Pattern Allowance in Casting Dr. S. R. Dulange (dulangesr@gmail.com) Associate Professor, Dept. of Mechanical Engineering Walchand Institute of Technology, Solapur. (www.witsolapur.org)

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

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

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 4

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

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

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

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

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 = 0.1875 inch =  0.2 inch For dimension 14 inch, allowance = 14 X 0.125 / 12 = 0.146 inch =  0.15 inch For dimension 8 inch, allowance   =  8 X 0.125 / 12 = 0.0833 inch =  0. 09 inch For dimension 6 inch, allowance   =   6 X 0.125 / 12 = 0.0625 inch =  0. 07 inch Walchand Institute of Technology, Solapur

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

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 6 - 0.07= 5.93inch Walchand Institute of Technology, Solapur

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

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

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

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 Walchand Institute of Technology, Solapur

Walchand Institute of Technology, Solapur Pattern Dimensions Walchand Institute of Technology, Solapur

Walchand Institute of Technology, Solapur Thank you Walchand Institute of Technology, Solapur