Modern Machine-Shop Practice Part 25

For cutting external or male threads by hand three cla.s.ses of tools are employed.

The first is the screw plate shown in Fig. 292. It consists of a hardened steel plate containing holes of varying diameters and threaded with screw threads of different pitches. These holes are provided with two diametrically opposite notches or slots so as to form cutting edges.

[Ill.u.s.tration: Fig. 292.]

This tool is placed upon the end of the work and slowly rotated while under a hand pressure tending to force it upon the work, the teeth cutting grooves to form the thread and advancing along the bolt at a rate determined by the pitch of the thread.

The screw plate is suitable for the softer metals and upon diameters of 1/8 inch and less, in which the cutting duty is light; hence the holes do not so rapidly wear larger.

The second cla.s.s consists of a stock and dies such as shown in Fig. 293.

For each stock there are provided a set of dies having different diameters and pitches of thread.

In this cla.s.s of tool the dies are opened out and placed upon the bolt.

The set screw is tightened up, forcing the dies to their cut, and the stock is slowly rotated and a traverse taken down the work.

[Ill.u.s.tration: Fig. 293.]

In some cases the dies are then again forced to the work by the set screw, and a cut taken by winding the stocks up the bolt, the operation being continued until the thread is fully developed and cut to the required diameter. In other cases the cut is carried down the bolt, only the dies being wound back to the top of the bolt after each cut is carried down. The difference between these two operations will be shown presently.

[Ill.u.s.tration: Fig. 294.]

The thread in dies which take successive cuts to form a thread may be left full clear through the die, and will thus cut a full thread close up to the head collar or shoulder of the work. It is usual, however, to chamfer off the half threads at the ends of the dies, because if left of their full _height_ they are apt to break off when in use. It is sometimes the practice, however, to chamfer off the first two threads on one side of the dies, leaving the teeth on the other side full, and to use the chamfered as the leading side in all cases in which the thread on the work does not require to be cut up to a shoulder, but turning the dies over with the full threaded teeth as the leading ones when the thread _does_ require to be carried up to a head or shoulder on the work.

To facilitate the insertion and extraction of the dies in and from their places in the stock, the Morse Twist Drill Co. employ the following construction. In Figs. 294 and 295 the pieces A, A' which hold the dies are pivoted in the stock at B, so as to swing outward as in Fig. 295, and receive the dies which are slotted to fit them. These pieces are then swung into position in the stock. The lower die is provided with a hole to fit the pin C, hence when that die is placed home C acts as a detaining piece locking the pieces A, A' through the medium of the bottom die.

[Ill.u.s.tration: Fig. 295.]

[Ill.u.s.tration: Fig. 296.]

In other dies of this cla.s.s the two side pieces or levers which hold the dies are pivoted at the corner of the angle, as in Fig. 296. In the bottom of the stock is a sliding piece beveled at its top and meeting the bottom face of the levers; hence, by pressing this piece inwards the side pieces recede into a slot provided in the stock, and leave the opening free for the dies to pa.s.s into their places, when the pin is released and a spring brings the side pieces back. Now, since the bottom die rests upon the bottom angle of the side pieces the pressure of the set screw closes the side pieces to the dies holding them firmly.

[Ill.u.s.tration: Fig. 297.]

In Fig. 297 is shown Whitworth's stocks and dies, the cap that holds the guide die _a_ and the two chasers _b_, _c_ in their seats or recesses in the stock being removed to expose the interior parts. The ends of the chasers _b_, _c_ are beveled and abut against correspondingly beveled recesses in the key _d_, so that by operating the nut _e_ on the end of the key the dies are caused to move longitudinally. The principles of action are more clearly shown in Fig. 298. The two cutting chasers B and C move in lines that would meet at D, and therefore at a point behind the centre or axis of the bolt being threaded; this has the effect of preserving their clearance. It is obvious, for example, that when these chasers cut a thread on the work it will move over toward guide A on account of the thread on the work sinking into the threads on A, and this motion would prevent the chasers B, C from cutting if they moved in a line pointing to the centre of the work. This is more clearly shown in Fig. 299, in which the guide die A and one of the cutting dies or chasers B is shown removed from the stock, while the bolt to be threaded is shown in two positions--one when the first cut is taken, and the other when the thread is finished. For the first cut the centre of the work is at E, for the last one it is at G, and this movement would, were the line of motion as denoted by the dotted lines, prevent the chaser from cutting, because, while the line of chaser motion would remain at J, pointing to the centre of work for the first cut, it would require a line at K to point to that centre for the last one; hence, when considered with relation to the work, the line of chaser motion has been moved forward, presenting the cutting edges at an angle that would prevent their cutting. By having their motion as shown in Fig. 299, however, the clearance of the chasers is preserved.

[Ill.u.s.tration: Fig. 298.]

[Ill.u.s.tration: Fig. 299.]

[Ill.u.s.tration: Fig. 300.]

Referring now to the die A, it acts as a guide rather than as a cutting chaser, because it has virtually no clearance and cannot cut so freely as B and C; hence it offers a resistance to the moving of the bolt, or of the dies upon the bolt, in a lateral direction when the chaser teeth meet either a projection or a depression upon the work. The guide principle is, however, much more fully carried out in a design by Bodmer, which is shown in Fig. 300. Here there is but one cutting chaser C, the bush G being a guide let into a recess in the stock and secured thereon by a pin _p_. The chaser is set in a stock, D also let into a recess in the stock, and this recess, being circular, permits of stock D swinging. At S are two set-screws, which are employed to limit the amount of motion permitted to D. the handle E screws through D, and acts upon the edge of chaser C to put on the cut. The action of the tool is shown in Fig. 301, where it is shown upon a piece of work. Pulling the handle E causes D to swing in the stock, thus giving the chaser clearance, as shown. When the cut is carried down, a new cut may be put on by means of E, and on winding the stock in the opposite direction, D will swing in its seat, and cant or tilt the chaser in the opposite direction, giving it the necessary clearance to enable it to cut on the upward or back traverse. Another point of advantage is that the cutting edges are not rubbed by the work during the back stroke, and their sharpness is, therefore, greatly preserved. A die of this kind will produce work almost as true as the lathe, and, in the case of long, slender work, more true than the lathe; but it is obvious that, on account of the friction caused by the pressure of the work to the guide G, the tool will require more power to operate than the ordinary stock and die or the solid die.

[Ill.u.s.tration: Fig. 301.]

In adjustable dies which require to take more than one cut along the bolt to produce a fully developed thread, there is always a certain amount of friction between the sides of the thread in the die and the grooves being cut, because the angle of the thread at the top of a thread is less than the angle at the bottom. Thus in Fig. 302 the pitch at the top of thread (at A, B) is the same as at the bottom (C, D). Now suppose that in Fig. 303 _a_ _b_ represents the axial line of a bolt, and _c_ _d_ a line at a right angle to _a_ _b_. The radius _e_ _f_ being equal to the circ.u.mference of the top of the thread, the pitch being represented by _b_; then _k_ represents the angle of the top of the thread to the axial line _a_ _b_. Now suppose that the radius _e_ _g_ represents the circ.u.mference at the bottom of the thread and to the pitch; then _l_ is the angle of the bottom of the thread to the axial line of the work, and the difference in angle between _k_ and _l_ is the difference in angle between the top and bottom of the thread in the dies and the thread to be cut on the work.

[Ill.u.s.tration: Fig. 302.]

Now the tops of the teeth on the die stand at the greatest angle _l_, in Fig. 303, when taking the first cut on the bolt, but the grooves they cut will be on the full diameter of the bolt, and will, therefore, stand at the angle _k_, hence the lengths of the teeth do not lie in the same planes as the grooves which they cut.

[Ill.u.s.tration: Fig. 303.]

In cutting [V]-threads, however, the angle of the die threads gradually right themselves with the plane of the grooves attaining their nearest coincidence when closed to finish the thread.

Since, however, the full width of groove is in a square thread cut at the first cut taken by the dies, it is obvious that a square thread cannot be cut by this cla.s.s of die, because the sides of the grooves would be cut away each time the dies were closed to take another cut.

[Ill.u.s.tration: Fig. 304.]

[Ill.u.s.tration: Fig. 305.]

Dies of this cla.s.s require to have the threaded hole made of a larger diameter than is the diameter of the bolt they are intended to thread, the reason being as follows:--

Suppose the threaded hole in the dies to be cut by a hob or master tap of the same diameter as the thread to be cut by the dies; when the dies are opened out and placed upon the work as in Fig. 304, the edges A, B will meet the work, and there will be nothing to steady the dies, which will, therefore, wobble and start a drunken thread, that is to say, a thread such as was shown in Fig. 253.

[Ill.u.s.tration: Fig. 306.]

Instances have been known in the use of dies made in this manner, wherein the workman using a right-hand single-threaded pair of dies has cut a right or left-hand double or treble thread; the teeth of the dies acting as chasers well canted over, as shown in Fig. 305. It is necessary to this operation, however, that the diameter of the work be larger than the size of hob the dies were threaded with.

In Fig. 306 is shown a single right-hand and a treble left-hand thread cut by the author with the same pair of dies.

All that is necessary to perform this operation is to rotate the dies from left to right to produce a right-hand thread, and from right to left for a left-hand thread, exerting a pressure to cause the dies to advance more rapidly along the bolt than is due to the pitch of the thread. A double thread is produced when the dies traverse along the work twice as fast as is due to the pitch of the thread in the dies, and so on.

[Ill.u.s.tration: Fig. 307.]

It is obvious, also, that a piece of a cylindrical thread may be used to cut a left-hand external thread. Thus in Fig. 307 is shown a square piece of metal having a notch cut in on one side of it and a piece of an external thread (as a tap inserted) in the notch. By forcing a piece of cylindrical work through the hole while rotating it, the piece of tap would cut upon the work a thread of the pitch of the tap, but a left-handed thread, which occurs because, as shown by the dotted lines of the figure, the thread on one side of a bolt slopes in opposite directions to its direction on the other, and in the above operation the thread on one side is taken to cut the thread on the other.

These methods of cutting left-hand threads with right-handed ones are mentioned simply as curiosities of thread cutting, and not as being of any practical value.

To proceed, then: to avoid these difficulties it is usual to thread the dies with a hob or master tap of a diameter equal to twice the depth of the thread, larger than the size of bolt the dies are to thread. In this case the dies fit to the bolt at the first cut, as shown in Fig. 308, C, D being the cutting edges. The relation of the circle of the thread in the dies to that of the work during the final cut is shown in Fig. 309.

[Ill.u.s.tration: Fig. 308.]

[Ill.u.s.tration: Fig. 309.]

There is yet another objection to tapping the dies with a hob of the diameter of the bolt to be threaded, in that the teeth fit perfectly to the thread of the bolt when the latter is threaded to the proper diameter, producing a great deal of friction, and being difficult to make cut, especially when the cutting edges have become slightly dulled from use.

Referring now to taking a cut up the bolt or work as well as down, it will be noted that supposing the dies to have a right-hand thread, and to be rotating from left to right, they will be pa.s.sing down the bolt and the edges C, D (Fig. 308) will be the cutting ones. But when the dies are rotated from right to left to bring them to the end of the bolt again, C, D will be rubbed by the thread, which tends to abrade them and thus destroy their sharpness.

[Ill.u.s.tration: Fig. 310.]

In some cases two or more pairs of dies are fitted to the same stock, as shown in Fig. 310, but this is objectionable, because it is always desirable to have the hole in the dies central to the length of the stock, so that when placed to the work the stock shall be balanced, which will render it easier to start the thread true with the axial line of the bolt.

[Ill.u.s.tration: Fig. 311.]

From what has been said with reference to Fig. 303, it is obvious that a square thread cannot be cut by a die that opens and closes to take successive cuts along the work, but such threads may be cut upon work that is of sufficient strength to withstand the twisting pressure of the dies, by making a solid die, and tapering off the threads for some distance at the mouth of the die, so as to enable the die to take its bite or grip upon the work, and start itself. It is necessary, however, to give to the die as many flutes (and therefore cutting edges), as possible, or else to make flutes wide and the teeth as short as will leave them sufficiently strong, both these means serving to avoid friction.

[Ill.u.s.tration: Fig. 312.]