Gear Cutting Tools

Machining with and
without coolant
The machining of steel materials
generates considerable quantities
of heat at the point of chip removal.
If the temperatures reach
excessive levels, the cutting edges
of the tool are rapidly destroyed.
In order to cool the tool and at the
same time to lubricate the cutting
edge, cooling lubricants have in
the past been applied to the contact
point between the cutting
edge and the material to be machined.
Cooling lubricants also
have the function of flushing away
the chips which are produced.
Cooling lubricants, however, have
considerable ecological, economic,
and in many cases also technological
disadvantages.
Cooling lubricants present an ecological
hazard since they impact
the environment in the form of oil
vapour and oil mist, and can
present a health hazard to humans.
Cooling lubricants are not economically
justifiable, because they
increase the production costs owing
to the very high costs of their
supply and disposal. Up to 16% of
the total gear production costs can
be saved by dry machining.
Furthermore, cooling lubricants
may pose disadvantages for technological
reasons. The use of cooling
lubricants in many hobbing operations
involving carbide cutting
edges, for example, may lead to
premature failure of the tool owing
to stress cracking (temperature
shock). For this reason, cutting
speeds are limited to 250 m/min
for wet machining (in comparison
with 350 to 450 m/min for dry machining).
The table shows the advantages
and disadvantages of
cooling lubricant with regard to
carbide hobbing.
The main problem with dry machining
lies in the increase in
cutting temperature. Up to 80% of
the heat which is generated is dissipated
with the chips, provided
attention has been paid to correct
tool design and suitable cutting
parameters are employed.
The configuration of the tool is dependent
upon the data of the gear
to be manufactured. A significant
influencing factor is the tip chip
thickness, which is derived from
the cutter design (number of
starts, number of gashes, diameter),
the workpiece geometry
(module, number of teeth, cutting
depth, helix angle) and the selected
feed. An important consideration
is that dry machining requires
observance not only of an upper
limit to the tip chip thickness, but
also of a minimum thickness value.
The greater the chip volume, the
greater the quantity of heat which
an individual chip can absorb. This
must be taken into account in order
to ensure that during dry machining,
the greater part of the machining
heat is dissipated by the
chips.
Advantages
Disadvantages
Machine ● Supports chip removal ● Aggregates (filters, pumps, etc.), therefore:
● Lower heating up of the machine
● Greater space requirements
● Additional operating expenditure (maintenance,
power, etc.)
Tool ● Cooling of the tool ● Lower tool life owing to the formation of cracks
perpendicular to the cutting edge (thermal shock)
● Lubrication of the friction zones
Workpiece ● Lower heating ● Cleaning necessary
● Lower dimensional deviations
● Protection against corrosion
Environment ● Binding of graphite dust ● Health risk
during cast iron machining
Further costs ● Tempering of the workpiece, ● Purchasing costs
thus faster measurement
● Inventory costs
Advantages and disadvantages of the use of cooling lubricant during hobbing
● Contaminated chips, therefore:
● Expensive recycling processes and
● Higher disposal costs
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