Assignment mathematics 3rd


























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“Pattern”- making, allowances and its types

“Pattern”- making, allowances and its types: Pattern: A pattern may be defined as a model of desired casting which when moulded in sand forms an impression called mould. The mould when filled with the molten metal forms casting after solidification of the poured metal. The quality and accuracy of casting depends upon the pattern making. The pattern may be made of wood, metal(cast iron, brass, aluminium and alloy steel.), plaster, plastics and wax. Pattern Allowances: A pattern is always made larger than the required size of the casting considering the various allowances. These are the allowances which are usually provided in a pattern. 1: shrinkage or contraction allowance: The various metals used for casting contract after solidification in the mould. Since the contraction is different for different materials, therefore it will also differ with the form or type of metal. 2: Draft allowance It is a taper which is given to all the vertical walls of the pattern for eas...
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Disadvantages of Cellular Layouts

• • Sometimes cells may not be formed because of inadequate part families. • Some cells may have a high volume of production and others very low. This results in poorly balanced cells. • When volume of production changes, number of workers are adjusted and workers are reassigned to various cells. To cope with this type of reassignments, workers must be multi-skilled and cross-trained. • Sometimes, machines are duplicated in different cells. This increases capital investment.
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Casting Defects

Casting Defects: The defects in a casting may be due to pattern and moulding box equipment, moulding sand, cores,gating system or molten metal. Some of the defects are: 1: Mould shift It results in a mismatching of the top and the bottom parts of the casting , usually at the parting line. 2: Swell It is an enlargement of the mould cavity by molten metal pressure resulting in localized or general enlargement of the casting. 3: Fins and Flash These are thin projections of the metal not intended as a part of casting. These usually occurs at the parting line of the mould. 4:   Sand   Wash It usually occurs near the in the gates as rough lumps on the surface of a casting. 5: Shrinkage It is a crack or breakage in the casting on the surface of the work piece, which results from un equal contraction of the metal during solidification. 6: Hot Tear It is an internal or external ragged discontinuously in the metal casting resulting just after the metal has s...
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