Modern Machine-Shop Practice Part 27

Another form of free cutting tap especially applicable to taps of large diameter has been designed by Professor Sweet. Its principles may be explained as follows:--

In the ordinary tap, with the taper four or five diameters in length, there are far more cutting-edges than are necessary to do the work; and if the taper is made shorter, the difficulty of too little room for chips presents itself. The evil results arising from the extra cutting edges are that, if all cut, then it is cutting the metal uselessly fine--consuming power for nothing; or if some of the cutting edges fail to cut, they burnish down the metal, not only wasting power, but making it all the harder for the following cutters. One plan to avoid this is to file away a portion of the cutting edges; but the method adopted in the Cornell University tap is still better. a.s.sume that it is desired to make three following cutters, to remove the stock down to the dotted line in Fig. 331. Instead of each cutter taking off a layer one-third the thickness and the full width, the first cutter is cut away on each side to about one-third its full width, so that it cuts out the centre to its full depth, as shown in Fig. 331, the next cutter cutting out the metal at A, and so on. This is accomplished by filing, or in any other way cutting away the sides of one row of the teeth all the way up; next cutting away the upper sides of the next row and the lower sides of the third, leaving the fourth row (if it be a four-fluted tap) as it is left by the lathe, to insure a uniform pitch and a smooth thread.

[Ill.u.s.tration: Fig. 333.]

Figs. 333, 334 and 335 represent an adjustable tap designed by C. R.

French, of Providence, R. I., to thread holes accurate in diameter.

The plug tap, Fig. 333, has at its end a taper screw, and the tap is split up as far as the flutes extend, a second screw binds the two sides of the tap together, hence by means of the two screws the size of the tap may be regulated at will. In the third or bottoming tap, Fig. 334, the split extends farther up the shank, and four adjusting screws are used as shown, hence the parallelism of the tap is maintained.

In the machine tap, Fig. 335, there are six adjusting screws, two of those acting to close the tap being at the extreme ends so as to strengthen it as much as possible.

[Ill.u.s.tration: Fig. 334.]

[Ill.u.s.tration: Fig. 335.]

In determining the number, the width, the depth, and the form of flutes for a tap, we have the following considerations. In a tap to be used in a machine and to pa.s.s entirely through the work, as in the case of tapping nuts, the flute need not be deep, because the taper part of the tap being long the cutting teeth extend farther along the tap; hence, each tooth takes a less amount of cut, producing less cuttings, and therefore less flute is required to hold them. In taps of this cla.s.s, the thread being given clearance, the length of the teeth may be a maximum, because they are relieved of friction; on the other hand, however, the shallower and narrower the flute the stronger the tap, so long as there is room for the cuttings so that they shall not become wedged in the flutes. Taps for general use by hand are frequently used to tap holes that do not pa.s.s entirely through the work; hence, the taper tap must have a short length of taper so that the second tap may be enabled to carry a full thread as near as possible to the bottom of the hole without carrying so heavy a cut as to render it liable to breakage, and the second or plug tap must in turn have so short a length of its end tapered that it will not throw too much duty upon the bottoming tap. Now, according as the length of the taper on the taper tap is reduced, the duty of the teeth is increased, and more room is necessary in the flute to receive the cuttings, and supposing the tap to be rotated continuously to its duty the flute must possess s.p.a.ce enough to contain all the cuttings produced by the teeth, but on account of the cuttings filling the flutes and preventing the oil fed to the tap from flowing down the flute to the teeth it is found necessary in hand taps (when they cannot pa.s.s through the work, or when the depth of the hole is equal to more than about the tap diameter), to withdraw the tap and remove the cuttings. On account of the tap not being accurately guided in hand-tapping it produces a hole that is largest at its mouth, and it is found undesirable on this account to give any clearance to hand taps, because such clearance gives more liberty to the tap to wobble in the hole and to enlarge its diameter at the mouth. It is obvious also, that the less of the tap circ.u.mference removed to form the flutes the longer the tap-teeth and the more steadily the tap may be operated. On the other hand, however, the longer the teeth the greater the amount of friction between them and the thread in the hole and the more work there is involved in the tapping, because the tap must occasionally be rotated back a little to ease its cut, which it is found to do.

[Ill.u.s.tration: Fig. 336.]

[Ill.u.s.tration: Fig. 337.]

Fig. 336 represents a form of flute recommended by Brown and Sharp. The teeth are short, thus avoiding friction, and the flutes are shallow, which leaves the tap strong. The inclination of the cutting edges, as A B (the cutting direction of rotation being denoted by the arrow), is shown by the dotted lines, being in a direction to curve the chip or cutting somewhat upward and not throw them down upon the bottom of the flute. A more common form, and one that perhaps represents average American practice, is shown in Fig. 337, the cutting edges forming a radial line as denoted by the dotted line. The flute is deeper, giving more room for the chips, which is an advantage when the tap is required to cut a thread continuously without being moved back at all, but the tap is weaker on account of the increased flute depth, the teeth are longer and produce more friction, and the flutes are deeper than necessary for a tap having a long taper or that requires to be removed to clear out the cuttings. Fig. 338 shows the form of flute in the Pratt and Whitney Company's hand taps, the cutting edges forming radial lines and the bottoms of the flutes being more rounded than is usual. It may here be remarked that if the flutes have comparatively sharp corners, as at C in Fig. 339, the tap will be liable to crack in the hardening process. The form of flute employed in the Whitworth tap is shown in Fig. 340; here there being but three flutes the teeth are comparatively long, and on this account there is increased friction. But, on the other hand, such a tap produces, when used by hand, more accurate work, the threaded hole being more parallel and of a diameter more nearly equal to that of the tap, it being observed that even though a hand tap have no clearance it will usually tap a hole somewhat larger than itself so that it will unwind easily. If a hand tap is given clearance not only will it cut a hole widest at the mouth, but it will cut a thread larger than itself in an increased degree, and, furthermore, when the tap requires to be wound back to extract it the fine cuttings will become locked in the threads and the points of the tap teeth are liable to become broken off. To ease the friction of long teeth, therefore, it is preferable to do so either as in Fig. 325 at A, B, C, or as in Fig. 341. In Fig. 325 the tops of the teeth are shown filed away, leaving each end full, so that the cuttings cannot get in, no matter in which direction the tap is rotated; but the clearance is not so complete as in Fig. 341, in which the teeth are supposed to be eased away within the area enclosed by dotted lines, which gives clearance to the bottom as well as to the tops and sides of the thread and leaves the ends of each tooth a full thread.

[Ill.u.s.tration: Fig. 338.]

[Ill.u.s.tration: Fig. 339.]

[Ill.u.s.tration: Fig. 340.]

[Ill.u.s.tration: Fig. 341.]

Concerning the number of flutes in taps, it is to be observed that the duty the tap is to be put to, has much influence in this respect. In hand tapping the object is to tap as parallel and straight as possible with the least expenditure of power. Now, the greater the number of flutes the less the tap is guided, because more of the circ.u.mferential guiding surface is cut away. But on the other hand, the less the number of flutes, and therefore the less the number of cutting edges, the more power it takes to operate the tap on account of the greater amount of friction between the tap and the walls of the hole. In hand tapping on what may be termed frame work (as distinguished from such loose work as nuts, &c.), the object is to tap the holes as parallel as possible with the least expenditure of power while avoiding having to remove the tap from the hole to clear it of the cuttings. Obviously the more flutes and cutting edges there are the more room there is for the cuttings and the less frequent the tap requires to be cleaned. If the tapping hole is round and straight the tapping may be made true and parallel if due care is taken, whatever the number of flutes, but less care will be required in proportion as there are less flutes, while, as before noted, more power and more frequent tap removals will be necessary. But if the hole is not round, other considerations intervene.

[Ill.u.s.tration: Fig. 342.]

Thus in Fig. 342 we have a three-flute tap in a hole out of round at A, and it is obvious that when a cutting edge meets the recess at A, all three teeth will cease to cut; hence there will be no inducement for the tap to move over toward A. But in the case of the four-flute tap in Fig.

343, when the teeth come to A there will be a strain tending to force the teeth over toward the depression A. How much a given tap would actually move over would, of course, depend upon the amount of clearance; but whether the tap has clearance or not, the three-flute tap will not move over, while with four flutes the tap would certainly do so. Again, with an equal width of flute there is more of the circ.u.mference tending to guide and steady the three-flute than the four-flute tap. If the hole has a projection instead of a depression, as at B, Figs. 344 and 345, then the advantage still remains with the three-flute tap, because in the case of the three flutes, any lateral movement of the tap will be resisted at the two points _c_ and D, neither of which are directly opposite to the location of the projection B; hence, if the projection caused the tap to move laterally, say, 1-100th inch, the effect at _c_ and D would be very small, whereas in the four-flute, Fig. 345, the effect at E would be equal to the full amount of lateral motion of the tap.

[Ill.u.s.tration: Fig. 343.]

[Ill.u.s.tration: Fig. 344.]

[Ill.u.s.tration: Fig. 345.]

In hand taps the position of the square at the head of the tap with relation to the cutting-edges is of consequence; thus, in Fig. 346, there being a cutting-edge A opposite to the handle, any undue pressure on that end of the handle would cause A to cut too freely and the tap to enlarge the hole; whereas in Fig. 347 this tendency would be greatly removed, because the cutting-edges are not in line with the handle. In a three-flute tap it makes but little difference what are the relative positions of the square to the flutes, as will be seen in Fig. 348, where one handle of the wrench comes in the most favorable and the other in the most unfavorable position. Taps for use by hand and not intended to pa.s.s through the work are sometimes made with the shank and the square end which receive the wrench of enlarged diameter. This is done to avoid the twisting of the shank which sometimes occurs when the tap is employed in deep holes, giving it much strain, and also to avoid as much as possible the wearing and twisting of the square which occurs, because in the course of time the square holes in solid wrenches enlarge from wear, and the larger the square the less the wear under a given amount of strain.

[Ill.u.s.tration: Fig. 346.]

Bra.s.s finishers frequently form the heads of their taps as in Fig. 349, using a wrench with a slot in it that is longer than the flat of the tap head.

[Ill.u.s.tration: Fig. 347.]

The thickness of the flat head at A is made equal for all the taps intended to be used with the same wrench. By this means one wrench may be used for many different diameters of taps.

[Ill.u.s.tration: Fig. 348.]

For gas, steam pipe, and other connections made by means of screw threads, and which require to be without leak when under pressure, the tap shown in Fig. 350 is employed. It is made taper and full threaded from end to end, so that the fittings may be entered easily into their places and screwed home sufficiently to form a tight joint.

[Ill.u.s.tration: Fig. 349.]

[Ill.u.s.tration: Fig. 350.]

The standard degree of taper for steam-pipe taps is 3/4 inch per foot of length, the taper being the same in the dies as on the taps. The threading tools for the pipes or casings for petroleum oil wells are given a taper of 3/8 inch per foot, because it was not found practicable to tap such large fittings with a quick taper, because of the excessive strain upon the threading tools. Ordinary pipe couplings are, however, tapped straight and stretch to fit when screwed home on the pipe.

Oil-well pipe couplings are tapped taper from both ends, and there is just enough difference in the taper on the pipe and that in the socket to show a bearing mark at the end only when the pipe and socket are tested with red marking.

PITCHES OF TAP THREADS IN USE IN THE UNITED STATES.

+-----------+---------+----------------+ | | | No. of Threads | | Diameter. | Length. | to Inch. | +-----------+---------+----------------+ | 1/4 | 2-3/4 | 16, 18 & 20 | | 5/16 | 2-7/8 | 16 & 18 | | 3/8 | 3-1/2 | 14 & 16 | | 7/16 | 3-13/16 | 14 & 16 | | 1/2 | 4-5/16 | 12, 13 & 14 | | 9/16 | 4-3/4 | 12 & 14 | | 5/8 | 5-1/8 | 10, 11 & 12 | | 11/16 | 5-3/8 | 11 & 12 | | 3/4 | 5-13/16 | 10, 11 & 12 | | 13/16 | 6 | 10 | | 7/8 | 6-1/8 | 9 & 10 | | 15/16 | 6-3/8 | 9 | | 1 | 6-13/16 | 8 | | 1-1/8 | 7-1/4 | 7 & 8 | | 1-1/4 | 8 | 7 & 8 | +-----------+---------+----------------+

Fig. 351 represents the form of tap employed by blacksmiths for rough work, and for the axles of wagon wheels. These taps are given a taper of 1/2 inch per foot of length, and are made with right and left-hand threads, so that the direction of rotation on both sides of a wagon wheel shall be in a direction to screw up the nuts and not to unscrew the nut, as would be the case if both ends of the axle were provided with right-hand threads.

[Ill.u.s.tration: Fig. 351.]

Taps that are used in a machine are sometimes so constructed that upon having tapped the holes to the required depth, the pieces containing the tap teeth recede from the walls of the hole, so that the tap may be instantly withdrawn from the hole instead of requiring to be rotated backwards. This is an advantage, not only on account of the time saved, but also because the cutting edges of the teeth are saved from the abrasion and its consequent wear which occur in rotating a tap backwards.

[Ill.u.s.tration: Fig. 352.]

Figs. 352 and 353 represent a collapsing tap that is much used in manufactories of pipe fittings.

[Ill.u.s.tration: Fig. 353.]

A is driven by the spindle of the machine, and drives B through the medium of the pin H. In B are three chasers C, fitting into the dovetail and taper grooves D. These chasers are provided with lugs fitting into an annular groove E sunk in A, so that if the piece H rises, the chasers will not rise with it, but will simply close together by reason of the lifting or rising of the core B, with its taper dovetail grooves; or, on the other hand, if the core B descends, the taper grooves in B force the chasers outward, increasing their cutting diameter.

When the tap is cutting, it is driven as denoted by the arrow, and the pin H is driven by the ends of the grooves, of which there are two, one diametrically opposite the other, inclined in the same direction. But when the tap has cut a thread to the required depth on the work, the handles H may be pulled or pushed the working way, pa.s.sing along the grooves I, and causing B to lift within A, and allowing the chasers to close away from the thread just cut, and the tap may be instantly withdrawn, and handles H pushed back to expand the chasers, ready for the next piece of work.

[Ill.u.s.tration: Fig. 354.]

Fig. 354 represents a collapsing tap used in Boston, Ma.s.sachusetts, at the Hanc.o.c.k Inspirator Works, in a monitor or turret lathe. It consists of an outer sh.e.l.l A carrying three chasers B, pivoted to A at C, having a small lug E at one end, and being coned at the inner end D. The inner sh.e.l.l F is reduced along part of its length to receive the lug E of the chaser, and permit the chasers to open out full at their cutting end. F has a cone at the end G, fitting to the internal cone on the chasers at D. At the other end of F is a washer H, against which abuts the spiral spring shown, the other end of this spring ab.u.t.ting against a shoulder provided in A. The washer H is bevelled on its outer or end face to correspond with the bevel on a notch provided in lever I, as is shown.

Within the inner tube F is the stem J, into the end of which is fitted the piece K, and on which is fixed the cone L. Piece K, and therefore L, is prevented from rotating by a spline in K, into which spline the pin M projects.

The operation is as follows. In the position in which the parts are shown in the engraving, F is pushed forward so that its coned end G has opened out the chaser to its fullest extent, which opening is governed by contact of the lug E with the reduced diameter of F. Suppose that the tap is operating in the work, then, when the foot N of K meets with a resistance (as the end of the hole being tapped), J, and therefore L, will be gradually pushed to the right, until, finally, the cone on L will raise the end of lever I until the notch on I is clear of H, when the spiral spring, acting against H, will force F to the right, and the shoulder on F, at X, will lift the end E of the chaser, causing the cutting end to collapse within A, the pivot C being its centre of motion. The whole device may then be withdrawn from the work. To open the chasers out again the rod J is forced, by hand, to the left, the cone-piece L meeting the face of H and pus.h.i.+ng it to the left until cone G meets cone D, when the chasers open until the end E meets the body of F, as in the cut. The rod J is then pulled to the right until L again meets the curved end of lever I and all the parts a.s.sume the positions shown in the cut. To regulate the depth of thread the tap shall cut, the body A is provided with a thread to receive the nut O, by means of which the collar P may be moved along A. This collar carries the pivots Q for levers I, so that, by s.h.i.+fting O, the position of I is varied, hence the point at which L will act upon the end of I and lift it to release H is adjustable.

When used upon steel, wrought iron, cast iron, copper, or bra.s.s, a tap should be freely supplied with oil, which preserves its cutting edge as well as causes it to cut more freely, but for cutting the soft metals such as tin, lead, &c., oil is unnecessary.

The diameters of tapping holes should be equal to the diameter of the thread at the root, but in the case of cast iron there is much difference of opinion and practice. On the one hand, it is claimed that the size of the tapping hole should be such as to permit of a full thread when it is tapped; on the other hand, it is claimed that two-thirds or even one-half of a full thread is all that is necessary in holes in cast iron, because such a thread is, it is claimed, equally as strong as a full one, and much easier to tap. In cases where it is not necessary for the thread to be steamtight, and where the depth of the thread is greater by at least 1/8 inch than the diameter of the bolt or stud, three-quarters of a full thread is all that is necessary, and can be tapped with much less labor than would be the case if the hole were small enough to admit of a full thread, partly because of the diminished duty performed by the tap, and partly because the oil (which should always be freely supplied to a tap) obtains so much more free access to the cutting edges of the tap. If a long tap is employed to cut a three-quarter full thread, it may be wound continuously down the hole, without requiring to be turned backwards at every revolution or so of the tap, to free it from the tap cuttings or shavings, as would be necessary in case a full thread were being cut. The saving of time in consequence of this advantage is equal to at least 50 per cent. in favor of the three-quarter full thread.

As round bar iron is usually rolled about 1/32 inch larger than its designated diameter, a practice has arisen to cut the threads upon the rough iron just sufficiently to produce a full thread, leaving the latter 1/32 inch above the proper diameter, hence taps 1/32 inch above size are required to thread nuts to fit the bolts. This practice should be discountenanced as destroying in a great measure the interchangeability of bolts and nuts, because 1/32 inch is too small a measurement to be detected by the eye, and a measurement or trial of the bolt and nut becomes necessary.