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operation than electro-discharge machining and is
capable of drilling holes up to a depth two hundred
times the diameter of the tube in use.
Capillary drilling
36. Similar in process to stem drilling but using
tubes produced from glass incorporating a core of
platinum wire (cathode). A twenty per cent nitric acid
solution is passed through the tube onto the
workpiece and is capable of producing holes as
small as 0.009 in. diameter. Depth of the hole is up
to forty times greater than the tube in use and
therefore determined by tube diameter.
37. Automation has also been added to the process
of electro-chemical machining (E.C.M.) with the introduction
of 360 degree E.G. machining of small
compressor blades, ref. fig. 22-12. For some blades
of shorter length airfoil, this technique is more cost
effective than the finished shaped airfoil when using
precision forging techniques. Blades produced by
E.C.M. employ integrated vertical broaching
machines which take pre-cut lengths of bar material,
produce the blade root feature, such as a fir-tree, and
then by using this as the location, fully E.C.M. from
both sides to produce the thin airfoil section in one
operation.
ELECTRO-DISCHARGE MACHINING (E.D.M.)
38. This type of machining removes metal from the
workpiece by converting the kinetic energy of electric
sparks into heat as the sparks strike the workpiece.
39. An electric spark results when an electric
potential between two conducting surfaces reaches
the point at which the accumulation of electrons has
acquired sufficient energy to bridge the gap between
the two surfaces and complete the circuit. At this
point, electrons break through the dielectric medium
between the conducting surfaces and, moving from
negative (the tool electrode) to positive (the
workpiece), strike the latter surface with great
energy; fig, 22-13 illustrates a typical spark erosion
circuit.
40. When the sparks strike the workpiece, the heat
is so intense that the metal to be removed is instantaneously
vaporized with explosive results. Away
Manufacture
238
Fig. 22-11 Electro-chemical machining.
from the actual centre of the explosion, the metal is
torn into fragments which may themselves be melted
by the intense heat. The dielectric medium, usually
paraffin oil. pumped into the gap between the tool
electrode and the workpiece, has the tendency to
quench the explosion and to sweep away metallic
vapour and molten particles.
41. The amount of work that can be effected in the
system is a function of the energy of the individual
sparks and the frequency at which they occur.
42. The shape of the tool electrode is a mirror image
of the passage to be machined in the workpiece and,
to maintain a constant work gap, the electrode is fed
into the workpiece as erosion is effected.
Manufacture
239
Fig. 22-12 Typical automated manufacture of compressor blades.
Fig. 22-13 Electro-discharge machining
circuit.
COMPOSITE MATERIALS AND SANDWICH
CASINGS
43. High power to weight ratio and low component
costs are very important considerations in the design
of any aircraft gas turbine engine, but when the
function of such an engine is to support a vertical
take-off aircraft during transition, or as an auxiliary
power unit, then the power to weight ratio becomes
extremely critical.
44. In such engines, the advantage of composite
materials allows the designer to produce structures
in which directional strengths can be varied by
directional lay-up of fibres according to the applied
loads.
45. Composite materials have and will continue to
replace casings which, in previous engines, would
have been produced in steels or titanium. By-pass
duct assemblies comprising of three casings are
currently being produced up to 4ft-7in. in diameter
and 2ft-0in in length using pre-cured composite
materials for the casing fabric. Flanges and mounting
bosses are added during the manufacturing process,
which are then drilled for both location and machined
for peripheral feature attachment on C.N.C.
machining centres, which at one component load,
completely machine all required features. Examples
of composite material applications are illustrated in
fig. 22-14.
46. Conventional cast and fabricated casings and
cowlings are also being replaced by casings of
sandwich construction which provide strength allied
with lightness and also act as a noise suppression
medium. Sandwich construction casings comprise a
honeycomb structure of aluminium or stainless steel
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