Master Pattern

Master Pattern :
         Master patterns are used for preparing the mouldsfor metal castings which are later used as patterns for further moulding work, called metal patterns. The masterpatterns are accurately finished wooden patterns, whichcarry double shrinkage allowance and the required machiningallowance. For example, an alluminium pattern is to bemade which is to be used further for making moulds forbrass castings. The alluminium pattern should, obviously,be larger than the desired brass casting by an amount equalto shrinkage that will take place during solidification ofthis casting. For making this alluminium pattern a woodenpattern is to be used which should be larger than thealluminium pattern by an amount equal to the alluminiumshrinkage, added with proper machining allowance forfinishing the alluminium casting. Mathematically, it can be represented thus :
Let Sb represent the size of the desired casting in brass.
And  Let Sa represent the size of aluminum pattern.
And  Let Cb represent the contraction allowance for brass.
Then  Sa=Sb+Cb
Again, let S represent the size of the master pattern.
And let Ca represent the contraction allowance for aluminum.
Also let Am represent the machining allowance required
to finish the aluminum casting to the required size of
pattern and to give smooth surface finish.
Then S = Sa+Ca+Am = Sb+Cb+Ca+Am
or
Size of master pattern = Size of the final casting to be Made + shrinkage allowance for the material of final casting + shrinkage allowance of the metal of which the pattern is to be made + Finishing allowance for the metal pattern.

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