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When the boring operation is accomplished, the
cutting edges move towards the cutter's center
to prevent scratching the cylinder surface on exit.
There are two common mechanisms (depending
on the machine): actuation by a linear draw bar,
which has only 2 positions (‘on’ during the boring
operation and ‘off’ during the feed out) and
actuation by a fully numerically controlled rotation
bar, which can change the tool diameter in real
time. For example, for producing conical, barrel
or other shaped holes, for internal grooving or for
bore diameter correction/compensation (due to
the insert's wear).

A key factor in the success of these operations
is the selection of the appropriate PCBN grade
related to the material being machined. The
correct balance between the hardness and
toughness of the grade has to be considered.
Although using PCBN with coolant is not
recommended, some automotive manufacturers
PCBN Inserts insist on a wet machining process. In these
cases, the machining environment (emulsion or
oil coolant, MQL, dry machining, etc.) has to be
considered. The cutting edge geometry derives
from the machined material, cutting parameters
and a depth of cut (T-land, E-land, S-land, sharp
or honed edge, etc.).

Valves
Gas exchange valves, particularly exhaust valves,
are always under intensive thermal loads.
As previously mentioned, the temperature of
the exhaust gases reach more than 900°C, this
constitutes a big challenge for valve materials
and can lead to excessive wear and
Gundrills premature fatigue. A few leading
companies have developed new
technologies to solve this problem.
One of these solutions is to gundrill
the valve stem up to its head and to
fill this cavity with sodium. During the
engine's operation, the sodium absorbs
the generated heat and melts.

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