Fig. 331Fig. 331.
Fig. 331.
Fig. 332Fig. 332.
Fig. 332.
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. Assume that it is desired to make three following cutters, to remove the stock down to the dotted line inFig. 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 inFig. 331, the next cutter cutting out the metal ata, 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.
Figs. 333,334and335represent an adjustable tap designed by C. R. French, of Providence, R. I., to thread holes accurate in diameter.
Fig. 333Fig. 333.
Fig. 333.
Fig. 334Fig. 334.
Fig. 334.
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.
Fig. 335Fig. 335.
Fig. 335.
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.
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 pass 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 class, 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 pass 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 toreceive the cuttings, and supposing the tap to be rotated continuously to its duty the flute must possess space 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 pass 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 circumference 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.
Fig. 336Fig. 336.
Fig. 336.
Fig. 337Fig. 337.
Fig. 337.
Fig. 338Fig. 338.
Fig. 338.
Fig. 339Fig. 339.
Fig. 339.
Fig. 340Fig. 340.
Fig. 340.
Fig. 341Fig. 341.
Fig. 341.
Fig. 336represents 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, asa 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 inFig. 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. 338shows 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 atcinFig. 339, the tap will be liable to crack in the hardening process. The form of flute employed in the Whitworth tap is shown inFig. 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 inFig. 325ata,b,c, or as inFig. 341. InFig. 325the 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 inFig. 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.
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 circumferential 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.
Fig. 342Fig. 342.
Fig. 342.
Fig. 343Fig. 343.
Fig. 343.
Fig. 344Fig. 344.
Fig. 344.
Fig. 345Fig. 345.
Fig. 345.
Thus inFig. 342we have a three-flute tap in a hole out of round ata, and it is obvious that when a cutting edge meets the recess ata, all three teeth will cease to cut; hence there will be no inducement for the tap to move over towarda. But in the case of the four-flute tap inFig. 343, when the teeth come toathere will be a strain tending to force the teeth over toward the depressiona. 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 circumferencetending to guide and steady the three-flute than the four-flute tap. If the hole has a projection instead of a depression, as atb,Figs. 344and345, 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 pointscandd, neither of which are directly opposite to the location of the projectionb; hence, if the projection caused the tap to move laterally, say,1⁄100th inch, the effect atcanddwould be very small, whereas in the four-flute,Fig. 345, the effect atewould be equal to the full amount of lateral motion of the tap.
Fig. 346Fig. 346.
Fig. 346.
Fig. 347Fig. 347.
Fig. 347.
Fig. 348Fig. 348.
Fig. 348.
In hand taps the position of the square at the head of the tap with relation to the cutting-edges is of consequence; thus, inFig. 346, there being a cutting-edgeaopposite to the handle, any undue pressure on that end of the handle would causeato cut too freely and the tap to enlarge the hole; whereas inFig. 347this 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 inFig. 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 pass 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.
Fig. 349Fig. 349.
Fig. 349.
Brass finishers frequently form the heads of their taps as inFig. 349, using a wrench with a slot in it that is longer than the flat of the tap head.
The thickness of the flat head atais 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.
Fig. 350Fig. 350.
Fig. 350.
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 inFig. 350is 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.
The standard degree of taper for steam-pipe taps is3⁄4inch 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 of3⁄8inch 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 tofit 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.
Fig. 351Fig. 351.
Fig. 351.
Fig. 351represents the form of tap employed by blacksmiths for rough work, and for the axles of wagon wheels. These taps are given a taper of1⁄2inch 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.
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.
Fig. 352Fig. 352.
Fig. 352.
Fig. 353Fig. 353.
Fig. 353.
Figs. 352and353represent a collapsing tap that is much used in manufactories of pipe fittings.
ais driven by the spindle of the machine, and drivesbthrough the medium of the pinh. Inbare three chasersc, fitting into the dovetail and taper groovesd. These chasers are provided with lugs fitting into an annular grooveesunk ina, so that if the piecehrises, the chasers will not rise with it, but will simply close together by reason of the lifting or rising of the coreb, with its taper dovetail grooves; or, on the other hand, if the corebdescends, the taper grooves inbforce the chasers outward, increasing their cutting diameter.
When the tap is cutting, it is driven as denoted by the arrow, and the pinhis 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 handleshmay be pulled or pushed the working way, passing along the groovesi, and causingbto lift withina, and allowing the chasers to close away from the thread just cut, and the tap may be instantly withdrawn, and handleshpushed back to expand the chasers, ready for the next piece of work.
Fig. 354Fig. 354.
Fig. 354.
Fig. 354represents a collapsing tap used in Boston, Massachusetts, at the Hancock Inspirator Works, in a monitor or turret lathe. It consists of an outer shellacarrying three chasersb, pivoted toaatc, having a small lugeat one end, and being coned at the inner endd. The inner shellfis reduced along part of its length to receive the lugeof the chaser, and permit the chasers to open out full at their cutting end.fhas a cone atthe endg, fitting to the internal cone on the chasers atd. At the other end offis a washerh, against which abuts the spiral spring shown, the other end of this spring abutting against a shoulder provided ina. The washerhis bevelled on its outer or end face to correspond with the bevel on a notch provided in leveri, as is shown. Within the inner tubefis the stemj, into the end of which is fitted the piecek, and on which is fixed the conel. Piecek, and thereforel, is prevented from rotating by a spline ink, into which spline the pinmprojects.
The operation is as follows. In the position in which the parts are shown in theengraving,fis pushed forward so that its coned endghas opened out the chaser to its fullest extent, which opening is governed by contact of the lugewith the reduced diameter off. Suppose that the tap is operating in the work, then, when the footnofkmeets with a resistance (as the end of the hole being tapped),j, and thereforel, will be gradually pushed to the right, until, finally, the cone onlwill raise the end of leveriuntil the notch oniis clear ofh, when the spiral spring, acting againsth, will forcefto the right, and the shoulder onf, atx, will lift the endeof the chaser, causing the cutting end to collapse withina, the pivotcbeing its centre of motion. The whole device may then be withdrawn from the work. To open the chasers out again the rodjis forced, by hand, to the left, the cone-piecelmeeting the face ofhand pushing it to the left until conegmeets coned, when the chasers open until the endemeets the body off, as in the cut. The rodjis then pulled to the right untillagain meets the curved end of leveriand all the parts assume the positions shown in the cut. To regulate the depth of thread the tap shall cut, the bodyais provided with a thread to receive the nuto, by means of which the collarpmay be moved alonga. This collar carries the pivotsqfor leversi, so that, by shiftingo, the position ofiis varied, hence the point at whichlwill act upon the end ofiand lift it to releasehis adjustable.
When used upon steel, wrought iron, cast iron, copper, or brass, 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 least1⁄8inch 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 about1⁄32inch 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 latter1⁄32inch above the proper diameter, hence taps1⁄32inch 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, because1⁄32inch is too small a measurement to be detected by the eye, and a measurement or trial of the bolt and nut becomes necessary.
A defect in taps which it has been found so far impracticable to eliminate is the alteration of pitch which takes place during the hardening process. The direction as well as the amount of this variation is variable even with the most uniform grades of steel, and under the most careful manipulation. Mr. John J. Grant, in reply to a communication upon this subject, informs me that, using Jones and Colver’s (Sheffield) steel, which is very uniform in grade, he finds that of one hundred taps, about 5 per cent. will increase in length, the pitch of the thread becoming coarser; 15 per cent. will suffer no appreciable alteration of pitch, and 80 per cent. will shrink in length, the pitch becoming finer, and these last not alike. But it must be borne in mind that with different steel the results will be different, and the greater the variation in the grade of the steel the greater the difference in the alteration of pitch due to hardening.
It is further to be observed that the expansion or contraction of the steel is not constant throughout the same tap; thus the pitches of three or four consecutive teeth may measure correct to pitch, while the next three or four may be of too coarse or too fine a pitch.
There is no general rule, even using the same grade of steel, for the direction in which the size of a tap may alter in hardening, as is attested by the following answers made by Mr. J. J. Grant to the respectivequestions:—
“Do the taps that shorten most in length increase the most in diameter?”
Answer.—“Not always; sometimes a tap that shortens by hardening becomes also smaller in diameter, while sometimes a tap will increase in length, and also in diameter from hardening.”
“Do taps that remain of true pitch after hardening remain true, or increase or diminish in diameter?”
Answer.—“They will generally be of larger diameter.”
“Do small taps alter more in diameter from hardening than large ones?”
Answer.—“No; the proportion is about the same, and is about .002 per inch of diameter.”
“What increase in diameter do you allow for shrinkage in hardening of hob taps for tapping solid dies?”
Answer.—“As follows:—
“Suppose a tap that had been hardened and tempered to a straw color contained an error1⁄1000inch both in diameter and in pitch, was softened again, would it when soft retain the errors, or in what way would softening affect the tap?”
Answer.—“We have repeatedly tried annealing or softening taps that were of long or short pitch caused by tempering, and invariably found them about the same as before the annealing. The second tempering will generally shorten them more than the first. Sometimes, however, a second tempering will bring a long pitch nearer correct.”
“Do you soften your taps after roughing them out in the lathe?”
Answer.—“Never, if we can possibly avoid it. Sometimes it is necessary because of improper annealing at first. The more times steel is annealed the worse the results obtained in making the tool, and the less durable the tool.”
The following are answers to similar questions addressed to the Morse Twist Drill and MachineCo.:—
“The expansion of taps during hardening varies with the diameter. A 1-inch tap would expand in diameter from1⁄1000to3⁄1000inch.”
“Taps above1⁄2inch diameter expand in diameter to stop the gauge every time.”
“The great majority of taps contract in pitch during the hardening, they seldom expand in length.”
“The shortening of the pitch and the expansion in diameter have not much connection necessarily, though steel that did not alter in one direction would be more likely to remain correct in the other.”
“There does not seem to be any change in the diameter or pitchof taps if measured after hardening (and before tempering) and again after tempering them.”
“Taps once out in length seem to get worse at every heating, whether to anneal or to harden.”
It will now be obvious to the reader that the diameter of a tap, to give a standard sized bolt a required tightness of fit, will, as a general rule, require to vary according to the depth of hole to be tapped, because the greater that depth the greater the error in the pitch. Suppose a tap, for example, to get of finer pitch to the amount of .002 per inch of length, then a hole an inch deep and tapped with that tap would err .002 in its depth, while a hole two inches deep would err twice as much in its depth.
Therefore a bolt that would be a hand fit (that is, screw in under hand pressure) in the hole an inch deep would require more force, and probably the use of a wrench, to wind it through the hole 2 inches deep; hence in cases where a definite degree of fit is essential, the reduction in diameter of the male screw or thread necessary to compensate for the error in the tap pitch must vary according to the depth of the hole, and the degree of error in the tap.
It is obvious that the longer a tap is the greater the error induced by hardening, and it often becomes a consideration how to tap a long hole, and obtain a thread true to pitch. This may be accomplished as follows. Several taps are made of slightly different diameters, the largest being of the required finished size. Each tap is made taper for a distance of two or three threads only, and is hardened at this tapered end, but left soft for the remainder of its length. The smallest tap is used first, and when it has tapped a certain distance, a larger one is inserted, and by continuing this interchange of taps and slightly varying the length of the taper, the work may be satisfactorily done.
Fig. 355Fig. 355.
Fig. 355.
To test the accuracy, or rather the uniformity, of a thread that has been hardened, a sheet metal gauge, such as atgor atg′(Fig. 355), may be used, there being ataandbteeth to fit the threads. If the edge of the gauge meets the tops of the threads, then their depth is correct. If it is desired to test only the pitch, then the gauge may be made as atg′, where, as is shown in thefigure, the edge of the gauge clears the tops of the threads, and in this way may be tried at various points along the thread length.
A method of truing hardened threads proposed by the author of this work in 1877, and since employed by the Pratt and Whitney Company to true their hardened steel plug-thread gauges, is as follows:—A soft steel wheel about 31⁄2inches in diameter, whose circumference is turned off to the shape of the thread, is mounted upon the slide rest of a lathe, and driven by a separate belt after the manner of driving emery wheels; this wheel is charged with diamond dust, which is pressed into its surface by a roller, hence it grinds the thread true.
The amount allowed for grinding is3⁄1000inch measured in the angles of the thread, as was shown inFigs. 280and281.
Fig. 356Fig. 356.
Fig. 356.
In charging the wheel with diamond dust it is necessary to use a roller shaped as inFig. 356, so that the axis of the rollerrand wheelwshall be at a right angle, as denoted by the dotted lines. If the roller is not made to the correct cone its action will be partly a rolling and partly a sliding one, and it will strip the diamond dust from the wheel rather than force it in, the reasons for this being shown inFigs. 57and58upon the subject of bevel-wheels.
Fig. 357Fig. 357.
Fig. 357.
Taps for lead and similar soft metal are sometimes made with three flat sides instead of grooves. The tapping holes may in this case be made of larger diameter than the diameter of the end of the tap thread, because the metal in the hole will compress into the tap thread, and so form a full thread. Taps for other metal have also been made of half-round section.Fig. 357represents a tap of oval cross section, having two flutes, as shown, but it may be observed that neither half-round nor oval taps possess any points of advantage over the ordinary forms of three or four fluted taps, while the former are more troublesome and costly to manufacture.
When it is required to tap a hole very straight and true, it is sometimes the practice to provide a parallel stem to the tap, asshown infigureatc. This stem is made a neat working fit to the tapping hole, so that the latter serves as a guide to the tap, causing it to enter and to operate truly.
Fig. 358Fig. 358.
Fig. 358.
Tap Wrench.—Wrenches for rotating a tap are divided into two principal classes, single and double wrenches. The former has the hole which receives the squared end of the tap in the middle of its length, as shown inFig. 358ate, there being a handle on each side to turn it by.