The V-shaped thread (Fig. 118a) is a thread having its sides at an angle of 60° to each other and perfectly sharp at the top and bottom. This thread is used mostly on screws designed for wood-working and for small brass work. The objections to its use are that the top, being very sharp, is injured by the slightest accident; and that in the use of taps and dies, the fine, sharp edge is quickly lost, causing constant variation in fitting.
The V-shaped thread is the strongest form of screw thread used in the making of bolts. But because the thrust between the screw and nut is not parallel to the axis of the screw, there is a tendency to burst the nut. Therefore this form of thread is unsuitable for transmitting power.
(a) V Thread.
(b) Whitworth Thread.
(c) United States Standard.
Fig. 118. - Standard Screw Threads.
The Whitworth's screw (Fig. 118b) is slightly rounded at the top and bottom. Compared with the American threads, the difference is in the angle between the sides, which is 55°. The French have a standard screw with the thread at an angle of 60°, with a flat top and bottom. Its pitch and diameter are given in millimeters. An international standard for metric screw threads was adopted at Zurich in October, 1898. This thread is based on the United States standard, which is an equilateral triangle truncated (cut) one-eighth of its height at top and bottom.
The United States standard thread (Fig. 118c), often called Seller's thread (from the man who first manufactured it), is also made with its sides at an angle of 60° to each other, but its top is cut off to the extent of one-eighth its pitch, and the same quantity is filled in at its bottom. The advantages claimed for this thread are that it is not easily injured, that the taps and dies retain their size longer, and that bolts and screws made with this thread are stronger and have a better appearance. As this thread has been recommended by the Franklin Institute of Philadelphia, it is sometimes called the "Franklin Institute Standard."
Since this thread is flattened or cut off at the point and root an amount equal to one-eighth of the pitch, it is only three-fourths as deep as a V thread of the same pitch. For example, a 1-in. bolt threaded with a United States standard thread will have a root circle .837 in. in diameter, while a V thread of the same pitch cut on a 1-in. bolt will have a root circle .784 in. in diameter. This shows that the V thread cuts into or "nicks" the bolt deeper, and therefore the bolt is not so strong as when threaded United States standard.