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is formed by a number of circular casings, ref. fig. 22-
1, which are assembled and secured together by
flanged joints and couplings located with dowels and
tenons. These engines use curvic and hurth
couplings to enable accurate concentricity of mating
assemblies which in turn assist an airline operator
when maintenance is required.
MANUFACTURING STRATEGY
9. Manufacturing is changing and will continue to
change to meet the increasing demands of
aeroengine components for fuel efficiency, cost and
weight reductions and being able to process the
materials required to meet these demands.
10. With the advent of micro-processors and
extending the use of the computer, full automation of
components considered for in house manufacture
are implemented in line with supply groups manufacturing
strategy, all other components being
resourced within the world-wide supplier network.
Manufacture
230
Fig. 22-1 Arrangements of a triple-spool turbo-jet engine.
11. This automation is already applied in the
manufacture of cast turbine blades with the seven
cell and computer numerical controlled (C.N.C.)
grinding centres, laser hardfacing and film cooling
hole drilling by electro-discharge machining (E.D.M.).
Families of turbine and compressor discs are
produced in flexible manufacturing cells, employing
automated guided vehicles delivering palletized
components from computerized storage to C.N.C.
machining cells that all use batch of one techniques.
The smaller blades, with very thin airfoil sections, are
produced by integrated broaching and 360 degree
electrochemical machining (E.C.M.) while inspection
and processing are being automated using the
computer.
12. Tolerances between design and manufacturing
are much closer when the design specification is
matched by the manufacturing proven capability.
13. Computer Aided Design (C.A.D.) and Computer
Aided Manufacture (CAM.) provides an equivalent
link when engine components designed by C.A.D.
can be used for the preparation of manufacturing
drawings, programmes for numerically controlled
machines, tool layouts, tool designs, operation
sequence, estimating and scheduling. Computer
simulation allows potential cell and flow line
manufacture to be proven before physical machine
purchase and operation, thus preventing equipment
not fulfilling their intended purpose.
14. Each casing is manufactured from the lightest
material commensurate with the stress and temperatures
to which it is subjected in service. For example,
magnesium alloy, composites and materials of
sandwich construction are used for air intake
casings, fan casings and low pressure compressor
casings, since these are the coolest parts of the
engine. Alloy si eels are used for the turbine and
nozzle casings where the temperatures are high and
because these casings usually incorporate the
engine rear mounting features. For casings
subjected to intermediate temperatures i.e. by-pass
duct and combustion outer casings, aluminium alloys
and titanium alloys are used.
FORGING
15. The engine drive shafts, compressor discs,
turbine discs and gear trains are forged to as near
optimum shape as is practicable commensurate with
non-destructive testing i.e., ultrasonic, magnetic
particle and penetrant inspection. With turbine and
compressor blades, the accurately produced thin
airfoil sections with varying degrees of camber and
twist, in a variety of alloys, entails a high standard of
precision forging, ret. fig. 22-2. Nevertheless
precision forging of these blades is a recognised
practice and enables one to be produced from a
shaped die with the minimum of further work.
Manufacture
231
Fig. 22-2 Precision forging.
Manufacture
232
Fig. 22-3 Method of producing an engine component by sand casting.
16. The high operating temperatures at which the
turbine discs must operate necessitates the use of
nickel base alloys. The compressor discs at the rear
end of the compressor are produced from creepresisting
steels, or even nickel base alloys, because
of the high temperatures to which they are subjected.
The compressor discs at the front end of the
compressor are produced from titanium. The higher
strength of titanium at the moderate operating temperatures
at the front end of the compressor,
together with its lower weight provides a considerable
advantage over steel.
17. Forging calls for a very close control of the
temperature during the various operations. An
exceptionally high standard of furnace control
　
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