ALTRACO VERNIER CALIPER Manual de Serviço descarregar pdf (Página 42)

D-42

■ About Long Calipers

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

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

resolution and accuracy. For details, refer to the values in our catalogue.

Resolution is constant whereas the accuracy obtainable varies dramatically

according to how the caliper is used.

The measuring method with this instrument is a concern since distortion of

the main beam causes a large amount of the measurement error, so accuracy

will vary greatly depending on the method used for supporting the caliper at

the time. Also, be careful not to use too much measuring force when using

the tips of the measuring faces of a long-jaw caliper.

■ Types of Vernier Scale

The Vernier scale is attached to the caliper’s slider and each division on this

scale is made 0.05 mm shorter than one main scale division of 1 mm. This

means that, as the caliper jaws open, each successive movement of 0.05 mm

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

convenience the scale is numbered in fractions of a mm). Alternatively, one

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

principle, and discrimination, is still the same.

0 1 2 3 4 5 6 7 8 9 10

0 2 4 6 8 10

0 10 20 30 40 30 40 50 60 70

0.05 mm

Reading 1.45 mm Reading 30.35 mm

■ Inside Measurement with a CM-Type Caliper

Because the inside measuring faces of a CM-type caliper (see illustration

at left) are at the tips of the jaws the measuring face parallelism is heavily

affected by measuring force, and this becomes a large factor in the

measurement accuracy attainable.

In contrast to an M-type caliper, a CM-type caliper cannot measure a very

small hole diameter because it is limited to the size of the stepped jaws,

although normally this is no inconvenience as it would be unusual to have

to measure a very small hole with this type of caliper. Of course, the radius

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.

■ Small Hole Measurement with an M-Type Caliper

A small error occurs when you measure the internal diameter of a small hole

with the inside jaws. This is because there is an offset (H) due to the thickness

of the jaws and the clearance between them.

øD : True internal diameter

ød : Indicated internal diameter

d: Measurement error (øD – ød)

True internal diameter (øD: 5 mm)

H 0.3 0.5 0.7

d 0.009 0.026 0.047

ød

øD

ød

øD

For inside only

For outside only

● Standard Vernier scale

(graduation 0.05 mm)

● Long Vernier scale (graduation

0.05 mm)

1 2 ... 37 38 39 40 41 42 43 44 45 46 47 ... 71 72

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ALTRACO VERNIER CALIPER Manual de Serviço Página 42

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