1. Pattern Allowance in Casting
Dr. S. R. Dulange
Associate Professor, Dept. of Mechanical Engineering
Walchand Institute of Technology, Solapur. (

2. Walchand Institute of Technology, Solapur
Learning Outcome
At the end of this session, Students will able to understand allowances and its application on pattern in casting process
Walchand Institute of Technology, Solapur

3. Walchand Institute of Technology, Solapur
Principle of Casting
Refractory mold  pour liquid metal  solidify, remove  finish
Walchand Institute of Technology, Solapur

4. Walchand Institute of Technology, Solapur 4
Pattern Allowance
1.Shrinkage or contraction allowance
2. Machining or finish allowance
3. Draft or taper allowance
4. Distortion or camber allowance
5. Rapping allowance
Walchand Institute of Technology, Solapur

5. Walchand Institute of Technology, Solapur
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, is provided in the mold.
Source: Virtual Machine Shop
Walchand Institute of Technology, Solapur

6. Walchand Institute of Technology, Solapur
Shrinkage Allowance
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.
Source: Virtual Machine Shop
Walchand Institute of Technology, Solapur

7. Walchand Institute of Technology, Solapur
Shrinkage Allowance
Table1 Rate of contraction for different materials
Source: D. C. Ekey and W. R. Winter, Foundry Technology, New York. McGraw-Hill, 1958
Walchand Institute of Technology, Solapur

8. Walchand Institute of Technology, Solapur
Exercise
Q. No.1 The pattern is to be made for the below drawing from wood material. The product material is cast iron. Find the dimensions of the pattern by considering shrinkage and machining allowance.
Walchand Institute of Technology, Solapur

9. Adding Shrinkage Allowance
The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet (as per  Table 1)
For dimension 18 inch, allowance = 18 X 0.125 / 12 = inch =  0.2 inch
For dimension 14 inch, allowance = 14 X 0.125 / 12 = inch =  0.15 inch
For dimension 8 inch, allowance   =  8 X / 12 = inch =  inch
For dimension 6 inch, allowance   =   6 X / 12 = inch =  inch
Walchand Institute of Technology, Solapur

10. Walchand Institute of Technology, Solapur
Reflection Spot- Think and Write
Question
What will be the dimension of hole after adding shrinkage allowance?
Pause this video now and answer this question
Walchand Institute of Technology, Solapur

11. Walchand Institute of Technology, Solapur
Question
What will be the dimension of hole after adding shrinkage allowance?
Answer
The shrinkage allowance of 0.07 inch is deducted from the hole diameter instead adding, hence
= 5.93inch
Walchand Institute of Technology, Solapur

12. Walchand Institute of Technology, Solapur
Casting Dimensions
Walchand Institute of Technology, Solapur

13. Necessity of Machining Allowance
For removing surface roughness, slag, dirt and other imperfections from the casting.
For obtaining exact dimensions on the casting.
To achieve desired surface finish on the casting.
Source: Virtual Machine Shop
Walchand Institute of Technology, Solapur

14. Walchand Institute of Technology, Solapur
Machining Allowance
Table 2
Source: D. C. Ekey and W. R. Winter, Foundry Technology, New York. McGraw-Hill, 1958
Walchand Institute of Technology, Solapur

15. Adding Machining Allowance
The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12 inch to 20 inch is 0.20 inch (as per Table 2)
For dimension 18 inch, allowance = 0.20 inch
For dimension 14 inch, allowance = 0.20 inch
For dimension 8 inch, allowance   = 0.12 inch
For dimension 6 inch, allowance   = 0.12 inch
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16. Walchand Institute of Technology, Solapur
Pattern Dimensions
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17. Walchand Institute of Technology, Solapur
Thank you
Walchand Institute of Technology, Solapur