Geometry tolerance in metrology

Ensuring compliance with the geometric specifications of mechanical parts is a major challenge for manufacturers. Checking these specifications relies on a variety of metrology methods. By selecting the method best suited to the specific characteristics of the part and the requirements of the application, manufacturers can ensure the quality and conformity of their products, while improving their operational efficiency.

Why checking geometrical specifications of a product?

In the industry, guaranteeing product quality is an obvious priority. Checking the shape parameters of many mechanical parts intended for integration into complex assemblies is of crucial importance to ensure their performance, reliability and compliance with the required standards. These include, for example, circularity, roundness, taper, straightness, and flatness. Strict compliance with these geometric tolerances is essential to ensure a proper fit between these parts and guarantee optimal assembly functionality. Various dimensional measurement methods are used to meet these requirements, each with its own advantages, limitations and specific applications.

Geometry tolerances and the pneumatic method

The pneumatic method relies on the use of compressed air to check the important shape parameters of parts. Through a combination of several dimensional measurements, it enables certain geometric specifications such as perpendicularity, parallelism, coaxiality, etc. to be checked. The measuring tools used, such as air plugs or rings, are positioned on the parts to be inspected, but with no need for physical contact.

This method offers the advantage of simple, fast and reliable measurement. It can be applied in any workshop, and therefore does not need to be carried out in a metrology laboratory. It is particularly well suited to industrial environments where speed is crucial.

Although this pneumatic method is effective for measuring parts with simple shapes, it may prove limited when it comes to checking parts with very small dimensions or complex shapes.

Geometry tolerances and touch probes

Metrology measurement using the tactile method is based on the use of LVDT (Linear Variable Differential Transformer) sensors to evaluate distances between different reference points. This method guarantees high measurement accuracy and reliability, making it the preferred choice for applications requiring critical dimensional accuracy, notably in the aerospace and automotive sectors.

However, the tactile method can be limited by the need for physical contact with the part, which can potentially cause surface defects if the right precautions are not taken. In addition, measurement points on the surface of parts must be accessible to allow contact with the sensor.

Geometry tolerances and industrial machine vision

Industrial Machine vision involves using cameras and advanced software to capture, record and analyze digital images of parts in an inspection process.

Although its use is often focused on detecting wrong assemblies or surface defects, it also offers a fast, non-invasive solution for measuring geometric parameters, making it ideal for complex, fragile parts.

The technology offers satisfactory accuracy and can be automated for high-throughput applications, improving the efficiency of manufacturing processes. However, machine vision can be affected by variations in lighting or visual obstructions, which can alter the quality of the images captured and therefore the accuracy of the measurements made.

Geometry tolerances and optical micrometry

Optical micrometry uses a similar approach to industrial machine vision but focuses on analyzing the shadow projected by the part being inspected. This method produces high-contrast pictures, which simplify their processing by imaging algorithms.

Like machine vision, optical micrometry enables rapid, non-invasive measurements, making it an ideal option for checking the geometry of parts without altering them. Often integrated into automated systems, it is particularly well suited to high-throughput applications. However, its capabilities are limited to checking the outlines of inspected parts.

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