and the work. In some cases it may be expeditious to clamp an angle-plate to the platen at one side of the work square with the spindle, so as to assist in locating the first hole and proving the work as we proceed. If holes are to be put in all of the different sides and the jig is clamped for locating the holes in the second side, the tool-maker can establish without trouble the correct relation between the holes by taking distances from the angle-plate to plugs inserted in the holes first bored, as per Fig. 19. When the distance from the first hole to the side of the jig is determined, we add the distance the jig is from the angleplate, and thus determine how far the first hole is from the angle

plate. With the rest of the work there are a number of ways to follow, but the most practical is to use the height-gauge to measure all distances. Another, that is almost as good, is to insert an arbor in the miller-spindle and feed the table forward until a piece of tissue paper will just draw out between the arbor and the angle-plate. Then by means of the dial on the longitudinal feed-screw run the table forward the required distance. When the screw on the machine has been determined to be correct, one can depend on the dial almost wholly for the vertical spacing, while the platen can be set by calipering to the arbor in the spindle.

In doing jig work on the plain-miller a parallel can often be clamped to the side of the jig, from which measurements may be taken. After the work has been located in place on the table a miller-vise may be clamped to the platen and a diamond-point tool clamped in it, with which the test arbor in the spindle may

be turned true, as shown in Fig. 20, finishing it to size convenient to use in locating the work both horizontally and vertically. Then again, a turning-tool may be clamped to the back edge of the table with a parallel spanning the distance to the first slot in

the table, and in this way true a piece of stock which may be held in a chuck in the spindle. Any tool-maker who has done much jig work on the miller will appreciate the advantage and the help in having a test piece in the spindle running perfectly true, and that in order to accomplish accurate work it is neces sary to have all conditions equally accurate and reliable as the job progresses.

It is sometimes necessary to bore a bushing-hole in a jig at an angle with one of its sides. To do this correctly on the plain miller we can set the jig body at the given angle with the angleplate-which has been first set square with the spindle-by a bevel protracter.

HANDLING LARGE JIG BODIES.

When work is to be handled that is larger than the capacity of the milling-machine platen, it is only necessary to provide an auxiliary platen almost as long as the machine table and about twice its width, and bolt it to the machine. This emergency

table should be provided with a number of slots or holes for fastening the work to it. Accurately made parallels which just fit the slots in the table are of great convenience in setting such large work, while a block with a tongue to fit the slot and nearly as wide as the table and with its edge milled accurately in line with the spindle axis is also a help.

After the jig is located and ready for letting in the bushinghole (whether on the lathe face-plate or on the table of the universal or plain milling machine), finishing should not be done with drill or reamer, for there will not be one chance in a thousand that the hole will be accurately located. The hole must be bored to a finish in order to do a correct job.

JIG FEET.

The proper feet for jigs is largely a matter of individual taste. There are, I believe, quite as many kinds of jig feet as there are jig designers. Some even go so far as to prefer having no feet at all on their jigs, and thus obviate the possibility of trouble with the drill-press table slots.

Figs. 21 to 30 show a number of different kinds of jig feet. Figs. 21 and 22 are flat-base types; Figs. 23 to 25, cast feet on

the base of jigs. Any of these make good feet, the one shown. in Fig. 23 being, of course, easier to make and just as good as

the others except where a foot of considerable length is necesWith steel feet all sorts and sizes are used and give satis

sary. faction.

Figs. 26 to 30 are types.

In concluding this chapter it will not be amiss to emphasize the advisability of becoming practically familiar with the instal

FIG. 23.

FIG. 24.

lation and operation of the interchangeable system of manufacturing. To demonstrate the necessity of mastering the details of the system, it is only necessary to point out that in the manufac turing machine-shop of the present day the efficiency of the machines or parts turned out can usually be judged by the use that is made of properly designed and constructed drilling and milling fixtures and jigs for the production in repetition of the most

FIG. 25.

accurate operations of the work. Although it has been, and is still, possible to obtain satisfactory results without a large outfit of such tools, no shop can produce interchangeable parts or du plicate machines in large quantities and sell them at a price which will compete in the open market, unless it has an adequate equipment of special jigs and fixtures, and a man at the head of it who thoroughly understands their design, construction, and use.

FIGS. 26 30.

CHAPTER IV.

Types of Simple and Inexpensive Drilling-Jigs ; Their Construction and Use.

IN order to discuss the subject of drilling-jigs exhaustively, I think it is best to follow up the chapter devoted to the fundamental principles for such work by first taking up the comparatively simple class of such tools which are used for the machining and duplication of parts in which great accuracy is neither essential nor desirable. As before stated, the main point to be always considered by the constructor of tools of this class is the degree of variation allowable in the work that is to be machined.

TWO TYPES OF VERY SIMPLE DRILLING-JIGS.

A

C

The

Fig. 31 is a plain casting with two ribs cast on one side. casting is first planed on the sides A A, and a cut is also taken off the ribs. It is then ready to be drilled. As the holes to be drilled are clearance holes for bolts and studs, no great accuracy in the jig is required. The jig for this casting is shown in three views in Fig. 32, and, as will be seen, is about as simple and inexpensive to construct as could be devised for the work. It consists of one body casting, D, with six projections on one side for the locating-points and fastening-screws. It is first planed on the top and then strapped on an The

Ω

Ва

B

OB

A

B C

с

с

B

Α

FIG. 31.

B

angle-plate on the miller-table, and the inside is milled.

inside of the projections F and E E are finished square with each other, as they are the locating-points. Holes are then drilled for the set-screws J and I I in the lugs G G and H respectively. These screws are case-hardened. In locating the