Technical sharing on form and position tolerances

    Geometric tolerances, also known as form and location tolerances, include form tolerances and location tolerances. Any part is composed of points, lines, and surfaces, which are called features. After machining, the actual features of the part always have errors relative to the ideal features, including form errors and location errors. Such errors affect the function of mechanical products. Corresponding tolerances should be specified during design and marked on the drawings according to the specified standard symbols.20Century50Around the turn of the century, industrialized countries already had geometric tolerance standards. InternationalStandards Organization(ISO)in1969year published geometric tolerance standards'1978'year recommended the detection principles and methods of geometric tolerances. China in1980year promulgated the standards for form and location tolerances, which include detection regulations. Form tolerances and location tolerances are combined and simply called geometric tolerances.

    Machined parts will have size tolerances, so the actual shapes or relative positions of the points, lines, and surfaces that constitute the geometric features of the parts differ from the shapes and relative positions specified by the ideal geometric body. This shape difference is the form tolerance, while the difference in relative position is the location tolerance. These differences are collectively referred to as geometric tolerances.(Geometric tolerances).

    Geometric tolerances include form tolerances and location tolerances, while location tolerances include orientation tolerances and position tolerances.

Geometric Tolerance Representation Methods:

Form Tolerances

1、Straightness: The symbol is a short horizontal line (-), which is an index that limits the amount of variation of the actual straight line relative to the ideal straight line. It is a requirement for the straightness of a line.

2、Flatness: The symbol is a parallelogram, which is an index that limits the amount of variation of the actual plane relative to the ideal plane. It is a requirement for the flatness of a plane.

3、Roundness: The symbol is a circle (○), which is an index that limits the amount of variation of the actual circle relative to the ideal circle. It is a requirement for the circular profile in a right section (a surface perpendicular to the axis) of parts with cylindrical surfaces (including conical surfaces and spherical surfaces).

4、Cylindricity: The symbol is a circle between two oblique lines (/○/), which is an index that limits the amount of variation of the actual cylindrical surface relative to the ideal cylindrical surface. It controls various form errors in the cross-section and axial section of the cylinder, such as roundness, straightness of the generatrix, and straightness of the axis. Cylindricity is a comprehensive index of various form errors of the cylinder./○/)

5、Profile of a line: The symbol is a convex curve (⌒), which is an index that limits the amount of variation of the actual curve relative to the ideal curve. It is a requirement for the shape accuracy of non-circular curves.

6、Profile of a surface: The symbol is a semicircle on top with a horizontal line below. It is an index that limits the amount of variation of the actual surface relative to the ideal surface. It is a requirement for the shape accuracy of surfaces.

Orientation Tolerances

1、Parallelism (∥): Used to control the directional deviation of the measured feature (plane or line) on the part relative to the datum feature (plane or line)0°requirement, i.e., the measured feature is equidistant from the datum.

2、Perpendicularity (⊥): Used to control the directional deviation of the measured feature (plane or line) on the part relative to the datum feature (plane or line)90°requirement, i.e., the measured feature is at90°。

3、Angularity (∠): Used to control the directional deviation of the measured feature (plane or line) on the part relative to the datum feature (plane or line) from a given angle (0°~90°) degree, i.e., the measured feature is at a certain angle (except90°) to the datum.

Position Tolerances

1、Coaxiality (◎）Used to control the degree of non-coaxiality between the measured axis and the datum axis that should theoretically be coaxial.

2、Symmetry: The symbol is three horizontal lines with a longer middle line, generally used to control the degree of non-coincidence between the measured feature (center plane, center line, or axis) and the datum feature (center plane, center line, or axis) that are theoretically required to be coplanar.

3、Position: The symbol is a circle with two mutually perpendicular straight lines, used to control the amount of variation of the measured actual feature relative to its ideal position, which is determined by the datum and the theoretically correct size.

Runout Tolerances

1、Circular Runout: The symbol is an oblique line with an arrow. Circular runout is the difference between the maximum and minimum readings obtained in a given direction by a position-fixed indicator when the measured actual feature rotates around the datum axis without axial movement for one revolution.

2、Total Runout: The symbol is two oblique lines with arrows. Total runout is the difference between the maximum and minimum readings obtained in a given direction by an indicator during the continuous rotation of the measured actual feature around the datum axis without axial movement, while the indicator moves continuously along the ideal feature line.

Form Errors

Refers to geometric errors that may occur in the geometric shape of geometric elements such as points, lines, and surfaces on parts during processing.For example, when processing a cylinder, the diameters of the various cross-sections of the axis may be different in size, or the cross-section of the axis may not be round, or the axis may not be straight, or the plane may be warped, etc.

Location Errors

Refers to the potential positional errors of structural elements on a part during machining.For example, the axes of rotation of a stepped shaft may be offset.There is a highly efficient method for measuring various geometric errors. It involves directly using a data acquisition instrument connected to various indicators, such as dial indicators. The data acquisition instrument automatically reads and analyzes the measurement data, eliminating manual measurement and data analysis and greatly improving the efficiency of mechanical measurement.

Measuring instruments: runout gauge, dial indicator (or other indicator), data acquisition instrument

Measurement principle: The data acquisition instrument can read data from the dial indicator in real time and perform geometric error calculation and analysis. Various geometric error calculation formulas are embedded in our data acquisition instrument software, completely eliminating the need for manual calculation of complex data and greatly improving measurement accuracy.