Pattern Allowances
The patterns are not made the exact size as the desired casting because such a pattern would produce undersize casting. When a pattern is prepared, certain allowances are given on the sizes specified in the drawing so that the finished and machined casting produced from the pattern will conform to the specified sizes. While designing pattern, the allowances commonly considered are discussed below. 1. Shrinkage Allowance

Generally metals shrink in size during solidification and cooling in the mould. So casting becomes smaller than the pattern and the mould cavity. Therefore, to compensate for this, mould and the pattern should be made larger than the casting by the amount of shrinkage. The amount of compensation for shrinkage is called the shrinkage allowance. Generally shrinkage of casting varies not only with material but also with shape, thickness, casting temperature, mould temperature, and mould strength. Therefore, it is better to determine the amount of shrinkage according to the past record obtained from many experiences. Table 1.2 shows an average amount of shrinkage for important cast metals. Table 1.2 Typical shrinkage allowances for important casting metals Type of metal Amount of shrinkage (%) Grey cast irons 0.55-1.00 White cast irons 2.10 Malleable cast irons 1.00 Steels 2.00 Manganese steel 2.60 Magnesium 1.80 Type of metal Zinc Brasses Bronzes Aluminium Aluminium alloys Tin Amount of shrinkage (%) 2.60 1.30-1.55 1.05-2.10 1.65 1.30-1.60 2.00

In practice, pattern makers use a special rule or scale, called the “pattern maker’s contraction rule”, which, after providing with necessary allowance, is slightly longer than the ordinary rule of the same length. The graduations are oversized by a proportionate amount, for example, when constructing a pattern for aluminium alloys, the pattern maker uses a contraction rule measuring about 12 mm longer per metre than the conventional rule because aluminium alloys shrinks 10 mm per metre. There may be different contraction rules for different casting metals, but generally one single ruler is used on each side of which there are two scale, the total number of scales being four for four commonly cast metals, e.g., cast irons, steels, brass and aluminium. Table 1.3 shows the use standards for contraction rule. Table 1.3 Use standards of contraction rule Contraction rule +8/1000 +9/1000 +10/1000 +12/1000 +14/1000 +16/1000 +20/1000 +25/1000 Materials used and place of use Cast iron in general, part of thin cast iron Cast iron products of high shrinkage, part of thin cast steel Same as above, and aluminium Aluminium alloys, bronze, cast steel (thickness 5-7 mm) High tension brass, cast steel Cast steel (thickness over 10 mm in general) Large cast steel Large cast iron

For mass-produced castings, a moulding die is sometimes made. In that case the master pattern must have allowances for the amount of shrinkage of the material of die and that of casting metal. This is called “double shrinkage allowance.” Example 1.1 Grey cast iron castings of dimension 80 mm are to be made in a metal mould made of aluminium alloy. The metal mould is to be made using a wooden pattern. Determine the correct dimension of the wooden pattern considering the solidification contraction only. Solution. Here the wooden pattern must have a double shrinkage allowance for the shrinkage of metal mould (aluminium) and the casting (cast iron). Allowance for aluminium = (80 mm) x (1.20/100 mm/mm) = 0.96 mm Allowance for cast iron = (80 mm) x (0.80/100 mm/mm) = 0.64 mm Therefore, total shrinkage allowance = 0.96 + 0.64 = 1.60 mm Hence, the dimension of the wooden pattern would be = 80 + 1.60 = 81.60 mm

2.

Machining Allowance

In case the casting designed to be machined, they are cast over-sized in those dimensions shown in the finished working drawings. Where machining is done, the machined part is made extra thick which is called machining allowance. Machining allowance is given due to the following reasons: 1. 2. 3. 4. Castings get oxidised inside mould and during heat treatment. Scale thus formed requires to be removed. For removing surface roughness, slag, dirt and other imperfections from the casting. For obtaining exact dimensions on the casting. To achieve desired surface finish on the casting.

The dimension of the pattern to be increased because of the extra metal required (i.e. finish or machining allowance) depends upon the following factors: 1. 2. 3. 4. Method of machining used (turning, grinding, boring, etc.). Grinding removes lesser metal than turning. Characteristics of metal (ferrous or non-ferrous, hard and easily machinable or soft). Ferrous metals get oxidised, aluminium does not. Method of casting used. Centrifugal casting requires more allowance on the inner side. Die castings need little machining, sand castings require more. Size and shape of the casting. For long castings, warpage is more and greater allowance is required. Thicker sections solidify late and impurities tend to collect there. This necessitates more machining allowance. Degree of finish required. A higher degree of finishing requires more machining allowance.

5.

The standard machining allowances for different metals and alloys are shown in Table 1.4. 3. Draft Allowance or Taper Allowance

When a pattern is drawn from a mould, there is always a possibility of damaging the edges of the mould. Draft is taper made on the vertical faces of a pattern to make easier drawing of pattern out of the mould (Fig. 1.3). The draft is expressed in milimetres per metre on a side or in degrees.

Table 1.4 Standards of general machining allowance Type of metal and alloys Cast irons (i) Large size castings (>1000 mm) (ii) Medium size castings (1000 mm) (ii) Medium size castings (1000 mm) (ii) Medium size castings (