Meaning
International standards providing specific rules for the indication of tolerances for non-rigid parts establish how to document and inspect components that deform significantly under their own weight. This document defines the conditions under which a part must meet its geometric specifications. For many plastic or thin-walled metal items, the shape measured in a free state differs from the shape when installed.
The standard allows designers to specify a restrained condition for measurement. This means the part can be clamped into a fixture that mimics its final assembly position. Any tolerances specified on the drawing only apply when the part is held in this defined state.
Without this standard, manufacturers and customers would struggle to agree on whether a flexible part is within specification. It provides a common language for managing the inherent elasticity of modern materials.
Restraint Condition
Specification of the forces and locations used to hold a part during inspection ensures that every measurement is repeatable and fair. Under the rules of iso 10579, a drawing must clearly state the clamping sequence and the amount of torque or pressure to be applied. These instructions allow the metrology lab to build a fixture that reproduces the stresses of the final assembly.
If a part is intended to be measured in its free state, the drawing must explicitly say so. This prevents confusion when a part sags or twists due to gravity. The standard also covers the use of supplemental weights or vacuum pressure to stabilize the component.
Accurate restraint is the only way to verify the true geometry of a flexible item.
Geometric Specification
Tolerances for non-rigid parts are often much larger in the free state than in the restrained state. This standard allows the engineer to provide two sets of requirements if necessary. One set ensures the part can be handled and shipped, while the other ensures it will function correctly once bolted down.
The standard uses the letter f in a circle to indicate that a tolerance applies only in the free state. This notation helps the inspector understand which features are critical for fit and which are merely a result of the material flexibility. Clear signaling on the technical drawing reduces the risk of expensive misunderstandings.
It also allows the manufacturer to focus on the dimensions that matter most for the end user.
Compliance Audit
Verification of parts following this standard requires a detailed inspection plan and specialized tooling. Quality auditors must check that the fixtures used in the lab match the specifications found on the engineering drawing. If the fixture is too stiff or applies pressure in the wrong place, it could mask a defect or create a false failure.
Calibration of the clamping forces is a necessary step in the audit process. Digital twins of the fixtures are often used to program coordinate measuring machines before the physical parts arrive. This preparation ensures that the measurement process is efficient and accurate.
Long-term data collection helps determine if the manufacturing process is stable or if the material properties are drifting. Correct application of the standard results in higher assembly yields and fewer field failures.
