Outside spring calipers	Inside spring calipers
Figure 1. Measuring with a caliper involves adjusting the tool so that the reference points of the measuring tool are the same as those of the feature of the part.

Calipers are used to measure features of a part that cannot be directly measured with a scale or micrometer measuring device. Of the two types mentioned, the inside caliper can be used to the greatest advantage. The important thing to develop is a sensitive "feel" of the caliper and workpiece. Always take a measurement at least twice to see that it is reliable. Calipers should be rocked over center (Figure 2). The feel that is provided as the tool passes over center is the limit of its sensitivity.

Figure 2. The lighter the feel, the more reliable the measurement.

When you use micrometers to get the size of the caliper, get the same "feel" between the micrometer and caliper as you had between the caliper and the workpiece. The caliper should be adjusted so that you have a bare minimum of pressure and contact. This will ensure the greatest accuracy of measurement. Only practice will make you proficient.

Small hole gages

These gages can be used to produce fast and accurate measurements (Figure 3). Small hole gages usually are purchased as a set of four gages, the range of which are: 0.125-0.500.
Figure 3. Small hole gages have a split ball for gaging the hole or slot width.

Once again, it is important to get that same "feel" between the micrometer and the hole gage (Figure 4). Always make at least two measurements, and try to hold the handle exactly in line with the center of the hole. 

Figure 4. After adjusting the small hole gage to the desired feel in the hole, the hole is measured with a micrometer.

In some instances you might use the small-hole gage for measuring slots, keyway, or other non-circular part features.

Telescoping gages

The telescoping gage can be used to measure very accurately if used correctly. The gage consists of a handle that has a knurled locking mechanism and two spring-loaded telescoping plungers which, when released by the lock, extend outward to make contact with the part feature (Figure 5).

Figure 5. Telescoping gages come in sizes which can measure from ½ inch to 6 inches.

Here again, these gages come in a variety of sizes, each size having its own operating range. To measure a hole, follow these steps (Figure 6):

Figure 6. Telescoping gages need to be rocked over center to size and center the telescoping gage.

1. Choose the proper size gage for the hole to be measured.
2. Push the plungers in and tighten the lock lightly. NEVER tighten the lock any more than is necessary to hold the plungers in place.
3. Place the gage in the hole as shown in Figure 6. Holding the gage tilted a little off center, release the lock, allowing the plungers to extend and contact the sides of the hole. Retighten the lock. Do not over-tighten.
4. Rock the gage over the center of the hole to the position shown in Figure 6.
5. Remove the gage from the hole carefully, and measure it with the proper micrometer. Use a light touch. Too much pressure with the micrometer could force the plungers in further, thus destroying the accuracy of the reading.
6. Do not go over center with the handle more than once. The tool is designed to be self-centering on the diameter to be measured. If you push the handle over center more than once, the micrometer reading will probably be smaller than the true size of the hole. Always make at least two measurements to ensure accuracy.
   When you use telescoping gages to measure straight-sided features, such as pockets or slots, you must move the gage about freely to ensure the fact that you have the shortest, most direct measurement.

Adjustable parallels

This is a useful tool, especially for measuring slots or keyways. The gages are manufactured in a variety of sizes and can be purchased either separately or in sets. They range in size from 3/8" up to about 2 1/2" commonly. The gages can be used together to measure features larger than your largest parallel. Most good adjustable parallels have a screw that can be tightened to hold the position that you have set. To use this type of gage:

1. Choose the properly sized parallel for the job.
2. Place the parallel in the feature to be measured. Press the ends of the parallels together tightly with your fingers. Do not force the parallels with a hammer or other item.
   If possible, leave a portion of the parallel exposed so that you can micrometer it without removing from the feature (Figure 8).
3. If you must remove the adjustable parallel from the workpiece to measure it, tighten the locking screw to maintain position and size, and remove it carefully.

Figure 8. Measuring a slot using an adjustable parallel and a micrometer

Adjustable parallels can also be purchased that have a radius on the contact edges so that they can be used to accurately measure the diameter of holes or other circular part features. As with any other gage or other measuring instrument, be sure that the feature is clean and free of dirt, oils, or machining burrs.

Planer gages

It can be set with the use of a micrometer, vernier caliper, or gage blocks (Figure 10). Once a dimension has been set, the planer gage can be used to transfer the dimension to another location (Figure 11). 

Figure 10. Setting the planer gage using a micrometer	Figure 11. Transferring the planer gage setting to the surface gage for layout

Very often a scribe is used in conjunction with the planer gage to produce very accurate layouts from a reference surface.

Radius gages

Too often radii on part or detail prints are regarded as unimportant. They are important, however, whether it is for strengths or as a firming or shaping consideration. Radius gages can be purchased in a variety of styles, sometimes as separate gages and sometimes attached together as a set (Figure 12). 

Figure 12. Radius gages

They are manufactured with great accuracy and can be used to measure both inside radii (fillets) and outside radii. They are used both in machining and inspection.