Machinery's Handbook, 31st Edition
1342 LAPS AND LAPPING kerosene. Soda water when used with alundum on the copper lap, gave the highest results of any of the lubricants used with that particular combination. Lapping Pressures.— Within the limits of the pressures used, that is, up to 25 pounds per square inch (172 kPa), the rate of cutting was found to be practically proportional to the pressure. The higher pressures of 20 and 25 pounds per square inch (138 and 172 kPa) are not so effective on the copper lap as on the other materials. Wet and Dry Lapping.— With the “wet method” of using a surface lap, there is a surplus of oil and abrasive on the surface of the lap. As the specimen being lapped is moved over it, there is more or less movement or shifting of the abrasive particles. With the “dry method,” the lap is first charged by rubbing or rolling the abrasive into its surface. All surplus oil and abrasive are then washed off, leaving a clean surface, but one that has embedded uniformly over it small particles of the abrasive. It is then like the surface of a very fine oilstone and will cut away hardened steel that is rubbed over it. While this has been termed the dry method, in practice, the lap surface is kept moistened with kerosene or gasoline. Experiments on dry lapping were carried out on the cast-iron, steel, and copper laps used in the previous tests, and on one of tin made expressly for the purpose. Carborundum alone was used as the abrasive and a uniform pressure of 15 pounds per square inch (103 kPa) was applied to the specimen throughout the tests. In dry lapping, much depends upon the manner of charging the lap. The rate of cutting decreased more rapidly after the first 100 revolutions than with the wet method. Considering the amounts ground off during the first 100 revolutions, and the best result obtained with each lap taken for comparison, it was found that with a tin lap, charged by rolling No. 150 carborundum into the surface, the rate of cutting, when dry, approached that obtained with the wet method. With the other lap materials, the rate with the dry method was about one-half that of the wet method. Summary of Lapping Tests.— The initial rate of cutting does not greatly differ for differ ent abrasives. There is no advantage in using an abrasive coarser than No. 150. The rate of cutting is practically proportional to the pressure. The wear of the laps is in the following proportions: cast iron, 1.00; steel, 1.27; copper, 2.62. In general, copper and steel cut faster than cast iron, but, where permanence of form is a consideration, cast iron is the superior metal. Gasoline and kerosene are the best lubricants to use with a cast-iron lap. Machine and lard oil are the best lubricants to use with copper or steel laps. They are, however, least effective on a cast-iron lap. In general, wet lapping is from 1.2 to 6 times as fast as dry lap ping, depending upon the material of the lap and the manner of charging. NONTRADITIONAL MACHINING AND CUTTING Introduction.— Manufacturers are constantly pursuing new methods to machine com- plex features faster, with more accuracy, and without imposing significant cutting forces on sensitive or thin parts. In addition, use of advanced composites and harder and brittle materials is increasing. These challenges create a growing need for nontraditional ma- chining processes, which can be categorized by material removal processes: chemical, electrochemical, mechanical erosion, and electro-thermal. Such modern methods tend to be automated, with sophisticated systems and software often used to control process variables. Shown in Table 1 are characteristics of some of the most commonly used nontraditional machining methods discussed in this section. Equipment improvements are ongoing and capabilities vary, so values given are representative rather than absolute. Table 2 contains some examples of cutting speeds for metals from industry literature. Actual speeds will vary, depending on equipment, setup, materials processed, and other factors.
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