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21/07/10 Tutorial - Surface Measurement Methods & Instruments
Visual Inspection
Do not underestimate the importance of initial visual inspection, it will help in deciding how to make the measurement. It can help in understanding the part being measured, save time, and can be a useful as a cross check with results shown on the screen or printout.
Using Comparison Plates
A basic method of surface finish measurement is by using a comparison plate. These plates consist of a range of metal blocks which have been machined to give various calibrated surface finish values.
A components surface can be compared against these machined blocks either visually or by touch. Obviously this method of measurement is a bit subjective. Using this method it is difficult to attach an absolute value to a surface.
This set consists of 30 comparison specimens, covering six commonly-used machining methods (6 turned, 6 end-milled, 6 horizontally milled, 6 surface-ground, 3 lapped, 3 reamed/drilled. A label gives the Ra values of each specimen in both metric and imperial units, but also identified with Roughness Grade Numbers, N12 to N1. These grades correspond to nominal preferred Ra values. N12 would be an Ra value of 50µm, N1 a value of 0.0125µm Ra.
Note: The N grades used in the USA are not identified in the same way as the European grades shown above. Germany, USSR, and Japan also have had such N systems.
Contact instruments
The contact type of instrument consists of a stylus which tracks across the surface under test. A gauge or pick-up which is a transducer that translates the movements of the stylus in the Z (height) direction as it tracks across the surface (X axis) into a usable electronic signal. This signal is then processed via software to present the operator with a value which represents the surface finish.
The traverse mechanism will also provide X co-ordinate positions of the surface data by using a grating which has a fixed spacing. The Form Talysurf instrument has data point spacing in the X axis of 0.08µm (0-15mm) 0.25µm 15-30mm & 1µm (30-200mm).
Non Contact Instruments
There are a number of non-contact gauges on the market that can be mounted on a standard surface measuring system.
Two Variants; 3D area measurement similar to looking through a microscope or 2D or 3D raster scanning, traditional measurement with translation of X stages (and Y to build up the 3D image).
