Principles of High-precision Measurements (Metrology)

1.18.
(i) Temperature of measurements needs to be clearly defined. An object placed at 21°C may
take more than 10 hours to attain room temperature of 20°C. This is so because temperature
decreases exponentially. The process can be quickened by placing the object on a large mass (say,
surface place which acts as a heat sink). Since heat also flows by radiation, careful attention must
be paid to prevent radiation due to lighting, sunlight, human bodies, and electric equipment from
affecting measurements.
If object’s temperature is at different temperature, then it must be monitored at all times,
and measurement must be corrected for difference between object’s temperature and reference
temperature by calculation.
(ii) Measurement force should be avoided (non-contact methods of measurement be
preferred).
(iii) The precision of the measuring instrument must be five to ten times higher than the
expected precision of the measured object.
(iv) Be aware of the instrument’s characteristics like undirectional approach to avoid
backlash effects, completing measurements quickly to avoid drift etc.
(v) Influence of work-affected layers on surface. The surface layer affected by machining
needs closer look for precision measurements. The presence of an affected layer will influence
residual stress (cause of changes with age), surface processability, reflectance, electrical conduc-
tivity, corrosion resistance, and abrasion resistance.
Work affected layer must consist of four or five layers as follows : Around 1 um thick
outermost layer consisting of a nearly amorphous structure, followed by 10 um fibrous layer which
has undergone severe plastic deformation and which resembles rolled steel in grain structure. Below
that is the plastically deformed layer consisting of crushed grains. Then comes the transgranular
slippage layer, in which the grains still retain their form but some internal slippage exists. Depth
of affected layer can be measured by:
(i) Ultrasonic microscope method, (ii) Corrosion method, (iii) microscopic analysis, (iv) X-ray
diffraction method, (v) hardness method, and (vi) recrystallisation method.