Surface metrology measurement

Surface metrology is the measurement of tiny features on surfaces and is a branch of metrology.Surface primary shape, surface fractal and surface finish (including surface roughness) are the parameters most commonly associated with this field.It is important to many disciplines and is known for machining precision parts and assemblies that contain mating surfaces or must operate under high internal pressures.Surface finish can be measured in two ways: contact and non-contact.The contact method involves dragging a measuring stylus over the surface; these instruments are called profilometers.Non-contact methods include: interferometry, digital holography, confocal microscopy, focal length variation, structured light, capacitance, electron microscopy, photogrammetry, and non-contact profilometers.

Overview

The most common method is to use a diamond stylus profiler.The stylus runs perpendicular to the direction of the surface.Probes usually follow a straight line on a plane or an arc around a cylindrical surface.The length of the path it traces is called the measurement length.The wavelength of the lowest frequency filter used to analyze data is usually defined as the sample length.Most standards recommend that the measurement length should be at least seven times longer than the sampling length,which according to the Nyquist-Shannon sampling theorem should be at least twice longer than the wavelength of the feature of interest.The evaluation length or evaluation length is the length of the data that will be used for analysis.Typically one sample length is discarded from each end of the measurement length.3D measurements can be made by scanning a 2D area of a surface using a profiler.

The disadvantage of a profiler is that it is inaccurate when the dimensions of the surface features are close to the dimensions of the stylus.Another disadvantage is that profilometers have difficulty detecting defects of the same general size as the surface roughness. Non-contact instruments also have limitations.For example, instruments that rely on optical interference cannot resolve features smaller than some fraction of the operating wavelength.This limitation makes it difficult to measure roughness accurately even on ordinary objects, since interesting features can be far below the wavelength of light.The wavelength of red light is about 650 nm,while the average roughness (Ra) of the grinding shaft may be 200 nm.

The first step in the analysis is to filter the raw data to remove very high frequency data (called "micro-roughness") as it is often attributed to vibrations or debris on the surface.Filtering out micro-roughness at a given cut-off threshold also enables closer roughness evaluation using profilometers with different stylus ball radii, e.g. 2 µm and 5 µm radii. Next, the data is divided into roughness, waviness and shape.This can be done using guide lines, envelope methods, digital filters, fractals or other techniques. Finally, summarize the data using one or more roughness parameters or graphs.In the past, surface finish was often analyzed manually.The roughness trajectory will be plotted on graph paper and it is up to the experienced machinist to decide which data to ignore and where to place the mean line.Today, measurement data is stored on computers and analyzed using signal analysis and statistical methods.