Gear Cutting Tools

Shorter machining times
The machining time (production
time) for the hobbing process is
determined on the one hand by
the gear width and number of
teeth and on the other by the
cutting speed, hob diameter,
number of starts, and axial feed.
The number of starts should always
be increased when the feed
is limited by the depth of the feed
markings before the maximum tip
chip thickness is reached.
The depth of the feed markings is
dependent upon whether the gear
is to be finish-hobbed or subsequently
shaved or ground.
The gear width and the number
of teeth are fixed geometric
values. The cutting speed is
largely dependent upon the gear
material, and its tensile strength
and machineability.
The machining time changes as a
function of the hob diameter,
however. With a small hob diameter
and with the cutting speed
unchanged, the hob spindle and
table speeds increase, and the
machining time is reduced. At the
same time, a reduction in hob diameter
results in a reduction in
the machining distance for axial
machining.
When selecting the hob diameter,
note that the number of gashes is
limited by this dimension, and
that a high number of gashes is
required for good tool life qualities
and lower cutting forces.
The cutter diameter should therefore
only be sufficiently small to
enable a specified cycle time to
be achieved. An unnecessarily
small cutter diameter impairs the
tool life and gear quality.
High axial feeds and multi-start
hobs reduce the machining time
considerably. However, they also
lead to higher tip chip thicknesses,
the increase in which is
influenced more strongly by the
number of starts than by the increased
axial feed.
A relatively high feed should be
selected, and the number of
starts kept as low as possible.
This combination produces the
lowest tip chip thickness. The
two variables are of equal importance
for calculation of the machining
time, i.e. the machining
time is determined by the product
of the feed and the number of
starts.
t h = z 2 · d a0 · π · (E + b + A)
z 0 · f a · v c · 1000
Machining time (production time) for hobbing
t h [min] = machining time
v c [m/min] = cutting speed
z 2
= number of teeth of the
gear to be machined
d a0 [mm] = tip circle diameter
E
b
A
z 0
[mm]
[mm]
[mm]
of the hob
= approach length
of the hob
= tooth width of the gear
to be machined
= idle travel distance of the hob
= number of starts of the hob
f a [mm/WU] = axial feed
δ y [mm] =
z 0
m n
α n
z 2
i =
d
δ y [mm] = π2 · z 2 0 · m n · sinα n
4 · z 2 · i 2
δy
=
=
=
=
envelop cut
deviation
number of starts
of the hob
normal module
profile angle
number of teeth
on the gear
number of gashes
of the hob
Envelop cut deviations
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