Roughness Tester

The outer shape of a surface or a body is determined, among other things, by the basic shape as well as its waviness and roughness. Waviness and roughness describe the relatively small deviations of the surface from the given ideal boundary lines of the respective shape. Roughness not only influences the appearance of the surfaces, but also the suitability of the products for use. Rough surfaces are quite desirable for certain applications, for example to prevent sliding. For this reason, sand is used on the pavements when it is icy or snowy, and floor coverings with a rough surface are used in the areas where an increased risk of slipping is to be expected. In order to achieve the specified roughness, the components are often specially processed, e.g. ground, polished, etched or coated. In this way, the roughness can be specifically reduced or increased.

Standards for describing and measuring the surface roughness

The two-dimensional surface finish of the technical components is described in the following standards, among others:

For the statements on the three-dimensional texture of the component surfaces, ISO 25178 (2016) "Geometrical product specification (GPS) - Surface texture: flat" can be used. There are also industries that work with the ANSI B 46.1 or JIS B 0601 standards.

Characteristic values to describe the roughness measured along a line
The roughness of a surface is usually not described by a single characteristic value but by several. Which characteristic values are relevant for your application is in most cases determined by the surface finish and the intended use of the tested profile.

R_a Mean roughness value, arithmetic mean roughness
The mean roughness value is a calculated average of the absolute individual values of a measured section that deviate upwards or downwards from the ideal profile line. From this value alone, no conclusions can be drawn about the maximum values and the distribution of heights and depths in the tested surface. The average roughness is measured on the sandblasted or polished surfaces, for example.

R_q Square mean roughness value
The root mean square roughness value is determined by summing the squared individual values, dividing by the number of the individual values and taking the square root from the result. This value is also called root mean square (RMS) or standard deviation of the individual measured distance.

R_z mean roughness depth
The roughness depth is the distance between the highest peak and the lowest valley of an individual measurement section. The average roughness depth is determined from the individual values of five individual roughness depths determined one after the other in a line. For this purpose, the five values are summed and divided by five.

R_z1max maximum roughness depth (peak-to-valley height)
The maximum peak-to-valley height of a total line consisting of five individual measured sections is the largest of the individually determined peak-to-valley heights.

R_t total height of the roughness profile
The total height of the roughness profile is the vertical distance between the upper and the lower extreme value of the total measuring section. The lowest valley and the highest peak can be located in different individual measurement sections.

Optical or tactile methods can be used to measure roughness. An optical roughness measuring device scans the surface of the component with light. The surface condition is determined by evaluating the reflected light rays. In tactile measurements, a cone-shaped probe tip is moved along a line over the examined surface. The tip radius and the cone angle of the probe tip influence when the probe tip is deflected upwards or downwards for peaks and valleys. This means that the type of the probe tip also determines how detailed a tactile roughness measuring device can detect peaks and valleys. For both optical and tactile measurements, the roughness tester performs many of the calculations to determine the various roughness parameters.

The standard ISO 3274 (1998) "Geometrical product specification (GPS) - Surface texture: profile method - Nominal characteristics of contact stylus instruments" was developed for the uniform characterisation of the tactile measuring instruments. This standard defines nominal values and basic characteristics of these devices and makes it possible to evaluate the influence of certain device specifications on the measurement result.

Measurement parameters and ranges Depending on which roughness parameters are to be measured, there are more or fewer instrument models to choose from. The next important specification is the measuring range in which the roughness measuring instrument can determine the respective parameter. Many device types can determine the parameters Rz, Rt, Ra, Rq. The measuring ranges of individual devices can differ considerably, for example

There are also the models available that show the parameters Rsm, Rsk, Rp, Rv and Rc. and those in the case of which for the measurements the characteristic values:

- Rmax, R3z, R3zmax, Rz1max, Rmr(c), Rdc and Rmr
- Ry(JIS) /Rz(JIS),
- Rp(ASME), Rpm(ASME), Rv(ASME)

or some others such as Rku, Mr1 and Mr2 can be determined.

Shape of the scanning needle
The shape of the scanning needle is one of the factors that determines how detailed the measurement results are. Common dimensions are:

Radius of the scanning tip: 2 μm / 5 μm / 10 μm
Angle of the cone tip: 60 ° / 90 °

Measuring distance, measuring filter on roughness tester
For many devices, 0.25 mm, 0.8 mm or 2.5 mm can be selected for the length of the individual measuring section. For the total measuring distance, a factor of four, three, two or one can be selected in deviation from the number of five times the individual distance specified in the standard. For some devices, the total sensing distance including pre- and post-travel is limited to 6 mm, so that a measurement of five times 2.5 mm is not possible with these devices.

The filtering of the measured values, which serves to differentiate between waviness and roughness, can be carried out according to various methods. In the roughness measuring device for demanding measuring tasks, it should be possible to set the required filter. Some devices offer different filter methods, for example Gaussian filter, RC filter, PC-RC filter and D-P filter.

Accuracy
As in the case of any measuring device, attention should be paid to the measuring accuracy of the roughness measuring device. The measuring accuracy of many devices is 10 percent or 15 percent, while the reproducibility is 6, 10 or 12 percent, for example. The accuracy can be checked by calibrations, i.e. measurements on workpieces of the known roughness under defined conditions.

Display and control panel
There are considerable differences between the different models, both in terms of the display and the control panel. The choice ranges from a device with a few keys and a display showing only a characteristic value and some additional information to a touch screen with a graphic display of the measured surface profile. It is also important to ensure that the roughness measuring device is easy to operate and easy to read under the prevailing conditions in the laboratory or in the production.

Memory, interfaces and software
For many applications it is helpful if the measurement data can be stored in the roughness measuring device or transferred directly to another device. Data interfaces offered include RS232, micro-USB and Bluetooth. For some models, there is special software that considerably simplifies the evaluation and storage of the measurement data.

Advantages of Bluetooth in the roughness measuring device
If the roughness measuring device is equipped with a Bluetooth interface, the measured values can be transmitted directly to a mobile device and conveniently evaluated, stored and, if necessary, forwarded to this device using an app. In a tactile roughness meter, it is also advantageous for some measurements that the parameters can be set via the associated app and the measurement can even be started via the app. This ensures that the scanning needle is not displaced from the starting point or deflected by the applied vibrations.

Calibration of the tactile roughness tester
ISO 12179 (2000) "Geometrical product specification (GPS) - Surface texture: stylus method - Calibration of tactile roughness gauges" describes how to control the accuracy of the gauges. Many devices can not only be calibrated, but if the deviations found during the calibration are too large, they can also be adjusted. Make sure that the roughness meter you want to use offers such a function. With some models, the roughness meter is already supplied with a roughness standard and a suitable calibration pad, so that the user can calibrate it himself / herself if necessary.

Each surface roughness tester bearing the PCE Instruments brand name is delivered manufacturer or factory calibrated. Calibration certificates are available optionally for purchase. PCE-branded roughness tester devices are equipped with easy-to-read LCD screens, making them well suited for a wide range of industrial applications—from materials testing in the research and development (R&D) lab to quality control on the manufacturing floor. PCE roughness testers offer a long service life due to their resilient, heavy-duty construction. PCE roughness testers come in a variety of measurement ranges, resolutions and accuracies. Most feature rechargeable batteries and chargers as well as a handy carrying case. A number of accessories, such as specialized software used to transfer measurement results to a PC or laptop for further analysis, are sold separately.