Pattern Allowances

Pattern allowances

Pattern Allowances

In the metal casting process, the pattern is used to manufacture a casting which is a final product of the desired dimensions. Generally, the pattern is not made dimensionally identical or similar to the castings to be made and some tolerances are provided onto the surface of pattern to overcome the problem of the shrinkage, called pattern allowances . These tolerances are provided for finishing purposes.

Pattern may be made from wood or metal and its color may not be same as that of the casting. The material of the pattern is not necessarily same as that of the casting.

Pattern carries an additional allowance to compensate for metal shrinkage.

  1. It carries additional allowance for machining, called Machining or Finishing allowance.
  2. It carries the necessary draft to enable its easy removal from the sand mass, called Draft or Taper allowance.
  3. It carries distortions allowance also. Due to distortion allowance, the shape of casting is opposite to pattern, also called cambered allowance.
  4. Pattern may carry additional projections, called core prints to produce seats or extra recess in mold for setting or adjustment or location for cores in mold cavity. It may be in pieces (more than one piece) whereas casting is in one piece.
  5. Sharp changes are not provided on the patterns. These are done on the casting with the help of machining. Surface finish may not be same as that of casting.

The size of a pattern is never kept the same as that of the desired casting because of the fact that during cooling the casting is subjected to various effects and hence to compensate for these effects, corresponding allowances are given in the pattern. These various allowances given to pattern can be classified as, allowance for shrinkage, allowance for machining, allowance for draft, allowance for rapping or shake, allowance for distortion and allowance for mould wall movement. These allowances are discussed as under follows-

Types of Pattern Allowances

THE VARIOUS PATTERN  ALLOWANCES ARE-

  1. Shrinkage or contraction allowance
  2. Machining or finishing allowance
  3. Draft of tapper allowances.
  4. Distortion or chamberred allowance.
  5. Shake or rapping allowance.

Also read: Sand Mould CastingPattern types, Metal Casting

1. Shrinkage or contraction allowance

In practice it is found that all common cast metals shrink a significant amount when they are cooled from the molten state. The total contraction in volume is divided into the following parts:

figure- Liquid and Solid shrinkage allowance
  1. Liquid Shrinkage

    It refers to the reduction in volume when the metal  changes from liquid state to solid state at the solidus  temperature.To account for this shrinkage;riser,which  feed the liquid metal to the casting,are provided in the  mold.

  2. Solid Shrinkage

    It refers to the reduction in volume caused when  metal loses temperature in solid state.To account for  this,shrinkage allowance is provided on the patterns.  Almost all cast metals shrink or contract volumetrically after solidification and therefore the  pattern to obtain a particular sized casting is made  oversize by an amount equal to that of shrinkage  or contraction.

2. Machining Allowance

It is a positive allowance given to compensate for the amount of material that is lost in machining or finishing the casting. If this allowance is not given, the casting will become undersize after machining. The amount of this allowance depends on the size of casting, methods of machining and the degree of finish. In general, however, the value varies from 3 mm. to 18 mm.

3. Draft or Taper Allowance

Draft allowance
figure- Draft or taper allowance

Taper allowance is also a positive allowance and is given on all the vertical surfaces of pattern so that the withdrawal of the pattern becomes easier. The normal amount of taper on the external surfaces varies from 10 mm to 20 mm/mt. On interior holes and recesses which are smaller in size, the taper should be around 60 mm/mt. These values are greatly affected by the size of the pattern and the molding method. In machine molding its, value varies from 10 mm to 50 mm/mt.

4. Rapping or Shake Allowance

Before withdrawing the pattern it is rapped and thereby the size of the mould cavity increases. Actually by rapping, the external sections move outwards increasing the size and internal sections move inwards decreasing the size. This movement may be insignificant in the case of small and medium size castings, but it is significant in the case of large castings. This allowance is kept negative and hence the pattern is made slightly smaller in dimensions 0.5-1.0 mm.

5. Distortion or Cambered Allowance

Distortion or cambered allowance
figure- Distortion or Cambered allowance

This allowance is applied to the castings which have the tendency to distort during cooling due to thermal stresses developed. For example a casting in the form of U shape will contract at the closed end on cooling, while the open end will remain fixed in position. Therefore, to avoid the distortion, the legs of U pattern must converge slightly so that the sides will remain parallel after cooling.

6. Mold wall Movement Allowance

Mold wall movement in sand moulds occurs as a result of heat and static pressure on the surface layer of sand at the mold metal interface. In ferrous castings, it is also due to expansion due to graphitisation. This enlargement in the mold cavity depends upon the mold density and mould composition. This effect becomes more pronounced with increase in moisture content and temperature. Copied from Introduction to Basic Manufacturing Processes and Workshop echnology by Rajender Singh

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