Is Optical Measurement of surface finish the best option?
Surface finish measurement was first developed by E.J. Abbot at the University of Michigan in the 1940’s. In recent times the importance of this measurement has increased dramatically as components have become more complex and tolerances of mating surfaces have been reduced to provide more functional products
The measurement of surface finish on machined components is a critical part of a manufacturing process. Surface finish is measured for two principle reasons: to try to predict the performance & functionality of the machined parts, such as engine pistons, fuel injection parts or ground mating surfaces, and to try to control the manufacturing process.
A surface consists of three basic components: form, waviness and roughness.
The traditional method to measure these surfaces are stylus-based surface-finish measuring systems: These use a sensitive, diamond-tipped stylus which is pulled across the surface to measure the surface finish. Stylus gauges can also be used for measurements of waviness and form, in addition to roughness. (see Fig1)
As the three basic forms of surface geometry are caused by different, they have a different affect to the performance of the part, and, it is common to separate them during feature extraction. This separation is achieved by the selection of filter with cut-off settings that allow the operator to select the degree of filtering that will be applied to the measured profile.
The irregularities of the machined surface consist of high and low spots created in by the tool bit or by a grinding wheel. These peaks and valleys can be measured and used to define the conditions and sometimes the performance of the surface. These are expressed by parameters, but for most cases, only a few are specified which in the case of Stylus Instruments are prefixed by the letter R. Each of the parameters has its own capabilities and limitations. Often one parameter is incapable of defining a surface adequately. Therefore, a complete definition of a surface often involves two or more parameters, and in some cases, the relationship or ratio of one parameter to another.
The most common parameter is Ra, or Arithmetic Average Roughness. It basically describes the average height of roughness component variations from a mean line and provides a simple value for accept/reject decisions. But Ra is not a good discriminator for different types of surfaces as it is incapable of differentiating between “spiky” and “scratched” surfaces having the same Ra. In some territories, such as Europe the more common parameter for roughness is Rz, or Mean Roughness Depth.
Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 – 2002, Surface Texture, and in International Standards, ISO 4287 and ISO 428 (will be replaced by the new iso21920 series currently in work)“
These parameters provide basic information about the surface; these were acceptable in the early days of surface measurement, however there are many limitations that arise from it.
Firstly, the information provided is 2D data and is provided by a diamond stylus which varies in size but could be as small as 2µm. From this it is not possible to describe a whole surface from such a small sample, the measurement will be correct in that position and therefore the measurement describes the line and not the surface.
Secondly a position as little as 0.5mm away from this line could easily have a completely different measurement, this means that the result is dependent both on the position and the operator. (see Figures 2&2A)
Thirdly completely different surfaces can provide the same “R” parameters although from a function perspective the 2 surfaces will operate completely differently. See Fig3. And finally the stylus can cause surface damage on soft materials.
Optical 3D Measurement.
Optical 3D measurement provides a valid alternative to Tactile measurement systems and provide a large amount of additional data not available or not easily available from Tactile methods.
The parameters are described as functional parameters and are described in ISO 25178: Geometric Product Specification (GPS)-Surface texture; areal.
The first item to note is that we are now describing Surface Texture rather than surface roughness. This is due to the requirement that we are measuring a 3D surface over an area rather than a 2D line along a single path and therefore extracting a dramatically increased level of data. (See Fig 4)
The process of surface data collection described here is based on the measuring principle of FocusVariation, first commercialised by Alicona Imaging (now Bruker Alicona) and a typical product is shown in (Fig 6)
Focus variation works on the principle of moving the focal plane of an optical system vertically over a defined surface at a controlled rate of movement. During this process sharp data is collected from the surface to provide a 3D natural colour view of the surface as a 3D model. Underlying this 3D representation is a 3D data set with up to 3.3 million data points and this is used to extract measurements from the surface. The dataset has a x-y size (depending on the objective used) of up to 2x2mm and across this whole surface measurements can be made.
This process offers many advantages to the user and the company using it.
- It is possible to measure both profile and areal measurements on the sample. A real advantage, if using profile measurements, is that the measurement line can be positioned where needed, when viewed on the 3D representation.
- The image data is saved in a database which can be recovered for QA purposes
- A full 3D height map view is available for evaluation.
- Full areal based parameters can be extracted at a single click. (see Fig 5)
Areal based parameters provide not only details of the surface finish but also data that can be used to assess the function that the surface operates in.
An example would be that from the ratio of 2 parameters, Vmc (core material volume) and Vvc (core void volume), it is possible to assess if a surface would be able to retain a lubricant, other parameters include Ssk (skewness) Sv (maximum pit height) Sz (Maximum height) Sku (skewness) Sku (Kurtosis) and Sp (maximum peak height). Although some of these parameters may not be used in a production environment for Quality and Design purposes these will be invaluable.
The traditional tactile methods are well established and work effectively in some industries and will continue to do so. The optical methods though provide the manufacturers with far more detailed information that can be used to improve functionality and reliability through the design and manufacturing process of components.
As manufactured products become more sophisticated and consumer demands for reliability and life span increase then the understanding of the surfaces that are manufactured will need to be improved. This cannot be achieved by the measurement of surfaces using single line profile gauges.
Designers also need to start producing designs where these functional parameters become the norm and the appropriate references are adopted.
To look at Optical Surface Finish measurement systems look at www.alicona.com