Method of Testing Internal Micrometer (Metrology)

The accuracy of the internal micrometer reading is checked in two parts, viz., checking the
accuracy of the traverse of the measuring head and the checking of the accuracy of the overall
lengths when the measuring head, set to zero, is associated with the various extension rods in turn.
The accuracy of the traverse of the measuring head is determined by clamping it in the
Vee-block with its axis in line with a sensitive indicator and the feeler (with a flat contact face) of
the indicator touching the rounded contact face of measuring head. The feeler is set initially such
that it reads zero when the micrometer head is also reading zero and a slip gauge of convenient
dimension has been introduced between the two faces. The
micrometer head is then set at same reading and correspond-
ingly the slip gauge size is reduced. Any error in the reading
is revealed by a corresponding departure of the indicator
pointer from its initial position. In this way the accuracy is
tested at various readings of the micrometer head. In order
to check both progressive and periodic errors, and in order
that both the type of errors may be revealed, the reading in
micrometer head of 0—25 mm range may be taken at follow-
ing values 0, 2.4, 5.1, 7.7, 10.3, 12.5, 14.9, 17.6, 20.2, 22.8,
25.0 mm. The accuracy of the overall length is determined
by using a vertical comparator. The indicator of the com-
parator is set at a height corresponding to that of the overall
length to be measured, from the flat base. After this setting
the internal micrometer is placed under it in the maximum
position and the error noted down.

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