Gear Cutting Tools

Carbide skiving hobs
Process and
range of applications
Skive hobbing is a machining process
in which skiving hobs are
used for cutting rough-milled and
hardened gears.
The main area of application is the
hobbing of straight and helical
spur gears. In addition, external
splines, roll profiles and a large
number of special profiles which
can be generated by the hobbing
method can be machined with the
skiving hob. There are various reasons
for using this process:
Finish-hobbing of gears
Skive hobbing eliminates hardening
distortion and improves the
quality of the gear.
The metal removal capacity is considerably
higher with skive hobbing
than with the usual grinding
processes. It is therefore economical
to replace grinding by skive
hobbing in the range of coarse and
medium gear tolerances.
Gear quality grade 6 to DIN 3962
can be quoted as an approximate
value for the attainable accuracy.
Profile- and flank modifications,
too, such as depth crowning, tooth
face setback or width crowning,
can be produced by suitable hob
profiles and corresponding machine
motions.
The tool
Design
The characteristic design feature
of skiving hobs is the negative tip
rake angle. The tip rake angle is
described as negative when the
cutting faces of the teeth lie, in the
direction of the cutting motion, in
front of the tool reference plane.
The tool reference plane is the
plane in which lie the tip cutting
edges of the axially parallel cutter
and cutter axis.
Due to the negative tip rake angle,
the flank cutting edges are inclined
in relation to the effective reference
plane (plane perpendicular to
the cutting motion) and in this way
produce a peeling cut.
The negative rake angle is greater
in the root area of the hob teeth
than in the tip area. The tip cutting
edges have no effective back rake
and cannot therefore generate a
curling cut. It therefore follows that
the skiving hobs should only produce
flank chips and that protuberance
cutters are used for
roughing the gears.
Tool material
Low chip thickness and hardened
gear materials make severe demands
on the edge strength of the
tool material. As the tool material
for skiving hobs, carbides of ISO
application groups K 05 to K 15
are used.
Designs
Depending on the module size and
the accuracy requirements, 3 skiving
hob designs can be basically
distinguished:
■ Solid carbide
up to and including module 4
FETTE Cat. no. 2028
■ Brazed-on carbide tips
for modules above 4
FETTE Cat. no. 2129
■ Indexable carbide inserts
for modules from 5 upwards
FETTE Cat. no. 2153
– γ
v c
λ s
-–γ γ = Kopfspanwinkel
tip rake angle
λ s = Neigungswinkel back rake of der the Flankenschneide
flank cutting edge
v Schnittgeschwindigkeit
c = cutting speed
Fig. 2
Preparation for grinding
For high gear quality requirements,
the gears are ground. The gear
cutting costs can be markedly reduced
if the hardening distortion is
before grinding removed by skive
hobbing, at the same time removing
material down to the necessary
grinding allowance. Grinding times
and costs are reduced while gaining
additional grinding capacity.
Fig. 1
40