_Answer._ The form is shown in fig. 201, and is similar to the V-thread, excepting that it is flattened at the top and bottom. QUESTION 342. _What are the reasons for the adoption of this form of thread?_ _Answer._ It has already been pointed out that if a screw is made with a “blunt” thread it will not fit a nut with very acute or “sharp” threads; or, if the thread of the bolt was acute and that in the nut obtuse, they would fit imperfectly. It is therefore necessary in a standard system to fix upon the angle which the sides of the thread shall bear to each other. This in the United States standard system was determined by Mr. Sellers at 60 degrees, because that angle is easily laid off without special instruments[81] and is perhaps as good as or better than any other form for the threads. [81] This can be done by drawing a circle of any diameter, and subdividing the circumference into six equal parts with the radius. Lines, drawn from the points of division to the centre will have an inclination of 60 degrees to each other. It is obvious that if a tool is ground with its sides at an angle of 60 degrees to each other, if the point is made sharp after a very little use it will be worn more or less so that the bottom of the thread will not be cut perfectly sharp, and therefore it will be difficult to make bolts and nuts with sharp threads fit each other accurately. It is also plain that the sharp edge of a thread gives very little strength to the screw, and yet diminishes that of the bolt very materially. It will also be impossible to measure the diameter of the screw at the bottom of the thread if it is made sharp, as its depth will vary as the point of the tool wears, and it is almost impossible to measure the diameter of such a screw accurately with ordinary calipers. A sharp-edged thread in a bolt is also very liable to be injured and bruised by coming in contact with other objects. To obviate these evils the standard threads are therefore made flat on the top, and it is evident that a similar shape at the bottom will give increased strength to the bolt as well as conform to and fit the thread in the nut. To give this form requires only that the point of the cutting tool shall be taken off, and then it is evident this form of thread can be cut in a lathe with the same tool and in the same manner as the sharp thread. The width of the flat top and bottom should of course bear a definite proportion to the size or pitch of the thread. [Illustration: Fig. 202.] QUESTION 343. _What are the proportions of the standard threads?_