Bonus Tolerance
Bonus tolerance is the additional geometric tolerance allowed under GD&T when a feature of size departs from its stated MMC or LMC. Calculated as the difference between actual size and MMC (for MMC-controlled features), it lets parts pass inspection if geometric error exceeds nominal, provided the feature size offers enough clearance.
On the shop floor, bonus tolerance is a practical tool used in CNC machining to relax inspection and fixturing demands, especially for patterns of holes, pins, or slots where assembly fit is functional but not strictly size-critical. When a hole is drilled slightly above its MMC limit, the extra clearance adds to the allowable position tolerance, preventing unnecessary scrap. Operators first measure the feature size, then add the available bonus to the stated GD&T tolerance to determine the true allowable error. This approach is valuable in high-volume production because it accommodates normal tool wear, spindle deflection, and thermal growth while preserving part function. Without the M or L modifier in the feature control frame, no bonus applies, so understanding when and how to apply it—correcting for hole versus pin direction—is critical for accurate first-article inspection and CMM verification.
When does bonus tolerance start?
Bonus tolerance starts immediately when the actual feature departs from the stated MMC or LMC in the favorable direction; the amount of departure becomes added geometric tolerance.
What tolerances can use bonus?
Commonly position and orientation controls such as perpendicularity, parallelism, and angularity, when the feature control frame includes a material condition modifier and the callout is on a feature of size.
Why is bonus tolerance valuable in machining?
It allows the process to use natural size variation as functional freedom, improving yield and lowering cost while maintaining fit and assembly performance.