Manufacture - Inspection

INSPECTION

Fig. 22-15 Advanced integrated manufacturing system (A.I.M.S.).

47. During the process of manufacture, component parts need to be inspected to ensure defect free engines are produced. Using automated machinery and automated inspection, dimensional accuracy is maintained by using multi-directional applied probes that record sizes and position of features.

Manufacture - machining (E.D.M.) Composite materials and sandwich casings

COMPOSITE MATERIALS AND SANDWICH CASINGS 

Fig. 22-14 Some composite material applications.

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.

Manufacture - Electro-discharge

ELECTRO-DISCHARGE MACHINING (E.D.M.) 
38. This type of machining removes metal from th workpiece by converting the kinetic energy of electric sparks into heat as the sparks strike the workpiece.

Fig. 22-13 Electro-discharge machining

circuit

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 instan- taneously vaporized with explosive results. Away 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. 

Manufacture - Capillary drilling

Capillary drilling

Fig. 22-12 Typical automated manufacture of compressor blades.

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 intro- duction 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.

Manufacture - Stem drilling

Stem drilling
35. This process consists of tubes (cathode) produced from titanium and suitably insulated to ensure a reaction at the tip. A twenty per cent solution of nitric acid is fed under pressure onto the blade producing holes generally in the region of 0.026 in. diameter. The process is more speedy in 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.

Manufacture - machining (E.C.M.)

33. Electrolytic grinding employs a conductive wheel impregnated with abrasive particles. The wheel is rotated close to the surface of the workpiece, in such a way that the actual metal removal is achieved by electro-chemical means. The by-products, which would inhibit the process, are removed by the sharp particles embodied in the wheel.
34. Stem drilling and capillary drilling techniques are used principally in the drilling of small holes, usually cooling holes, such as required when producing turbine blades.

Manufacture - Electro-chemical

ELECTRO-CHEMICAL MACHINING (E.C.M.)

Fig. 22-11 Electro-chemical machining.

30. This type of machining employs both electrical and chemical effects in the removal of metal. Chemical forming, electro-chemical drilling and elec- trolytic grinding are techniques of electro-chemical machining employed in the production of gas turbine engine components.

31. The principle of the process is that when a current flows between the electrodes immersed in a solution of salts, chemical reactions occur in which metallic ions are transported from one electrode to another (fig. 22-11). Faraday's law of electrolysis explains that the amount of chemical reaction produced by a current is proportional to the quantityof electricity passed.
32. In chemical forming, (fig. 22-11), the tool electrode (the cathode) and the workpiece (the anode) are connected into a direct current circuit. Electrolytic solution passes, under pressure, through the tool electrode and metal is removed from the work gap by electrolytic action. A hydraulic ram advances the tool electrodes into the workpiece to form the desired passage.

Manufacture - Electron beam welding (E.B.W.)

Electron beam welding (E.B.W.)

Fig. 22-9 Electron beam welding.

Fig. 22-10 Examples of T.I.G. and E.B. welds.

29. This system, which can use either low or high voltage, uses a high power density beam ofelectrons to join a wide range of different materials and of varying thickness. The welding machine ref. fig. 22-9, comprises an electron gun, optical viewing system, work chamber and handling equipment, vacuum pumping system, high or low voltage power supply and operating controls. Many major rotating assemblies for gas turbine engines are manufac- tured as single items in steel, titanium and nickel alloys and joined together i.e., intermediate and high pressure compressor drums. This technique allows design flexibility in that distortion and shrinkage are reduced and dissimilar materials, to serve quite different functions, can be homogeneously joined together. For example, the H.P. turbine stub shafts requiring a stable bearing steel welded to a material which can expand with the mating turbine disc. Automation has been enhanced by the application of computer numerical control (C.N.C.) to the work handling and manipulation. Seam tracking to ensure that the joint is accurately followed and close loop under bead control to guarantee the full depth of material thickness is welded. Focus of the beam is controlled by digital voltmeters. See fig. 22-10 for weld examples.

Manufacture - Tungsten inert gas (T.I.G.) welding

Tungsten inert gas (T.I.G.) welding 

Fig. 22-7 Typical tungsten inert gas 

welding details

Fig. 22-8 Tungsten inert gas welding.

28. The most common form of tungsten inert gas welding, fig, 22-7, in use is the direct current straight polarity i.e., electrode negative pole. This is widely used and the most economical method of producing high quality welds for the range of high strength/high temperature materials used in gas turbine engines. For this class of work, high purity argon shielding gas is fed to both sides of the weld and the welding torch nozzle is fitted with a gas lens to ensure maximum efficiency for shielding gas coverage. A consumable four per cent thoriated tungsten electrode, together with a suitable non-contact method o! arc starting is used and the weld current is reduced in a controlled manner at the end of each weld to prevent the formation of finishing cracks. All welds are visually and penetrant inspected and in addition, weld associated with rotating parts i.e., compressor and/or turbine are radiologically examined to Quality Acceptance Standards. During welding operations and to aid in the control of distortion and shrinkage the use of an expanding fixture is recommended and, whenever possible, mechanised welding employed together with the pulsed arc technique is preferred. A typical T.I.G. welding operation is illustrated in fig. 22-8.

Manufacture - Welding

WELDING
27. Welding processes are used extensively in the fabrication of gas turbine engine components i.e., resistance welding by spot and seam, tungsten inert gas and electron beam are amongst the most widely used today. Care has to be taken to limit the distortion and shrinkage associated with these techniques.

Manufacture - Forging

FORGING

Fig. 22-2 Precision 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.

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 creep- resisting steels, or even nickel base alloys, because of the

Manufacture - Fabrication

FABRICATION

Fig. 22-6 Wide chord fan blade

construction

25. Major components of the gas turbine engine i.e. bearing housings, combustion and turbine casings, exhaust units, jet pipes, by-pass mixer units and low pressure compressor casings can be produced as fabricated assemblies using sheet materials such as stainless steel titanium and varying types of nickel alloys.

26. Other fabrication techniques for the manufacture of the low pressure compressor wide chord fan blade comprise rolled titanium side panels assembled in dies, hot twisted in a furnace and finally hot creep formed to achieve the necessary configu- ration. Chemical milling is used to recess the centre of each panel which sandwiches a honeycomb core, both panels and the honeycomb are finally joined together using automated furnaces where an activated diffusion bonding process takes place, ref. fig. 22-6.

Manufacture - Casting

CASTING

Fig. 22-3 Method of producing an engine component by sand casting.

Fig. 22-4 Automatic investment casting.

20. An increasing percentage of the gas turbine engine is produced from cast components using sand casting, ref. fig. 22-3, die casting and investment casting techniques; the latter becoming the foremost in use because of its capability to produce components with surfaces that require no further machining. It is essential that all castings are defect free by the disciplines of cleanliness during the casting process otherwise they could cause component failure.

21. All casting techniques depend upon care with methods of inspection such as correct chemica composition, test of mechanical properties, radiolog- ical and microscopic examination, tensile strength and creep tests. 22. The complexity of configurations together with accurate tolerances in size and surface finish is totally dependent upon close liaison with design, manufacturing, metallurgist, chemist, die maker, furnace operator and final casting.

Manufacture - Manufacturing strategy

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 manufac- turing strategy, all other components being resourced within the world-wide supplier network.
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.

Manufacture - Introduction

INTRODUCTION 
1. During the design stages of the aircraft gas turbine engine, close liaison is maintained between design, manufacturing, development and product support to ensure that the final design is a match between the engineering specification and the man- ufacturing process capability. 

Fig. 22-1 Arrangements of a triple-spool turbo-jet engine.

2. The functioning of this type of engine, with its high power-to-weight ratio, demands the highest possible performance from each component. Consistent with this requirement, each component must be manufactured at the lowest possible weight and cost and also provide mechanical integrity through a long service life. Consequently, the methods used during manufacture are diverse and are usually determined by the duties each component has to fulfil.
3. No manufacturing technique or process that In any way offers an advantage is ignored and most available engineering methods and processes are employed in the manufacture of these engines, In some instances, the technique or process may appear by some standards to be elaborate, time consuming and expensive, but is only adopted after confirmation that it does produce maximized  component lives comparable with rig test achieve- ments.

Rolls-Royce RB168 Mk202/Mk203 | Rolls-Royce RB39 Clyde

Rolls-Royce RB168 Mk202/Mk203

Rolls-Royce RB39 Clyde

Encouraged by results obtained from the Trent, Rolls-Royce decided to go ahead with an engine designed from the start as a turbo- prop. Named the Clyde it utilized the axial compressor from the Metrovick F2 as first stage and a scaled up supercharger impeller from a Merlin as second stage. First running in August 1945 at 2000 shp, later engines produced up to 4200 shp.