Question: Explain Pattern allowances.
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Mumbai University > Mechanical Engineering > Sem 3 > Production Process 1

Marks: 5M

Year: May 2015

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modified 2.6 years ago  • written 2.6 years ago by gravatar for aksnitald aksnitald0
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Pattern is the replica of the casting to be produced. Replica means that shape of the pattern remains same as that of the shape of casting to be produced.

                              Pattern size=casting size±allowance

Pattern allowances are of five types

a. Shrinkage allowance

b. Machining allowance

c. Draft allowance

d. Shake allowance

e. Distortion allowance

a. Shrinkage allowance

                 Pouring temperature=Melting point+150℃ to 200℃ (degree of superheat)

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Shrinkage of material takes place in all three stages. Shrinkage taking place in first two stages is called liquid shrinkage and shrinkage taking place in the last stage is called solid shrinkage.

As the liquid shrinkages are measurable in terms of the volume, liquid shrinkages are always specified as a percentage over volume.

Solid shrinkages taking place in the casting process will be compensated by providing shrinkage allowance on the pattern.

Total shrinkage (liquid+solid) together will be highest in case of steel.

b. Machining Allowance

Extra dimensions provided on the pattern in the casting which will be removed by machining after casting process is called as machining allowance.

Machining allowance will be provided because

  • Since the casted components have poor surface finish and most of the engineering applications require excellent surface finish, so the machining of casting has to be done. Hence excess material must be provided on the casting.

  • To accommodate the variation in shrinkage taking place due to variation in room temperature at the place of cooling.

                                        Machining allowance=x mm/side
    

The amount of machining allowance to be provided depends on the complexity of the casting to be produced.

c. Draft allowance

Making the vertical surfaces of the pattern into inclined surfaces is called as draft allowance. This is provided for easy removal of the pattern form.

Draft allowance is not provided if the patterns are made of wax and mercury.

d. Shake allowance

After preparing the mould, because of the presence of adhesiveness of moulding sand, the moulding sand may get adhered to the walls of the pattern. During removal of the pattern from the mould, the adhered moulding sand will try to come along with the pattern and damages the mould walls. To avoid this, the pattern will be shaken before removal of the pattern from the mould.

Due to shaking, the adhered moulding sand will be separated but the size of the cavity produced will become greater than the size of the pattern and thus size of the casting increases.

To maintain the required casting size, the original pattern dimensions must be reduced by an amount called shake allowance.

e. Distortion allowance

The distortion allowance is not required to be provided on all castings but it is provided on U or V shaped castings to be produced.

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