BS 1134-1:1988 Title: Assessment of surface texture. Methods and instrumentation
Surface Finish Notes
BS EN ISO 3274:1998 Geometric product specifications (GPS). Surface texture. Profile method. Nominal characteristics of contact (stylus) instruments
BS EN ISO 4287:2000 Geometrical product specification (GPS). Surface texture. Profile method. Terms, definitions and surface texture parameters
BS EN ISO 4288:1998 Geometrical product specifications - Surface texture: profile method - Rules and procedures for assessment of surface texture.
BS EN ISO 1302:2002 Geometrical product specifications (GPS). Indication of surface texture in technical product documentation
The accurate characterization of surface texture has become increasingly important in
the instrument, computing, data storage, automotive and biomedical industries.
Surface texture is a key consideration affecting the
function and reliability engineering components.
Measuring Surface Finish
The surface texture is the local deviations of a surface from its ideal shape e.g perfect flat shape,
perfect cylindrical shape, spherical shape etc.
The measure of the surface texture is generally determined in terms of its roughness,
waviness and Form .
The roughness of a manufactured surface generally results from the production process and not
specifically from the machining process if any. The action of the cutting tool,
chemical action, polishing, lapping, and the structure of the material all contribute to the roughness of the surface.
Waviness, the more widely spaced repetitive deviations, usually relate to the characteristics
of an individual machine or to external environmental factors. It may result from such factors as
machine or work deflection, vibration, chatter, heat treatment, or warping strains. Because both
process and machine induced irregularities occur simultaneously,roughness is superimposed over waviness.
The form of a surface is the profile of the surface under consideration ignoring variations due to roughness and waviness.
Deviations from the desired form result from clamping marks or sliding marks machining guide errors etc.
In the past surface texture has been assessed by the judgment of the inspector either
by eye or even fingernail. The evaluation was done by comparing the surface
to be measured with standard surfaces
Surface Texture Parameters
A modern typical surface measuring instrument will consist of a stylus with a small tip (diamond)
a gauge or transducer, a traverse datum and a processor. The surface is
measured by moving the stylus across the surface. As the stylus moves up and down along the surface, the transducer converts this
movement into a signal which is then exported to a processor which converts this
into a number and usually a visual profile.
The stylus must be moved in a straight line to give accurate readings
On an expensive fixed instrument the necessary guide is of some form of datum bar that is usually
lapped or precision ground to a high straightness tolerance. On the normal portable units
the guide is generally based on a skid which slides over the general surface ensuring
that the linked stylus moves in a straight line parallel to the local surface
The identification of the surface texture used a number of parameters. These are
different depending on the standard used and on the issue of the relevant standard.
The notes below relate to the standards as listed above
R a - Average Roughness....
The average roughness is the area between the roughness profile and its mean line,
or the integral of the absolute value of the roughness profile height over the
evaluation length Graphically, the average roughness is the area (shown below) between
the roughness profile and its center line divided by the evaluation length (normally five sample
lengths with each sample length equal to one evaluation length)
This is the parameter that has been used universally for many years
The European and ISO standards now more generally use R z:
R t(R y), R p, and R v ....
The peak roughness R< sub>p is the height of the highest peak in the roughness profile over
the evaluation length Similarly, Rv is the depth of the deepest
valley in the roughness profile over the evaluation length .
The total roughness, R t(R y), is the sum of these two, or the vertical distance from the
deepest valley to the highest peak.:
R z is the sum of the height of the highest peak plus the lowest valley depth
within a sampling length ( lr ). R z is gnerally used, in European documents,
to indicated the roughness surface texture
Note: The default evaluation length "ln" consists of 5 sample lengths "lr."
Modern surface texture measuring instruments record the stylus movement over the evaluation
length electronically and can produce output readings to a significant level of detail.
The instruments measure the roughness variations and the waviness variations .
Selecting a roughness filter will remove waviness elements, leaving the roughness
profile for evaluation. Selecting a waviness filter will remove roughness
elements,leaving the waviness profile for evaluation.
The signal filters operate on the digital information. Separating the
roughness and waviness is achieved by using filter cut-offs.
lc = the long wave filter. In the roughness requirement
this value is the sampling length. This value may be used as the basis for the Waviness surface texture indication
ls = the short wave filter used for the lower band of the roughness requirement.