Fig. 333.
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. 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 to receive 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.