Gear Cutting Tools

The enormous increases in tool life
over that of uncoated tools can be
attributed to the physical friction
and the chemical characteristics,
in addition to the high hardness.
The low chemical affinity of the TiN
to the hot steel chip leads to lower
friction and in turn to less frictional
heat, thereby reducing the wear.
The coating acts as a barrier which
protects the underlying substrate
against wear.
Toll life travel per cutter tooth (m)
10
8
6
4
2
0
TiN TiN / reground Uncoated
The higher cutting and feed
speeds possible with coated tools
are particularly advantageous for
the user. A chief factor is not only
the longer tool life, but also the reduction
in production times. Coated
hobs thus recoup their coating
costs within a very short space of
time.
Workpiece: Sun wheel
Material: 17CrNiMo6
Tool:
HSS hob
Dimensions: d 90 x 80 mm
Module: 3 mm
Number of starts: 1
Number of gashes: 12
Quality grade: AA
Cutting data
Cutting depth:
Cutting speed:
Axial feed:
Tip chip thickness:
Shift length:
6,808 mm
65 m / min
3 mm / WU
0,224 mm
54,3 mm
The tool life of an HSS hob used
for the manufacture of a sun wheel
was increased five-fold from 100
to 502 finished wheels by the application
of a TiN coating. Following
regrinding, the tool was not recoated,
and was therefore coated
only on the flank, and not on the
cutting face. It nevertheless attained
an average tool life of 251
finished wheels in this condition.
Over a total of 22 regrinding cycles,
a total of 2300 wheels were
manufactured with the uncoated
hob and 6024 with the coated hob,
i.e. 2.6 times the number. The relatively
low additional cost of the TiN
coating was therefore recouped
with ease.
Workpiece
Coating
Higher hardness
+ Lower friction
+ Reduced diffusion
= Lower wear
Wedge
Chip
Coating
Re-coating following regrinding (of
the cutting face) of a worn HSS
tool is also economically viable.
Use of the ion plating process for
TiN coating presents no problems.
The tool can be re-coated several
times; alternatively, the coating
can also be removed chemically in
a bath.
The combination of TiN on carbide
is somewhat more complicated,
although repeated coating is still
possible. Removal of the coating
from carbide in the bath is however
difficult, owing to the chemical
affinity of the carbides and the TiN
coating.
Re-coating of the grey-violet TiCN
presents greater problems, since
TiCN has a multi-layer structure.
Structures of this kind cannot simply
be "stacked" one upon the
other without difficulty. Removal of
the coating from HSS by immersion
in a bath is possible, but still
more complex than with TiN. On
carbide, the problems described
above are also encountered.
The gold TiCN Plus is an interesting
coating type. Essentially, this is
a TiCN multi-layer coating with
high resistance to abrasion. A pure
TiN surface coat is however deposited
at the end of the coating
process. As a result, the friction
behaviour of the chip on the tool is
influenced chiefly by the TiN- surface
layer, the abrasion resistance
by the underlying TiCN. TiCN Plus
is more conducive to re-coating
than TiCN.
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