Computer tomography in gear measurement technology

This article highlights the application of computed tomography (CT) in gearing measurement technology and pays particular attention to the direct comparison of gearing parameters with tactile gearing measuring devices. The aim is to demonstrate the impressive possibilities of CT technology in the area of gear measurement technology and to show the major advantages of eXact computer tomographs in gear measurement technology.

Industrial computed tomography has had a firm place in dimensional measurement technology for several years now. Especially when determining complex geometries that are otherwise inaccessible to optical or probing methods, this is often the only way to check the geometric tolerance of components. A major advantage of the technology is that, in addition to geometric testing, the material of the component is also tested for errors in order to also avoid related component failure.

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Analysis of the ability of CT technology to measure on gears
The possible application of computed tomography in gear measurement technology, however, is still relatively unknown. The reason for this was the previously unavailable option of being able to evaluate CT measurement data in accordance with existing gear measurement technology standards. This resulted in a lack of repatriation. The lack of proof of competence has prevented use in quality assurance up to now. For this reason, WENZEL, as a long-standing manufacturer of high-precision gear measuring machines and fast computed tomography measuring devices, has recently focused on combining the techniques.

Through direct comparison between gearing parameters, determined through tactile measurements and CT scans, it is now possible for the first time to analyse the ability of CT technology to measure gears. This article is intended to inform quality managers in gear measurement technology about the current state of the art and thus contribute to strengthening the role of CT as a measurement technology for gears.

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Advantages and difficulties of plastic gears
Due to energy-saving aspects, the industrial focus on materials for machine components is increasingly shifting away from heavier metal alloys towards lightweight plastics. However, in addition to checking the geometric product specification, this also requires additional testing of the potential durability of the non-metallic components. It is generally known that the durability of machine components made of plastics suffers more from material defects than their counterparts made from harder metal alloys. WENZEL's industrial computer tomographs make it possible that this does not always have to involve an increase in costs.

Industrial computed tomography using X-rays is ideal for non-destructive testing of plastics. In order to establish CT technology as a reliable method for measuring gears, different measurements of transmission parameters are compared with each other. The CT scans are assessed using tactile reference measurements. The presented method for traceable dimensional measurement of gears with CT is based on the proven ISO 10360 guidelines in general and VDI/VDE 2612/13 specifically for measurements on threads and gears.

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Investigations carried out
A diameter straight gearing with a normal module of 1.2 mm made of plastic was selected as the test object.

The size of a measurement object is a very important parameter for computed tomography. The eXact computer tomography series from WENZEL is designed to precisely measure very small or, optionally, large components. A decisive advantage in terms of cost savings is that the eXact L 225 used, with its outstanding output of max. 225 kV, also offers the option of measuring several objects at the same time

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This is possible when the size of the measurement object is in a certain relationship to its characteristic sizes, or vice versa. In the case of gears, it is the diameter of the gear and the module. The selected gear wheel with a diameter of approx. 45 mm and a module of 1.2 mm meets these requirements and can therefore be used in the future for further tests on multi-part pallet measurements.

The GT 450 gear measuring device from WENZEL used as a reference is a high-precision gear measuring machine. Your advantages lie in the accurate and rapid measurement of gears in transmissions. Another special feature of the GT 450 is the WM | Gear software used. The software enables both the control of the machine and the evaluation of the contact points. WM | Gear handles the threading of the mechanical probe into the gearing, recording the contact points and evaluating the tactile point cloud in accordance with standards.

Any gearing, straight, angled and screw gearing can be analyzed in their internal or external variants. The software has the exclusive WENZEL interface called the “virtual coordinate measuring machine”. This makes it possible to no longer carry out the measurements via the CNC control of a WENZEL coordinate measuring machine, but instead to carry them out virtually 1:1 in the memory of a computer using CT data from a WENZEL computer tomograph.

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The illustrations show the results of the test reports for plastic straight gearing. The blue lines represent the measurement points, while the black straight lines represent the theoretical outline. In this setting, a black square on the check of the report paper corresponds to a deviation of 0.25 µm. The measurement results show very good agreement.

Overall, a maximum deviation of less than 5 micrometers was found for all gearing parameters. The average deviations are even less than three micrometers. Although these are initial results, the accuracy achieved is excellent. The results prove that the WENZEL EXACT series is suitable as a measuring device against plastic gears.

The usual required quality classes of 5 - 10 for plastic wheels can be reliably tested using the CT method. This is primarily due to the fact that, in contrast to the tactile area, in the CT method, the probe size can be virtually as small as you like.

Conclusion: WENZEL eXact computer tomographs enable fast and highly accurate measurement of gears and other plastic components. Compared to traditional methods, the devices offer significant time savings and some highlights: Inspection of internal and external gears, testing of material density and structure, and creation of 3D models for further analyses. WENZEL's CT technology enables companies in the gearing industry to identify product defects at an early stage, increase product quality and make the manufacturing process more efficient.

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Author: Dr. Uwe Hilpert, CT Product Manager

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