Engineering Guide 16

Probability And Six Sigma Thinking In Tolerance Bands

Original practical guidance for using shaft and hole tolerance calculations in real mechanical design work.

Probability And Six Sigma Thinking In Tolerance Bands is a practical topic because tolerance codes are not decoration on a drawing. They define how a shaft and hole will assemble, how much movement is expected, and how difficult the parts will be to inspect. A useful rule is to document assumptions so manufacturing and inspection teams can reproduce the decision.

For many metric assemblies, a fit such as H7/g6 gives a starting point, but the calculator should be used with the real basic size, unit, tolerance grade, material, and service temperature. A 20 mm joint and a 180 mm joint can share the same code while producing different absolute tolerance bands.

Practical checks

Confirm the basic size first, then choose whether the design is hole-basis or shaft-basis. Use the maximum clearance and minimum clearance from the calculator to decide whether assembly behavior is clearance, transition, or interference. If the result is close to zero, review surface finish, coating thickness, and measurement uncertainty before releasing the drawing.

Manufacturing route matters. Tight grades usually need turning and finish grinding, stable workholding, and a written inspection plan. If the part is low-cost or non-critical, a looser IT grade may reduce scrap without changing the product's real performance.

Documentation note

Record the selected fit, the intended function, inspection temperature, and any assumptions about coating or heat treatment. This makes the tolerance choice easier to audit later and helps suppliers avoid interpreting the same code differently.