Dial calipers is one of three essential tools to an auto technician or machinist. The dial indicator is a three-dimensional tool for measuring thickness, depth and inside diameter with an accuracy of .001 or an inch or millimeter. The micrometer measures small items in thickness only to an accuracy of five ten-thousandths because the dial indicator measures run out. The two large fixed jaws on the calipers measure outside circumferences, while the short jaws on the opposite side make inside measurements. The long slider bar that extends when the dial is moved is made for depth measurements. Lastly, the dial is the final part of the equation taking the measurement into the thousandths of an inch or millimeter.
Instructions
1. Lay the masking tape on the table. Open the large jaws on the calipers by using the thumb wheel below the dial indicator. The thumb wheel should always be used to get an accurate reading. Ensure the calipers are extremely clean. Sweat on your hands or dust on the calipers can distort the reading when dealing with this type of accuracy.
2. Place the jaws over the masking tape in preparing to measure the thickness. With the thumb wheel, close the jaws on the object -- in this case, the masking tape -- ensuring the jaws are perfectly perpendicular to your work and not angled. The jaws must make complete contact along their length. Tighten the locking screw slightly clockwise to keep the caliper from moving as it is removed.
3. Remove the calipers and read the longitudinal scale on the long bar. Notice that if you follow it inside of the large movable jaw connected to the dial, you will see an edge adjacent to the dial's mounting pad. Read the number next to this edge as the first part of the measurements. These are the "whole" numbers. If you are reading the millimeter scale each division is 1 millimeter. The opposite inch scale has divisions of one tenth of an inch. If the edge is on the second mark after the 2 on the millimeter scale, it would be read as 22 millimeters. To convert inches to millimeters, multiply the number of inches by 25.4. For millimeters to inches, divide the total millimeter measurement by 25.4.
4. Read the dial. For illustration, if the needle is 2 1/2 increments past 8 on the millimeter dial, you would read this as .8 millimeters for the number and the increments would be .01 per increment. The last would be an interpolation or estimate, since it is between increments. Call it .005. The number would then be the bar number of 22 mm plus the dial numbers of .8 and two increments at 1 per increment equals 2 plus the 1/2 increment estimated at .005. The answer is 22.825 millimeters, converted to inches: 22.825 divided by 25.4 equals 0.898622 inches.
5. Measure the masking tape using the inches scale. Start with measuring thickness. Place the large jaws over the tape and close them. Look at the bar scale and read the last number visible next to the dial bracket. If it is less than an inch, use the dial indicator only. If there is a number, write this down followed by a period. Read the numbers in the dial. Start with the whole number and write it down after the period. Count the increments after the whole number and write this down. The final part is to determine if the dial is located on an increment, or if it is between. Since this instrument is accurate to .001 it is necessary to interpolate where the needle lies between the two marks or increments. Use a sliding scale of 1 through 10 with 5 situated in the middle. Each of the numbers represents 1/10 of an inch. Each of the marks between the numbers is .010 inch. For example, 5.567 equates to 5 inches and 567 hundredths of an inch.
6. Measure the inside diameter of the tape by using the small jaws. Place the jaws inside the lip of the tape and open them until they contact the sides of the object and read the results. Depth is determined with the use of the extending bar at the bottom of the caliper bar. Just place the end of the caliper bar on the surface and open the calipers extending the bar until it contacts the item. Read the scale in the same manner.
