Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

394 Molesworth’s pocket-book Teeth of Wheels—continued. (Method communicated by Mr. Aubrey Ohren.) From the radial line at the edge of the tooth on the pitch line lay off the line H K at an angle of 75° with the radial line; on this line will be the centres of the root A B and the point EF. The lines struck from these centres are shown in thick, lines. Circles drawn through centres thus found will give the lines in which the remaining centres will be. The radius I) A for striking the root AB is = pitch + the thickness of the tooth. The radius C E for striking the point of the tooth hf = the pitch. TO DESCRIBE THE ANGLE OF 75° WITHOUT A PROTRACTOR. Describe a quarter of a circle M N with any radius L M, and from N with a radius = LM, strike a portion of a circle intersecting the circle MN at P; then from the points P and M as centres with any equal radii strike out two portions of circles intersecting each other at Q; the line joining LQ will be at an angle of 75° with L N.