What are pattern allowances in casting and why are they necessary?

Pattern allowances are modifications made to pattern dimensions to compensate for changes during casting. Key allowances include: Shrinkage allowance (compensates for metal contraction during cooling), Draft allowance (taper for easy pattern removal), Machining allowance (extra material for finishing), Distortion allowance (for irregular shapes), and Shake allowance (for pattern withdrawal). These ensure the final casting meets design specifications.