Into the interior walls of this triangle there is fitted a cylindrical bush B, it being obvious that this bush is held axially true or central to the triangle, and it is secured in place by screws _y_, _y_, _y_, passing through its flange and into bars A.
At one end of the bush B, is a cylindrical part D, whose diameter is 2 inches or equal to the length of one side of an equilateral triangle circumscribed about a circle whose diameter is 1.1547 inches, as shown in Fig. 278 and through this bush B passes a pin P, having a nut N. A small triangle is then roughed out, and its bore fitting to the stem of pin P, and by means of nut N, the small triangle is gripped between the under face of D and the head of P. The large triangle is then held to an angle-plate upon a machine while resting upon the machine-table, and the uppermost edge of the small triangle is dressed down level with the cylindrical stem D, which thus serves as a gauge to determine how much to take off each edge of the small triangle to bring it to correct dimensions.
The truth of the angles of the small triangle depends, of course, also upon the large one; thus with face H resting upon the machine-table, face G is cut down level with stem D; with face F upon the table, face E is cut down level with D; and with face L upon the table, face K is dressed down level with D. And we have a true equilateral triangle produced by a very ingenious system of chuckings, each of which may be known to be true.
The next operation is to cut upon the small triangle the flat representing the top and bottom of the United States standard thread, which is done by cutting off one-eighth part of its vertical height, and it then becomes a test piece or standard gauge of the form of thread. The next step is to provide a micrometer by means of which tools for various pitches may be tested both for angle and for width of flat, and this is accomplished as follows:--
[Illustration: _VOL. I_ =MEASURING AND GAUGING SCREW THREADS.= _PLATE III._
In Fig. 278 F is a jaw fixed by a set screw to the bar of the micrometer, and E is a sliding jaw; these two jaws being fitted to the edges of the triangle or test piece T in the figure which has been made as already described. To the sliding jaw E is attached the micrometer screw C, which has a pitch of 40 threads per inch; the drum A upon the screw has its circumference divided into 250 equidistant divisions, hence if the drum be moved through a space equal to one of these divisions the sliding jaw E will be moved the 1-250th part of 1-40th of an inch, or in other words the 1-10,000th of an inch. To properly adjust the position of the zero piece or pointer, the test piece T is placed in the position shown in Fig. 278, and when the jaws were so adjusted that light was excluded from the three edges of the test piece, the pointer R, Fig. 277, was set opposite to the zero mark on the drum and fastened.