Figure 14-25.--Milling machine set up for gashing and hobbing of a worm wheel.
blueprints of both the worm and the worm wheel and
The cutter you select to gash the hob should be 1/8
any information such as bore size for the shell-type hob.
to 1/4 inch thick at the periphery, depending on the pitch
of the hob thread. The width of the gash at the periphery
of the hob should be about 0.4 times the pitch of the
CUTTING WORM WHEEL TEETH
flutes. The depth of the gash should be about 3/16 to 1/4
ON A MILLING MACHINE
inch below the root of the thread.
You can cut the teeth of a worm gear on a milling
There are three types of hobs:
machine, usually in two operations. The first is called
Shell. This has a straight bore with a keyway to
hold the arbor that drives the hob.
spur-gear cutter of the correct pitch and number
Straight shank This is the integral part of the
according to the number of teeth and pitch of the worm
shaft. It is used between centers.
gear. Set the milling machine table at an angle equal to
the lead or helix angle of the worm thread. Be sure to
Tapered shank. This has a milling machine taper
center the gear blank under the cutter. To perform the
(Brown and Sharpe). The outside end is supported by
operation, raise the table a distance equal to the whole
either a line or a dead center.
depth of the tooth. Use the graduated vertical feed dial
Since the hob is a cutting tool, the top of the tooth
to get a uniform depth for each tooth. Index each tooth,
on the hob is a dedendum. The bottom of the hob tooth
using the dividing head with a dog clamped to the
forms the top of the worm wheel tooth and must be
mandrel to drive the gear blank.
given a clearance. Therefore, it also equals a dedendum
of the gear you are cutting. The working depth of the
view B). First, mount the hob on a cutter arbor. Then set
hob (cutting portion) is the addendum plus the
the table back to zero, or at a right angle to the machine
dedendum of the gear you are cutting.
spindle. Remove the dog so the gear blank can rotate
The nomenclature of the hob is the same as that of
freely. Line up the gear blank so the hob meshes with
the worm; chordal thickness (normal), linear pitch, lead,
pressure angle, and pitch diameter. The outside
the gashed slots. When you start the machine, the
diameter of the hob has two clearances, and both are
rotating hob will rotate the gear blank. As the hob and
larger than the worm's outside diameter:
gear blank rotate, raise the table gradually until the teeth
are cut to the correct depth. To get the correct
WPD + 2 DED
center-to-center distance, use the worm that you will use
with the worm gear before you remove the worm gear
Hobs can be bought commercially, or made by a
from the milling machine.
machinist. If you order a hob, furnish drawings or