Pattern allowances in order to produce a casting of proper size and shape depend partly on product design, mould design, shrinkage and contraction characteristics of the metal being cast, etc. Actually, it is very difficult to predict precisely any rule mathematically, and a process of trial and error is best to achieve adjustment of pattern dimensions to obtain desired results.
1. Shrinkage Allowance:
When any metal cools, it naturally shrinks in size. The total contraction of a casting comprises of three elements viz., the contraction of liquid from pouring temperature to freezing temperature, contraction on account of change from liquid to solid and lasting contraction of solid casting from freezing temperature to surroundings. While, first two contractions are taken care of in the design of castings, last effect is taken care of by designing pattern of bigger size.
If the actual object itself is used for the pattern, the resulting casting would be slightly smaller than desired. To compensate for this possibility, a shrink rule is used in laying out of measurements for the pattern. A shrink rule for cast iron is 10 mm per metre (the average shrinkage for cast iron) longer than the standard rule.
Typical shrinkage allowances for important casting materials in mm/metre are as under: Grey cast iron—7 to 10.5, white cast iron—21, malleable iron—15, steel—20, brass—14, aluminium—18, aluminium alloys—13 to 16, bronze—10.5 to 21, magnesium—18. In actual practice shrinkage allowance varies with the casting design, thickness and other dimensions of casting, type of metal, pouring temperature, and resistance of the mould to normal contraction of the casting caused by projecting lugs, cores, design and intricacy of casting, moulding material used and its hardness, method of moulding etc.
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When metal patterns are to be cast from the original patterns, double shrinkage must be allowed.
2. Draft Allowance:
When a pattern is drawn out from a mould the tendency to tear away the edges of the mould in contact with the pattern is greatly decreased, if the surfaces of the pattern are given a slight taper in a direction parallel to which it is being withdrawn, (Refer Fig. 3.15).
This tapering of the sides of the pattern, known as draft, is done to provide a slight clearance for the pattern as it is lifted up. The amount of draft required depends upon the shape and size of the casting, (length of the vertical side of pattern), moulding method, the method of production, intricacy of pattern, and whether moulded by hand or machine. Machine moulding requires minimum draft.
The amount of draft on exterior surfaces is about 10 to 20 mm per metre. On interior holes which are fairly small, the draft should be around 30 mm per metre. In green-sand moulding, interior surfaces usually require more draft than exterior surfaces.
3. Finishing Allowance:
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When a draftsman draws up the details of a part to be made, each surface to be machined is indicated by a finish mark. The mark indicates that additional metal must be provided at this point so that there will be some metal to machine. The amount that is to be added depends upon the size, shape of casting, but in general, the allowance for small castings and average sized castings is 3 mm for ferrous castings and 1.5 mm for non-ferrous castings.
Wherever possible, surfaces to be machined should be cast in the drag side of the mould. However, where finished surfaces are cast in the cope, an extra allowance should be made. Finishing or machining allowance depends on method of machining used (turning, boring, grinding, etc.), characteristics of metal (ferrous, non-ferrous, soft, easily machinable), method of casting used (e.g. centrifugal casting requires more allowance on the inner side), size and shape of the casting (e.g. more allowance is required at top surface which collects impurities, long castings have tendency to warp), degree of finish required.
4. Distortion or Camber Allowance:
This allowance applies only to those castings of irregular shapes such as U-shape or those having large flat areas, which are distorted in the process of cooling as a result of metal shrinkage. For instance, casting of U-shape will be distorted with the legs divergent, instead of parallel. To compensate this, the pattern is made such that legs converge, but on cooling the casting legs straighten. Such an allowance depends on the judgment and experience of the pattern maker, who understands the shrinkage characteristics of the metal.
5. Rapping or Shaking Allowance:
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When a pattern is trapped in the mould before it is withdrawn, the cavity in the mould is slightly increased. In an average sized casting, this increase in size can be ignored. In large castings or in one that must fit without machining, however, shake allowance must be considered by making the pattern slightly smaller.