If the leaf must be fastened solidly, and the amount of time consumed is not of great importance, some form of screw clamp may be used; but if the work must be handled rapidly, the clamping device is generally operated by some form of cam. However, a screw clamp may be designed to work quite rapidly, and such a one is illustrated in Fig. 291. This screw clamp consists of a screw with a hole drilled through it to receive a pin that is used as a lever to operate the screw. The screw is necked 1/16 inch deep, the necking being 1/8 inch wide; a fiat washer is attached to the leaf of the jig by a small screw, as shown. A slot the width of the screw is cut in this washer to allow it to slide back and forth, and in the end of the washer is a slot the width of the bottom of the necking in the screw. The other end of the washer is turned up, as shown, to furnish a means of pushing back and forth. When the jig leaf is closed, the washer is pushed forward and the ends engage in the slot in the screw. One turn of the screw binds it very tightly. When the screw is given one turn to loosen it, the washer may be pushed back and the jig leaf raised.
Fig. 291. Sorew Clamp for Jig.
Where it is not necessary to use much power, but extreme rapidity of action is desired, a hinged cam lever of the design shown in Fig. 292 may be used. The cam lever is pivoted to the base of the jig by means of a pin as shown. The lever passes into a slot in the leaf, and the bearing surfaces on the under part of the head come in contact with the inclined surfaces at the end of the leaf. Bushings. Bushings of hardened tool steel are made as a permanent guide for the cutting tools. The hole in the bushing is made to fit the cutting tool that is to be guided. There are various forms of bushings; the plain straight form, Fig. 293, is sometimes used, but is objectionable because it may be pushed into the jig if the cutting tool is too large to pass through the hole. To overcome this tendency, bushings are sometimes made tapering on the outside, as shown in Fig. 294; but as this is an expensive form, and as it is an extremely difficult operation to bore the bushing hole in the jig, it is not generally used for permanent bushings.
Fig. 292. Hinged Cam Lever for Jig.
The most common form of bushing is straight, with an enlarged portion or head. When no allowance is made for grinding on the outside, the bushing is usually made in the form shown in Fig. 295. If the shoulder under the head is square, it is likely to crack at the sharp corner, or the head may be broken off when being forced into position. In order to avoid these difficulties, a fillet is left under the head, as shown in Fig. 296.
Fig. 293. Bushing with Straight Outside.
Fig. 294. Bushing with Tapering Outside.
When it is essential that the location of the drilled hole or portion of the piece being machined in the jig be exact, the tool must fit well in the bushing; and as the size and shape of the bushing are likely to change in the hardening, it is advisable to leave enough stock to grind to size, both inside and out. It is essential that the outside of the bushing be exactly concentric with the inside. After the hole is ground and lapped to size, the bushing may be placed on a mandrel which runs true, and the outside ground to size. When machining a bushing which is to be ground on the outside, it is necessary to neck in, under the head, as shown in Fig. 297, in order that the emery wheel may pass entirely over the part being ground and insure a straight surface. The under side of the head which rests on the upper surface of the jig should be ground so that it may be true with the surface of the jig.
Fig. 295. Common Bushing.
Fig. 296. Bushing with Fillet under Head.
Fig. 297. Necked Bushing.
When grinding a bushing, a mandrel should be used which is straight or of very slight taper and has been tested for trueness. If the taper is considerable one end of the hole in the bushing will not fit, and the outside of the bushing will not be concentric with the hole. Consequently, no matter how careful the tool-maker may be in laying out his work and in boring the holes for the bushings, the jig will not be accurate.
The outside diameter of a bushing is often determined by the design of the jig; for instance, two holes are often located so near each other that it is impossible to make the bushings much larger than the holes through them. Whenever possible, the outside diameter should be made enough larger than the hole to leave a reasonably thick wall. A bushing with thin walls is likely to close in when being pressed to its seating; also, if a cutting tool binds in a bushing with thin walls, the bushing turns in the jig.
It is sometimes advisable to perform two or more operations in the same jig. After a hole has been drilled, it may be that it will be considered good practice to counter-bore or tap it, or, possibly, it may be better to do the three operations while the work is seated in the jig. In such cases the bushing having a hole the size of the drill must be removed, and one inserted that has a hole fitting the tool to be used.
A very simple way of making a removable bushing consists in boring the hole in the jig large enough to receive a hardened bushing with a hole the size of the outside of the bushing to be used. If the hole in the large stationary bushing and the outside surface of the removable bushing are lapped smoothly after grinding, they may be used for a long period before wearing enough to affect appreciably the location.
Fig. 298. Threaded.
Fig. 299. Removable Bushing, Threads Run Entire Length.
Tapered removable bushings are sometimes used, but on account of the expense of producing them, and the fact that chips and dirt readily throw them out of their true locations, they are not very common and their use is not advised.
Fig. 298 shows a form of removable bushing threaded on the outside to fit a threaded hole in the jig. If the thread on the outside of the bushing runs the entire length, Fig. 299, the process of screwing it in and out of the jig is necessarily very slow; consequently it is advisable to have but a few threads. The balance of the length may be made to fit a bearing in the jig. If it is advisable to thread the entire length, the hole should be ground true with the thread to prevent change of shape in hardening. As it is not well to attempt to grind between the lands of the thread with the facilities in the ordinary machine shop, it is necessary to grind the hole true with the thread. This can be done satisfactorily by placing a piece of stock in a chuck on the lathe having a grinding attachment. After drilling and boring the hole to tapping size, the thread should be chased so that the bushing is a good fit in the hole. It can then be screwed in, and the hole ground to size.