Gauge design explained: Inspection fixtures for quality assurance

Gauges in mechanical engineering and modern manufacturing play a central role in meeting the highest quality requirements and standards. Test gauges guarantee fast and accurate test processes without having to rely on complicated measuring technology. Inspection fixtures for material testing machines are easily adaptable to individual components. But what types of gauges and test fixtures are there? How are these structured and what components and materials are suitable when creating such a test fixture? Learn all about this in this article.

What is a test gauge?

In testing technology, a gauge is a device or a testing instrument for checking components. The gauge can be used to determine whether a workpiece meets the required tolerances. The gauge defines the target condition that a workpiece must have to meet the requirements. The actual condition of the workpiece is compared to this target condition. The difference between measuring and gauging is that a physical value is determined by means of a measuring device during measurement, while gauging checks whether a feature is within the specified tolerance—without determining the exact measured value. In our article Fit Types and Tolerances - An Overview you will find further information on the topic of fits.

An inspection fixture is a specialized, often individually designed device that fixes, orients, and, if necessary, automatically or semi-automatically performs certain test processes. It is used in particular when complex geometries, multiple test points, or large numbers of test pieces are involved. Unlike a conventional gauge, the inspection fixture can also be combined with sensor technology, camera technology, or pneumatic elements to speed up test processes and enable more precise evaluation. When designing an inspection fixture, the following questions must be clarified:

• What should be checked?
• How true to shape should the test fixture be positioned? Shape retention is important, for example, for sheets that bend easily.
• At how many points should it be inspected and supported? B-pillars of motor vehicles have many freeform shapes, for example.

The actual test device consists of an interaction of several coordinated components such as limit plug gauges, form support points, base construction, positioning pins and clamping technology. In the example shown here, all components of a Body-in-White part gauge available from MISUMI are colored yellow.

When using gauges and inspection fixtures, sources of error caused by simple measurements are reduced. It can be clearly and reproducibly determined whether the inspected workpiece meets the previously defined requirements or not. At the same time, it is ensured that quality control can be performed systematically and easily.

What additional function can a test gauge and test fixture perform?

• Sorting out defective parts: Especially in series production, defects can be quickly detected and documented. The rejected part can be directly removed and subsequent costs avoided.
• Measurement, parallelism, and flatness testing: By probing at several different measuring points, the component geometry as well as the surface finish can be checked. Irregularities can be detected and corrected faster.
• Testing of positions: Critical locations such as mounting points can be checked for their position. This ensures that the component can also be installed correctly later.

For more information on common measurement methods, see our article on Measuring Technology – Quality Control Through Measuring Methods.

What types of gauges are there?

There is a variety of gauges that can be used to test different properties. Form gauges are used to check whether certain specified parameters correspond to the nominal condition. Geometric contours and profiles such as angles, radii, etc. are checked. The gauge represents the nominal geometry, against which the test piece is held for comparison. Dimensional gauges check that the dimensions of a workpiece are within the specified limits. For example, lengths, widths, or diameters are checked. Using a defined inspection limit, simple yes/no statements can be made about the dimensional accuracy. Classic examples are plug gauges or snap gauges.

For this purpose, the maximum and minimum dimensions are used. One side of the limit gauge represents the optimum, the other the reject side. The test piece is then compared and evaluated.

Limit gauges check whether a workpiece is within specified limit dimensions. For this purpose, the maximum dimension (reject gauge) and the minimum dimension (good gauge) are used. A typical gauge for outer diameters is the limit snap gauge: The good side of the gauge must be able to slide over the testpiece; the reject side must not fit. For internal dimensions such as bores, for example, a plug gauge is used. Here too, the passing of both sides indicates whether the test piece meets the tolerance requirements.

Functional gauges check whether the component can perform its intended function in the later assembly state. Features such as position, shape and dimensions are controlled simultaneously. The gauge simulates the installation situation and ensures that the component can be installed without rework. Gauges compare target and actual values with the aid of measuring probes, dial gauges, etc. Test equipment.

Design of gauges

A central component of any test gauge is the measuring fixture or Inspection fixture that ensures that a component is precisely fixed before tactile or optical measurement. Measuring fixtures/inspection fixtures are individually adapted to the respective test object and allow repeatable series measurements due to the defined orientation. The basis for this is the so-called reference point system (RPS), which determines how the component must be positioned in space based on six degrees of freedom (translations and rotations). Additional practical tips for part orientation are available in the article Position determination of workpieces in positioning devices.

An inspection fixture is usually created for the inspection of exactly one component. Positioning is usually done by using locating pins and stops on a base construction.

In series testing, test devices are often supplemented by proximity sensors, mechanical probes or automatic clamping systems in order to automate and monitor the test and measurement process. Modern gauges are increasingly relying on modular measuring and clamping technology, with which different component variants can be tested quickly and efficiently.

Materials used

The following also applies to material selection: Depending on the application, industry and required precision, the material selection is also adjusted.

Heavy-duty and highly accurate test gauges are usually made of hardened steels. Aluminum, among other things, can be a good choice as a material for the base body and underbody of the device. Finding a middle ground between cost, precision, longevity and efficiency is important. Ideally, the test equipment can be built with a modular design in such a way that it can be reused at least partially after completion of the series production.

In order to evaluate the inspection fixture for the first time, the prototype of the component to be tested is required. Using this prototype, the inspection fixture is checked for dimensional accuracy, function, and shape fidelity. It is also examined whether the test device provides for checking all important functional characteristics of the test specimen or whether further test steps are necessary. The prototype can consist of a wide variety of form-stable materials. In addition to traditional subtractive manufacturing processes, additive manufacturing processes such as 3D printing are also available here. Often, complex geometries can be realized quickly and sufficiently precisely with 3D printing technologies in order to be able to use them as test samples and thus keep the costs as low as possible.

Components used

As already mentioned, various components are used in the construction of test gauges: Positioning pins, shim washers, mounting plates. See our overview of components for clamping and measuring fixtures. Below is a selection of components that can help you create an inspection gage:

What is NAAMS?

NAAMS stands for North American Automotive Metric Standards. NAAMS is a metric-based standardization of stamping and assembly tools used by a variety of automotive manufacturers and their suppliers.

The NAAMS standard originated from the search for ways to reduce the cost of manufacturing stamping tools for the automotive industry. The main focus was on the standardization of components and on consistent quality and compatibility. Many manufacturers worldwide adhere to the NAAMS standard for producing parts with a uniform technical specification. The NAAMS standard includes two specification areas: one for components (e.g. positioning pins, angle brackets) and one for assemblies. The part specification defines specific sizes, shapes, and other characteristics, while the assembly specification provides information on assembling multiple parts. With NAAMS components, test gauges can also be structured and modularly constructed, ensuring high-quality and cost-effective manufacturing.