Aerospace & Defense Technology - October 2024 - 19

Making the Rounds
in Aerospace:
Tooling and Techniques
for Discs, Blisks and Rings
A
erospace engine components
like discs, blisks and rings are
engineered to perform in
extreme operating environments.
They need to withstand intense
heat and stress and be as lightweight as
possible to meet exacting specifications.
These parts are also notoriously difficult
to machine, and manufacturers who
work with them must meet serious challenges
of their own. Holding tight tolerances,
maintaining predictable tool life
and accounting for internal material
stress relief from material removal can be
especially difficult when profiling complicated
features such as thin-walled
flanges, undercut pockets and seal fins.
The challenges are also evolving.
Advances in material science are leading
to new exotic materials that are stronger,
more heat-resistant and, as a result, even
more difficult to machine. Meanwhile,
machinists are asked to do more than ever
while navigating ever increasing complexities
in parts, machines and processes.
In this environment, the biggest challenge
shops face is to become complacent
- using the same tools and techniques
they've been using for decades.
Keeping pace with change isn't easy, but
taking advantage of the latest solutions
can lead to greatly increased productivity,
process security and quality when
making these complex components.
Here, we'll look at some ways shops can
overcome common challenges with the
latest tools and techniques.
Part Conformity: Maintaining
Precision When the Material Wants
to Move
Machining discs, blisks and rings for
aircraft engines takes exceptional precision,
because any flaw or deviation can
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affect the engine's performance. These
parts may need to be machined to tolerances
within one-thousandths of an
inch or more. Adding to the challenge,
internal stresses in the material may be
released after unlocking the workpiece
from the machining fixture, causing the
component to flex or " spring back. "
Accounting for these stress releases is
crucial to maintaining precise operations
and hold tolerances for blisks, discs and
rings, especially when removing material
for thin-walled features. Understanding
how metal removal and the order of
operations affect stress relief in the component
is essential.
To address this, it's best to " sneak up "
on the material with multiple roughing
operations and setups. If there is any
deformation, you still have enough
material to bring it back into size. When
roughing for a thin-walled feature, consider
leaving extra material on the component,
then do a semi-finish cut with a
smaller cutting tool nose radius and less
tool pressure. After that, remove the part
from the fixturing to release any built-up
stresses and flip it to rough the other
side. This back-and-forth process may
need to be repeated several times before
finishing. It takes more time, requires
more setups and may seem counterintuitive,
but it's the most effective way to
accurately machine these parts to spec.
When milling out pockets for blisk
blades, shops need to contend with an
extreme blade wall thickness-to-length
ratio that makes machining even more of
an issue - not just for the immediate
pocket being milled but also the adjacent
pocket. Here, the order of operations is
particularly important. One effective strategy
is to skip over every other pocket all
the way around the blisk and then contin19
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Aerospace & Defense Technology - October 2024

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