ROOT—The surface of the thread corresponding toidentify the necessary dimensions. Figure 4-14 showsthe minor diameter of an external thread and the majorgear nomenclature, and the terms in the figure arediameter of an internal threadexplained in the following list:DEPTH—The distance from the root of a thread tothe crest, measured perpendicularly to the axis.PITCH—The distance from a point on a screwthread to a corresponding point on the next thread,measured parallel to the axis.LEAD—The distance a screw thread advances onone turn, measured parallel to the axis. On asingle-thread screw the lead and the pitch are identical;on a double-thread screw the lead is twice the pitch; ona triple-thread screw the lead is three times the pitchGEARSWhen gears are drawn on machine drawings, thePITCH DIAMETER (PD)—The diameter of thepitch circle (or line), which equals the number of teethon the gear divided by the diametral pitchDIAMETRAL PITCH (DP)—The number of teethto each inch of the pitch diameter or the number of teethon the gear divided by the pitch diameter. Diametralpitch is usually referred to as simply PITCH.NUMBER OF TEETH (N)—The diametral pitchmultiplied by the diameter of the pitch circle (DP x PD).ADDENDUM CIRCLE (AC)—The circle over thetops of the teeth.OUTSIDE DIAMETER (OD)—The diameter ofdraftsman usually draws only enough gear teeth tothe addendum circle.Figure 4-14.—Gear nomenclature.4-5