Pattern allowance is a essential feature because it affects the dimensional characteristics of the casting. when the pattern is produced, certain allowances should be given on the sizes specified in the finished component drawing in order that a casting with the specific specification may be made. The choice of accurate allowances greatly allows to reduce machining costs and avoid rejections.

The allowances generally considered on patterns and center boxes are as follows:

  1. Shrinkage/Contraction allowance
  2. Draft or taper allowance
  3. Machining or end allowance
  4. Distortion or camber allowance
  5. Rapping allowance.

  1. Shrinkage/Contraction allowance

All most all cast metals shrink or contract volumetrically on cooling. The steel shrinkage is of  types:
i. Liquid Shrinkage: it refers back to the reduction in volume when the steel changes from liquid state to solid state on the solidus temperature. To account for this shrinkage; riser, which feed the liquid steel to the casting, are provided in the mold.

ii. Solid Shrinkage: it refers back to the reduction in quantity caused when steel loses temperature in stable state. To account for this, shrinkage allowance is provided at the patterns. The rate of contraction with temperature is depending on the material.

For example metal contracts to a better degree compared to aluminum. To compensate the solid shrinkage, a shrink rule should be used in laying out the measurements for the pattern. A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of four inch in diameter turned into planned to provide out of cast iron, the shrink rule in measuring it 4 inch could actually measure 4 -1/24 inch, therefore compensating for the shrinkage. The numerous rate of contraction of various materials are given in Table below.

rate of contraction of various materials

2. Draft or Taper Allowance

By draft is supposed the taper supplied through the pattern maker on all vertical surfaces of the pattern so that it may be removed from the sand with out tearing away the sides of the sand mold and with out excessive rapping through the molder.
In the following figure, till the pattern is completely lifted out, its sides will remain in touch with the walls of the mold, thus tending to break it.

Pattern Having No Draft on Vertical Edges

Pattern Having No Draft on Vertical Edges

Pattern Having Draft on Vertical Edge

Draft allowance varies with the complexity of the sand job. But in general internal details of the pattern require higher draft than outer surfaces. 

Pattern Having Draft on Vertical Edges

The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material. 

3. Machining or Finish Allowance

  • The finish and accuracy completed in sand casting are generally poor and consequently when the casting is functionally required to be of good surface finish or dimensionally accurate, it’s far normally completed by subsequent machining. 
  • Machining or finish allowances are consequently added in the pattern dimension.
  • The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or steel mold casting.
  • The amount of machining allowance is also affected by the dimensions and shape of the casting; the casting orientation; the steel; and the degree of accuracy and finish required.
Machining or Finish Allowance

4.Distortion/Camber Allowance

This is provided on patterns whose castings have a tendency to distort on cooling.
The pattern itself is distorted certainly to yield a right formed casting. Required for flat, long, ‘U’ and ‘V’ formed castings.

5. Shake/Rapping Allowance

To withdraw the pattern from the rammed sand without difficulty without damaging the mold walls of mold cavity pattern is Shaked/ rapped. During this manner the mold cavity is enlarged. To compensate this pattern length is reduced. Important for big sized and precision castings; Amount of allowance depends on Extent of rapping, Degree of compaction of sand, length of mold; maximum of this are difficult to evaluate.

Corner and fillets allowance:

The intersection of surfaces in castings need to be smooth and form no sharp angles. For this, the external and inner corners of patterns are suitably rounded. Fillets facilitate the removal of the pattern from the mold, prevent the formation of cracks and shrink holes in the casting.

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