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Aviation History
1961
1961 - 0187.PDF
FLIGHT, 10 February 1961 187 pie Industry ELECTROCHEMICAL MACHINING ELECTROCHEMICAL machining offers the production.E- engineer a useful method of processing difficult-to-machine me:ils. Surface atoms of the material are dislodged by electricalenergy and then swept away by a rapid controlled flow of an electrolytic solution. Various aspects of the technique aredescribed in the February issue of our sister journal Aircraft Production, and further information is due to be given in theMarch issue. Known as the Sifco electro-shaping process, it was developed at the Battelle Memorial Institute, Columbus, Ohio,through research sponsored by the Steel Improvement and Forge Co, of Cleveland, Ohio. It is operated in this country byMetachemical Processes Ltd, Crawley, Surrey. The technique consists basically of taking away the negativeelectrical charges that bind the surface atoms to their substrate associates in a metal crystal. These dislodged surface-atoms arepositively charged and are attracted away into the dissolving solu- tion. The metal workpiece (the anode) and another conductor (thetool, or cathode) are submerged in a chemical solution and the workpiece is connected to the positive terminal of a d.c. source.The negative terminal is connected to the cathode immersed in the solution. Current requirements, at 3V to 12V, are as high as l,500amp/sq in of workpiece surface-area. The cathode is profiled, so that at the start of the operation some portions of it are nearer to thework than others. Currents passing across such zones are there- fore higher, with the result that the anode dissolves more rapidly.In so doing, it initiates the shaping action. As the anode acquires the shape of the cathode, it is fed inwards until eventually thework is of mating configuration. The rate at which metal is removed increases directly with thecurrent passed per unit of area, but for most applications the rate is restricted to an infeed of the cathode of 0.006 to 0.008in/min.This rule applies only as long as the solution can supply the reactants to dissolve the metal at the rate at which it is electrolytic-ally dislodged. Maximum speed is obtained when the amount of electrical energy is so great as completely to deplete the reactantsand/or saturate the solution. Therefore, if current is permitted to flow for an extended period, more material is removed at pointsof greatest flow. Generally speaking, current flow is maintained at 200 to500amp/sq in, at which values metal is removed at rates from 0.05 to 0.1 cu in/min. The upper limit to the rate of removal isdetermined by the maximum current fhat can be conducted into the metal, or passed without boiling the solution. The chemical condition of most importance to electro-machining is a film about 0 003in thick on the work surface. The dislodged metal enters this film, which must be removed andreplaced by a fresh film at a high rate of exchange. Practical applications of the process have shown that a turbine- Waspalhy turbins-blade electroshaped by the Sifco process. The finished blade (top) has edge radii of 0.005'm blade forging-die requiring some 7hr to produce by conventionalmethods can oe electro-mecrnnically machined in 50min, this with t'.ie material in the annealed or fully hardened condition. In the irnin, turbine and compressor blades are produced bycombining basic processes, such as a combination of casting and forging techniques, or a combination of forging and rolling. Asturbine blades are normilly produced from the higher alloyed materials designed to withstand deformation at operating tempera-tures of the order of 1,400 to 2,000°F, the fabricating problems experienced tend to be of greater magnitude th:n those encoun-tered in the manufacture of compressor blades produced in rela- tively simpler grades of material. The Steel Improvement Co and Metachemical Processes Ltdhave succeeded in developing a method of electrochemical shaping of aerofoil sections which, it is claimed, not only offerssubstantial savings in time and cost, but also provides a potential for greatly improved dimensional accuracy. This development hasnow been carried to the point where some 15,000 blades, of 16 different designs, have been produced. In addition to the produc-tion work carried out on blades, the organization has developed the process to the stage where tooling can also be produced bysimilar techniques. The approach followed at Steel Improvement has been toproduce a slightly oversize forgin?, allowing a b.nd of material for subsequent removal by electrochemical techniques. This tech-nique permits the use of considerably wider dimensional tolerances on the forging, and provides increased die-life. Inspection of theforged product is appreciably reduced, compared with that neces- sary for precision forging.While the work carried out has so far been confined to the electrochemical shaping of oversize forged bhdes, the new processis readily applicable to cJstinTs or bar-stock. The overall tooling cost is stated to bs. considerably less than that of precision forging,and the life of the electrochemical shaping tools appears to be of considerable duration. It is also claimed that good surface-finishesare being consistently obtained and stress problems ehminned, and it is thought that warp, bow and twist tolerance requirementcan be drastically reduced. Aerofoil sections with leading- and trailing-edges as thin as 0.005in hive been produced. The processcan be used with success for most forms of metal-removal machin- in?, including the deep drilling of holes as small as 0.012in diam. Blade materials machined by the Sifco process include stainless-steels, both pearhtic and austenitic grades; S-816; Waspalloy; Udimet 700; Nimonic alloys; A-286; and V-57. Materials pro-cessed in this manner for other applications include tungsten and molybdenum. IN BRIEF Mr P. T. Stephens has been appointed chairman of the SaundersValve Co Ltd following the death of Mr J. C. Billingham. Mr A. L. Trump and Mr Stephens will continue to act as joint managing directors. The publicity division of Solartron Electronic Group have moved tome new group headquarters at Victoria Road, Farnborough, Hants Famborough 3000). Following tests by their medical branch, BEA have recommendedChance Crookes sunglasses for use by thsir pilots, to diminish eye-strain caused by sun reflection on clouds, without reducing range of vision. Mr T. K. Singer has been made general sales manager of James BoothAluminium Ltd. Mr Singer, formerly an executive with Kaiser Aluminum and Chemical Corporation, has been appointed following the resignau'onof Mr D. McL. Burnell. Mr Frank J. Mills continues as sales manager. Kelvin Hughes Mk 5F ultrasonic flaw-detection equipment was usedto inspect wing-root areas of CAA Viscount 700s following reports of cracks in two of these aircraft Flight, January 13 and 27). BOAC usethe Mk 5 equipment for routine testing of wheel-hubs and undercarriage components on their Britannias, Comets, 707s and DC-7Cs. Guest speaker at the George Kent Ltd annual apprentice prizegivingand exhibition of work, held at Luton recently, was Mr W. F. Stephenson, principal of Luton College of Technology. He emphasized industry'sgrowing need for qualified technical specialists and therefore the ereat importance of theoretical studies, but referred to the Government WhitePap-r outlining parallel opportunities available to those more suited to specialist careers of a practical nature. Prizes were presented bythe company chairman, Cdr P. W. Kent, RN. Mr F. Vincent Everard, guest of honour on the Rolls-Royce apprentices' awards day, presenting awards and certificates to (from right) trade apprentices B. Poxon, J D Rowland and M. Rowley, on completion of th?ir training with distinction. In the background of the picture is Lt-Col C. £. Lone, the company's apprentice recruitment officer
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