Gear Cutting Tools

Cutting conditions
Cutting speed
The cutting speed depends on the
module size and on the hardness
of the gear. As an approximate value,
a cutting speed of 36 m/min
can be quoted for module 30 and
of 110 m/min for module 2.
For the lower modules, higher values
between 140 and 160 m/min
are also possible. These high
cutting speeds do however reduce
the service life of the skiving hob
and the workpiece structure is increasingly
affected.
For workpiece hardness values
from HRC 62 upwards, the cutting
speed should be limited initially to
70 m/min and then optimized in
consideration of the cutting result
and the service life of the tool.
Feed
The structure of surfaces machined
with hobs is affected by the
depth of the feed markings. The
depth of the feed mark increases
quadratically with the value of the
feed. It is therefore logical to distinguish
between feeds for finishing
and for roughing.
Approximate value for the feed:
For the finishing cut
1.5 to 2 mm/workpiece rotation
for the roughing cut
up to 4 mm/workpiece rotation
Climp hobbing method
Climb hobbing for skive hobbing is
preferred since this yields the best
service life of the skiving hobs.
Removal per flank
To maintain a reasonable service
life of the hobs, not more than
0.15 ÷ 0.20 mm/flank should be
removed in one cut.
For high quality requirements,
hobbing must always be done in
several cuts. For the last cut, a removal
of 0.1 mm/flank should be
aimed at, to affect the structure of
the gear material as little as possible.
Cooling
Intensive cooling of the tool, workpiece,
holding fixture and machine
with the cutting oils usual for hobbing,
the temperature-dependent
error values are reduced and the
service life of the skiving hobs is
extended.
Wear and tool life
values
Wear mark width
The wear mark width on the skiving
hobs should not exceed 0.15
mm.
Cutting forces increase with greater
wear mark width and with very
thin chips deflection of the hob
cutting edges will occur.
This may have the following consequences:
quality losses, chipping of carbide
cutting edges and excessive
structural changes through tempering
and re-hardening processes
on the gears.
Uniform wear through
shifting
Wear only occurs on the tooth
flanks of the skiving hobs. The
wear marks are relatively short
and follow the contour of the engagement
lines.
By shifting the hob in the axial direction
after hobbing a gear or set
of gears, the wear is distributed
evenly over the flank cutting edges
and over the entire cutting edge
length of the hob. This process is
further facilitated if the hobbing
machine is equipped with a synchronous
shifting arrangement.
This arrangement ensures that the
machine table makes an additional
turn when the tangential slide is
moved. The relative position of the
hob motion then remains as set
during centering.
Tool life between regrinds
The life between regrinds of a hob
equals the sum of the lengths of all
hobbed workpiece teeth between
two regrinds of the hob.
The calculation of the life between
regrinds, the tool requirement, the
proportional tool costs etc. is
based on the life between regrinds
per cutter tooth. This depends on
the module value and on the hardness
of the material being cut. Experience
has shown the tool life
between regrinds to lie between 2
and 4 m per cutter tooth for skive
hobbing.
Gear cutting quality
The gear quality when skive hobbing
depends on the interaction of
a large number of components and
parameters, such as:
■ skiving hob (cutting material,
correctly sharpened, sufficent
accuracy)
■ rigid hobbing machine
■ accurate and stable clamping
of hob and workspiece
■ hob aligned with an absolute
minimum of runout
■ accurate centering
■ correct selection of cutting
speed, feed and metal removal
per flank
■ adherence to the maximum
wear mark width
■ material, preparation and heat
treatment of the workpieces
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