PATTERN ALLOWANCES
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- Q1: What is Allowance?Ans: Allowance is the extra dimension given to a pattern to compensate for changes that occur during casting.
- Q2: What is Pattern Allowance?Ans: Pattern allowance is the additional size or shape modification given to a pattern so that the final casting comes out with correct dimensions.
- Q3: State the types of Pattern Allowance.Ans: The types of pattern allowance are:
- ➔ Shrinkage (Contraction) Allowance
- ➔ Distortion Allowance
- ➔ Draft (Taper) Allowance
- ➔ Machining (Finishing) Allowance
- ➔ Rapping (Shake) Allowance
- Q4: Briefly explain Contraction or Shrinkage Allowance.Ans: Shrinkage allowance is provided to compensate for the reduction in size of metal as it cools and solidifies after casting.
- Q5: Briefly explain Distortion Allowance.Ans: Distortion allowance is given to prevent bending or warping of the casting during cooling.
- Q6: Briefly explain Draft or Taper Allowance.Ans: Draft allowance is a slight taper given on vertical surfaces of the pattern to allow easy removal from the mould.
- Q7: Briefly explain Finishing or Machining Allowance.Ans: Machining allowance is the extra metal provided on the casting so that it can be removed during machining to obtain a smooth surface.
- Q8: Briefly explain Shake or Rapping Allowance.Ans: Rapping allowance is a negative allowance given because the pattern cavity becomes slightly larger when the pattern is shaken before removal.
- Q9: What is the shrinkage allowance for Cast Iron?Ans: The shrinkage allowance for cast iron is about 10 mm per meter (or 1%).
- Q10: What is the shrinkage allowance for Steel?Ans: The shrinkage allowance for steel is about 20 mm per meter (or 2%).
- Q11: What is the shrinkage allowance for Brass?Ans: The shrinkage allowance for brass is about 15 mm per meter (or 1.5%).
- Q12: State the factors affecting Machining Allowance.Ans: The factors are:
- ➔ Type of metal
- ➔ Size and shape of casting
- ➔ Surface finish required
- ➔ Method of casting
- Q13: Quote the price of Machining Allowance.Ans: The machining allowance generally ranges from 1 mm to 5 mm depending on the size and type of casting.
- Q14: Quote the price of Rapping Allowance.Ans: The rapping allowance is usually small and negative, about 0.5 mm to 1 mm depending on the pattern size.
