In contrast, a few typical conditions that occur on technical surfaces are shown in the diagram in Fig. 15-14. These examples illustrate the limitations of average-roughness as a representative value for functionally significant surface conditions. Because of these limitations of the single-average value, the inspection of critical surfaces is often carried out with the aid of stylus type instruments by producing chart tracings, also termed graphs, as the magnified portrayal of the traced surface.

Fig. 15-14. Diagrams showing certain types of limitations of the stylus type, average-roughness measuring method.

(1) The relationship between the actual digressions from the theoretical profile and the indicated average values.

(2) A functionally important condition of the surface texture that is not revealed by average-roughness indications.

(3) The selective assessment of surface variations by skid-guided tracing as compared to the datum-guided, surface-texture tracing method.

The representation of an inspected surface element by means of a chart tracing, however, requires interpretation for providing any specific type of information. In addition to the basic variables of all recorded tracings, namely, the different scales of vertical and horizontal magnifications, there are also other variables, which are related to characteristics of the applied methods and instruments. Examples of these types of variable factors are:

a. The datum for the translational movement of the pickup;  

b. The degree of fidelity of representation, as determined by the relationship between the speed of response of the instrument system and the speed of the trace (scanning) movement; and

c. The type of electronic filtering used, if any, in processing the signals supplied by the pickup.

The existence of these variables was pointed out in the preceding discussions; at this point, their effect on the produced chart tracing and on the representation of the surface conditions will be considered.

For this purpose, Table 15-6 reviews several currently used systems of portraying, graphically, the traced surface element, together with various methods applied for the assessment of the resulting chart tracings.

Only a few of the listed methods are defined in some foreign standards, and none in the current American Standard. However, the valuable information such tracings can supply on certain critical surface characteristics explains the expanding application of surface-textureinspection by means of chart tracings. The survey in Table 15-6 should be helpful in the selection of a suitable system for any particular set of conditions and functional requirements.

TABLE 15-6. SURFACE-TEXTURE ANALYSIS BY CHART RECORDING—EXAMPLES OF CHART TYPES AND ASPECTS OF ASSESSMENT