D-37DAbout 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.Vernier scaleThis is a short auxiliary scale that enables accurate interpolation between the divisions of a longer scale without using mechanical magnication. The principle of operation is that each vernier scale division is slightly smaller than a main scale division, so that successive vernier graduations successively coincide with main scale graduations as one is moved relative to the other. Specically, n divisions on a vernier scale are the same length as n‑1 divisions on the main scale it works with, and n denes the division (or interpolation) ratio. Although n may be any number, in practice it is typically 10, 20, 25, etc., so that the division is a useful decimal fraction. The example below is for n = 10. The main scale is graduated in mm, and so the vernier scale is 9 mm (10 divisions) long, the same as 9 mm (9 divisions) on the main scale. This produces a difference in length of 0.1 mm (1) as shown in gure A (the 1st vernier graduation is aligned with the rst main scale graduation). If the vernier scale is slid 0.1 mm to the right as shown in gure B, the 2nd graduation line on the vernier scale moves into alignment with the 2nd line on the main scale (2), and so enables easy reading of the 0.1 mm displacement. Some early calipers divided 19 divisions on the main scale by 20 vernier divisions to provide 0.05 mm resolution. However, the closely spaced lines proved difficult to read and so, since the 1970s, a long vernier scale that uses 39 main scale divisions to spread the lines is generally used instead, as shown below.0123456789100246810010203040304050607039 mm19 mm0.05 mm Scale reading 1.45 mm Scale reading 30.35 mmCalipers were made that gave an even finer resolution of 0.02 mm. Theserequired a 49-division vernier scale dividing 50 main scale divisions. However,they were difficult to read and are now hard to find since Digital calipers withan easily read display and resolution of 0.01 mm appeared.Inside 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:Measured diametert1, t2: Thickness of the inside jawC: Distance between the inside jawsd: Measurement error (øD–ød)True internal diameter (øD: 5 mm) Unit: mmt1+t2+C0.30.50.7d0.0090.0260.047For inside onlyFor outside only• 19 mm Vernier scale• 39 mm vernier scale (long vernier scale)ødt2t1CøDødøD0123456789100246810010203040304050607039190.05mm9 mm (9 graduation lines)(1) 0.1 mm0.1 mmDivided into10 equal divisionsAB(2) Aligned
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