SURFACE-TEXTURE MEASUREMENT WITH STYLUS TYPE INSTRUMENTS

The principles of this general category of surface-texturemeasuring processes may be described in the following manner. On the surface to be inspected, an element is selected in a location and orientation that is considered to provide a dependable representation of the texture of that surface. A needle type instrument member, the stylus, is moved along the selected surface element. The stylus is connected with a pickup that is maintained in a controlled level, yet that permits the stylus to follow intimately the physical surface of the part. Variations of the surface in relation to the level of the pickup translation cause the stylus to deflect from a reference position. The mechanical deflections of the stylus during its scanning movement along the part’s surface are translated by the pickup into electrical signals. These signals either reflect the velocity of the stylus deflections or, more frequently, produce a voltage or current proportional to the amplitude of those deflections. The electrical signals are electronically amplified and either displayed as an average value on a meter or, in the case of displacement sensing pickups, are channeled into a recorder to produce a magnified chart tracing of the scanned surface element. The level in which the pickup is held during its translational movement can be either an envelope surface of the part, or may be provided by an extraneous datum that has been established essentially parallel with the general direction of the inspected surface element.  

Considering the above-mentioned variables, namely, the selection of the reference plane to which the departures of the surface are related, and the indications of the sensed surface irregularities, the use of a stylus type instrument does not exclusively define the process. Actually, there are several systems of stylusinstruments used for surface-texture measurements that differ in operating principles and also in the information they supply.  

In many cases it is possible for the values indicated by different types of instruments to be correlated, and with the concepts of the ASME B46.1 standard. However, even under optimum conditions, as provided by a workpiece with essentially uniform texture, such correlations are only approximations and by no means comparable with, say, the excellent agreement between values of repeated length measurements. The correspondence of indications originating from different types of instruments becomes poorer when inconsistently occurring departures characterize the surface being examined.  

The complexity of surface conditions, which are designated collectively as texture, and the diversity of their effects on specific operational requirements, warrant the efforts of instrument manufacturers toward devising and building different systems of surface-texturemeasuringinstruments. Although a complete listing of all the available systems and methods of surface-texturemeasurement by stylus type instruments would be difficult to accomplish in view of continued progress in this field, the major systems and the basic characteristics of the information that they supply are reviewed in Table 15-3.

TABLE 15-3. OPERATIONAL CHARACTERISTICS OF DIFFERENT SYSTEMS OF STYLUS TYPE

SURFACE-TEXTURE MEASURING INSTRUMENTS

Subsequent sections will review: (a) the major elements and functions of the instrumentation, as well as their various forms of execution; (b) characteristic examples of diverse types of stylusinstruments; and (c) the assessment of indications obtained by means of different, currently used stylus-tracing processes.