Machinery's Handbook, 31st Edition
CENTRIFUGAL CASTING
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Fig. 14. True Centrifugal Casting There are numerous advantages of centrifugal casting. The force used to distribute the molten metal as it is poured is so strong that it displaces lighter oxides and impurities to the center of the hollow cylinder; it is then easy to remove these impurities during machin- ing. Blowholes, sand residue, hard spots, cavities, and porosity are virtually eliminated, and the resulting uniformity means machining time and material waste are significantly reduced. Due to the finer grain and denser structure of parts cast with centrifugal force, the part’s usage life, endurance, and ability to withstand impact without fracturing are increased. The centrifugal process also allows economical production of a wide range of sizes, shapes, and quantities. Thus, centrifugal casting produces quality parts, reduces manufacturing costs, and pro- vides production flexibility. There are three types of centrifugal casting: true centrifugal casting, semicentrifugal casting, and centrifuge casting. True Centrifugal Casting.—In this technique, molten metal is poured into a rotating mold. The axis of rotation usually is horizontal but can be vertical for short workpieces. (One setup is illustrated in Fig. 14.) Molds are made of steel, iron, or graphite and may be coated with a refractory lining to increase mold life. Mold surfaces can be shaped so that pipes with various external designs can be cast. Cylindrical parts ranging from 0.6 in. to 10 ft. (15 mm to 3 m) in diameter and 52 ft. (16 m) in length can be cast centrifugally, with wall thicknesses ranging from 0.24 to 5 in. (6 to 127 mm). True centrifugal casting produces parts of good quality, dimensional accuracy, and ex- ternal surface detail; better mechanical properties can be achieved than in conventional static casting. Non-metallic impurities segregate toward the bore and can be machined off. There is less loss of metal as compared with conventional sand casting. The casting is relatively free from defects, there are no parting lines, and production rates are high. This process is used to produce hollow cylindrical parts, such as pipes, gun barrels, bushings, bearing rings, and streetlamp posts; bimetallic tubes can be made. Consider how fast the mold must rotate in horizontal centrifugal casting for the process to work successfully. Centrifugal force acting on a rotating body is defined by the follow - ing equation: (9) where F c = centrifugal force; m = mass; v = velocity; and R = inside radius of the mold. Gravitational force is its weight: (10) where F g = gravitation force; m = mass; and g = acceleration of gravity. Velocity v can be expressed as: (11) where N = rotation speed. F mv R c = 2 F mg g = v RN RN = = 2 60 30 π π
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