Shape and position tolerances
From a motor that works reliably, to the precise functioning of a clock, to the structural integrity of a solid bridge: The applications of shape and position tolerances are diverse and we often encounter them without even realizing it. They are the invisible but crucial factors that ensure that the products and structures around us not only function, but also meet safety and quality standards.
table of contents
- What are shape and position tolerances?
- Why is it so important to know the shape and position tolerance?
- Measurement and evaluation of form and position tolerances in Quality Analysis
- Shape and position tolerance and its relationship to dimensional tolerance
- Measurement and specification of form and position tolerances
- Technical drawings and the specification of tolerances
What are shape and position tolerances?
Shape and position tolerances are specific types of geometric tolerances in technical production. They define the extent to which the shape and position of a manufactured part may deviate from the ideal dimensions and orientation specified in the technical drawings.
These are therefore geometric product specifications that determine how far the shape and position relationship may deviate in order to still be functional. They are important aspects for geometric dimensioning and tolerancing. Shape and position tolerances ensure precision in the manufacture of components and their correct interaction in assemblies. They are therefore essential for the quality and functionality of manufactured products.
Why is it so important to know the shape and position tolerance?
Shape and position tolerances are the key benchmarks for evaluating the geometric precision of components. This is because deviations from the target shape and position can occur during the manufacturing process, which can impair the function and interchangeability of entire assemblies. These unavoidable deviations are tolerable as long as they are within the limits defined by shape and position tolerances.
The deviations can arise for various reasons during the manufacturing process. Possible causes include manufacturing errors, cutting force, machine vibrations or temperature differences. Knowledge of shape and position tolerances is therefore essential for standardization in the manufacturing industry.
Measurement and evaluation
shape and position tolerances in quality analysis
Modern industrial manufacturing demands ever faster production speeds, which is why rapid measurement and validation of component tolerances is becoming increasingly important. At Quality Analysis, we offer you fast and reliable geometric dimensional inspections. Using tactile and optical measurement technology as well as industrial computer tomography, we measure high-precision shape and position tolerances, geometries, undercuts and free-form surfaces of small to large components
- Measurement using tactile and optical measurement technology and industrial computed tomograph
- Tactile and optical dimensional inspection of shape and position tolerances
- Surface comparisons (nominal-actual comparison/actual-actual comparison) to determine form deviations
- High-precision measurement of shape and position tolerances of small to large components
- Measurement of undercuts and geometries in the component
- Fast and precise analysis of your 3D data with the software ZEISS Inspect Pro, Calypso, Mitutoyo or Volume Graphics
Shape and position tolerance and their relationship to dimensional tolerance
Measurement tolerance, shape tolerance and position tolerance are three essential components of the dimensioning and tolerancing of products. Taken together, they ensure the precision and functionality of manufactured components.
The dimensional tolerance (size tolerance) refers to the permissible deviations from the dimensions of a component. These tolerances define how large or small a component may be compared to the specified dimensions. Dimensional tolerances are therefore also fundamental to the accuracy of fit of components and have a direct impact on their interchangeability.
Shape tolerance, on the other hand, determines the permissible deviations in the shape of a component from the ideal geometry. It includes aspects such as roundness, straightness, flatness, line shape and cylindricity. Shape tolerances are crucial to ensure that a component can fulfill its functional role within an assembly, regardless of its dimensions.
The position tolerance defines how precise the position of a component or feature must be relative to other parts or reference points. This includes tolerances for position, coaxiality, alignment, parallelism and angularity. Positional tolerances are critical for the correct assembly and interaction of components in an assembly.
The relationship between the types of tolerance
To ensure the quality and functionality of components, it is important to consider the tolerance types together. While dimensional tolerances define basic dimensions, shape and position tolerances ensure that the parts function correctly if they are within the dimensional tolerances.
However, there is no correlation when it comes to defining them. The principle of independence applies, as they define different specification aspects - meeting one tolerance does not mean that the others have been met. For example, a component may be within its dimensional tolerances, but without adequate form and position tolerances it could still be non-functional as it may not fit properly into the overall structure or have the required structural integrity.
Different measuring devices are required for dimensional tolerance and form and position tolerances. While a caliper gauge or micrometer screw, which is measured between two points, is used for dimensional tolerance, specialized measuring instruments are used to measure form and position tolerances.
Measurement and specification of shape and position tolerances
There are various methods to ensure that shape and position tolerances are measured to ensure that components meet the requirements for precision and quality.
The Tactile metrology
Tactile metrology using a 3D coordinate measuring machine impresses with its accuracy. Here, the component is scanned point by point using a stylus. The scanned points are used to evaluate the shape and position of the component, including the determination of position, surface shape and other geometric properties. Tactile measurement thus enables maximum accuracy in the thousandths of a millimeter range. This makes it ideal for inspecting workpieces where only the smallest tolerances are acceptable. Compared to measurement using computer tomography, the tactile method also makes it possible to measure in a constrained position and to measure large or very heavy objects.
The Optical metrology
In addition to tactile metrology with a 3D coordinate measuring machine, there are other advanced methods for recording shape and position tolerances. Optical metrology systems, such as laser scanners, are ideal for measuring the surfaces of components quickly and without contact. This method is therefore particularly suitable for complex geometries and sensitive materials, as it does not physically affect the component during the measurement.
Industrial computed tomography
Shape and position can also be measured effectively using computer tomography (CT). This method is ideal for measuring complex components made of flexible or reflective materials and – in contrast to optical metrology technology – also captures internal geometries. Industrial computed tomography therefore makes it possible to measure components both internally and externally in a completely non-destructive manner.
Analysis and valuation
Special software can be used to evaluate the measurement data, such as ZEISS Inspect Pro, which takes over the data evaluation after the measurement with an optical measuring device or a computer tomograph. The software makes it possible to inspect and visualize the shape and position tolerance, generate inspections and apply alignment methods and reference systems. For example, surface shapes, roundness and concentricities can be evaluated and visualized.
The results are usually documented in a measuring protocol and often displayed in technical drawings or CAD models together with the tolerance specifications. This integration enables a clear and precise representation of the specifications and requirements.
The documentation plays a decisive role in quality assurance and acts as proof that the components comply with the specified tolerances. VDA-compliant logging can also contribute to compliance with certain standards in the automotive industry. VDA standards specify how measurement results are to be documented in order to ensure uniform and standard-compliant reporting. Such logging not only facilitates quality assurance, but also serves as legal proof of compliance with specified tolerances.
Technical drawings and indication of tolerances
Technical drawings are an essential tool for planning and controlling the design and production of components. Tolerances are specified on these drawings using drawing entries and symbols that are regulated by the ISO 1101 standard.
Shape and position tolerances are typically specified in a tolerance frame on the drawings, which defines the entire width of the tolerance zone. On technical drawings, they can be indicated symmetrically or asymmetrically. These tolerance indications, standardized by specific symbols and notations, provide precise instructions for engineers and technicians to ensure the required accuracy when manufacturing and testing components.
Reference arrows are used to link the tolerance frame to the toleranced element. The position of the arrowhead determines whether the surface or the center plane is meant.
Observing these tolerance specifications ensures that the manufactured parts meet the specified standards and therefore satisfy the highest quality requirements.
