A pattern is defined as a model of casting, constructed in such a way that it can be used for forming an impression or cavity into mould. The first step in making a casting is to prepare a model, known as pattern. A pattern is replica of casting but it has slightly different dimensions than casting. These differences are known as pattern allowances. Pattern allowances are given to the pattern to compensate metal shrinkage, to provide sufficient metal for machining, to facilitate moulding, etc... To produce casting of proper size and shape, pattern allowance are given to the pattern and these pattern allowances are depend partly on product design, mould design, shrinkage and contraction characteristics of metal being cast, etc... Due to these wide factors, it is difficult to predict precise mathematical rule for pattern allowances but by trial and error method, it is possible to achieve pattern dimensions to obtain desired results.
Letís consider two factors which are affected to the shape and size of casting and due to these factors pattern allowances are given to the pattern.
Shrinkage and contraction characteristics of metal being cast
When any metal cools from liquid to solid state, it naturally shrinks in size. If the actual object is used for the pattern, resulting casting size would be slightly smaller than desired size. To overcome this shrinkage problem, shrinkage allowance is considered while designing a pattern. Average shrinkage of cast iron is 10 mm for 1 metre.
Letís consider a sphere is required of 100 mm diameter and material of sphere is cast iron. Sphere is manufacture by casting process. If pattern is design of 100mm diameter then the resulting casting have near about 99 mm diameter. To obtain desired result, pattern must have larger diameter than required diameter near about 101 mm then resulting casting will be near about 100 mm diameter.
Machining or Finishing processes
Note: Here only one factor is considered of pattern allowances, which is machining and finishing process, not shrinkage and other factors.
Another factor is considered while designing a pattern which is machining or finishing processes. Casting is not a final or finish product. To convert casting into finish product, casting is going through some machining or finishing operations. In machining operation some amount of material is removed from casting, this factor is considered while designing a pattern and finishing allowance or machining allowance is given to the pattern.
Letís consider a well super finished cube is required of 50mm length, 50 mm breadth and 50mm height. A cube is manufactured by casting process. Here only one factor is considered of pattern allowances, which is machining and finishing process. If pattern is design of 50mm length, 50 mm breadth and 50mm height then resulting casting will have same dimensions but this cube is not well super finished. If casting is going through finishing processes then resulting cube will have slightly lower dimensions than required dimensions. It will depend upon finishing process; consider finishing process required 0.5 mm for finishing one side. Then the resulting object will have 49 mm length, 49 mm breadth and 49 mm height. To obtain desired dimensions of a cube, pattern must have larger dimensions than required dimensions that are 51 mm length, 51 mm breadth and 51 mm height.
These are the few factors which are considered while designing a pattern. There are also some factors which are considered while designing a pattern and according to these factors pattern allowances are divided into different types and these are as follow.
Types of Pattern Allowances
- Shrinkage Allowance
- Draft Allowance
- Finishing Allowance or Machining Allowance
- Distortion or Camber Allowance
- Rapping or Shaking Allowance
Article in Video Format