The basic profile is the section of a tooth of the basic rack, i.e. with rectilinear flanks, and which determines the shape of the generating tool.
The involute gear is obtained by rolling with tools of which the tooth profile reproduces a basic rack (so with rectilinear flanks) (see table BASIC PROFILES pag. 36).
The angle formed by a tooth flank and the centre line of a tooth is called “pressure angle”. This pressure angle is normalized; it is, after all, one of the main features of a gear. Following values are subject to standardization:
a = 14°30 ‘ (Belgium and Anglo-Saxon countries)
a = 15° (Germany)
a = 20° (all countries)
a = 25° (USA -current standardization)
The “size” of a gear is determined by the “pitch“: distance between the centre lines of two successive teeth; likewise by the ratio of “pitch – height”.