Sandvik Coromant Heat resistant super alloys - Manual (Page 86)

84
When milling using ceramic inserts, cool-
ant should normally not be used. In most
operations the use of coolant has a nega-
tive effect on tool life. This is due to the
increase in thermal shock with cooling and
heating of the cutting zone as the insert
enters and then exits the workpiece. This
increases the chance of top slice of the
ceramic. However, a small amount of MQL (Minimum
Quantity Lubrication) could have a positive
effect when reducing heat in the machining
process.
Below is an example of the effect on wear/
tool life of ceramic with and without cool-
ant. Both inserts have been machined with
the same cutting parameters and for the
same amount of time.
Coolant and ceramic milling
Effect of cutting speed, v
c
Without coolant With coolant
When milling in ceramics, due to intermit-
tent cutting, it is a much cooler operation
than turning. For this reason speeds of
700-1000 m/min when milling are adopted
compared with 200-300 m/min for turning.
The high cutting speed used when milling
with ceramic inserts increases the tem-
perature of the chip making the chip highly
sheared. The cutting speed should be balanced to
create enough heat in the cutting zone but
not too high to unbalance the ceramic. Too
low cutting speed can result in top slice of
the insert, too high cutting speed can result
in insert failure.
Top slice Even flank wear
Ceramic cutter, Insert grade CC6060,
z
n
4, D
3
63 mm, v
c
700 m/min, a
e
32 mm,
f
z
0.1 mm/tooth, a
p
1.5 mm, Material: Waspalloy
Ceramic cutter, Insert grade CC6060,
z
n
4, D
3
63 mm, v
c
1000 m/min, a
e
32 mm,
f
z
0.1 mm/tooth, a
p
1.5 mm, Material: Waspalloy
Recommendation – 1000 m/min cutting speed gives most balanced speed.