Fig. 327.
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 the top 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. In [Fig. 326] is a piece of work with a tap a, having a tapered thread, and a tap b, in which the taper is given by turning off the thread. In the case of a the 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 of b the 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 by b than by a, because a greater length of cutting edge is in operation and more metal is being removed, and as a result b requires 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 on b decreases as the tap enters, because it gets less width and area of groove to cut, while the cutting duty on a increases 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 at a), the cutting edge of each tooth does not cut a groove sufficiently large in diameter to permit the tooth itself to pass through. In [Fig. 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 line a b. Now take the tooth o p, which stands lengthwise, in the plane c d. Its cutting edge is at p, but the diameter of the tap at p is less than it is at o, while o has to pass through the groove that p cuts. To obviate this difficulty the tap is given clearance, as shown in [Fig. 324], the amount being slightly more than the difference in the diameter of the tap at o and at p in 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 in [Fig. 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 of 1⁄3 or 1⁄2 the 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 in [Fig. 328]. The threads are made parallel at the roots.