Machinery's Handbook, 31st Edition
1340 LAPS AND LAPPING plates are being ground in, they should be carefully tested and any high spots which may form should be reduced by rubbing them down with a smaller block. To charge cylindrical laps for internal work, spread a thin coating of prepared abrasive over the surface of a hard steel block, preferably by rubbing lightly with a cast-iron or copper block; then insert an arbor through the lap and roll the latter over the steel block, pressing it down firmly to embed the abrasive into the surface of the lap. For external cylindrical laps, the inner surface can be charged by rolling-in the abrasive with a hard steel roller that is smaller in diameter than the lap. The taper cast-iron blocks sometimes used for lapping taper holes can also be charged by rolling-in the abrasive, as described; there is usually one roughing and one finishing lap, and when charging the former, it may be necessary to vary the charge according to any error that might exist in the taper. Rotary Diamond Lap.— This style of lap is used for accurately finishing very small holes, which, because of their size, cannot be ground. While the operation is referred to as lapping, it is, in reality, a grinding process, the lap being used the same as a grinding wheel. Laps employed for this work are made of mild steel, soft material being desirable because it can be charged readily. Charging is usually done by rolling the lap between two hardened steel plates. The diamond dust and a little oil is placed on the lower plate, and as the lap revolves, the diamond is forced into its surface. After charging, the lap should be washed in benzine. The rolling plates should also be cleaned before charging with dust of a finer grade. It is very important not to force the lap when in use, especially if it is a small size. The lap should just make contact with the high spots and gradually grind them off. If a diamond lap is lubricated with kerosene, it will cut freer and faster. These small laps are run at very high speeds, the rate depending upon the lap diameter. Soft work should never be ground with diamond dust because the dust will leave the lap and charge the work. When using a diamond lap, it should be remembered that such a lap will not produce sparks like a regular grinding wheel; hence, it is easy to crowd the lap and “strip” some of the diamond dust. To prevent this, a sound intensifier or “harker” should be used. This is placed against some stationary part of the grinder spindle, and indicates when the lap touches the work, the sound produced by the slightest contact being intensified. Grading Diamond Dust.— The grades of diamond dust used for charging laps are desig nated by numbers, the fineness of the dust increasing as the numbers increase. The dia mond, after being crushed to powder in a mortar, is thoroughly mixed with high-grade olive oil. This mixture is allowed to stand 5 minutes and then the oil is poured into another receptacle. The coarse sediment is left is removed and labeled No. 0, according to one system. The oil poured from No. 0 is again stirred and allowed to stand 10 minutes, after which it is poured into another receptacle and the sediment remaining is labeled No. 1. This operation is repeated until practically all of the dust has been recovered from the oil, the time the oil is allowed to stand being increased as shown by the following table. This is done to obtain the smaller particles that require a longer time for precipitation: To obtain No. 1 — 10 minutes To obtain No. 4 — 2 hours To obtain No. 2 — 30 minutes To obtain No. 5 — 10 hours To obtain No. 3 — 1 hour To obtain No. 6 — until oil is clear The No. 0 or coarse diamond which is obtained from the first settling is usually washed in benzine, and re-crushed unless very coarse dust is required. This No. 0 grade is sometimes known as “ungraded” dust. In some places the time for settling, in order to obtain the various numbers, is greater than that given in the table. Cutting Properties of Laps and Abrasives.— In order to determine the cutting proper ties of abrasives when used with different lapping materials and lubricants, a series of tests was conducted, the results of which were given in a paper by W. A. Knight and A. A. Case, presented before the American Society of Mechanical Engineers. In connection with these tests, a special machine was used, the construction being such that quantitative results could be obtained with various combinations of abrasive, lubricant, and lap mate- rial. These tests were confined to surface lapping.
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