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
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
through the unilateral height
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.