Steel sufficiently large should be selected, the decarbonized portion removed, and the shank turned to size. . The square should be milled for a wrench and the size and number of threads per inch stamped on the shank. The cutting end of the tap is turned to size, the necessary amount of taper given the tap, and then the threads are cut.
The tool used for cutting square threads is similar in form to a cutting-off (parting) tool, except for its angle side rake. It should be made of the proper thickness at the point, but should be somewhat narrower back of the cutting end, Fig. 119, in order that it may clear when cutting.
Fig. 119. Gage Thread Tool.
The thickness of the cutting end should be one-half the distance from the edge of one thread to the corresponding edge of the next thread. For a square thread of 1/2-inch pitch, the land and space together would be 1/2 inch, while the land and space would each be i inch wide. The point of the tool should be 1/4 inch thick.
The sides of the tool from AtoB, Fig. 120, must be inclined to the body as shown, the amount of the inclination depending upon the pitch of the thread and the diameter of the tap to be cut. This may be determined by the method shown in Fig. 121. Draw the line AB and at right angles to it draw CD, whose length must be equal to the circumference of the thread to be cut, measured at the bottom or root of the thread. On AB lay off from the point C a distance EC equal to the pitch of the thread to be cut, and draw the line DE. The angle CDE will represent the angle of the side of the thread; the angle of the side of the cutting tool must be sufficiently greater to give the necessary clearance. It is advisable to cut the thread first with a tool somewhat narrower than the required width, and to finish with a tool of the proper thickness.
Square-thread taps may be fluted according to directions given for V-thread taps. If a tap is intended to cut a full thread, it must be well backed off, in order to avoid the necessity of using so much force that the tap would be broken. When a tap is to be used to size a hole whose thread has been cut by a smaller tap, very little clearance is necessary. Left-Hand Thread. Taps are made with left-hand thread for tools requiring such thread. Many times fixture jaws are made in pairs, that is, two jaws are made to hold the work, and are opened and closed by turning a screw which passes through a threaded portion in each. One jaw has a right-hand thread tapped in it while the other has a left-hand thread. The screw is made with a righthand thread on one portion and a left-hand thread on the other. If the pitch is the same on both threads the jaws will open and close uniformly and will accurately center pieces of various sizes.
Fig. 120. End View of Gage Thread Tool.
Fig. 121. Diagram Showing Angle of Thread Tool.
It is necessary, of course, to back off the cutting lips of a left-hand threaded tap on the opposite side of the end from that backed off on one that is right-hand threaded.
Left-hand threaded taps are stamped with an L to prevent confusion, for while it is possible to detect the difference in the way the threading runs, in the case of coarse pitches, yet without a distinguishing mark the workman would often waste valuable time trying to use a left-hand tap for a right-hand tap.