The rule depth gage is a graduated rule with a sliding head designed to bridge a hole or slot and to hold the rule perpendicular to the surface on which the measurement is taken. This type has a measuring range of 0 to 5 inches. The sliding head has a clamping screw so it may be clamped in any position. The sliding head has a flat base that is perpendicular to the axis of the rule and ranges in size from 2 to 2 5/8 inches in width and from 1/8 to 1/4 inch in thickness. The micrometer depth gage consists of a flat base attached to the barrel (sleeve) of a micrometer head. These gages have a range from 0 to 9 inches, depending on the length of extension rod used. The hollow micrometer screw (the threads on which the thimble rotates) itself has a range of either 1/2 or 1 inch. Some have a ratchet stop. The flat base ranges in size from 2 to 6 inches. Several extension rods are normally supplied with this type of gage. To measure the depth of a hole or slot with reasonable accuracy, use a depth gage as shown in figure 2-48, view A. Hold the body of the depth gage against the surface from which the depth is to be measured and extend the scale into the hole or slot. Tighten the set-screw to maintain the setting. Withdraw the tool from the work and read the depth on the scale. To measure the depth of a hole or slot with more accuracy than is possible with an ordinary depth gage, place a vernier depth gage over the slot as shown in figure 2-48, view B. Notice the clamping screws are at X and Y; the horizontal adjusting screw nut is at Z. With X and Y loose, slide the scale down into the slot being measured until it is almost in contact. Then tighten X to make Z operative. With Z, adjust the scale to the proper feel and secure the setting with Y. By proper feel we mean the adjustment at which you first notice contact between the end of the scale and the bottom of the slot. Then read the setting as described under “Reading a Vernier Scale.” To set the vernier depth gage to a particular setting, loosen both setscrews at X and at Y and slide Figure 2-48.—Using depth gages. the scale through the gage to the approximate setting. Tighten the sets crew at X, turn the knurled nut at Z until the desired setting is made, and tighten the setscrew at Y to hold the setting. To measure the depth of a hole or slot, as shown in figure 2-48, view C, with more accuracy than is possible with either an ordinary depth gage or a vernier depth gage, place a micrometer depth gage over the slot and adjust the thimble until the contact of the spindle causes the ratchet stop to slip. Remove the micrometer from the work and read the micrometer. Remember, if extension rods are used, the total depth reading will be the sum of the length of the rods plus the reading on the micrometer. DIAL INDICATOR The dial indicator is used in several different ways to measure the amount of deviation (or runout) in revolving or rotating parts. Accurate shaft-to-shaft alignment is impossible without using a dial indicator. Dial indicators are supplied with various fittings, links, and adapters. Additionally, special application hardware is available. Figure 2-49 illustrates the basic hardware of a dial indicator set and two different types of dial heads. The dial scale is usually graduated in thousandths of an inch and has an adjustable bezel around it. The scale of a dial indicator usually reads plus numbers to the right of zero and minus numbers to the left of zero. The typical setup for checking the trueness of a shaft, using a dial indicator and vee blocks (or roller blocks), is to place the shaft in the vee blocks, mount the magnetic base or clamp attachment (whichever is the most appropriate) with the swivel post and dial head attached to a solid surface. Adjust the mounting linkage to a convenient angle that permits ease in reading the dial (but does not interfere with the task being performed). Bring the sensor button into contact with the shaft, loosen the swivel post clamp screw, raise and lower the sensor button to determine what the full travel of the indicator is. After the extent of travel has been determined, set the pointer at mid travel, secure the swivel post clamp screw, and then zero the dial with the adjustable bezel. Rotate the shaft slowly and observe the pointer for deviation. The combined deflection (plus and minus sides of the scale) is the total indicator reading (TIR). CARE OF PRECISION INSTRUMENTS Special treatment is required for precision instruments if they are to serve their intended purpose. The following precautions will help ensure their accuracy. 2-30