8. Rim diameter

b. Use the following formulas to solve for all

normal tooth dimensions:

a. To find the rim diameter for single- and

double-start worms, multiply the linear

pitch by the constant 0.4775 and add the

result of the throat diameter.

b. To find the rim diameter for three or more

starts, multiply the linear pitch by the

constant 0.3183 and add the result to the

throat diameter.

9. Throat radius

a. To find this radius, subtract one addendum

(normal) from the pitch radius of the worm.

a worm with a l-inch linear pitch.

b. This dimension is taken from the worm but

5. Length of the worm (LOW)

is machined on the worm wheel blank.

a. It is found by using the following formula:

10. Blank width

a. To find the blank width for single- and

b. The worm is longer than is required for

double-start worms, multiply the linear

complete meshing between the worm and

pitch by the constant 2.38 and add the result

the worm wheel.

to the constant 0.250.

6. Worm wheel pitch diameter (WWPD)

b. To find the blank width for three or more

a. You learned in spur gearing that for every

starts, multiply the linear pitch by the

tooth in the gear there is a circular pitch on

constant 2.15 and add the result to the

the pitch circle, and for every tooth on the

constant 0.20.

gear there is an addendum on the pitch

diameter.

b. By using this theory, we can derive the

11. Tooth dimensions. These are the same as those

following formulas:

of the worm. The linear pitch and the circular pitch are

(1) WWPC D (real)

of equal value.

12. Number of teeth (NT). Multiply the number of

starts by the ratio of the worm to the worm wheel.

= *LP *× 0.3183 × *NT*

(2) *ADD (real) *= *LP *× 0.3183

7. Throat diameter

a. It is found by adding the worm wheel pitch

When you machine the throat radius of a worm

diameter and twice the addendum normal.

wheel, select a two- or four-lip end mill with a radius

smaller than the calculated throat radius. You need the

smaller radius because as you swivel the cutter from its

b. It is measured at the base of the throat

vertical position to a desired angle, the radius being cut

increases.

radius.

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