CIRCULAR  PITCH  (CP)—The  length  of  the  arc  of the pitch circle between the centers or corresponding points  of  adjacent  teeth. ADDENDUM (A)—The height of the tooth above the pitch circle or the radial distance between the pitch circle and the top of the tooth. DEDENDUM (D)—The length of the portion of the tooth from the pitch circle to the base of the tooth. CHORDAL  PITCH—The  distance  from  center  to center of teeth measured along a straight line or chord of the pitch circle. ROOT DIAMETER (RD)—The diameter of the circle at the root of the teeth. CLEARANCE (C)—The distance between the bottom of a tooth and the top of a mating tooth. WHOLE DEPTH (WD)—The distance from the top of the tooth to the bottom, including the clearance. FACE—The working surface of the tooth over the pitch line. THICKNESS—The width of the tooth, taken as a chord of the pitch circle. PITCH CIRCLE—The circle having the pitch diameter. WORKING DEPTH—The greatest depth to which a tooth of one gear extends into the tooth space of another  gear. RACK TEETH—A rack may be compared to a spur gear that has been straightened out. The linear pitch of the rack teeth must equal the circular pitch of the mating gear. HELICAL  SPRINGS There are three classifications of helical springs: compression, extension, and torsion. Drawings seldom show a true presentation of the helical shape; instead, they usually show springs with straight lines. Figure 4-15 shows several methods of spring representation including  both  helical  and  straight-line  drawings.  Also, springs  are  sometimes  shown  as  single-line  drawings  as in figure 4-16. FINISH MARKS The military standards for finish marks are set forth in ANSI 46.1-1962. Many metal surfaces must be finished with machine tools for various reasons. The acceptable roughness of a surface depends upon how the Figure 4-15.—Representation of commm types of helical springs. Figure 4-16.—Single line representation of springs part will be used.  Sometimes only certain surfaces of a part need to be finished while others are not. A modified symbol (check mark) with a number or numbers above it is used to show these surfaces and to specify the degree of  finish.  The  proportions  of  the  surface  roughness symbol are shown in figure 4-17. On small drawings the  symbol  is  proportionately  smaller. The number in the angle of the check mark, in this case 02, tells the machinist what degree of finish the surface  should  have.  This  number  is  the root-mean-square  value  of  the  surface  roughness  height in  millionths  of  an  inch.  In  other  words,  it  is  a measurement  of  the  depth  of  the  scratches  made  by  the machining  or  abrading  process. Wherever possible, the surface roughness symbol is drawn  touching  the  line  representing  the  surface Figure 4-17.—Proportions for a basic finish symbol. to 4-6