When screw-threads are to be cut, the pitch used depends upon the outside diameter of the bar. A standard which has been generally adopted in the United States, is known as the United States Standard. Table III gives the outside diameter of the screw from 1/4 inch to 6 inches in diameter, with the number of threads per inch to be cut.
When setting the tool for any form of thread, the tool point must be exactly level with the work axis, and a line at right angles to the axis of the lathe must bisect the angle of the tool point. In order that these conditions may be fulfilled, a thread or center gage, Fig. 162, is used. In this tool, the angles A, B, and C are made exactly 60 degrees. The two opposite sides are parallel. The angles A, B, and C are used when grinding and setting the tool.
Fig. 161. Internal Threading Tool.
Fig. 162. Thread or Center Gage.
The aides of ike former are made to touch all along the edge of the tool.' For setting the tool, the upper parallel side is held against the face of the work in a horizontal position. The tool is then set so that its sides touch along the edges of the notch B. The angle C may be used to gage the thread after it is cut.
The pitch measurement of fine threads is a difficult matter where an ordinary rule is used and the threads between the inch marks are counted; for this purpose pitch gages, Fig. 163, are very often used. The gages are short screw-sections on thin sheets of metal. To ascertain the pitch of any thread, set the gages over it successively until one is found that exactly fits. The figures stamped thereon will give the number of threads per inch.