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Precision components often have to be produced with an accuracy of a few micrometers. Even the most modern processing machines reach their limits. A new type of optical sensor from the Fraunhofer IPM ensures the necessary component quality: the holo-cut sensor records 3D structures over a large area and with micrometer precision directly in the machine tool.
In the manufacture of precision components, accuracies are required that even the most modern processing machines often cannot reliably deliver. Even different trajectories, slightly worn tools or incorrectly calibrated sensors can lead to component geometries that are outside the desired specification – a few micrometers are often decisive.
For quality control, the components must therefore be measured with appropriate precision. This is typically done with coordinate measuring machines in special measuring rooms outside the machine tool. This tactile measurement is cumbersome, slow and only possible in random samples. And: After the test has been carried out, the workpiece must be set up again in the machine for any reworking that may be necessary.
Measure component surfaces over a wide area
The digital-holographic measuring system enables real 100% quality control in the production process for the first time. It uses digital multi-wavelength holography for 3D measurement. The system records the topography of even rough object surfaces with interferometric accuracy. It measures the surfaces of components without contact and with high precision and works so quickly and robustly that it can be integrated directly into the machine tool to measure there. The workpiece can remain set up during the measurement, so the measured values can flow directly into a control loop for post-processing.
The greatest user benefit of the sensor comes from the timely, micrometer-precise detection of the entire machining surface, so that the actual data obtained can be fed back into the machining process immediately. This allows the production process to be monitored on the one hand, and the quality of each individual workpiece to be 100% guaranteed on the other. Iterative control mechanisms, which were previously only possible on a random basis through time-consuming 3D measurements in separate measuring rooms, can now take place between the individual processing steps in the machine.
Faster introduction of new products
The components do not first have to go through the entire manufacturing process until the expensively manufactured component is finally identified as defective. This accelerates the introduction of new products and at the same time minimizes waste. Another advantage is the cost-effective integration into existing manufacturing processes: Costly handling processes are almost completely eliminated by using it directly in the machine tool.
To prove the performance and practical relevance of the sensor, the IPM has already carried out extensive tests. It shows exemplary 3D data from two similarly milled test surfaces – one with optimally functioning temperature control (a) and one with a temperature sensor that was incorrectly calibrated due to mechanical influences (b). In both milling processes, the machine tool completed the machining without an error message. The machining was completely identical, both the machining parameters and the milling tools were exactly the same.
The results speak for themselves: By precisely recording the faulty processing on the test surface (b), this can be corrected afterwards – without having to set up the workpiece again. It is important to note that the color scales represent different structural depths when comparing the 3D measurement data: For the detailed area (a), measured values from -3 to +3 µm are shown color-coded; for the faulty detailed area (b), the measured values extend over the range from – 9 to +9 µm.
100% control in the machine
For the first time, the digital-holographic measuring system allows real 100% quality control in a machine tool. The sensor captures macroscopic topographies with axial resolution down to the sub-micron range. Roughness measurements are also possible on functional surfaces that are difficult to access. For the first time, It offers the possibility of high-precision process control and post-processing – without having to set up a workpiece again.
Due to the short measurement time, inline measurements in an industrial environment are completely non-critical. The measuring system can thus be the decisive building block for an automated inspection of high-precision components. The bottom line is that the enormous advantages of eliminating complicated handling procedures reduce the testing costs for the entire process.
Identify waste early
In this way, the sensor detects rejects at an early stage, so that different process errors can be identified directly in the processing result and corrected immediately by feeding them back into the manufacturing process. In this way, milling parameters such as infeed or cutting speed as well as the trajectory of the milling head can be optimized. The wear and tear of the tool is also precisely detected so that it can be replaced at the optimum time.
The sensor is designed in such a way that it can be easily integrated into a wide variety of machine tool types. In the current version, the sensor head weighs 7.5 kg and measures 140 mm × 235 mm × 215 mm. With a typical measuring field of 20 mm × 20 mm, the measuring system achieves a lateral resolution of less than 7 µm; the working distance is around 300 mm. Using the example of a heat sink for laser assembly, It shows which dimensions and surface parameters are available in fractions of a second for feedback in the machining process. The reproducibility of the height measurements is better than a micrometer.
Digital holography is an interferometric process that can capture the macroscopic topography of a component surface with microscopic accuracy (see additional information on the topic).
In practice, such methods often fail due to complex structures such as inclines, deep grooves, high edges and holes. In the case of steep object edges, the calculated height relief can usually no longer be evaluated unequivocally.
This problem is solved with the sensor using several narrow-band lasers with different measurement wavelengths. The process is therefore called digital multi-wavelength holography. So-called synthetic wavelengths are generated from several wavelengths in the computer in order to cover a larger measuring range – depending on the roughness of the surface from the (sub-)micrometer to the millimeter range. This allows the sensor to capture macroscopic topographies with microscopic accuracy.
The digital-holographic reconstruction of the topography of the test object is very computationally intensive. Just a few years ago, the computing time was a limiting factor for the duration of the measurement. Due to the computing power of modern graphics cards, however, the evaluation time has receded into the background: 3D measurement systems now evaluate more than 100 million 3D measurement points per second and thus have a unique position worldwide in terms of accuracy and speed.
