Machinery's Handbook, 31st Edition
LAPS AND LAPPING 1339 The tendency of lead laps to lose their form is an objectionable feature. They are, how ever, easily molded, inexpensive, and quickly charged with the cutting abrasive. A more elaborate form for holes is composed of a steel arbor and a split cast-iron or copper shell which is sometimes prevented from turning by a small dowel pin. The lap is split so that it can be expanded to accurately fit the hole being operated upon. For hardened work, some toolmakers prefer copper to either cast iron or lead. For holes varying from 1 ∕ 4 to 1 ∕ 2 inch (6.35 − 12.7 mm) in diameter, copper or brass is sometimes used; cast iron is used for holes larger than 1 ∕ 2 inch (12.7 mm) in diameter. The arbors for these laps should have a taper of about 1 ∕ 4 or 3 ∕ 8 inch per foot (20.83 or 31.25 mm/m). The length of the lap should be some what greater than the length of the hole, and the thickness of the shell or lap proper should be from 1 ∕ 8 to 1 ∕ 6 its diameter. External laps are commonly made in the form of a ring, with an outer ring or holder and an inner shell which forms the lap proper. This inner shell is made of cast iron, copper, brass, or lead. Ordinarily the lap is split and screws are provided in the holder for adjust ment. The length of an external lap should at least equal the diameter of the work, and may be longer. Large ring laps usually have a handle for moving them across the work. Laps for Flat Surfaces.— Laps for producing plane surfaces are made of cast iron. In order to secure accurate results, the lapping surface must be a true plane. A flat lap that is used for roughing or “blocking down” will cut better if the surface is scored by narrow grooves. These are usually located about 1 ∕ 2 inch (12.7 mm) apart and extend both lengthwise and crosswise, thus forming a series of squares similar to those on a checker-board. An abrasive of No. 100 or 120 emery and lard oil can be used for charging the roughing lap. For finer work, a lap having an unscored surface is used, and the lap is charged with a finer abrasive. After a lap is charged, all loose abrasive should be washed off with gasoline, for fine work, and when lapping, the surface should be kept moist, preferably with kerosene. Gasoline will cause the lap to cut a little faster, but it evaporates so rapidly that the lap soon becomes dry and the surface caked and glossy in spots. Loose emery should not be applied while lapping, for if the lap is well charged with abrasive in the beginning, is kept well moistened and not crowded too hard, it will cut for a considerable time. The pressure upon the work should be just enough to insure constant contact. The lap can be made to cut only so fast, and if excessive pressure is applied it will become “stripped” in places. The causes of scratches are: Loose abrasive on the lap; too much pressure on the work, and poorly graded abrasive. To produce a perfectly smooth surface free from scratches, the lap should be charged with a very fine abrasive. Grading Abrasives for Lapping.— For high-grade lapping, abrasives can be evenly graded as follows: A quantity of flour-emery or other abrasive is placed in a heavy cloth bag, which is gently tapped, causing very fine particles to be sifted through. When a suffi cient quantity has been obtained in this way, it is placed in a dish of lard or sperm oil. The largest particles will then sink to the bottom and in about one hour the oil should be poured into another dish, care being taken not to disturb the sediment at the bottom. The oil is then allowed to stand for several hours, after which it is poured again, and so on, until the desired grade is obtained. Charging Laps.— To charge a flat cast-iron lap, spread a very thin coating of the prepared abrasive over the surface and press the small cutting particles into the lap with a hard steel block. There should be as little rubbing as possible. When the entire surface is apparently charged, clean and examine for bright spots; if any are visible, continue charging until the entire surface has a uniform gray appearance. When the lap is once charged, it should be used without applying more abrasive until it ceases to cut. If a lap is over-charged and an excessive amount of abrasive is used, there is a rolling action between the work and lap which results in inaccuracy. The surface of a flat lap is usually finished true, prior to charg - ing, by scraping and testing with a standard surface-plate, or by the well-known method of scraping-in three plates together, in order to secure a plane surface. In any case, the bearing marks or spots should be uniform and close together. These spots can be blended by covering the plates evenly with a fine abrasive and rubbing them together. While the
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