Vernier caliper

The vernier caliper is a precision instrument that is made to measure in thousandths of an inch. Instead of using a screw, like the micrometer, vernier tools have a sliding scale. The vernier caliper may be used for taking both inside and outside measurements. Graduations on one side are for inside measurements and those on the opposite side are for outside measurements.

The instrument consists of an L-shaped frame with a main scale engraved on the shank. The length of this main scale determines the size of the caliper. Smaller sizes are generally 6 inches. The commonly used ones are 12 and 18 inches, but they may be manufactured in sizes up to 48 inches.

The main scale is divided into inches. Each inch is divided into 40 parts, like the sleeve on the micrometer. Each division is equal to 0.025 inch. Every fourth division is numbered 1, 2, 3 and so on, indicating 0.100, 0.200, 0.300 inch, and so forth.

The vernier scale is attached to a movable jaw that matches the fixed jaw and slides along the main scale bar. The vernier scale has 25 divisions that equal 24 divisions on the main scale. The 24 divisions on the main scale are equal to a distance of 0.600 (24 x 0.025 = 0.600) inch. Thus, the value of one vernier division or space equals 0.600/25 = 0.024. Therefore, the difference between a space on the main scale (0.025) and a space on the vernier scale (0.024) is 0.025 ! 0.024 = 0.001. It is this difference in the spaces between the main scale divisions and the vernier divisions that makes it possible to measure to one-thousandth of an inch.

Click on image to enlarge.

Vernier Caliper

Read the number of full inches that show from zero on the main scale to zero on the vernier. In the figure, this would be 1.000 inch.

Read the number of divisions on the main scale beyond the last full inch to the zero of the vernier scale. The figure shows 0.425. Note that the zero line of the vernier is between the 0.425 and 0.450 lines on the main scale.

To read the vernier scale, read along the scale until you find the line on the vernier scale that lines up exactly with a line above on the main scale. In the figure (at right), the eleventh line is correct. This means that 0.011 is to be added to the main scale reading.

The sum of the readings in the paragraphs above is the correct reading of the caliper. In the example in the figure, it would be 1.000 + 0.425 + 0.011 = 1.436 inches.

Vernier calipers are also available with dial indicators and even digital liquid crystal display (LCD) heads which do not require interpolation (direct reading) and some convert to metric at the touch of a button.

Click on image to enlarge.

Reading a Vernier Caliper

Click on images to enlarge.
   

Dial and Digital Vernier Calipers

The feeler or thickness gage is used to determine space or clearance between two objects. Although this tool does not give a precise measurement, the measurement is acceptable for dimensions recommended to be obtained by use of the feeler gage.

The common feeler gage consists of a group of flat steel blades or leaves ranging in thickness from 0.001 to 0.035 (one thousandth to thirty-five thousandths) inch. Combining blades allows the user to obtain almost any size needed. These blades, or leaves, are encased with a single screw in one end to hold the leaves together. A knurled thumbnut allows locking of one or more blades away from the remainder of the pack.

The feeler gage is available in various lengths to satisfy the needs for which it is used.

How to use a common feeler gage

Before using a feeler gage, remove any foreign matter from the blades.

Insert various blades or combinations of blades between two surfaces until a snug fit is obtained.

The thickness of the individual blade or the total thickness of all the blades used is the measurement between the surfaces.

Do NOT force the blades into openings that are too small for them. Some blades are very thin and can be bent or kinked easily.

Click on image to enlarge.

Feeler Gage

The stepped or go/no-go feeler gage is more convenient to use in many cases than the common or flat blade type gage. The blades of the stepped feeler gage are of two thicknesses.

If the desired clearance is 0.011 inch, select the blade labeled ".010 - .012." One-half inch of the tip of a 0.012-inch blade is ground to a thickness of 0.010 inch.

If the 0.010-inch tip will enter the opening, but the blade stops when it reaches the 0.012-inch area, the clearance is between 0.010 inch and 0.012 inch or very close to the measurement desired, 0.011 inch.

Telescoping gage

A telescoping gage is used somewhat like an inside caliper but it is simpler to use. Greater accuracy can be obtained when the gage is used with an outside micrometer or vernier caliper. Five different sizes are required to measure holes from 0.5 to 6 inches because the capacity of each is limited to a short distance.

The gage consists of two rods, one of which slips inside the other. One part of the rod is fixed to a handle, while the movable part of the rod is forced outward by a spring. A lockscrew through the handle holds the movable part in any location.

Click on image to enlarge.

Stepped or Go/No-Go Feeler Gage

To measure an inside diameter, loosen the lockscrew, compress the telescope pin, tighten the lockscrew, and place the gaging head in the hole.

Hold the handle parallel with the hole and loosen the lockscrew so that the telescoping rod can expand in the hole. Carefully align the handle to the vertical position and tighten the lockscrew. Check carefully for feel and contact before removing the gage.

Measure across the two parts of the rod with a micrometer or vernier caliper.

Click on image to enlarge.

Telescoping Gage