Pattern Allowances in Metal Casting

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Introduction to Casting Casting is one of the earliest manufacturing processes known to human beings. In casting, molten metal is solidified to the desired shape and size. Casting is often the cheapest and most direct way of shaping the metal. Practically any intricate shape can be produced. With normal sand casting processes, dimensional accuracies and surface finish are often poor. Defects are inevitable. Steps in Casting Pattern Making Mould and Core Making Melting and Pouring of metal Taking the casting out of mould Cleaning (Fettling) and Inspection Pattern & Allowances A pattern is the replica of the casting to be made, with certain modifications. Pattern size = Casting size $\pm$ Allowances. Allowances: Shrinkage Allowance Machining Allowance Draft Allowance Shake Allowance Distortion Allowance Shrinkage Allowance Shrinkage of metal during casting takes place in three stages: Shrinkage of molten metal when reducing from pouring temperature to freezing temperature. Shrinkage of molten metal during freezing. Shrinkage of solid metal when reducing from freezing temperature to room temperature. Pouring Temp. = Melting Temp. + $(150-200)^\circ C$. Liquid Shrinkage Liquid shrinkage is always specified by percentage over volume. Highest liquid shrinkage = Aluminium (6.60%). Liquid shrinkage is compensated by providing a riser during mould making. Metal in the riser should solidify last. Riser volume must be sufficient for compensating shrinkage in casting. Solid Shrinkage Shrinkage allowance is provided on the pattern to compensate for solid shrinkages. Solid shrinkage $\propto \alpha$, where $\alpha$ = coefficient of thermal expansion of metal. Shrinkage allowance is specified as: $x$ mm/m over each linear dimension. In the case of internal dimensions, the material tends to contract towards the center (e.g., internal diameter of a cylinder is to be increased in the pattern). Shrink Rules Special scales where unit dimensions shown are actually longer than the original unit dimension by a measure equal to the shrinkage allowance. Different shrink rules are used for different materials. Example: For a plain carbon steel casting of dimensions $200 \text{mm} \times 150 \text{mm} \times 100 \text{mm}$, the pattern size using a shrink rule will be $200 \text{mm} \times 150 \text{mm} \times 100 \text{mm}$. Machining Allowance Extra dimension provided on the pattern to compensate for subsequent machining required on the casting. Purpose: To achieve the desired finish on the casting product. To accommodate variation in dimensions due to temperature variation during solidification. Machining allowance is specified by '$x$' mm/side. Draft Allowance Provision of inclination to the vertical surfaces of the pattern. It is provided for easy removal of the pattern from the mould. Draft allowance varies with the complexity of the job. Inner details of the pattern require higher draft than outer surfaces. If the pattern is made using wax or mercury, no draft allowance is required. Shake Allowance Moulding sand adheres to the pattern walls during moulding. Shake allowance is the wrapping done all around the vertical faces of the pattern. It is provided to avoid damages caused by adhering moulding sand to the pattern walls. It is highly dependent on the mould making person. As it reduces the dimension of the pattern, it is taken as a negative allowance. If the pattern is made using materials like wax, mercury, or polystyrene, no shake allowance is provided. Distortion Allowance Not required on all castings, mainly in 'U' or 'V' shaped castings. In 'V' or 'U' shape castings, due to differential shrinkages at different locations of the cavity, the legs may bend outwards, producing inclined legs. The shape of the pattern itself is given a distortion of equal amount in the opposite direction of the likely distortion direction. Done by trial-and-error basis to determine the distortion amount.