The width of a die or chaser should never be less than the diameter of bolt it is to thread, so that it may contain as many threads as are contained in a length of bolt equal to the bolt diameter. Now the 11⁄4-inch chaser equals in width the diameter of bolt it is to cut, viz. 11⁄4 inches; but if the chaser for 1⁄4-inch bolts was threaded parallel and left its full width it would be five times as wide as the diameter of the bolt and the thread cut would be imperfect, because the chasers alter their pitches in the hardening process, as was explained with reference to taps, and it is found that the error induced in the hardening varies in amount and sometimes in direction: thus of the four chasers three may expand and become of coarser pitch, each varying in degree from the other two, and the other may remain true, or contract and become of finer pitch.
Fig. 1858.
As a rule the dies expand, but do not so equally. The more teeth there are in the die the more the pitch error from the hardening; or in other words, there is obviously more error in an inch than there is in half an inch of length. Suppose then that we have a die for 20 threads per inch, and as the chaser is 11⁄4 inches wide, it will contain 25 teeth, and the amount of pitch error due to 11⁄4 inches of length; and this amount not being equal in all the chasers, the result is that the dies cut the sides of the thread away, leaving it sharp at the top but widened at the bottom, as shown in [Fig. 1858], weakening it and impairing its durability while placing excessive duty on the dies and on the machine.
Fig. 1859.
