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Aviation History
1952
1952 - 1578.PDF
686 BLADES BY An Ingenious Contribution to the Solution of a Gas Turbine Production Problem IT has long been appreciated that, except for the smallest parts, forging to shape is an inherently better manufac turing process than machining. Not only does it afford savings in raw material and manufacturing time, but the forging causes the metal to flow smoothly until it assumes the contours of the finished part, thus conferring optimum strength. Although forging is still usually performed after furnace heating, this method may introduce severe disadvantages. Omes Ltd., of Barnes, London, S.W.13, have long been known for their.electro- forging methods which, by heating the workpiece rapidly and uniformly, overcomes many of the usual furnace troubles. The company has also developed a series of upsetting machines which utilize electric-resistance heating to produce a controlled upset at any point on bar stock. In recent years, this method has been increasingly applied to the manufacture of turbine and axial-compressor blades in any standard blade material. Both rotor and stator blades are in pro duction by the method, with any type of root end or shrouding. Stainless steel, Nimonic alloys, and aluminium- or beryllium- bronze are the principal materials used. Incidentally, pure alumin ium has been upset regularly as has—with difficulty—magnesium alloy. For blade production, the upset is of the order of 20 or 25 diameters. Raw material consists of centreless-ground bar of minimum diameter, this dimension being determined by blade width, but being of the order of Jin. The stock is cut into accurate lengths of about 1 ft each, chamfered at one end; this end is then placed against an anvil on the upsetting machine and the bar gripped fairly firmly in a hydraulic vice. The brass vice-jaws are water-cooled electrodes (both are anodes) and the similarly-cooled anvil is the cathode. A heavy low-voltage current is passed through that portion of the stock between the vice and anvil; a hydraulic ram then pushes the stock ANVIL UPSET VICE ELECTRODES The ram pushes the workpiece through the vice-electrode to "feed" the upset. to form the upset, the anvil retracting to allow for the growui. By varying the speed of ram and anvil, corresponding variation can be effected in the shape of the upset. Current is tapped from the Omes-designed transformer and the machine uses a self-contained water or oil system for ram opera tion. Once set, the upsetting is automatic—thus permitting opera tion by semi-skilled workers—and a typical cycle time is 15 sec. The temperature coefficient of resistance of most blade alloys does not vary greatly over the forging range; usual upsetting tempera ture is 1,050-1,150 deg C, or about 880 deg C for al-bronze. The rapid heating at one end permits the piece to be hand-held in transit to a vertical 500-ton forging press which forms a fiat head from the upset. A 10 per cent tungsten, pre-heated die is used, with one forging blow. After a slight re-heat to the appro priate temperature, the piece (which now resembles a mushroom valve) is placed on a horizontal die and forged flat—again at one blow—into a semi-finished blade and root. A combined coining and trimming operation, usually carried out cold, is then followed by vapour blasting (shot-blasting for stainless steel) and, possibly, electro-polishing. The final forging blow produces a blade contour held within an envelope of ± o-ooo5in. Advantages claimed for this process may be listed as follows: (1) Rapid and uniform heating is obtained throughout, scaling and carburizing being almost eliminated. (2) Precise volumetric control of the workpiece prevents waste and greatly increases die life. (3) Very little is demanded of the operator. (4) The upsetting opens up any minute shake, inclusion or other flaw in the stock; if this becomes apparent, the piece is rejected without further time being spent on it. Equally, the production of a good upset largely FLIGHT, 6 June 1952 UPSETTING Stages in the manufacture of an aluminium-bronze compressor blade: (left to right) centreless-ground chamfered bar; after upsetting; after vertical forging; after re-heat and horizontal forging; and after combined coining and trimming the flash. (Below) A typical Omes 15 kV A upsetting machine working on com pressor blading. The electrodes are water-cooled and the hydraulic rams are operated by a self-contained water or oil system. guarantees a perfect blade. (5) The whole process affords a very great reduction in the overall time required to produce a blade— the sequence of upset, forge, heat, forge and trim comparing very favourably with the many operations, with re-heats, otherwise required. (6) Perfect grain-flow is produced throughout the blade and root. (7) Material grain-size cannot grow in the brief heating period. (8) The process is carried out entirely with self-contained machines and working conditions are very favourable by comparison with furnace heating. Further developments of the Omes method include the large- scale production of shrouded blades with a precision upset at both ends and—possibly the ultimate in blade production—the up setting of a continuous aerofoil-section strip. It would appear that complete turbine or compressor blades will eventually be produced in a fraction of a minute, starting with continuous strip. Among firms stated to be using, or interested in, Omes equip ment, may be mentioned Rolls-Royce (Barnoldswick works), the Utica Drop Forge and Tool Co., Avro Canada, and Bofors A.B., Sweden. While details are restricted, it may be said that consider able blade production for many important engines is being carried out by this method. This noteworthy process is to be described in detail in the July issue of our associated journal Aircraft Production.
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