ISO 4288:1996 pdf download – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Rules and procedures for the assessment of surface texture
5.1 Areas on the feature to be inspected
The surface texture of the workpiece under inspection can appear homogeneous or be quite different over various areas. This can be seen by visual examination of the surface. In cases where the surface texture appears homogeneous, parameter values determined over the entire surface shall be used for comparison with the requirements specified on the drawings or in the technical product documentation. If there are separate areas with obviously different surface texture, the parameter values which are de- termined on each area shall be used separately for comparison with the requirements specified on the drawings or in the technical product documentation. For requirements specified by the upper limit of the parameter, those separate areas of the surface shall be used which appear to have the maximum par- ameter value.
5.2 The 16 %-rule
For requirements specified by the upper limit (see IS0 1302:1992, 6.2.3) of a parameter, the surface is considered acceptable if not more than 16 % of all the measured values (see notes 1 and 2) of the selected parameter, based upon an evaluation length, exceed the value specified on the drawings or in the technical product documentation. For requirements specified by the lower limit of the surface parameter, the surface is considered accept- able if not more than 16 % of all the measured values (see notes 1 and 2) of the selected parameter, based upon an evaluation length, are less than the value specified on the drawings or in the technical product documentation. To designate the upper and the lower limits of the par- ameter, the symbol of the parameter shall be used without the “max.” index.
NOTES 1 Annex A provides simplified practical guidance for com- paring measured values with upper and lower limits.
2 In cases where the values of the roughness profile par- ameter of the surface being inspected follow a normal dis- tribution, the determination of the upper limit as a limit which may be exceeded by 16 % of the measured values of the roughness profile parameter conforms with the limit determined by the value p + a, where p is the arithmetic mean value of the roughness profile parameter and c is the standard deviation of the values. The greater the value of o, the further from the specified limit (the upper value) the mean value of the roughness profile parameter needs to be. (See figure 1.)
5.4 Uncertainty of measurement
For proving conformance or non-conformance with specification, measured values of parameters shall be compared with the specified limiting values taking into account the uncertainty of measurement according to the rules given in IS0 14253-1. In case of comparing results of measurement with upper and lower limits, the uncertainty of measurement shall be estimated without taking into account the inhomogeneity in the surface which is already accounted for in the 1 6 % allowance.