Profile Measurements (Metrology)

7.11.
Profile is that part of the contour of a component, which is observed as its boundary line
in a functionally significant cross-sectional plane. The inspection and measurement of thein a functionally significant cross-sectional plane. The inspection and measurement of thethe
profile can be accomplished in many different manners, depending on the size of the part, the
required functional properties and level of accuracy of the profile, its form characteristics, etc.
Various methods of profile measurement are described below in brief.
(i) Direct comparison with a washer having the inverse form of the normal contour. The
gaps between specimen contour and washer can be observed by light transmission directly.
(ii) Limit gauges (in pairs for GO and NOT GO sizes) for the entire contour of the part.
The sizes are determined by the maximum and minimum material conditions resulting from
the compounded effects of the form and size
tolerances.
(iii) Mounting indicators side by side
in the plane which contains the profile to be
inspected.
The indicators need to be located
judiciously and must be set with the aid of
a washer representing the nominal profile.

Fig. 7.60. Measurement of profile.
(iv) Measuring the coordinate positions of characteristic contour points. This is possibly
by inspecting the size and form of features bounded by essentially straight lines.
(v) If a contour can be defined by polar coordinates as in case of cam, then functionally
essential contour characteristics can be specified, laid out and inspected by individual points
whose positions are defined by the means of dimensioning in a system of polar coordinates.
(vi) Magnification of part and comparison with a graticule carrying reference lines of
normal form and size. The silhoutte image of the inspected object appears in the focal plane of
a microscope where a transparent graticule with reference marks can be mounted.
(vii) Projection of magnified image on a special screen with charts having single or double
contour lines of the object to be inspected.
(viii) Optical profile comparison by light sectioning. In this method, a narrow band of light
is projected obliquely around the edge of the curved specimen, to illuminate a section which
by means of optical correction, appears as a normal section in the field of view of the special
microscope, to be observed concurrently with a washer drawing of the specimen.