5 TYPES OF ALLOWANCES IN MANUFACTURING

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 SELECTION OF MANUFACTURING PROCESS WILL DEPENDS ON

  1. Shape and size of the object
  2. accuracy and surface finish required by the object
  3. Properties required by the object
  4. cost of the object.
ALLOWANCES
    To get the exact shape and size of the object, the allowance is provided to the object.

TYPES OF ALLOWANCES
  1. Shrinkage or Contraction allowance
  2. Draft and Taper allowance
  3. Machining or Finishing allowance
  4. Rapping or Shake allowance
  5. Distortion allowance
SHRINKAGE ALLOWANCE:
    Contraction of the liquid from the pouring temperature to the freezing temperature.
   The contraction associated with the change of phase freezing temperature
    
    Tf = Freezing Temperature
    Tp = Tm + Tl
    Tp = Pouring Temperature
    At pouring, temperature flow metal has maximum fluidity.
   By reducing the energy density decreases and due to this the liquid metal gas converted into solid metal.
    If we allow the liquid metal into the cavity during the solidification process, the material will get shrink and this will take place in three different phases:
  1. The amount of shrinkage in the liquid state is known as liquid shrinkage i.e, from pouring temperature to freezing temperature.
  2. The amount of shrinkage that takes place during phase transformation is known as solidification shrinkage.
  3. The amount of shrinkage which takes place in a solid-state is known as solid shrinkage i.e., Freezing temperature to ambient temperature.
    Liquid and Solidification shrinkage can be compensated by providing the riser and these shrinkage values are expressed in terms of % of shrinkage, Volume of casting.
    At the same temperature, Shrinkage takes place due to LHF(Latent Heat of Fusion)
    Solid shrinkage can be compensated by increasing the size of the pattern at in the form of shrinkage allowances and these values are expressed in terms of linear dimensions.

    Shrinkage value for different material
    
        Bismuth         - negligable
        White metal   - 5 mm/m
        Cast iron        - 10 mm/m
        Aluminium    - 15 mm/m
        Brass              - 16 mm/m
        Copper           - 17 mm/m
        Steel               - 20 mm/m
        lead and Zinc - 26 mm/m
    These are solid shrinkage because these are expressed in linear dimension.
    More size of the pattern means more shrinkage allowance.

DRAFT AND TAPER ALLOWANCE:
    For easy removal of the pattern from the mold in the mold, cavity patterns are provided some amount of draft due to this continuous contact of the vertical surface of the pattern will be reduced with the surface of the mold.
 
MACHINING ALLOWANCE:
    Machine bed -  provide a base to the surface.
    Casting objects are not having a better surface finish.
    To get a better surface finish on the casting machining is required due to this the size of the casting will be reduced.
    To overcome this machining allowance will be provided on the surface of the pattern

RAPPING OR SHAKE ALLOWANCE:
    It is a negative allowance.
    It depends on the skill of the operator and also on the shape and size of the pattern.
    For easy removal of the pattern from the mould, the pattern will be shaken in a different direction due to which some clearance will be generated between the pattern and the mould surface.
    During the process, the size of the pattern will be slightly increased.
    To overcome this pattern size will be reduced by considering rapping allowance.

RAPPING PLATES
    It is a plate in which several threads are provided in form of different pores and it can be used by both hands or more than one person at a time


DISTORTION ALLOWANCE:
    Distortion depends on l and also depending on the shape and size of the object due to the difference in shrinkage rates, Shrinkage stresses will be developed due to this distortion of the casting will take place.
    To overcome this distortion allowance is provided on the pattern opposite to the direction of distortion.



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