After a lead lap has been expanded two or three times in this manner, its surface will be uneven, and a new lap must then be made. The work or the lap must rotate at a fairly high speed during the lapping process. While the lapping can be done in a grinding machine, it is usually more convenient to use a hand lathe.

FIG. 1

FIG. 2

A

FIG. 3

External Lap Holders.-Forms of holders for external laps are shown in Figs. 4 and 5. Fig. 4 is a solid ring with one expanding screw a, and two closing screws b that affect the diameter of the bushing, or lap, of lead, brass, or cast iron. Fig. 5 is opened by loosening the screw a and tightening the screw b and is closed by loosening the screw b and tightening the screw a. The holder may be split and a screw provided for closing in the

When

lap when it has become so worn that it will not cut. much external lapping is to be done, the ring may be furnished with a handle about 15 in. long for the sake of convenience in using it.

Lapping Plane Surfaces.-Cast iron is thought to be the most satisfactory for plane surfaces. The face of the lap is planed as true as possible and covered with oil and abrasive, after which the work is rubbed over it, changing the work around frequently and rubbing it in all directions. Great care is required to prevent crowning the work—that is, lapping the edges away faster than the center. The lap must be planed off frequently, as it wears out of true quite rapidly.

Charging the Lap.-To charge cylindrical laps for internal lapping, spread a thin coating of the prepared abrasive over the surface of a hard steel block, and then roll the lap over the steel block by means of its arbor, pressing down firmly at the same time, until the abrasive is embedded in the surface of the lap.

To charge cylindrical laps for external work, spread a thin coating of the abrasive on the inner surface of the lap, and press it into the lap by a hard steel roller somewhat smaller than the lap.

To charge a flat cast-iron lap, the abrasive is spread thinly over the surface, and pressed into the lap by means of a hard steel block. Do not rub any more than is absolutely necessary in charging the lap. The surface, when finished, must have a uniform gray appearance, and all high spots removed.

JIGS AND FIXTURES

Jigs are devices which are used to guide cutting tools and support the work in such a manner that the work produced by their use becomes alike in all essential features, independently of the skill of the operator. Fixtures are those devices which support the work but do not guide the cutting tools.

JIGS

All jigs consist of guides for the cutting tools; the body, which supports the guides and the work; the stops, or gauges, which

locate the work correctly in reference to the guides and to one or more points or surfaces of the work; the clamping arrangement, which serves to hold the work to the body; and the supporting surface or surfaces, which rest on the table of the machine and insure parallelism of the axes of the guides with the axis of the spindle that carries the cutting tool.

Forms of Jigs.Two general types of jigs are in common use. The one type, called the

clamp jig, is intended for work where the center lines of all holes that are cut by the aid of the jig are parallel. The holes

need not necessarily be located in the same plane, nor must they be drilled from the same side of the jig. Jigs of this type frequently resemble some form of a clamp, although in some cases there is little resemblance between the jig and a clamp. The other type of jig is intended for work that

requires the holes that are to be cut through it, or into it, to be at various angles to one another. Since jigs intended for holes at angles to one another most frequently resemble some form of a box, the name of box jig is commonly applied to them.

The

Clamp Jig. A simple form of clamp jig is shown in Fig. 1. The work is held by the clamps a and the thumb nuts b. drilling is done through the bushings c.

Box Jig.-In Fig. 2 is shown a box jig designed for drilling holes in three different directions. The shape of the work is shown in (a). The jig (b) is made in two parts, the part a with legs b, carrying all the bushing and stops, and a cover that fits over the end of the jig and has a setscrew similar to that shown at C. By turning the jig on its different faces, holes can be drilled in the desired directions.

Combination Jig.-By means of a combination jig, as shown in Fig. 3, the work h may be drilled, reamed, counterbored, and tapped at one setting. The spotting drill a, which has the

same diameter as the twist drill g, reamer b, and counterbore c is guided in the bushing by the collars d, e, and f, respectively, which are parts of the tools.

FIG. 3

Collars used to guide reamers or other tools, generally have several lubricating grooves, preferably spiral, cut lengthwise. The grooves serve to collect the fine dust and small chips that otherwise would crowd in between the bushing and collar and cause the bushing to cut the collar.

Guide Bushings.-The guides for the cutting tools, which are usually drills, reamers, or taps, are generally hardened steel bushings set into the jig body. The hole in the bushing is made to fit the shank of the tool closely; the outside of the bushing is exactly concentric with the inside.

FIG. 4

The bushings are made in various forms. Common forms of plain bushings, intended to be driven into holes in the jig body, are shown in Figs. 4 and 5. In Fig. 4 the bushing is straight inside and outside, except that the end is rounded out to allow the drill to enter easily. This plain bushing is the cheapest bushing to make, and, if well fitted to the hole, is thoroughly satisfactory.