Manufacture
Manufacture
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.
ELECTRO-CHEMICAL MACHINING (E.C.M.)
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 quantity of 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.
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
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.
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 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.
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.
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