To obviate these difficulties, an excellent form of chaser holder is shown in [Figs. 1010] and [1011]. Its top face c being made of such a height that when the holder rests on the surface of the slide rest and is in the tool box, c will stand horizontally level with the horizontal centre of the work, as denoted by the horizontal line d e; then the tool proper may have long teeth as denoted by a, and the surface of the teeth may always be brought up level with the top surface of the tool holder as tested with a straight-edge. This is a ready and accurate mode of adjustment. A top view of the tool holder is shown in [Fig. 1011], in which a is the tool holder, b the threading tool, with a clamp to hold b, and a screw to tighten the clamp.
Fig. 1012.
It may now be pointed out that a common sharp V-chaser may be used to cut a United States standard thread by simply grinding off the necessary flats at the points of the teeth, because when the chaser has entered the work to the proper depth it will leave the necessary flat places at the top of the thread, as is shown in [Fig. 1012].
In cutting internal, inside, or female threads (these terms being synonymous) the diameter of the bore or hole requires to be made of the diameter of the male thread at the root.
Since, however, it is impracticable to measure male threads at the root, it becomes a problem as to the proper size of hole to bore for any given diameter and pitch of thread. This, however, may be done by the following rules:—
To find the diameter at the roots or bottom of the thread of United States standard threads:
Rule.—Diameter - (1.299 ÷ pitch) = diameter at root.
Example.—What is the diameter at the root of a United States standard thread measuring an inch in diameter at the top of the thread and having an 8 pitch?