Gear Cutting Tools

Carbide types and
coatings
The carbide types generally used
are those of the main machining
groups K and P. The types present
advantages and disadvantages
according to their material composition
(alloying elements and components)
and their grain size.
Whereas K carbides, owing to the
tendency of chips to bond to the
uncoated substrate, can only be
employed fully coated, P carbides
can also be employed in uncoated
form. There is therefore no need
for the cutting face to be re-coated
following regrinding. This reduces
the maintenance costs for P carbide
hobs considerably.
In addition, P carbides are less
sensitive to temperature, and the
strong progressive increase in
wear which takes effect from a
flank wear of approximately 0.2
mm onwards is considerably lower.
The substrate reacts more favourably.
By contrast, fine-grain carbides
have as yet only been developed
for the K types. Fine-grain carbides
permit very high hardness
values and consequently a high resistance
to wear, combined with
excellent toughness.
Consequently, fully coated K substrates
generally permit higher tool
life qualities when compared with
hobs manufactured from P carbides,
which lose their cutting face
coatings at the first regrind at the
latest. P carbide hobs must therefore
be changed more frequently.
TiN, manufactured by means of
PVD, continues to be the main
substance employed for the hard
material layer of hobs. TiN possesses
excellent chemical resistance
to the hot steel chips. In addition
to its hardness of 2200 HV,
its relatively high toughness makes
it particularly attractive for hobs.
The logistics aspect represents a
decisive advantage. TiN is the
coating which, owing to its low
pressure characteristics, can be
re-coated more easily. This is essential
following grinding of the
cutting face of hobs with a K type
substrate.
Newly developed coatings such as
TiCN and (TiAIN)
can attain longer tool life travel for
a given application, but have yet to
be accepted by the market, particularly
with regard to the re-coating
aspect.
Advantages:
Disadvantages:
● Re-coating not necessary following regrinding
● Low maintenance costs (regrinding only)
● Shorter tool life in the reground condition, therefore:
● More frequent tool changes required
● Shorter maintenance times, consequently:
● Fewer tools in circulation (lower capital investment)
● Lower progressive rise in wear when the coating is penetrated,
consequently:
● Lower risk of built-up edges
Use of coated solid carbide hobs with P type substrate
Maintenance process: regrinding (flank coated, cutting face uncoated)
Advantages:
Disadvantages:
● Generally longer tool life, therefore:
● Less frequent tool changing
● Fine-grain grades possible, therefore:
● Greater toughness and greater hardness
● Cannot be employed uncoated, i.e. removal of the coating and
re-coating is required, therefore:
● Higher maintenance costs
● Longer maintenance times, therefore:
● More tools in circulation (greater capital investment)
● Strongly progressive increase in wear following penetration of
the coating, consequently:
● Greater risk of built-up edges
Use of coated solid carbide hobs with K type substrate
Maintenance process: removal of coating - regrinding - re-coating (flank and cutting face coated)
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