If A is not known but B and C are given, according to the formula below the table A BCF. Then if B

=

=

==

=

3.134", C = 1", and the angle is 30 degrees, as before, A B+CF 3.134′′ + (.75′′ X 1.1547) = 4′′, whence Z can be found, as already shown. If the corners of the dovetail are flat, as shown in Fig. 9 at I and G, and the dimensions I and H and the angles are known, it will be found from the formulas below the table that A also I+HF; 3.8557", H = ", and the angle 30 degrees, then 3.8557" + (.125" X 1.1547) = 4", from which Z is

=

so that if I =
A = I + H F
found as before.

=

=

SHOP AND DRAWING ROOM STANDARDS

STANDARD JIG PARTS

Drill Bushings

WHEN drilling and reaming operations are to be performed in the same jig, two slip bushings, one for the drill and the other for the reamer, should be used; if the jig is to be used for a large number of parts, the hole for the bushings should in turn be bushed with a steel lining to prevent wearing. The soft cast-iron will wear rapidly if this is not done, and the jig will soon have to be rebored and rebushed.

Three different styles of bushings with their dimensions are shown in Figs. 1, 2 and 3. These can be blanked out in quantities and finished to required sizes as needed, and should be made of tool steel. Allowances should be made in the blanks for grinding and

lapping after hardening. Fig. 1 shows a slip bushing; Fig. 2 a stationary bushing, and Fig. 3 a stationary bushing where tools with stop collars are to be used. Such bushings as shown in Figs. 2 and 3 are also used for linings for slip bushings.

Binding Screws

Binding-screws should be made in various sizes and with threads to conform to the standard taps with which the shop is provided. When drills of a very large size are used, a screw with a square or hexagon head is best, as the work requires firm clamping. If the drills used are small, a winged screw will be sufficient and more convenient, as it will require less time to manipulate. Some good screws for clamping straps are shown in Figs. 4 and 5. Of course the screws can be made of any length desired.

When the work is to be held against the seat or a stop by means of a set-screw, such screws as shown in Figs. 6 and 7 will be found very useful. If, however, the work is very light, a wing screw can be used.

Figs. 8 and 9 show screws that are useful in supporting work against the thrust of drills when the work is of such a nature that it cannot be supported otherwise.

Locking Screws

This

A convenient hinge-cover locking screw is shown in Fig. 10. screw, when used, should be adjusted so that only a quarter turn will be needed to clamp or release the cover, which should be slotted to admit the head of the screw.

The different sizes of the styles of screws shown are not only used with drilling jigs, but are equally useful with other jigs and fixtures. These screws should be made of screw stock and case-hardened.

The

A convenient strap to use with these jigs is shown in Fig. II. straps should be made of bessemer steel and case-hardened after finishing. The slot G can be located in the proper position and made of such dimensions as to allow the strap to be slipped back out of the way when work is being placed in and taken from the jig.