T-74
equivalent to a one-tooth gear; and two-threads equivalent to two-teeth, etc. Referring to
Figure 9-4, for a lead angle g, measured on the pitch cylinder, each rotation of the worm
makes the thread advance one lead.
There are four worm tooth profiles in JIS B 1723, as defined below.
Type I Worm: This worm tooth profile is trapezoid in the radial or axial plane.
Type II Worm: This tooth profile is trapezoid viewed in the normal surface.
Type III Worm: This worm is formed by a cutter in which the tooth profile is trapezoid
form viewed from the radial surface or axial plane set at the lead angle. Examples are
milling and grinding profile cutters.
Type IV Worm: This tooth profile is involute as viewed from the radial surface or at
the lead angle. It is an involute helicoid, and is known by that name.
Type III worm is the most popular. In this type, the normal pressure angle an has the
tendency to become smaller than that of the cutter, ac.
Per JIS, Type III worm uses a radial module mt and cutter pressure angle ac = 20° as
the module and pressure angle. A special worm hob is required to cut a Type III worm gear.
Standard values of radial module, mt, are presented in Table 9-1.
Table 9-1 Radial Module of Cylindrical Worm Gears
Fig. 9-4 Cylindrical Worm (Right-Hand)
1
6.30
1.25
8.00
1.60
10.00
2.00
12.50
2.50
16.00
3.15
20.00
4.00
25.00
5.00
––
pd
d
g
L = pd tang
ax
b
px
pn
pn
an
at
pt
