Gear Cutting Tools

Coated solid-type hobs with a high
number of gashes are ideally suited
to high-performance hobbing
of straight spur gears. Solid-type
hobs are more stable than any
type of composite hob. The high
number of gashes permits a high
rate of chip removal, and the tool
life is increased substantially by
the coating and, where applicable,
re-coating.
Compared to conventional hobs,
high-performance hobs are required
to have:
■ A higher tool life quality;
■ Shorter machining times;
■ At least equal if not superior
gear quality.
These requirements are interrelated,
such that measures which for
example reduce the machining
time may have a detrimental effect
upon the tool life or the gear
quality.
Hobs can be optimized only in
consideration of the machining environment.
Based upon the geometry
and the material and quality
characteristics of the gear in
question, the hob design and
cutting parameters must be
matched such that the requirements
are broadly fulfilled.
Tip chip thickness
The tip chip thickness is an important
criterion for hob design and
optimization.
The tip chip thickness is the theoretical
maximum chip thickness
which can be removed by the hobs
teeth.
The following hob characteristics
and cutting parameters are taken
into account during calculation of
the tip chip thickness:
■ Module
■ Number of teeth
■ Helix angle
■ Profile displacement
■ Cutter diameter
■ Number of gashes
■ Number of starts
■ Axial feed
■ Cutting depth.
Increased tool life quality
An increase in the number of
gashes is a design measure with a
decisive, positive effect upon the
tool life quality. The increase in the
number of gashes results in the
volume to be machined being distributed
over a greater number of
cutter teeth, and the tip chip thicknesses
being reduced.
Smaller tip chip thicknesses require
smaller cutting forces, which
reduce the stresses placed upon
the cutting edges of the hob and
lead to lower wear. Lower tip chip
thicknesses enable higher tool life
qualities to be achieved.
Assuming that the hob diameter
remains unchanged, however, an
increase in the number of gashes
reduces the number of regrinds
which are possible. If the number
of gashes is selected so that only
one to three regrinds are possible,
the hob is described as an superfine-tooth
cutter.
Hobs with 20 to 30 gashes and a
useful tooth length for approximately
10 regrinds are described
as multi-tooth cutters.
Whether multi-tooth or superfine
tooth hobs are the ideal tools for a
specific gear hobbing task must
be determined by means of a cost
analysis. The cost structure and
capacity exploitation of the user's
installation are also decisive factors.
Developments over recent years
have shown that in the majority of
cases, the multi-tooth cutter is the
most suitable tool.
A cutter with a high number of
gashes also generates a denser
envelope network, i.e. the profile
form of the gear is improved. This
is particularly significant for workpieces
with a small number of
teeth.
δ x [mm] =
2
fa
·
cos β 0
sin α n
4 · d a0
δ x
f a
δ x [mm] = depth of the feed marking
f a [mm/WU] = axial feed
β 0
= helix angle
α n
= pressure angle
d a0 [mm] = tip circle diameter of the hob
d
Depth of the feed markings
20