Tolerance (Machining)

Tolerance is the permissible dimensional variation specified on an engineering drawing, defining the acceptable range of sizes that a part dimension can vary while still functioning correctly in its assembly. Standard CNC machining centers achieve tolerance of ±0.001 to ±0.005 inches routinely, with tighter tolerance (±0.0005 inches or better) achievable through precision boring, reaming, or secondary grinding operations.

Tighter tolerance specifications increase cost and cycle time because they require slower feeds, careful tool selection, excellent fixturing, and additional setup time for programmers and operators to verify and optimize. Engineers specify tight tolerance only where necessary, on critical dimensions like bearing bores or fastener holes, while allowing loose tolerance on non-critical dimensions to reduce cost.

Interpreting tolerance specifications, especially in combination with GD&T symbols, is essential for programmers and inspectors to ensure parts are manufactured and verified correctly.