process development
R&D embracing a number of OEMs identifies point grinding as the
most fruitful way forward in turbine blade root slot production
By David Aspinwall, Leung Soo & David Curtis - Machining Research
Group, University of Birmingham
New routes to roots
Sample broached fir
tree root slot in
aeroengine disc - footnote
reference [1]
TRADITIONALLY, small diameter (< 25mm)
grinding wheels, employing both conventional and
superabrasive grit systems, are used for internal
grinding and a range of de-burring operations where
the tools tend to be referred to as grinding points, pins,
burrs or pencil grinders. Applications encompass many
industries engaged in precision engineering. These
include the mould, die & press tool sector, clockmaking,
woodworking, and in aerospace manufacture for
composites. In addition to a wide range of single layer
electroplated and vitrified superabrasive products from
manufacturers including Saint-Gobain Abrasives, and
Wendt, vitrified bond CBN wheels as small as 1mm
diameter are available from companies such as Meister
Abrasives, while Haefeli Diamantwerkzeugfabrik AG
suggests it has a world first, with a sintered pencil
grinder having an active wheel of just 0.2mm in
diameter.
The perception, whether right or wrong, is that over
the past 30 years relatively little has changed in this
branch of the technology, in contrast to mainstream
grinding wheel products and associated process
developments with VIPER, HEDG etc. However, the use
of precision shaped grinding points for the
manufacture of turbine blade root slots in aeroengine
compressor discs as an alternative to broaching, is in
anyone’s book leading edge technology, and is
questioning current approaches to grinding wheel
design/fabrication and use in a number of applications.
Most aeroengines, whether for commercial or
military use, employ detachable rotating blades. These
are typically located and held in position on a series of
disks using either a generic fir tree or dovetail design
ground into the blade root, with the equivalent profile
in the disc. The rotor assembly for the engine
compressor and turbine, will, depending on size,
contain many hundreds of blades of varying size and
shape with similar variation in the scale and nature of
the root attachment or slot. Currently, the machining
of axial/oblique root slots in individual discs is almost
universally accomplished using broaching, both in this
country by manufacturers such as Rolls-Royce plc, and
by their counterparts around the world.
The process is mature, and in all fairness does the job
in providing complex slots of appropriate geometrical
size (within 10μm), quality (workpiece Ra ≤ 2μm) and
integrity, with roughing and finishing possible within
one stroke. On the downside however, is the high
36 MWP january 2008
capital cost of the machines, and their large footprint.
These factors, together with their inflexibility, costly
tooling, lengthy set-up/validation/changeover times,
high cutting forces (up to 10,000N) and relatively long
manufacturing times, have fuelled the call for change.
Finding a potential alternative process has not been
easy, not least because the re-entrant nature of disc slots
appears to preclude the use of conventional grinding
wheel arrangements. Makino NCMT however has
recently suggested that a modified VIPER wheel
configuration can be employed, at least for larger fir
tree roots, although details of the approach are scant.
Grinding point application involving electroplated
single layer superabrasive wheels with a full or partial
form, appears to offer much greater scope for
machining a wide range of slot shapes and sizes. When
used in combination with a suitable HSM machine tool
platform incorporating a high speed spindle (up to
90,000revs/min), rapid tool changing, laser/contact
probe technology for tool setting and tool/workpiece
inspection etc, the process appears to offer significant
versatility.
With such potential in mind, machine tool
manufacturer Matsuura has designed a flexible point
grinding machining centre (H.Plus 300a) incorporating
a 60,000revs/min horizontal spindle which can
accommodate discs up to 750mm diameter. As
Technical Director of Matsuura Machinery UK, David
Edwards is keen to emphasise the company’s
innovative/leading edge approach, which entails the
combination of advanced grinding techniques and
HSM technology for machining a range of aeroengine
components. On paper at least, point grinding looks to
be a winner. However, translating the approach into
practice within an industry whose overriding
consideration is passenger safety, has taken time and
money and continues to be the subject of intensive
research rather than a production reality, for the
moment at any rate.
Collaborative research on point grinding involving
the University of Birmingham, Rolls-Royce, Matsuura
& Saint-Gobain Abrasives has been on-going for
several years (starting with EPSRC Grant No.
GR/M66790/01) with feasibility work using plain
points reported in MWP as far back as October 2001.
Key elements in the work have been the development of
appropriate machine tool/tool holding technologies,
abrasive systems, fluid application, grinding point
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