Since the part quality requirements are always rising, the check of geometric dimensions including comprehensible documentation is essential in the manufacturing process nowadays. Before acquiring a new 3D measuring system, the fundamental question arises as to which technology is the most suitable for the corresponding measuring task. Is a tactile 3D measuring system needed that captures all relevant measuring points with a measuring probe? Or is an optical 3D measuring system preferable that digitizes entire surfaces in a contactless manner? This article explains the basic functions of both methods and investigates the advantages, differences and application fields in the automotive industry.
Tactile Measurement Technology – Point-Based Acquisition
In the field of dimensional workpiece testing, coordinate measuring machines (CMMs) are the most known systems in traditional measurement technology. CMMs work with tactile or scanning measuring systems. For the measurement, the probe is positioned at the desired measuring point. Optionally, a controlled rotation table can be used for rotating a part. The connected measuring software computes the geometrical elements from the captured individual points and derives the actual values from these for the object characteristics that have to be inspected.
Tactile Measurement Technology – First Choice For High-Precision Parts
The tactile measuring technology convinces especially with its very high absolute accuracy and is therefore still the first choice for measuring high-precision parts. A stationary CMM can measure points with an accuracy of a thousandth millimeter. As of today, such a high accuracy cannot yet be achieved with optical 3D measurement technology.
Optical 3D measurement technology is very well suited if the accuracy requirements are only within a hundredth millimeter range. If you want to acquire a new measuring system and are not sure whether you want a tactile or optical one, finding out which accuracies are needed is the first thing to do. A rule of thumb says that the system accuracy of the measuring system should always be increased by a factor of five to ten than the highest required tolerance to be measured. This means: If the tolerance of a characteristic is, for example, 0.1 mm, the measuring device should have an accuracy of at least 0.02 mm.
In the automotive industry, gears, crankshafts and engine blocks are the classic candidates for tactile measurements: The tolerances and accuracies to be adhered to of these parts require the highest possible degree of precision. A gear from the automotive industry normally requires an accuracy of 1 µ or higher. This accuracy is currently barely given with optical measuring systems.
Tactile Measurements – Lengthy and Not Full-Field
What speaks against tactile measurement technology is the high time investment if higher data densities are required: Probing hundreds of measuring points on one workpiece can take a long time, sometimes several hours. Therefore, a complete check is barely possible in production – because of the time investment and because many CMMs often cannot be placed directly in production. To save time, the number of measuring points can be reduced, but this is at the expense of the data density. Here, the ratio of time investment and data density always has to be carefully weighed.
No matter how many measuring points are captured under highest care: It is not possible to measure the entire surface of the measuring object. This is where the optical measurement technology comes into play: Optical measurement technology is not only faster but also creates a digital image of the entire measuring object and therefore delivers more detailed quality information than tactile measurement technology.
Optical Measurement Technology – Full-Field Acquisition of Object Surfaces
Optical measuring systems (e.g. laser scanner, photogrammetry or fringe projection systems) measure the measuring object with optical sensors. The measurement is non-contact; the measuring sensor is never touching the surface of the measuring object. The non-contact measuring principle has a decisive advantage on sensitive object surfaces: Damage on the measuring object can be safely prevented. Wear and tear as known from tactile measuring systems also cannot occur.
Optical Measurement – Fast With High Data Density
The measuring procedure with optical 3D measuring systems is very simple: The measuring object is placed in front of the sensor – either manually or robot-guided. Then, the image acquisition begins: The measuring sensor captures step by step every side of the measuring object. To capture the entire surface, either the workpiece is moved, so that the sensor can capture all areas, or the sensor itself is moved around the workpiece. Then, the connected measuring software automatically transforms all individual measurements into a common coordinate system. This results in a complete 3D point cloud of the object’s surface. The generated measuring data allows various inspections, for example, full-field nominal-actual comparisons of the object geometry or the check of GD&T elements. Based on color deviation representations, problematic areas can be easily recognized and therefore enable a targeted improvement of the manufacturing process. In this way, unnecessary iteration loops can be avoided.
Another advantage of optical 3D measurement technology: The measuring procedure is extremely fast. The digitization of complex workpieces only takes a few minutes – sometimes only seconds.
Examples for the use of optical 3D measurement technology can be found galore in the automotive industry: from process planning for machine capability analyses to automated quality control in pressing plants and car body manufacturing as well as the check of cast, forged, and plastic parts up to process optimization in the final assembly.
Surface Preparation Necessary In Some Cases
High-gloss parts in engine manufacturing (e.g. gears, crank cases, cylinder heads), mirroring milling parts or translucent parts made out of glass, synthetic resin or very light plastics are a challenge for optical sensors. The reason is simple: A measurement with light does not work or only works restrictedly on translucent, reflecting or mirroring materials, unless a workaround is used: spraying the measuring object with a special scanning spray before measuring. The spray lays itself on the object’s surface and therefore makes scanning with an optical measuring system possible. This trick turns optical measurement technology into a valid alternative to tactile measurement metrology even for demanding surfaces and serial production: Application examples have shown in many places that optical measurements including automatic spraying using a robot and a subsequent cleaning of the measuring object are still faster than measurements with a tactile measuring system.
Combining Both Methods
By now, more and more measuring systems are coming into the market that combine both measuring methods: For accelerating the measurement and making it possible the measure touch-sensitive surfaces, CMMs can be equipped with an optical sensor. On the other hand, optical measuring systems can be extended with a probe so that it is possible to capture areas of a workpiece that are optically hard to reach, such as deep bore holes, pockets or undercuts. In this regard, one important aspect has to be kept in mind: The system accuracy of optical 3D measuring systems cannot be increased by an additional probe – it is just possible to capture additional object characteristics on complex structures.
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