Fig. 354.
[Fig. 354] represents a collapsing tap used in Boston, Massachusetts, at the Hancock Inspirator Works, in a monitor or turret lathe. It consists of an outer shell a carrying three chasers b, pivoted to a at c, having a small lug e at one end, and being coned at the inner end d. The inner shell f is reduced along part of its length to receive the lug e of the chaser, and permit the chasers to open out full at their cutting end. f has a cone at the end g, fitting to the internal cone on the chasers at d. At the other end of f is a washer h, against which abuts the spiral spring shown, the other end of this spring abutting against a shoulder provided in a. The washer h is bevelled on its outer or end face to correspond with the bevel on a notch provided in lever i, as is shown. Within the inner tube f is the stem j, into the end of which is fitted the piece k, and on which is fixed the cone l. Piece k, and therefore l, is prevented from rotating by a spline in k, into which spline the pin m projects.
The operation is as follows. In the position in which the parts are shown in the [engraving], f is pushed forward so that its coned end g has opened out the chaser to its fullest extent, which opening is governed by contact of the lug e with the reduced diameter of f. Suppose that the tap is operating in the work, then, when the foot n of k meets with a resistance (as the end of the hole being tapped), j, and therefore l, will be gradually pushed to the right, until, finally, the cone on l will raise the end of lever i until the notch on i is clear of h, when the spiral spring, acting against h, will force f to the right, and the shoulder on f, at x, will lift the end e of the chaser, causing the cutting end to collapse within a, the pivot c being its centre of motion. The whole device may then be withdrawn from the work. To open the chasers out again the rod j is forced, by hand, to the left, the cone-piece l meeting the face of h and pushing it to the left until cone g meets cone d, when the chasers open until the end e meets the body of f, as in the cut. The rod j is then pulled to the right until l again meets the curved end of lever i and all the parts assume the positions shown in the cut. To regulate the depth of thread the tap shall cut, the body a is provided with a thread to receive the nut o, by means of which the collar p may be moved along a. This collar carries the pivots q for levers i, so that, by shifting o, the position of i is varied, hence the point at which l will act upon the end of i and lift it to release h is adjustable.
When used upon steel, wrought iron, cast iron, copper, or 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 least 1⁄8 inch than the diameter of the bolt or stud, three-quarters of a full thread is all that is necessary, and can be tapped with much less labor than would be the case if the hole were small enough to admit of a full thread, partly because of the diminished duty performed by the tap, and partly because the oil (which should always be freely supplied to a tap) obtains so much more free access to the cutting edges of the tap. If a long tap is employed to cut a three-quarter full thread, it may be wound continuously down the hole, without requiring to be turned backwards at every revolution or so of the tap, to free it from the tap cuttings or shavings, as would be necessary in case a full thread were being cut. The saving of time in consequence of this advantage is equal to at least 50 per cent. in favor of the three-quarter full thread.
As round bar iron is usually rolled about 1⁄32 inch larger than its designated diameter, a practice has arisen to cut the threads upon the rough iron just sufficiently to produce a full thread, leaving the latter 1⁄32 inch above the proper diameter, hence taps 1⁄32 inch above size are required to thread nuts to fit the bolts. This practice should be discountenanced as destroying in a great measure the interchangeability of bolts and nuts, because 1⁄32 inch is too small a measurement to be detected by the eye, and a measurement or trial of the bolt and nut becomes necessary.
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 respective questions:—