76 teeth × 31⁄2 inches, pitch = 266 inches in circum. = 7 ft. 011⁄16 in. diam. = 3 ft. 611⁄32 in. radius; with this measurement as represented on scale, draw line P P on drawing. This is called the pitch line.
Draw next diameter line, produce or extend this diameter line for pinion, and with radius of 1019⁄32 (19 teeth × 31⁄2) from pitch line of wheel, draw pitch line of pinion.
Take any point in this pitch line of wheel, mark off 31⁄2 inches as represented on scale, mark this around the pitch line, it will be the center of each of the 76 teeth; then the breadth of thickness of each tooth (= pitch × 0.475) must be marked from these centers, then mark from P L, length of tooth to point (= pitch × 0.35) and P L to root (= pitch × 0.4), draw circles for outside of teeth N and root of tooth O; now with compass set to the pitch (31⁄2) of the wheel, draw the outer portion from pitch line of tooth.
The radius will center in the pitch line of next tooth where thickness of tooth has been marked; after finishing outer portion of both sides of teeth, set the compass from center of tooth with radius to the thickness marked on pitch line and draw the portion of tooth from pitch line to root.
Now mark off with dividers and draw thickness of rim (= pitch × 0.5), divide this line into six parts, draw radii for centers of arms; draw the bore hole 7″ and the thickness of metal for hub same as pitch.
On radii lines of arms, draw the breadth of arm at rim (= pitch and thickness of tooth), increase in breadth approaching the center (1″ per foot), draw the thickness of feather of arm (= pitch × 0.35); draw web on inside of rim (= pitch × 0.375); fill in arcs for the joining of arms in rim and hub (radii = pitch × 0.8) and feather to rim and hub (radii = pitch × 0.37).
Proceed in similar manner, completing the teeth of pinion, and when pencil lines are all in, ink the drawing, erasing all needless lines.
P P shows the pitch line; B, thickness of tooth; c, breadth of space; A, the pitch; E, clearance at root; N, the addendum of tooth; O, the root of tooth; H, length of tooth from pitch line to point; I, length of tooth pitch line to root; G, whole length of tooth; F, thickness of rim; J, web or feather on rim; K, breadth of arm; L, thickness of feather; M, hub, or thickness round the eye.
Note.—It must be remembered that no fixed standard has ever been agreed upon for these proportions, and workshops differ considerably in practice.
