Measuring Surface Finish
Measuring Surface Finish
Measuring surface finish can be accomplished in one of two common ways. The oldest method is to use a stylus profiler, which has been in use in various forms for more than 100 years. Non-contact methods, such as optical profilers from Zygo, date to concepts developed in the 1980's.
Stylus profilers are contact profilers in that they physically touch the surface being measured, typically with a diamond tip on a delicately balanced arm. The stylus is dragged in contact with the surface, and as the diamond tip encounters peaks and valleys, the tip is raised and lowered from an otherwise straight line. The amount to which the stylus is raised or lowered at any given point is recorded. Sometimes the variation is compared to a reference surface, or sometimes the motion is detected in relation to some reference point in comparison to the stylus arm's position.
Measuring Surface Finish - Crosshatch Machining Marks
Measuring Surface Finish - Prosthetic Knee Joint Surface
The result from a stylus measurement is often expressed as a single parameter (Ra), but this number is calculated from a linear data set, which is normally graphed as a line - in a profile plot.
Non-contact methods of measuring roughness or surface finish are also common, such as Coherence Scanning Interferometry (CSI), which is used by Zygo's lines of Optical Profilers; confocal microscopy techniques; focus variation; structured light techniques such as Zygo's Moire interferometry; or using capacitors as a proximity sensor, measuring induced fluctuations in an electrical field.
Reasons non-contact surface finish measurements may be preferred or required:
• If a material is soft, fragile, pristine, or chemically sensitive and won't tolerate contact methods
• If material lay is a factor, and a roughness measurement in more than one direction is desired
• If consumables, such as fragile diamond stylus tips, are difficult to maintain in inventory - they aren't needed for an optical profilometer
• If volumetric data is required, such as when measuring wear of materials or the volume of an engineered micro structure
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