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Types of Vernier Scale Small Hole Measurement with an M-Type Caliper
The Vernier scale is attached to the caliper’s slider and each division on this A small error occurs when you measure the internal diameter of a small hole
scale is made 0.05 mm shorter than one main scale division of 1 mm. This with the inside jaws. This is because there is an offset (H) due to the thickness
means that, as the caliper jaws open, each successive movement of 0.05 mm of the jaws and the clearance between them.
brings the succeeding vernier scale line into coincidence with a main scale line
and so indicates the number of 0.05 mm units to be counted (although for D : True internal diameter
convenience the scale is numbered in fractions of a mm). Alternatively, one d : Indicated internal diameter
vernier division may be made 0.05 mm shorter than two divisions of the main
scale to make a long vernier scale. This makes the scale easier to read but the d: Measurement error (D – d)
principle, and discrimination, is still the same.
True internal diameter (D: 5 mm)
Standard Vernier scale Long Vernier scale (graduation H 0.3 0.5 0.7
(graduation 0.05 mm) 0.05 mm) d 0.009 0.026 0.047
0 10 20 30 40 30 40 50 60 70
0.05 mm
0246 8 10 0 1 2 3 4 5 6 7 8 9 10
19 39 t1 D
H
Reading 1.45 mm Reading 30.35 mm
About Long Calipers d
D
Steel rules are commonly used to roughly measure large workpieces but if a
little more accuracy is needed then a long caliper is suitable for the job. A long Inside Measurement with a CM-Type Caliper
caliper is very convenient for its user friendliness but does require some care in
use. In the first place it is important to realize there is no relationship between Because the inside measuring faces of a CM-type caliper (see illustration
resolution and accuracy. For details, refer to the values in our catalogue. at left) are at the tips of the jaws the measuring face parallelism is heavily
Resolution is constant whereas the accuracy obtainable varies dramatically affected by measuring force, and this becomes a large factor in the
according to how the caliper is used. measurement accuracy attainable.
The measuring method with this instrument is a concern since distortion of In contrast to an M-type caliper, a CM-type caliper cannot measure a very
the main beam causes a large amount of the measurement error, so accuracy small hole diameter because it is limited to the size of the stepped jaws,
will vary greatly depending on the method used for supporting the caliper at although normally this is no inconvenience as it would be unusual to have
the time. Also, be careful not to use too much measuring force when using to measure a very small hole with this type of caliper. Of course, the radius
the tips of the measuring faces of a long-jaw caliper. of curvature on the inside measuring faces is always small enough to allow
correct hole diameter measurements right down to the lowest limit (jaw
closure).
Mitutoyo CM-type calipers are provided with an extra scale on the slider
for inside measurements so they can be read directly without the need for
calculation, just as for an outside measurement. This useful feature eliminates
the possibility of error that occurs when having to add the inside-jaw-
thickness correction on a single-scale caliper. d
D
For inside only
For outside only
D-42
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