Fig. 292Fig. 292.
Fig. 292.
The first is the screw plate shown inFig. 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.
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 upondiameters of1⁄8inch and less, in which the cutting duty is light; hence the holes do not so rapidly wear larger.
Fig. 293Fig. 293.
Fig. 293.
The second class consists of a stock and dies such as shown inFig. 293. For each stock there are provided a set of dies having different diameters and pitches of thread.
In this class 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.
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.
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 fullheightthey 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 threaddoesrequire to be carried up to a head or shoulder on the work.
Fig. 294Fig. 294
Fig. 294
Fig. 295Fig. 295
Fig. 295
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. InFigs. 294and295the piecesa,a′which hold the dies are pivoted in the stock atb, so as to swing outward as inFig. 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 pinc, hence when that die is placed homecacts as a detaining piece locking the piecesa,a′through the medium of the bottom die.
Fig. 296Fig. 296
Fig. 296
In other dies of this class the two side pieces or levers which hold the dies are pivoted at the corner of the angle, as inFig. 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 pass 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.
Fig. 297Fig. 297.
Fig. 297.
Fig. 298Fig. 298.
Fig. 298.
Fig. 299Fig. 299.
Fig. 299.
InFig. 297is shown Whitworth’s stocks and dies, the cap that holds the guide dieaand the two chasersb,cin their seats or recesses in the stock being removed to expose the interior parts. The ends of the chasersb,care beveled and abut against correspondingly beveled recesses in the keyd, so that by operating the nuteon the end of the key the dies are caused to move longitudinally. The principles of action are more clearly shown inFig. 298. The two cutting chasersbandcmove in lines that would meet atd, and therefore at a point behind the centre or axis of the bolt being threaded; this has the effect ofpreserving their clearance. It is obvious, for example, that when these chasers cut a thread on the work it will move over toward guideaon account of the thread on the work sinking into the threads ona, and this motion would prevent the chasersb,cfrom cutting if they moved in a line pointing to the centre of the work. This is more clearly shown inFig. 299, in which the guide dieaand one of the cutting dies or chasersbis 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 ate, for the last one it is atg, 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 atj, pointing to the centre of work for the first cut, it would require a line atkto 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 inFig. 299, however, the clearance of the chasers is preserved.
Fig. 300Fig. 300.
Fig. 300.
Fig. 301Fig. 301.
Fig. 301.
Referring now to the diea, it acts as a guide rather than as a cutting chaser, because it has virtually no clearance and cannot cut so freely asbandc; 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 inFig. 300. Here there is but one cutting chaserc, the bushgbeing a guide let into a recess in the stock and secured thereon by a pinp. The chaser is set in a stock,dalso let into a recess in the stock, and this recess, being circular, permits of stockdswinging. Atsare two set-screws, which are employed to limit the amount of motion permitted tod. the handleescrews throughd, and acts upon the edge of chasercto put on the cut. The action of the tool is shown inFig. 301, where it is shown upon a piece of work. Pulling the handleecausesdto 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 ofe, and on winding the stock in the opposite direction,dwill 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 guideg, the tool will require more power to operate than the ordinary stock and die or the solid die.
Fig. 302Fig. 302.
Fig. 302.
Fig. 303Fig. 303.
Fig. 303.
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 inFig. 302the pitch at the top of thread (ata,b) is the same as at the bottom (c,d). Now suppose that inFig. 303a brepresentsthe axial line of a bolt, andc da line at a right angle toa b. The radiuse fbeing equal to the circumference of the top of the thread, the pitch being represented byb; thenkrepresents the angle of the top of the thread to the axial linea b. Now suppose that the radiuse grepresents the circumference at the bottom of the thread and to the pitch; thenlis the angle of the bottom of the thread to the axial line of the work, and the difference in angle betweenkandlis the difference in angle between the top and bottom of the thread in the dies and the thread to be cut on the work.
Now the tops of the teeth on the die stand at the greatest anglel, inFig. 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 anglek, hence the lengths of the teeth do not lie in the same planes as the grooves which they cut.
In cuttingV-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 class of die, because the sides of the grooves would be cut away each time the dies were closed to take another cut.
Dies of this class 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 asfollows:—
Fig. 304Fig. 304.
Fig. 304.
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 inFig. 304, the edgesa,bwill 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 inFig. 253.
Fig. 305Fig. 305.
Fig. 305.
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 inFig. 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.
Fig. 306Fig. 306.
Fig. 306.
InFig. 306is 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.
Fig. 307Fig. 307.
Fig. 307.
It is obvious, also, that a piece of a cylindrical thread may be used to cut a left-hand external thread. Thus inFig. 307is 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.
Fig. 308Fig. 308.
Fig. 308.
Fig. 309Fig. 309.
Fig. 309.
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 inFig. 308,c,dbeing 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 inFig. 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 beingdifficult 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 passing down the bolt and the edgesc,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,dwill be rubbed by the thread, which tends to abrade them and thus destroy their sharpness.
Fig. 310Fig. 310.
Fig. 310.
In some cases two or more pairs of dies are fitted to the same stock, as shown inFig. 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.
From what has been said with reference toFig. 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.
Fig. 311Fig. 311.
Fig. 311.
Fig. 312Fig. 312.
Fig. 312.
The teeth for adjustable dies, such as shown inFig. 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 inFig. 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 finishing 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 passage through the dies. If the hob is made parallel and a full thread from end to end, as inFig. 312, the dies must traverse up and down the hob, or the hob through the dies to form a full thread.
The third class of stock and die is intended to cut a full thread at one passage 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.
Fig. 313Fig. 313.
Fig. 313.
InFig. 313is 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 flange 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 is1⁄32inch. The dies may be taken out and ground up to sharpen.
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 passage 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. 314Fig. 314.
Fig. 314.
Fig. 315Fig. 315.
Fig. 315.
Fig. 314represents 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 torelease a collet, or close thereon and grip it, as may be required when inserting or extracting the same. The dies are formed as shown inFig. 315, in whicha,aare the dies, andbthe collet. To open the dies within the collet, the screwseare loosened and the screwsdare tightened, while to close the diesd,dare loosened andeare tightened; thus the adjustment to size is effected by these four screws, while the screwsdalso serve to hold the dies to the colletb. The collets are provided with a collar having a boref, through which the work passes, 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.
Fig. 316Fig. 316.
Fig. 316.
InFig. 316is shown Stetson’s die, which cuts a full thread at one passage, is adjustable to take up its wear, and has a guide to steady it upon the work and assist it in cutting a true thread. The guide piece consists of a hub (through which the work passes) having a flange fitting into the dies and being secured thereto by the two screws shown. The holes in the flanges 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 pass through one die in a plain hole and screw into the other.
Fig. 317Fig. 317.
Fig. 317.
InFig. 317is 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 passage 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.
The hardening process usually increases the thickness of these dies, making the pitch of the thread coarser. 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 quantity can be given.[12]
[12]See alsopage 108.
[12]See alsopage 108.
Fig. 318Fig. 318.
Fig. 318.
Stocks and dies for pipe work are made in the form shown inFig. 318, in whichbis the stock having the detachable handles (for ease of conveyance)a,h, the latter being shown detached. The solid screw-cutting diescare placed in the square recess atb, and are secured inbby the capd, which swings over (upon its pivoted end as a centre) and is locked by the thumbscrewe. To guide the stocks and cause them to cut a true thread, the bushesfare provided. These fit into the lower end ofband are locked in position by four set screwsg. The bores of the bushesfare made an easy fit to the outside of the pipe to be threaded, there being a separate bush for each size of pipe.
Fig. 319Fig. 319.
Fig. 319.
The dies employed in stocks for threading steam and gas pipes by hand are sometimes solid, as inFig. 318atc, and at others adjustable. InFig. 319is 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.
Fig. 320Fig. 321Fig. 322Fig. 320.Fig. 321.Fig. 322.
Fig. 320.Fig. 321.Fig. 322.
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 isthe first to be inserted, and (when the hole to be threaded passes entirely through the work) rotated until it passes through the work, thus cutting a thread parallel in diameter through the full length of the hole. If, however, the hole does not pass 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.
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 passed 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 circumstances 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.
Fig. 323Fig. 323.
Fig. 323.
When the tap is intended to pass 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 inFig. 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.
Fig. 324Fig. 324.
Fig. 324.
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 inFig. 324, in whicha arepresents a washer, andb atap in the same, the back of the teeth receding atc,d,e, from the true circle of the bore ofa 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 passes 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.
Fig. 325Fig. 325.
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 inFig. 325ata,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 pass 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.
Fig. 326Fig. 326.
Fig. 326.
Fig. 327Fig. 327.
Fig. 327.
Fig. 328Fig. 328.
Fig. 328.
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 thetop 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. InFig. 326is a piece of work with a tapa, having a tapered thread, and a tapb, in which the taper is given by turning off the thread. In the case ofathe 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 ofbthe 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 bybthan bya, because a greater length of cutting edge is in operation and more metal is being removed, and as a resultbrequires 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 onbdecreases as the tap enters, because it gets less width and area of groove to cut, while the cutting duty onaincreases 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 ata), the cutting edge of each tooth does not cut a groove sufficiently large in diameter to permit the tooth itself to pass through. InFig. 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 linea b. Now take the tootho p, which stands lengthwise, in the planec d. Its cutting edge is atp, but the diameter of the tap atpis less than it is ato, whileohas to pass through the groove thatpcuts. To obviate this difficulty the tap is given clearance, as shown inFig. 324, the amount being slightly more than the difference in the diameter of the tap atoand atpin 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 inFig. 328, is wrong in principle, and from the unsteady manner in which it operates is undesirable, even though its thread be given clearance.
In some cases the thread is made parallel at the tops and turned taper for a distance of1⁄3or1⁄2the 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.
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 inFig. 328. The threads are made parallel at the roots.
The Pratt and Whitney taper taps for use in machines are of the followingform:—
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.
Fig. 329Fig. 329.
Fig. 329.
Fig. 330Fig. 330.
Fig. 330.
A form of tap that cuts very freely on account of the absence of friction on the sides of the thread is shown inFig. 329. The thread is cut in parallel steps, increasing in size towards the shank, the last step (fromdtoein the figure) being the full size. The end of the tap atabeing the proper size for the tapping hole, and the flutes not being carried througha, 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 (fromatob) is below the diameter ofa, so as to relieve the sides of the thread offriction 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 inFig. 330, in whichnrepresents a nut andtthe tap, both in section. The stepcholds the tap to its work, and it is obvious that, as the toothbenters, 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 class, 27⁄8inches diameter, with a 4 pitch, and 10 inches long, will tap a hole 5 inches deep, passing 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.
Another form of free cutting tap especially applicable to taps of large diameter has been designed by Professor Sweet. Its principles may be explained asfollows:—