Support Instructions Measuring Geometric tolerances

Geometric tolerances

Geometric tolerances are a critical part of technical product documentation and are used to define the precise geometric characteristics of parts and components. Tolerances ensure that product parts meet the required quality and functionality standards, thereby reducing the risk of errors and incompatibilities during assembly and use.

Importance of Geometric Tolerancing

Geometric tolerances are an essential component of the Geometrical Product Specifications (GPS) system, providing a standardized language for defining and communicating the geometry of products in technical product documentation, such as engineering drawings. GPS enables the unambiguous documentation and communication of product geometry requirements, reducing the risk of errors and misunderstandings in design, manufacturing, and inspection.

ISO GPS and ASME Y14.5

There are two significant international systems for geometric tolerancing:

ISO GPS (Geometrical Product Specifications):

  • Widely used in Europe and many other countries.
  • Defines geometric tolerances using a series of standards, the most important being ISO 1101, which determines form, orientation, location, and run-out tolerances.

ASME Y14.5:

  • Strong position especially in North America.
  • Provides a similar yet slightly different system for defining geometric tolerancing.

Classes of Geometric Tolerances

Geometric tolerances are divided into four main classes: form, orientation, location, and run-out tolerances. Each class includes different tolerances that describe various geometric characteristics of the part. 

Form Tolerances

Form tolerances specify the shape of individual geometric elements without reference to other elements. They indicate how accurately a certain element must adhere to the defined geometry.

  • Straightness: Determines how straight a line or axis of an element must be. Straightness tolerance is applied either to a line or a surface.
  • Flatness: Determines how flat a surface must be. Flatness tolerance limits the deviation of a surface, requiring it to stay within a certain permissible area.
  • Circularity: Determines how round a particular cross-sectional circle must be, considering both the wave and symmetrically distributed deviations.
  • Cylindricity: Determines how cylindrical an element must be, involving a combination of circularity, straightness, and flatness.
  • Profile of a Line: Specifies the shape of a certain line on a particular surface.
  • Profile of a Surface: Determines the three-dimensional accuracy of a surface's shape.

Orientation Tolerances

Orientation tolerances define the relative directions of geometric elements. These tolerances compare one element to another or to a specific reference point.

  • Parallelism: Determines how well an element (e.g., a line or surface) is parallel to another reference point.
  • Perpendicularity: Determines how perpendicular an element is with respect to another reference point. This pertains to the 90-degree angle between two surfaces.
  • Angularity: Determines a particular element's angle in relation to another reference point. The angle can be any degree, not just 90 degrees.
  • Profile of a Line: Specifies the shape of a certain line on a particular surface concerning a specified reference.
  • Profile of a Surface: Determines the shape accuracy of a surface concerning a specified reference.

Location Tolerances

Location tolerances establish the positions of geometric elements relative to each other. These tolerances ensure that elements are precisely placed in relation to one another.

  • Position: Determines the exact location of geometric elements in relation to defined reference points.
  • Concentricity: Specifies how well the center of an element is aligned with the common center of another element.
  • Symmetry: Determines the symmetry of an element concerning a central line or point.
  • Profile of a Line: Specifies the shape of a line concerning a particular position and reference.
  • Profile of a Surface: Specifies the shape of a surface concerning a particular position and reference.

Run-out Tolerances

Run-out tolerances define the position and orientation of an element as it rotates around an axis. These tolerances ensure the balance and accuracy of the element in rotating application situations.

  • Circular Runout: Determines an element's deviation as it rotates around an axis, considering both element orientation and position.
  • Total Runout: A broader measure of runout, determining the entire surface deviation of an element as it rotates around an axis, including all orientation and position deviations.

Geometric Tolerance Annotations

Geometric tolerance annotations are standardized symbols depicted in drawings within a rectangular tolerance frame. The tolerance frame states the following information in order:

  • Tolerance identifier: For example, the symbol for straightness “| |”.
  • Numerical tolerance value: For example, “0.1 mm”.
  • Additional specifications (if necessary): Such as material requirements.
  • Reference points: Define other elements to which the toleranced element is related.
  • The tolerance frame is connected to the toleranced element with a leader line, ending typically with an arrow or dot, pointing precisely to the toleranced feature.

Dimensional Tolerances and Geometric Tolerance

Geometric tolerances complement dimensional tolerances. Dimensional tolerances (ISO 14405) define the permissible limits for the dimensions of a part, whereas geometric tolerances (ISO 1101) specify the precise geometric characteristics of certain elements, such as straightness, flatness, and parallelism. Geometric tolerances provide detailed information on the tolerances of a component's functional parts, which dimensional tolerances alone cannot describe.

Measurement Techniques and Geometric Tolerances

Measurement methods used to determine and validate geometric tolerances must adhere to strict standards and guidelines. Various measurements, such as with coordinate measuring machines (CMM), can precisely determine and verify geometric tolerances. Calibration of measuring devices and reporting measurement uncertainties are essential components of the GPS standard requirements, ensuring measurements are reliable and traceable to international or national standards.

Summary

Geometric tolerances are indispensable in technical drawings and product documentation, providing an accurate and unambiguous way to define and communicate the geometric features of a product. Geometric tolerances are standardized in international systems like ISO GPS and ASME Y14.5, defining the symbolic language and requirements for geometric accuracy and measurements. When used properly, geometric tolerances improve product quality, reduce errors, and ensure compatibility of components, thereby facilitating the entire manufacturing process and the use of final products.

Geometric tolerances are widely used in CAD/CAM systems and measurement device software that support ISO GPS. They enable the implementation of precise and consistent quality control practices in various production processes. Properly employed, geometric tolerances reduce quality issues, enhance the compatibility and reliability of products and parts, and support compliance with technical standards and regulations.

Thus, geometric tolerances are essential across all sectors of technology and manufacturing industries, where they help manage and ensure the geometric precision and quality of a product or component.