Quick Guide to Precision Measuring Instruments

PG20About Long CalipersSteel 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 rst place it is important to realize there is no relationship between resolution and accuracy. For details, refer to the values in our catalog.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 outside measuring faces as they are furthest away from the main beam so errors will be at a maximum here. This precaution is also necessary when using the tips of the outside measuring faces of a long-jaw caliper.Types of Vernier ScaleThe Vernier scale is attached to the caliper’s slider and each division on this scale is made 0.05mm shorter than one main scale division of 1 mm. This means that, as the caliper jaws open, each successive movement of 0.05mm brings the succeeding vernier scale line into coincidence with a main scale line and so indicates the number of 0.05mm units to be counted (although for convenience the scale is numbered in fractions of a mm). Alternatively, one vernier division may be made 0.05mm 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 graduation, is still the same.0123456789100246810010203040304050607039190.05mm Reading 1.45mm Reading 30.35mmInside Measurement with a CM-type CaliperBecause the inside measuring faces of a CM-type caliper 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 caliperA structural error d occurs when you measure the internal diameter of a small hole.øD:True internal diameterød:Indicated internal diameterd: Measurement error (øD – ød)True internal diameter (øD: 5mm)H0.30.50.7d0.0090.0260.047ødt1øDødHøDFor inside onlyFor outside only●Standard Vernier scale (graduation 0.05mm)●Long Vernier scale (graduation 0.05mm)Calipers

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