Die hobs are finish taps for sizing the thread in screw cutting dies. The several flutes are narrower than those of an ordinary tap, and the lands are correspondingly wider. The tap shown in Fig. 110 has eight flutes. The increased number and broader lands support the tap while running through dies whose clearance holes are drilled, in order to remove burrs thrown in the threads when drilling. It is customary to give screw die hobs from six to ten flutes.
When hobs are used for solid dies, they must be of exact size. When intended for tapping adjustable dies, such as are ordinarily used for cutting threads in screw machine work, the hobs are made from .003 to .005 inch above the size of the screw to be cut. The extra size gives relief to the threads of the die.
While it is generally considered advisable to run one or more taps through a die before the hob, some tool-makers consider it better to make a hob that will do all the cutting, claiming that no two taps can be made and hardened so that the pitch will be exactly the same. In such cases a hob is made that will cut a full thread by passing through the die, Fig. 111.
Fig. 110. Screw Die Hob.
Courtesy of Wiley and Russell Manufacturing Company, Greenfield, Massachusetts.
Some manufacturers cut the thread tapering for about three-quarters of its entire length, leaving the balance straight for use in sizing the die. Others cut the thread straight and taper the outside for three-quarters of its length. If the threads are cut tapering, they must be relieved back of the cutting edges.
When hardening large hobs, those, say, 3 inches in diameter and larger, it is a good plan to fill the threads with the mixture of charred leather, flour, and salt, used for hardening twist drills. After this dries, the taps may be heated and hardened. Best results follow if they are hardened in a bath of lukewarm brine.
Fig. 111. Hob for Cutting Full Threads.
Courtesy of S. W. Card Manufacturing Company, Mansfield, Massachusetts.