Page:Miscellaneous Papers on Mechanical Subjects.djvu/41

Rh On the other hand, finer threads on large bolts besides being weaker and less durable, would render it difficult to unfix them when occasion required.

It will be remembered that the threads, of which the preceding table shows the average, are used in cast iron as well as wrought; and this circumstance has had its effect in rendering them coarser than they would have been, if restricted to wrought iron.

The variation in depth among the different specimens was found to be greater proportionally than in pitch. The angle made by the sides of the thread will afford a convenient expression for the depth. The mean of the variations of this angle in 1-inch screws was found to be about 55°, and this was also pretty nearly the mean of the angle in screws of different diameters. As it is for various reasons desirable that the angle should be constant, more especially with reference to general uniformity of system, the angle of 55° has been adopted throughout the entire scale. A constant proportion is thus established between the depth and the pitch of the thread. In calculating the former, a deduction is to be made for the quantity rounded off, amounting to one-third of the whole depth—that is, one-sixth from the top, and one-sixth from the bottom of the thread. Making this Rh