LO 5-3 Identify and describe common instruments and techniques to make linear precision measurements

LO 5-03: Identify and describe common instruments and techniques to make linear precision measurements

Vernier Calipers

Vernier calipers are a type of sliding caliper that make more precise measurements than a simple sliding caliper with a single main graduated scale. Like a simple caliper, vernier calipers are also constructed with a fixed, movable jaw and a bar (also called a beam or frame ) displaying the main graduated measuring scale. What is unique to a vernier caliper is the addition of a second graduated scale positioned on the moving jaw frame. Finer graduations or markings on the second scale, called the vernier scale , are used to calculate work piece dimensions by combining its smaller, higher-resolution readings with main scale graduations FIGURE 9. Measuring devices with higher resolution means a smaller, more accurate measurement is made having more significant digits or numbers. Vernier scales are also on micrometers and other measuring tools to enable a device to achieve extra decimal points in its measurement. To do this, divisions or markings on the graduated vernier scale are made at intervals of one or two divisions shorter than main scale divisions. This means the vernier scale will provide tenths or hundredths of a measuring unit for additional resolution. If the caliper measures in inches, the main scale divisions are typically tenths of inches, but finer resolution vernier calipers can have 20 divisions for each inch FIGURE 10. Metric scales use 1 mm divisions on the main scale with 0.5 or smaller divisions on the vernier scale FIGURE 11. When the movable jaw is adjusted while taking an inside, outside, or depth measurement, the vernier scale moves with the moveable jaw. This means that when the caliper jaws are moved, a graduation line on the vernier scale will align with a single line on the main scale but all other lines are slightly misaligned. To then calculate a measurement, the main scale graduation is added to the vernier scale graduation marking that aligns with the main scale FIGURE 12.

FIGURE 05-9: On short sliding calipers, the vernier scale is made shorter than one main scale division. Longer calipers use two main scale divisions to equal one vernier scale division.

FIGURE 05-10: A sliding-type vernier caliper with vernier scales in 1/10". The main scale reading of 0.6" is added to the vernier scale of 0.05" to sum to 0.65" diameter.

FIGURE 05-11: The reading of 16.15 mm measurement is read by adding the mark at line for the main scale graduation to the 0.15 mm mark where the vernier and main scale graduations align.

FIGURE 05-12: The vernier scale is a device that is used for gaining an extra decimal point in the accuracy of a measurement.

When the vernier scale on the caliper frame is replaced by a dial indicator or digital display, technically it is no longer a vernier caliper because it does not use a vernier-type scale. When a dial is used, rather than adding a main scale reading to a vernier scale, the main scale reading is instead added to the measurement on the dial or dial indicator FIGURE 13.

To measure the dimensions of the work piece, technicians must:

1. Locate on the vernier scale where the zero position aligns with the main scale (11.9 in the illustration) plus an unknown distance measured by the vernier.
2. Identify the line on the vernier scale that matches most closely to a line on the main scale. In this illustration, the main and vernier scales align at the number 0.03, which has divisions measured in hundredths.
3. Add the two dimensions together. In this case 11.9 + 0.03 = 12.12 units.

FIGURE 05-13: The reading of 16.13 mm is read by adding the main scale graduation mark to the reading on the dial scale.

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