the diamond cone penetrator. These loads are applied by means of proportional weights which are suspended from the end of the weighing-lever and are under the control of the operator. Provision is made for reducing the major load to 60 kg, and special penetrators for making comparative tests can be supplied if desired. All major loads, i.e., 60, 600, and 150 kg are inclusive of the 10 kg minor load initially applied.
This is an extremely accurate dial instrument giving direct readings in terms of two standard penetration scales, "B" and "C." Scale "B" relates to the 1/16in. diameter ball penetrator and 100 kg load; and "scale "C" to the diamond cone and 150 kg. load.
Graduations range from zero up to 100 for one complete revolution of the main indicating hand, and a 0.2 mm. movement of the penetrator.