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Computer Tomography Use In Plastic Gear Metrology Evaluation

Industrial computed tomography (CT) has been firmly established in dimensional metrology for several years now. It is often the only way to check the geometric tolerance of components, particularly when determining complex geometries that are not accessible by other optical or tactile methods. A major advantage of the technology is that, in addition to the geometric inspection, the material of the component can also be checked for defects in order to avoid component failures.

The following article examines the use of computed tomography in gear metrology, with a particular focus on the direct comparison of gear parameters with tactile gear measuring devices. The aim is to demonstrate the impressive possibilities of CT technology in the field of gear metrology and to show the great advantages of exaCT computed tomography in gear metrology.

Analysis of Measuring Capability of CT Technology on Gears

The possible application of computer tomography in gear metrology is still relatively unknown. The reason for this is that it has not yet been possible to evaluate CT measurement data in accordance with existing gear metrology standards. This resulted in a lack of traceability. The lack of proof of capability has so far prevented its use in quality assurance. 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 technologies.

By directly comparing the gear parameters determined by tactile measurements and CT scans, it is now possible for the first time to analyze the measurement capability of CT technology on 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.

Advantages and Difficulties of Plastic Gears

Due to energy-saving aspects, the industrial focus in terms of 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 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 of harder metal alloys. The industrial computer tomographs from WENZEL make it possible to show that this does not always have to be associated with an increase in costs.

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

Tests Performed

A spur gear with a normal modulus 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 computed tomography series from WENZEL is designed to precisely measure very small or optionally also large components. A decisive cost-saving advantage is that the exaCT 225 used, with its outstanding performance of max. 225 kV, also offers the possibility of measuring several objects simultaneously.

This is always possible if the size of the object to be measured is in a certain ratio to its characteristic values, 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 fulfills these conditions and can therefore be used in the future for further tests with multi-part pallet measurements.

Tactile gear measuring device GT 450 and Computed tomography exaCT L 225

The GT 450 gear measuring machine from WENZEL used as a reference is a high-precision gear measuring machine. Its advantages lie in the precise and fast measurement of gears in gearboxes. 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 probing points. WM | Gear takes over the threading of the mechanical probe element into the gearing, the recording of the probing points and the standard-compliant evaluation of the tactile point cloud.

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

Plastic Gear Measurement Results between GT450 (left) and exaCT L 225 (right)

The figures show the results of the measurement reports for the plastic spur gearing. The blue lines represent the measuring points, while the black straight lines represent the theoretical contour. A black square on the check of the report paper corresponds to a deviation of 0.25 µm in this setting. The measurement results show very good agreement.

Overall, a maximum deviation of less than 5 micrometers was determined for all gearing parameters. The average deviations are even less than three micrometers. Although these are initial results, the accuracy achieved is outstanding. The results prove the suitability of the WENZEL exaCT series as a measuring device for plastic gears.

The usual required quality classes for plastic wheels of 5 – 10 can be reliably tested using the CT method. This is mainly due to the fact that there is no physical limit for a tactile probe used in the tactile method.


WENZEL exaCT computer tomographs enable the fast and highly accurate measurement of gears and other plastic components. Compared to traditional methods, the devices offer significant time savings and several highlights: Inspection of internal and external gears, inspection of material density and structure, and creation of 3D models for further analysis. WENZEL’s CT technology enables companies in the gear industry to detect product defects at an early stage, improve product quality and make the manufacturing process more efficient.

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