Calibration of Stylus Type Average Surface-Roughness Measuring Instruments

In order to assure the compliance of the instrument’s indications with the concepts and dimensional relationships of the standard, periodic verifications are required. For that purpose a reference specimen is traced with the instrument at standard speed and in a direction normal to the lay. The average-roughness indications of the instrument should agree with the value marked on the reference specimen.

Essentially, such a calibrationmaster (see Fig. 15-6) is a flat block, on one surface of which a field is covered with consecutive parallel grooves of specific shape and size. The basic cross-sectional shape of the grooves is that of an isosceles triangle with 150 degrees included angle, as specified in the standard. The peakto- valley height of the triangles is four times the average roughness size they represent, that is, 0.0005 inch for 125-microinch average roughness.

Fig. 15-6. Cali-Block reference specimen for the calibration of stylus type, surface-roughness measuring instruments with average indications. The block is made of nickel by an electroforming process, from a goldmaster, produced with a very accurate ruling machine. The block has two fields representing 20- and 125-microinch average roughness, respectively.

The 125-microinch average-roughness value has been selected for a widely used type of reference master, as the scale value for calibrating average roughness measuring instrument. The same reference specimen also has a second field, with finer grooves, having a spacing that theoretically represents 20-microinch average roughness. However, the tracer instrument, when calibrated at the 125-microinch scale does not indicate the nominal value of a theoretically dimensioned array of closely spaced grooves. The stylus of the instrument has a finite radius on its tip, which cannot penetrate into the sharp bottom of the valleys, consequently, the span of the actual stylus excursions will be smaller than the distance between the peaks and valleys of the reference specimen.

The ratio between the actually traced and the theoretical values will be close to one for the wider spaced grooves, but it will decrease as the spacing becomes narrower. There is a geometric relationship between the stylus radius and the reduced indications due to incomplete penetration of the stylus tip into the bottom part of the groove valleys. This relationship is utilized for determining the effective radius of a particular stylus tip in the calibration procedure by:

1. First calibrating the instrument on the 125-microinch scale, and then

2. Tracing the finer, 20-microinch scale, and observing the difference on the meter, between the actually indicated and the nominal scale values.

These principles of stylus type instrumentcalibration are illustrated in Fig. 15-7.

Fig. 15-7. Concepts related to the calibration of stylus type, average-roughness measuring instruments.

(A) The basic profile of the calibration specimen in the cross-sectional plane of the stylus traverse, and the development of the original peak-to-valley height h, through the unilateral height m , into Y height representing the assigned Ra value of the surface.

(B) The geometric conditions limiting the penetration of the stylus of finite radius to the full depth of the valley, thereby reducing the actual range of stylus excursion in relation to the peak-to-valley height of the theoretical profile.

(C) Curve representing the relationship between stylus radius and the indicated average roughness on the 20-microinch scale; the decrease resulting from the reduced stylus excursions.