Modern Machine-Shop Practice Part 29

To insure that a tap shall tap a hole straight, the machinist, in the case of hand tapping, applies a square to the work and the tap, as shown in Fig. 361, in which W represents a piece of work, T a tap, and S S two squares. If the tap is a taper one the square is sighted with the shank of the tap, as shown in position 1, but if the thread of the tap is parallel, the square may be applied to the thread of the tap, as in position 2. If the tap leans over to one side, as in Fig. 362, it is brought upright by exerting a pressure on the tap wrench handle B (on the high side) in the direction of the arrow A, while the wrench is rotated; but if the tap leans much to one side it is necessary to rotate the tap back and forth, exerting the pressure on the forward stroke only.

[Ill.u.s.tration: Fig. 366.]

[Ill.u.s.tration: Fig. 367.]

It is necessary to correct the errors before the tap has entered the hole deeply, because the deeper the tap has entered the greater the difficulty in making the correction. If the pressure on the tap wrench be made excessive, it is very liable to cause the tap to break, especially in the case of small taps, that is to say, those of 5/8 inch or less in diameter. The square should be applied as soon as the tap has entered the hole sufficiently to operate steadily, and should be applied several times during the tapping operation.

When the tap does not pa.s.s through the hole it may be employed with a guide which will keep it true, as shown in Fig. 363, in which W is a piece of work, T the tap, and S a guide, the latter being bolted or clamped to the work at B. In this case the shank of the tap is made fully as large in diameter as the thread. In cases where a number of equidistant holes require tapping, as in the case of cylinder ends, this device saves a great deal of time and insures that the tapping be performed true, the hole to receive the bolt B and that to receive the tap being distant apart to the same amount as are the holes in the work.

In shops where small work is made to standard gauge, and on the interchangeable system, devices are employed, by means of which a piece that has been threaded will screw firmly home to its place, and come to some definite position, as in the following examples. In Fig. 364 let it be required that the stud A shall screw in the slide S; the arm A to stand vertical when collar B is firmly home, and a device such as in Fig. 365 may be employed. P is a plate on which is fixed a chuck C to receive the slide S. In plate P is a groove G to hold the head H at a right angle to the slideway in C, there being a projection beneath H and beneath C to fit into G. The tap T is threaded through H, but not fluted at the part that winds through H when the tapping is being done, so as not to cause the thread in H to wear. H acts as a guide to the tap and causes it to start the thread at the same point in the bore of each piece S, and the stem will be so threaded that the screw starts at the same point in the circ.u.mference of each piece.

[Ill.u.s.tration: Fig. 368.]

A second example of uniform tapping is shown in Figs. 366, 367, and 368.

The piece, Fig. 366, is to have its bore A tapped in line with the slot C, and the thread is to start at a certain point in its bore. In Fig.

367 this piece is shown chucked on a plate D. F is a chuck having a lug E fitting into the slot (C, Fig. 366) of the work. This adjusts the work in one direction. The face D of the plate adjusts the vertical height of the work, and the alignment of the hole to the axis of the tap is secured in the construction of the chuck, as is shown in Fig. 369. A lug K is at a right angle to the face B of the chuck and stands in a line with lug E, as denoted by the dotted line _g_ _g_, and as lug K fits into the slot G, Fig. 367, the work will adjust itself true when bolted to the plate.

[Ill.u.s.tration: Fig. 369.]

Fig. 368 shows a method of tapping or hobbing four chasers (as for a bolt cutter), so that if the chasers are marked 1, 2, 3 and 4, as shown, any chaser of No. 1 will work with the others, although not tapped at the same operation. C is a chuck with four dies (A, B, C, D) placed between the chasers. By tightening the set-screws S, the dies and chasers are locked ready for the tapping. N is a hub to receive a guide-pin P, which is pa.s.sed through to hold the chasers true while being set in the chuck, and it is withdrawn before the tapping commences; _d_ _e_ _f_ are simply to take hold of when inserting and removing the dies. It is obvious that a chuck such as this used upon a plate, as in Fig. 365, with the hob guided in the head H there shown, would tap each successive set of chasers alike as a set, and individually alike, provided, of course, that the hob guide or head H is at each setting placed the same distance from the face of the chuck, a condition that applies to all this cla.s.s of work. In the case of work like chasers, where the tap or hob does not have much bearing to guide it in the work, a three-flute hob should be used for four chasers, or a four-flute hob for three chasers, which is necessary so that the hob may work steadily and tap all to the same diameter.

CHAPTER V.--FASTENING DEVICES.

Bolts are usually designated for size by their diameters measured at the cylindrical stem or body, and by their lengths measured from the inner side of the head to the end of the thread, so that if a nut be used, the length of the bolt, less the thickness of the nut and washer (if the latter be used), is the thickness of work the bolt will hold. If the work is tapped, and no nut is used, the full length of the bolt stem is taken as the length of the bolt.

A _black_ bolt is one left as forged. A finished bolt has its body, and usually its head also, machine finished, but a finished bolt sometimes has a black head, the body only being turned.

A square-headed bolt usually has a square nut, but if the nut is in a situation difficult of access for the wrench, or where the head of the bolt is entirely out of sight (as secluded beneath a f.l.a.n.g.e) the nut is often made hexagon. A machine-finished bolt usually has a machine-finished and hexagon nut. Square nuts are usually left black.

[Ill.u.s.tration: Fig. 370.]

The heads of bolts are designated by their shapes, irrespective of whether they are left black or finished. Fig. 370 represents the various forms: _a_, square head; _b_, hexagon head; _c_, capstan head; _d_, cheese head; _e_, snap head; _f_, oval head, or b.u.t.ton head; _g_, conical head; _h_, pan head; _i_, countersink head.

The square heads _a_ are usually left black, though in exceptional cases they are finished. Hexagon heads are left black or finished as circ.u.mstances may require; when a bolt head is to receive a wrench and is to be finished, it is usually made hexagon. Heads _c_ and _d_ are almost invariably finished when used on operative parts of machines, as are also _e_ and F. Heads _g_ are usually left black, while _h_ and _i_ are finished if used on machine work, and left black when used as rivets or on rough unfinished work.

The heads from _e_ to _i_ a.s.sume various degrees of curve or angle to suit the requirements, but when the other end of the bolt is threaded to receive a nut, some means is necessary to prevent them from rotating in their holes when the nut is screwed up, thus preventing the nut from s.c.r.e.w.i.n.g up sufficiently tight. This is accomplished in woodwork by forging either a square under the head, as in Fig. 371, or by forging under the head a t.i.t or stop, such as shown in Figs. 372 and 373 at P.

Since, however, forging such stops on the bolt would prevent the heads from being turned up in the lathe, they are for lathe-turned bolts put in after the bolts have been finished in the lathe, a hole being subsequently drilled beneath the head to receive the pin or stop, P, Fig. 372, which may be tightly driven in. A small slot is cut in the edge of the hole to receive the stop.

[Ill.u.s.tration: Fig. 371.]

[Ill.u.s.tration: Fig. 372.]

[Ill.u.s.tration: Fig. 373.]

Bolts are designated for kinds, as in Fig. 374, in which _k_ is a machine bolt; _l_ a collar bolt, from having a collar on it; _m_ a cotter bolt, from having a cotter or key pa.s.sing through it to serve in place of a nut; _n_ a carriage bolt, from having a square part under the head to sink in the wood and prevent the bolt from turning with the nut; and _o_ a countersink bolt for cases where the head of the bolt comes flush.

[Ill.u.s.tration: Fig. 374.]

The simple designation "machine bolt" is understood to mean a black or unfinished bolt having a square head and nut, and threaded, when the length of the bolt will admit it, and still leave an unthreaded part under the bolt head, for a length equal to about four times the diameter of the bolt head. If the bolt is to have other than a square head it is still called a machine bolt, but the shape of the head or nut is specially designated as "hexagon head machine bolt," this naturally implying that a hexagon nut also is required.

In addition to these general names for bolts, there are others applied to special cases. Thus Fig. 375 represents a patch bolt or a bolt for fastening patches (as plate C to plate D), its peculiarity being that it has a square stem A for the wrench to screw it in by. When the piece the patch bolt screws into is thin, as in the case of patches on steam boilers, the pitch of the thread may, to avoid leakage, be finer than the usual standard.

In countersink head bolts, such as the patch bolt in Fig. 375, the head is very liable to come off unless the countersink in the work (as in C) is quite fair with the tapped hole (as in D) because the thread of the bolt is made a tight fit to the hole, and all the bending that may take place is in the neck beneath the head, where fracture usually occurs.

These bolts are provided with a square head A to screw them in by, and are turned in as at B to a diameter less than that at the bottom of the thread, so that if screwed up until they twist off, they will break in the neck at B.

[Ill.u.s.tration: Fig. 375.]

[Ill.u.s.tration: Fig. 376.]

Instead of the hole being countersunk, however, it may be cupped or counterbored, as in Fig. 376, in which the names of the various forms of the enlargement of holes are given. The difference between a faced and a counterbored hole is that in a counterbored hole the head or collar of the pin pa.s.ses within the counterbore, the use of the counterbore being in this case to cause the pin to stand firmly and straight. The difference between a dished and a cupped is merely that cupped is deeper than dished, and that between grooved and recessed is that a recess is a wide groove.

Eye bolts are those having an eye in place of a head, as in Fig. 377, being secured by a pin pa.s.sing through the eye, or by a second bolt, as in the figure. When the bolt requires to pivot, that part that is within the eye may be made of larger diameter than the thread, so as to form a shoulder against which the bolt may be screwed firmly home to secure it without gripping the eye bolt.

[Ill.u.s.tration: Fig. 377.]

[Ill.u.s.tration: Fig. 378.]

Fig. 378 represents a foundation bolt for holding frames to the stone block of a foundation. The bolt head is coned and jagged with chisel cuts. It is let into a conical hole (widest at the bottom) in the stone block, and melted lead is poured around it to fill the hole and secure the bolt head.

[Ill.u.s.tration: Fig. 379.]

[Ill.u.s.tration: Fig. 380.]

Another method of securing a foundation bolt head within a stone block is shown in Fig. 379; a similar coned hole is cut in the block, and besides the bolt head B a block W is inserted, the faces of the block and bolt being taper to fit to a taper key K, so that driving K locks both the bolt and the block in the stone. When the bolt can pa.s.s entirely through the foundation (as when the latter is brickwork) it is formed as in Fig. 380, in which B is a bolt threaded to receive a nut at the top. At the bottom it has a keyway for a key K, which abuts against the plate P. To prevent the key from slackening and coming out, it has a recess as shown in the figure at the sectional view of the bolt on the right of the ill.u.s.tration, the recess fitting down into the end of the keyway as shown.

[Ill.u.s.tration: Fig. 381.]

Another method is to give the bolt head the form at B in Fig. 381, and to cast a plate with a rectangular slot through, and with two lugs A C.

The plate is bricked in and a hole large enough to pa.s.s the bolt head through is left in the brickwork. The bolt head is pa.s.sed down through the brickwork in the position shown at the top, and when it has pa.s.sed through the slot in the plate it is given a quarter turn, and then occupies the position shown in the lower view, the lugs A C preventing it from turning when the nut is screwed home. The objection to this is that the hole through the brickwork must be large enough to admit the bolt head. Obviously the bolt may have a solid square head, and a square shoulder fitting into a square hole in the plate, the whole being bricked in.

[Ill.u.s.tration: Fig. 382.]

[Ill.u.s.tration: Fig. 383.]

Figs. 382 and 383 represent two forms of hook bolt for use in cases where it is not desired to have bolt holes through both pieces of the work. In Fig. 382 the head projects under the work and for some distance beneath and beyond the washer, as is denoted by the dotted line, hence it would suspend piece A from B or piece B from A. But in Fig. 383 the nut pressure is not beneath the part where the hook D grips the work, hence the nut would exert a pressure to pull piece B in the direction of the arrow; hence if B were a fixed piece the bolt would suspend A from it, but it could not suspend B from A.

In woodwork the pressure of the nut is apt to compress the wood, causing the bolt head and nut to sink into the wood, and to obviate this, anchor plates are used to increase the area receiving the pressure; thus in Fig. 384 a plate is tapped to serve instead of a nut, and a similar plate may of course be placed under the bolt head.

[Ill.u.s.tration: Fig. 384.]

The Franklin Inst.i.tute or United States Standard for the dimensions of bolt heads and nuts is as follows. In Fig. 385, D represents the diameter of the bolt, J represents the short diameter or width across flats of the bolt head or of the nut, being equal to one and a half times the diameter of the bolt, plus 1/16 inch for finished heads or nuts, and plus 1/8 inch for rough or unfinished heads or nuts. K represents the depth or thickness of the head or nut, which in finished heads or nuts equals the diameter of the bolt minus 1/16 inch, and in rough heads equals one half the distance between the parallel sides of the head, or in other words one half the width across the flats of the head.

H represents the thickness or depth of the nut, which for finished nuts is made equal to the diameter of the bolt less 1/16 inch, and therefore the same thickness as the finished bolt head, while for rough or unfinished nuts it is made equal to the diameter of the bolt or the same as the rough bolt head. I represents the long diameter or diameter across corners, which, however, is a dimension not used to work to, and is inserted in the following tables merely for reference:--

[Ill.u.s.tration: Fig. 385.]