Pattern allowances and pattern size

Pattern allowances

Pattern length

Length of the casting, including the machining allowance on both faces:

$$L_{c} = 450 + 2\times 3 = 456\ \text{mm}$$

The pattern must be larger by the shrinkage allowance (1 percent):

$$L_p = 456\,(1+0.01) = 460.56\ \text{mm}$$

Core diameter

For an internal surface, the machining allowance makes the cast hole smaller, because boring will enlarge it. With 2.5 mm removed from the radius (5 mm from the diameter), the cast hole diameter is

$$d_{cast} = 80 - 2\times 2.5 = 75\ \text{mm}$$

As the casting cools, the hole contracts along with the rest of the metal, so the core must be made larger by the shrinkage allowance:

$$d_{core} = 75\,(1+0.01) = 75.75\ \text{mm}$$

Answer: pattern length $= 460.56$ mm (about 460.6 mm); core diameter $= 75.75$ mm.