Vernier calipers are designed around the principle of a graduated bar with a stationary jaw and a movable jaw. The primary scale is scribed on the beam of the stationary jaw. To the movable jaw, a vernier scale is added. The caliper is normally equipped with a fine-adjustment feature that permits a more uniform "feel" of pressure on the tips of the jaws. On standard vernier calipers, both inside and outside readings are taken from the same set of jaws. On others, two sets of jaws are provided. Verniers are available with both inch and centimeter units. Vernier plates are available with either a 25-or 50-division scale. Figure 1 shows a vernier caliper.
In Figure 1, the upper scale is used in conjunction with measurements with the inner jaws. The lower scale is for measurements with the outer jaws. The lower scale is explained; reading the upper scale is similar.
The basic reading on the beam is at the zero point of the vernier. When this zero does not coincide with a division on the beam, the graduation value on the beam immediately preceding the vernier zero is supplemented by a value obtained from the vernier. The supplementary value is the value of the vernier that exactly aligns with any of the graduations on the beam. Thus, the total value of the vernier is equal to the value of one graduation on the beam, as shown in Figure 2. The principles used in reading the vernier scale will apply to any vernier instrument.
The steps in reading an English vernier caliper are as follows:
Read the number of whole inches on the main scale that appears to the left of 0 on the vernier.
Read the highest numbered graduation on the main scale that lies to the left of the index (0) on the vernier scale. Read these graduations as even one hundred mil (0.100, 0.200, etc.). Add to the whole inches from step one (1.100, 2.100, etc).
Read the highest number of whole minor divisions to the left of the index. Read these graduations as even twenty five mil (0.000, 0.025, 0.050, and 0.075 in.). Add this to the sum of steps 1 and 2 (1.125, 1.150 in., etc).
Now find the vernier graduation that most perfectly coincides with any graduation on the main scale. That is the nearest mil and can be any whole number from zero to 25 (0.000, 0.001,.... 0.024, 0.025). It should be noted that only when the vernier is at zero will two lines, 0 and 25 for the vernier scale, coincide with the main scale.
The principle is the same for metric verniers, as shown in Figure 3. Some instruments have both inch and metric scales. These provide the best of both worlds.
The steps in reading a metric vernier caliper are as follows:
- To obtain a reading, count the number of mm between the zero line on the bar and the zero line on the vernier.
- Each bar graduation = 1.00 mm
- Each vernier graduation = 0.02 mm
- Add the vernier plate reading to the number of mm on the bar.
These instruments are more delicate than the steel rule and therefore require more care. When using a vernier caliper, make sure they are kept clean so that your reading is accurate and also to prevent dirt from getting on the slide or in the gears.
- Before you take a reading, make sure that all the locking screws are loose.
- Slide jaws to contact surfaces being measured.
- Set clamp screw.
- Turn fine adjustment nut.
- Remove the vernier caliper from the item and tighten main clamp screw.
- Take the reading, from the largest to the smallest, and then total them.
Dial calipers are very similar to vernier calipers except that the vernier is replaced with a dial indicator. One revolution of the dial indicator is the equivalent of the vernier scale. Therefore, the reading of the dial indicator is added to the reading of the beam, as shown in Figure 4.
Vernier calipers and dial calipers are general-purpose length measurement devices. Their sensitivity is 0.001 inch or .02mm. Dial calipers give best accuracy of about 0.001 inch per 6 inch length of measurement. These devices provide for relatively fast measurements of lengths, diameters, widths, and heights. They are suitable devices for any of these features having a tolerance of 0.004 inch or more.
The manufacturing tolerances of the instruments establish the maximum possible accuracy and precision. Manufacturing tolerances are most critical for the width and surface finish of the contact portion of the jaws, parallelism of the faces of the jaws, perpendicularity of the faces of the jaws to the beam, scale linearity, width of graduation lines, and the intervals of the dial indicator.
Super smooth knife-edges of the jaws provide essentially a point contact with the boundaries of the dimension to be measured. This is particularly important when such boundaries have a radius. The dial indicator adds a new source of error referred to as hysteresis.
Hysteresis refers to a lagging of movement when direction of motion is changed. It affects both accuracy and precision.
Other sources of error are operator and procedure oriented. Positional consistencies and full and accurate reading of scale are important to operator techniques and precision. Standardization of calibration within a laboratory and from one organization to another is important.