ISO Geometric Tolerances - What You Need to Know
Complete guide to geometric tolerances according to ISO. Flatness, perpendicularity, concentricity, and more - how to interpret them?
Geometric tolerances (GD&T - Geometric Dimensioning and Tolerancing) are an advanced system for specifying requirements regarding the shape and position of part features. Unlike dimensional tolerances, they define permissible deviations from geometric form. Understanding this system is crucial for technologists and quality inspectors.
Form Tolerances
Form tolerances specify permissible deviations from ideal geometric form. These include: straightness (—), flatness (▱), circularity (○), and cylindricity (⌭). They do not require a datum reference because they concern the element itself. For example, a flatness tolerance of 0.05 means that all points of the surface must lie between two parallel planes 0.05 mm apart.
Orientation Tolerances
Orientation tolerances specify the orientation of an element relative to a datum. These include: parallelism (//), perpendicularity (⊥), and angularity (∠). They require a defined datum reference. A perpendicularity of 0.02 relative to datum A means that the axis or surface must lie within a tolerance zone perpendicular to datum A with a width of 0.02 mm.
Location Tolerances
Location tolerances include: position (⊕), concentricity (◎), and symmetry (≡). Position specifies where an element should be located relative to datums. Concentricity defines the permissible deviation of an axis from the datum axis. The MMC modifier (Ⓜ) allows for additional tolerance at maximum material condition.
Runout Tolerances
Runout (↗) and total runout (↗↗) are composite tolerances that control both circularity and concentricity simultaneously. A radial runout of 0.03 relative to axis A-B means that during rotation around this axis, indicator variations cannot exceed 0.03 mm. Often used for shafts and bushings.
Feature Control Frame
Geometric tolerances are written in rectangular frames divided into compartments. The first compartment contains the tolerance symbol, the second the tolerance value (with optional modifier), and subsequent compartments contain datum designations. Datums are designated with letters (A, B, C) and their order matters - the first datum is primary.
Summary
Geometric tolerances allow for precise specification of quality requirements that cannot be expressed with dimensional tolerances alone. The Technolog CNC system automatically recognizes GD&T symbols on drawings and includes them in technological analysis, helping with proper interpretation of requirements.
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