RIEHLE TESTER

Figure 1-28.—Riehle portable hardness tester. numbers in the outer circle are black, and the inner numbers are red. The Rockwell tester is equipped with a weight pan, and two weights are supplied with the machine. One weight is marked in red. The other weight is marked in black. With no weight in the weight pan, the machine applies a major load of 60 kilograms. If the scale setup calls for a 100-kilogram load, the red weight is placed in the pan. For a 150-kilogram load, the black weight is added to the red weight. The black weight is always used in conjunction with the red weight; it is never used alone. Practically all testing is done with either the B-scale setup or the C-scale setup. For these scales, the colors may be used as a guide in selecting the weight (or weights) and in reading the dial. For the B-scale test, use the red weight and read the red numbers. For a C-scale test, add the black weight to the red weight and read the black numbers. In setting up the Rockwell machine, use the diamond penetrator for testing materials that are known to be hard. If in doubt, try the diamond, since the steel ball may be deformed if used for testing hard materials. If the metal tests below C-22, then change to the steel ball. Use the steel ball for all soft materials-those testing less than B-100. Should an overlap occur at the top of the B scale and the bottom of the C scale, use the C-scale setup. Before the major load is applied, the test specimen must be securely locked in place to prevent slipping and to properly seat the anvil and penetrator. To do this, a load of 10 kilograms is applied before the lever is tripped. This preliminary load is called the "minor load." The minor load is 10 kilograms regardless of the scale setup. When the machine is set up properly, it auto- matically applies the 10-kilogram load. The metal to be tested in the Rockwell tester must be ground smooth on two opposite sides and be free of scratches and foreign matter. The surface should be perpendicular to the axis of penetration, and the two opposite ground surfaces should be parallel. If the specimen is tapered, the amount of error will depend on the taper. A curved surface will also cause a slight error in the hardness test. The amount of error depends on the curvature–the smaller the radius of curvature, the greater the error. To eliminate such error, a small flat should be ground on the curved surface if possible. RIEHLE TESTER The Riehle hardness tester is a portable unit that is designed for making Rockwell tests comparable to the bench-type machine. The instrument is quite universal in its application, being readily adjustable to a wide range of sizes and shapes that would be difficult, or impossible, to test on a bench-type tester. Figure 1-28 shows the tester and its proper use. It may be noted that the adjusting screws and the penetration indicator are set back some distance from 1-38