Modern Machine-Shop Practice Part 26

The teeth for adjustable dies, such as shown in Fig. 293, are cut as follows:--There is inserted between the two dies a piece of metal, separating them when set together to a distance equal to twice the depth of the thread, added to the distance the faces of the dies are to be apart when the dies are set to cut to this designated or proper diameter. The tapping hole is then drilled (with the pieces in place) to the diameter of the bolt the die is for. The form of hob used by the Morse Twist Drill & Machine Company, to cut the thread, is shown in Fig.

311. The unthreaded part at the entering end is made to a diameter equal to that of the work the dies are to be used in; the thread at the entering end is made sunk in one half the height of the full thread, and is flattened off one half the height of a full thread, so that the top of the thread is even with the diameter of the unthreaded part at the entering end. The thread then runs a straight taper up the hob until a distance equal to the diameter of the nut is reached, and the length of hob equal to its diameter is made a full and parallel thread for finis.h.i.+ng the die teeth with. The thread on the taper part has more taper at the root of the thread than it has at the top of the same, and the diameter of the full and parallel part at the shank end of the thread is made of a diameter equal to twice the height or depth of a full thread, larger than the diameter at the entering end of the hob.

The hob thus becomes a taper and relieved tap cutting a full thread at one pa.s.sage through the dies. If the hob is made parallel and a full thread from end to end, as in Fig. 312, the dies must traverse up and down the hob, or the hob through the dies to form a full thread.

The third cla.s.s of stock and die is intended to cut a full thread at one pa.s.sage along the work, while at the same time provision is made, whereby, to take up the wear due to the abrasion of the cutting edges, which wear would cause the diameter of thread cut to be above the standard.

In Fig. 313 is shown the Grant adjustable die made by the Pratt & Whitney Company. It consists of four chasers or toothed cutting tools, inserted in radial recesses or slots in an iron disc or collet encircled by an iron ring. Each chaser is beveled at its end to fit a corresponding bevel in the ring, and is grooved on one of its side faces to receive the hardened point of a screw that is inserted in the collet to hold the chaser in its adjusted position. Four screws extend up through the central f.l.a.n.g.e or body of the collet, two of which serve to draw down the ring, and by reason of the taper on the ring move the chasers equally towards the centre and reduce the cutting diameter of the die, while the other two hold the ring in the desired position, or force it upward to enlarge the cutting diameter of the die. The range of adjustment permitted by this arrangement is 1-32 inch. The dies may be taken out and ground up to sharpen.

[Ill.u.s.tration: Fig. 313.]

The object of cutting grooves in the sides of the chasers is that the fine burrs formed by the ends of the set screws do not prevent the chasers from moving easily in the collet during the process of adjustment; the groove also acts as a shoulder for the screw end to press the chaser down to its seat. These chasers are marked to their respective places in the collet, and are so made that if one chaser should break, a new one can be supplied to fit to its place, the teeth of the new one falling exactly in line with the teeth on the other three, whereas under ordinary conditions if one chaser breaks, a full set of four new ones must be obtained.

In this die, as in all others which cut a full thread at one pa.s.sage along the work, the front teeth of the chasers are beveled off as shown in the cut; this is necessary to enable the dies to take hold of or "bite" the work, the chamfer giving a relief to the cutting edge, while at the same time forming to a certain extent a wedge facilitating the entrance of the work into the die.

Fig. 314 represents J. J. Grant's patent die, termed by its makers (Wiley and Russel) the "lightening die." In this, as in other similar stocks, several collets with dies of various pitches and diameters of thread, fit to one stock. The nut of the stock is split on one side, and is provided with lugs on that side to receive a screw, which operates to open and enlarge the bore to release a collet, or close thereon and grip it, as may be required when inserting or extracting the same. The dies are formed as shown in Fig. 315, in which A, A are the dies, and B the collet. To open the dies within the collet, the screws E are loosened and the screws D are tightened, while to close the dies D, D are loosened and E are tightened; thus the adjustment to size is effected by these four screws, while the screws D also serve to hold the dies to the collet B. The collets are provided with a collar having a bore F, through which the work pa.s.ses, so that the dies may be guided true when starting upon the work; but if it is required to cut a thread close up to a head or shoulder, the stock is turned upside down, not only to have the collet out of the way of the head or shoulder, but also because the thread of the dies on the collet side are chamfered off (as is necessary in all solid dies, or dies which cut a full thread at one traverse down the work) so as to enable them to grip or bite the work, and start the thread upon it as before stated.

[Ill.u.s.tration: Fig. 314.]

[Ill.u.s.tration: Fig. 315.]

In Fig. 316 is shown Stetson's die, which cuts a full thread at one pa.s.sage, is adjustable to take up its wear, and has a guide to steady it upon the work and a.s.sist it in cutting a true thread. The guide piece consists of a hub (through which the work pa.s.ses) having a f.l.a.n.g.e fitting into the dies and being secured thereto by the two screws shown.

The holes in the f.l.a.n.g.es are slotted to permit of the dies being closed (to take up wear) by means of the small screws shown at the end of the die, which screws pa.s.s through one die in a plain hole and screw into the other.

[Ill.u.s.tration: Fig. 316.]

In Fig. 317 is shown Everett's stocks and dies. In this tool the dies are set up by a cam lever, the dies being set to standard size when the lever arm stands parallel with the arm of the stock. By turning the straight side of the cam lever opposite to the dies, the latter may be instantly removed and another size of die inserted. The dies may be used to cut on their pa.s.sage up and down the bolt or by operating the cam.

When the dies are at the end of a cut the dies may be opened, lifted to the top of the work and another cut taken, thus saving the time necessary to wind the stock back. When the final cut is taken the dies may be opened and lifted off the work.

[Ill.u.s.tration: Fig. 317.]

The hardening process usually increases the thickness of these dies, making the pitch of the thread coa.r.s.er. The amount of expansion due to hardening is variable, but increases with the thickness of the die. The hob as a rule shortens during the tempering, but the amount being variable, no rule for its quant.i.ty can be given.[12]

[12] See also page 108.

Stocks and dies for pipe work are made in the form shown in Fig. 318, in which B is the stock having the detachable handles (for ease of conveyance) A, H, the latter being shown detached. The solid screw-cutting dies C are placed in the square recess at B, and are secured in B by the cap D, which swings over (upon its pivoted end as a centre) and is locked by the thumbscrew E. To guide the stocks and cause them to cut a true thread, the bushes F are provided. These fit into the lower end of B and are locked in position by four set screws G. The bores of the bushes F are made an easy fit to the outside of the pipe to be threaded, there being a separate bush for each size of pipe.

[Ill.u.s.tration: Fig. 318.]

The dies employed in stocks for threading steam and gas pipes by hand are sometimes solid, as in Fig. 318 at C, and at others adjustable. In Fig. 319 is shown Stetson's adjustable pipe die containing four chasers or toothed thread-cutting tools. These are set to cut the required diameter by means of a small screw in each corner of the die, while they are locked in their adjusted position by four screws on the face.

The tap is a tool employed to cut screw threads in internal surfaces, as holes or bores. A set of taps for hand use usually consist of three: the taper tap, Fig. 320; plug tap, Fig. 321; and bottoming tap, Fig. 322.

(In England these taps are termed respectively the taper, second, and plug tap.) The taper tap is the first to be inserted, and (when the hole to be threaded pa.s.ses entirely through the work) rotated until it pa.s.ses through the work, thus cutting a thread parallel in diameter through the full length of the hole. If, however, the hole does not pa.s.s through the work, the taper tap leaves a taper-threaded hole containing more or less of a fully developed thread according to the distance the tap has entered.

[Ill.u.s.tration: Fig. 319.]

[Ill.u.s.tration: Fig. 320.]

[Ill.u.s.tration: Fig. 321.]

[Ill.u.s.tration: Fig. 322.]

To further complete the thread the plug tap is inserted, it being parallel from four or five threads from the entering end of the tap to the other end. If the work will admit it, this tap is also pa.s.sed through, which not only saves time in many cases, by avoiding the necessity to wind the tap back, but preserves the cutting edge which suffers abrasion from being wound back. To cut a full thread as near as possible to the bottom of a hole the bottoming tap is used, but when the circ.u.mstances will admit, it is best to drill the hole rather deeper than is actually necessary, to avoid the trouble incident to tapping a hole clear to the bottom.

On wrought iron and steel, which are fibrous and tough, the tap, when used by hand, will not (if the hole be deeper than the diameter of the tap) readily operate by a continuous rotary motion, but requires to be rotated about half a revolution back occasionally, which gives opportunity for the oil to penetrate to the cutting edges of the tap, frees the tap and considerably facilitates the tapping operation, especially if the hole be a deep one.

[Ill.u.s.tration: Fig. 323.]

When the tap is intended to pa.s.s entirely through the work with a continuous rotary motion, as is the case, for example, in tapping nuts in a tapping machine, it is made of similar form to the taper hand tap, but longer, as shown in Fig. 323, the thread being full and parallel at the shank end for a distance at least equal to the full diameter of the tap measured across the tops of the thread.

If the thread of a tap be in diametral section a full circle, the sides of the thread rub against the grooves cut by the teeth, producing a friction which augments as the sharp edge of the teeth become dulled from use, but the tap cuts a thread of great diametral accuracy.

To reduce this friction to a minimum as much as is consistent with maintaining the standard size of the tapped hole, taps are sometimes given clearance in the thread, that is to say, the back of each tooth recedes from a true circle, as shown in Fig. 324, in which A A represents a washer, and B A tap in the same, the back of the teeth receding at C, D, E, from the true circle of the bore of A A, the tap cutting when revolved in the direction of the arrow. The objection to this is that when the tap is revolved backwards, as it must be to extract it unless the hole pa.s.ses clear through the work, the cuttings lodge between the teeth and the thread in the work, rendering the extraction of the tap difficult, unless, indeed, the clearance be small enough in amount to clear the sides of the thread in the work sufficiently to avoid friction without leaving room for the cuttings to enter. If an excess of clearance be allowed upon taps that require to be used by hand, the tap will thread the hole taper, the diameter being largest at the top of the hole. This occurs because the tap is not so well steadied by its thread, which fails to act as a guide, and it is impossible to revolve the tap steadily by hand. Taps that are revolved by machine tools may be given clearance because both the taps and the work are detained in line, hence the tap cannot wobble.

[Ill.u.s.tration: Fig. 324.]

[Ill.u.s.tration: Fig. 325.]

In some cases clearance is given by filing or cutting off the tops of the threads along the middle of the teeth, as shown in Fig. 325 at A, B, C, which considerably reduces the friction. If clearance were given to a tap after this manner but extended to the sides and to the bottom of the thread, it would produce the best of results (for all taps that do not pa.s.s entirely through the hole), reducing the friction and leaving no room for the cuttings to jam in the threads when the tap is being backed out. The threads of Sir Joseph Whitworth's taper hand taps are made parallel, measured at the bottom of the thread, and parallel at the tops of the thread for a distance equal to the diameter of the tap at the shank end; thence, to the entering end of the tap, the tops of the thread are turned off a straight taper, the amount of taper being slightly more than twice the depth of the thread: hence, the thread is just turned out at the entering end of the tap, and that end is the exact proper size for the tapping hole.

This enables the tap to enter the tapping hole for a distance enveloping one or perhaps two of the tap threads, leaving the extreme end of the tap with the thread just turned out. In the practice of some tap makers the diameter of the thread at the top is made the same as in the Whitworth system, but there is more depth at the root of the thread and near the entering end of the tap, hence the bottoms of the thread at that end perform no cutting duty. This is done to enable the tap to take hold of, and start a thread in, the work more readily, which it does for the following reasons. In Fig. 326 is a piece of work with a tap A, having a tapered thread, and a tap B, in which the taper is given by turning off the thread. In the case of A the teeth points cut a groove that is gradually widened and deepened as the tap enters, until a full thread is finally produced. In the case of B the teeth cut at first a wide groove, leaving a small projection, that is a part of the actual finished thread, and the groove gets narrower as the tap enters; so that in the one case no part of the thread is finished until the tap has entered to its full diameter, while in the other the thread is finished as it is produced. On entering, therefore, more cutting duty is performed by B than by A, because a greater length of cutting edge is in operation and more metal is being removed, and as a result B requires more power to start it, so that in practice it is necessary to exert a pressure upon it, tending to force it into the hole while rotating it.

The cutting duty on B decreases as the tap enters, because it gets less width and area of groove to cut, while the cutting duty on A increases as the tap enters, because it gets a greater width and area of groove to cut. In the latter case the maximum of pressure falls on the tap when it has entered the hole deepest, and hence can be operated steadiest, which, independent of its entering easiest, is an advantage. When, however, the bottom of a thread is taper (as must be the case to enable it to cut as at A), the cutting edge of each tooth does not cut a groove sufficiently large in diameter to permit the tooth itself to pa.s.s through. In Fig. 327, for example, is shown a tap which is taper and has a full thread from end to end (as is necessary for pipe tapping). Its diameter increases as the thread proceeds from the end towards the line A B. Now take the tooth O P, which stands lengthwise, in the plane C D.

Its cutting edge is at P, but the diameter of the tap at P is less than it is at O, while O has to pa.s.s through the groove that P cuts. To obviate this difficulty the tap is given clearance, as shown in Fig.

324, the amount being slightly more than the difference in the diameter of the tap at O and at P in that figure. It follows, therefore, that a tap having taper from end to end and a full thread also, as shown in the lower tap in Fig. 328, is wrong in principle, and from the unsteady manner in which it operates is undesirable, even though its thread be given clearance.

[Ill.u.s.tration: Fig. 326.]

[Ill.u.s.tration: Fig. 327.]

In some cases the thread is made parallel at the tops and turned taper for a distance of 1/3 or 1/2 the length of the tap, the root of the thread at the taper part being deepened and the tops being given a slight clearance. This answers very well for shallow holes, because the taper tap cuts more thread on entering a given depth so that the second tap can follow more easily, but the tap will not operate so steadily as when the taper part is longer.

[Ill.u.s.tration: Fig. 328.]

It is on account of the tops of the teeth performing the main part of the cutting that a tap taper may be sharpened by simply grinding the teeth tops. In the Pratt and Whitney taps, the hand taper tap is made parallel at the shank end for a distance equal in length to the diameter of the tap.

The entering end of the taper tap is made straight or parallel for a distance equal in length to one half the diameter of the tap, the diameter at this end being the exact proper size of tapping hole. The parallel part serves as a guide, causing the tap to enter and keep axially true with the hole to be tapped. The plug and bottoming taps are made parallel in the thread, the former being tapered slightly at and for two or three threads from the entering, as shown in Fig. 328. The threads are made parallel at the roots.

The Pratt and Whitney taper taps for use in machines are of the following form:--

The entering end of the tap is equal in diameter to the diameter of the tapping hole into which the tap will enter for a distance of two or three threads. The thread at the shank end is parallel both at the top and at the root for a distance equal, in length, to twice the diameter of the tap. The top of the thread has a straight taper running from the parallel part at the shank to the point or entering end, while the roots of the thread are made along this taper twice the taper that there is at the top of the thread, which is done to make the tap enter and take hold of the nut more easily.

[Ill.u.s.tration: Fig. 329.]

A form of tap that cuts very freely on account of the absence of friction on the sides of the thread is shown in Fig. 329. The thread is cut in parallel steps, increasing in size towards the shank, the last step (from D to E in the figure) being the full size. The end of the tap at A being the proper size for the tapping hole, and the flutes not being carried through A, insures that the tap shall not be used in holes too small for the size of the tap, and thus is prevented a great deal of tap breakage. The bottom of the thread of the first parallel step (from A to B) is below the diameter of A, so as to relieve the sides of the thread of friction and cause the tap to enter easily. The first tooth of each step does all the cutting, thus acting as a turning tool, while the step within the work holds the tooth to its cut, as shown in Fig.

330, in which N represents a nut and T the tap, both in section. The step C holds the tap to its work, and it is obvious that, as the tooth B enters, it will cut the thread to its own diameter, the rest of the teeth on that step merely following frictionless until the front tooth on the next step takes hold. Thus, to sharpen the tap equal to new, all that is required is to grind away the front tooth on each step, and it becomes practicable to sharpen the tap a dozen times without softening it at all. As a sample of duty, it may be mentioned that, at the Harris-Corliss Works, a tap of this cla.s.s, 2-7/8 inches diameter, with a 4 pitch, and 10 inches long, will tap a hole 5 inches deep, pa.s.sing the tap continuously through without any backing motion, two men performing the duty with a wrench 4 feet long over all, the work being of cast iron.

[Ill.u.s.tration: Fig. 330.]

[Ill.u.s.tration: Fig. 331.]

[Ill.u.s.tration: Fig. 332.]