Comprehensive 3D Machine Tool Measurement

The manufacture of precision components requires accuracies that even the most modern processing machines cannot reliably deliver. Quality control therefore involves high-precision measurement of the components. This is typically done with coordinate measuring machines in special measuring rooms away from the machine tool. This tactile measurement is cumbersome, slow and possible only in random samples. And what is even worse: After the test has been carried out, the workpiece must be set up again in the machine tool for any necessary reworking.

Alternative To Coordinate Measuring Machine

The optical sensor HoloTop NX developed by Fraunhofer IPM provides a remedy: It measures the topography of freshly machined component surface directly in the machine tool. During measurement, the workpiece can remain set up. The optical sensor detects the surface and depth of milling paths over a large area and reveals inaccurate tool settings with micrometer precision.

Process errors are thus identified directly in the processing result and corrected immediately by feedback to the production process. For the first time ever, the HoloTop NX measuring system enables true 100-percent quality control in the production process.

Multi-Wavelength Holography For High-Precision Topographical Measurements

The system uses digital multi-wavelength holography and by using a flexible mechanical interface, it can be integrated into many existing machine tools. Measuring fields of a size of 12,5 x 12,5 mm² can be measured. The surface topography is recorded with high precision from the micrometer to the millimeter range, and with unprecedented measuring speed and robustness.

The HoloTop sensor family also enable fast and highly accurate 3D measurement of component surfaces directly in the production line. Systems are currently available for detecting surfaces between 15 × 15 mm² and 200 × 150 mm² with lateral resolutions between 3 and 30 µm and accuracies down to less than 0.2 µm (3σ). In addition HoloTop systems can be used in the quality control of precision metallic surfaces (especially sealing surfaces) or electronic components (microbump structures or high-current circuit boards).

At the Control exhibition, earlier this year, a ready-to-use HoloTop 9M18 system was presented, which enabled the measurement of an area of 18 × 18 mm² with 9 million 3D points in less than 60 ms. Single point repeatability of less than 1 µm (3σ) achieved under production conditions.

Measuring Macroscopic Topography With Microscopic Accuracy

The tactile measurements or optical probes, which are currently commonplace, are severely limited by the number of measuring points and the significant measuring time that this entails. Furthermore, these methods have a very limited capacity to measure complex structures such as gradients, deep grooves, high edges or holes. Optical alternatives generally require a separate measurement system, meaning that workpieces must be repeatedly set up after inspection. HoloTop NX sets new standards here – the sensor is fully integrable and boasts a very large working distance, a large measuring range and high measuring accuracy.

Digital multi-wavelength holography is based on the principle of interferometry, in which the light of a laser is split into a measuringbeam and a reference beam. While the measuring beam strikes the surface of the sample object, the reference beam is run through a precisely defined optical path within the sensor. Using a camera, the measuring beam and the reference beam are then superimposed. The emerginginterference pattern contains information on the topography of the sample object. With the appropriate numerical methods, the shape of the technical surface can be calculated using the interference pattern in just a fraction of a second. Not only that: Since digital holography captures both the intensity as well as the phase of the measuring beam accurately and comprehensively, it is also possible to numerically calculate its spatial propagation. In this way, the object surface can be measured, even if it is not displayed in high resolution on the camera chip. By using multiple lasers with various wavelengths, HoloTop is able to carry out unambiguous measurements down to the centimeter as well as down to the submicrometer.

For more information: www.ipm.fraunhofer.de

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