Measuring Roughness with Coordinate Measuring Machines
The measurement of roughness close to production is becoming increasingly important. In connection with the measurement of gears, roughness parameters are of great interest, for example, as they not only describe the surface topography in the micro- and nanometer range, but also characterize the running behavior of gearboxes. In addition to roughness, waviness also plays a role. This differs from roughness primarily in the spatial frequency range under consideration. The spatial frequency ranges are separated according to DIN EN ISO 21920 (formerly DIN EN ISO 4287 and DIN EN ISO 13565) by applying profile-based Gaussian filters. The question arises as to what extent classic coordinate measuring machines can also be used to measure waviness and roughness, as well as the corresponding parameters.
In addition to tactile measurement applications, CMMs have been upgraded in recent years, particularly for optical measurement tasks. In the course of this, the already solid machine base was further optimized to meet the increasing requirements in terms of accuracy, measurement speed, and multi-sensor technology. This created a basis that can be expanded to include roughness as an additional feature.
With tactile, profile-based roughness measuring systems, a basic distinction is made between two types of systems: sliding block systems and cantilever systems. With sliding block systems, the object surface is morphologically pre-filtered using a sliding block (see roughness measurement on WENZEL measuring machines with the REVO RFP2 sensor from Renishaw). The roughness measuring tip is supported by the roughness measuring skid, resulting in a very small measuring circle. The use of sliding skid systems drastically reduces the requirements for the positioning base measuring system. The control behavior of the machine and external vibration influences are less relevant, as a slight temporal deviation in position can be averaged out by the referenced sliding block. The shape of the surface is already removed by the morphological filter (the sliding block). The roughness measuring tip thus follows the contour of the profile to be measured. The prerequisite is that the CMM can perform sufficiently accurate pre-positioning. Standard-compliant evaluation of common roughness parameters, such as Ra or Rz, is possible, for example. However, the interaction is accompanied by an undesirable distortion of the measurement result, especially in the medium and long-wave spatial frequency range. The filter effect distorts the detection of waviness and shape, so that standard-compliant evaluation, for example of the W and P parameters according to DIN EN ISO 21920, is no longer possible without restriction. However, the W parameters in particular are decisive for noise behavior (see FFT analysis by WENZEL Metrology GmbH). An evaluation of Abbott-based parameters such as Rv, Rp, and Rk (formerly DIN EN ISO 13565) is also no longer possible without further ado.
Upper figures: Waviness and roughness of a standard (Ra = 1.6 µm) and profile-based FFT analysis
In order to map these parameters as accurately as possible, sliding blocks should not be used; instead, cantilever roughness measuring systems should be employed. For this reason, WENZEL Metrology GmbH relies on a cantilevered measuring system in the form of the WM | RS-T sensor as an alternative to the sliding block system. The cantilevered roughness sensor does not use a sliding block and reproduces the measurement profile of the surface as accurately as possible. The only deviations result from the unavoidable interaction of the roughness measuring tip with the measuring object and the control behavior of the machine. The primary profile, which forms the basis of the measurement, can be derived from the coordinate measuring machine or the sensor itself. When the machine is in motion, the requirements for the machine base are much higher and depend in particular on its design. The quality of the machine base and the interaction between the machine (mechanics) and the controller are therefore crucial. When measuring with the WM | RS-T sensor using the , the machine is held in a fixed, controlled position. This reduces the requirements for the control behavior of the CMM, which is why CMMs with a large measuring circle can also be used for measurements with this type of sensor. The feed movement required to measure the primary profile (as the basis for waviness and roughness analysis) is achieved by a feed axis integrated in the sensor. This ensures high positional accuracy through homogeneous movement of the roughness measuring tip. In contrast to competing systems, the use of a sensor-side reference ball to support the measuring system has been deliberately omitted. Supporting the system with such spheres is intended to reduce the measuring circle and minimize machine influences. To a certain extent, it represents a compromise between a cantilevered system and a sliding skid. However, WENZEL Metrology GmbH does not see the need for such an aid, especially since there is one decisive argument against its use: accessibility.
By dispensing with the use of a sensor-side reference ball, the sensor technology can also be used to measure objects under difficult geometric conditions. Such geometric difficulties arise, for example, in radial waviness and roughness measurement on gear flanks with particularly small modules (module 2 or smaller). In such a measurement task, the measuring range is further restricted by the reference sphere attached to the side, taking into account the required stroke of the roughness needle. In addition, there is a decisive criterion that is particularly important for WENZEL Metrology GmbH: The WM | RS-T roughness sensor has two axes of rotation (R- & T-axis), which can be moved continuously in the range of 0° to 360° or -90° to +90° in order to achieve the best possible positioning relative to the workpiece. T-axis) that can be moved continuously in the range from 0° to 360° or -90° to +90° in order to achieve the best possible positioning relative to the workpiece. Combined with a rotary table axis (C) and the standard coordinate axes (XYZ), this results in a total of 6 positioning axes that cover all degrees of spatial freedom – perfect for gear-related measuring tasks on GT machines, among other things. The WM | RS-T sensor is not treated as a sensor by the software, but as a measuring head that can be automatically exchanged for a scanning measuring probe (e.g., Renishaw SP600) using a specially developed exchange interface. In order to calibrate such a measuring head in the coordinate system of the measuring machine, it is necessary to calibrate the system with different swivel positions on a calibration sphere. The procedure is completely analogous to already known measuring heads such as the Renishaw PH10-M or PHS-2. For this purpose, the WM | RS-T sensor has, among other things, the ability to record individual measuring points.
The WM | RS-T easily transforms a coordinate measuring machine into a roughness measuring device. Integration into the WM | Quartis R2025-2 user software is a matter of course and opens up a wide range of possibilities for roughness-based measuring tasks in the near future.
Author: Dr.-Ing. François Torner Head of System Development – Wenzel Metrology GmbH
For more information: www.wenzel-group.com
