An electronic caliper (see Fig. 2.10) is more expensive ($100 - $200) than a dial caliper. However, electronic calipers can do a great deal more with greater precision [±0.013 mm (±0.0005 in)]. A measurement can be locked in by pressing a button; so, a measurement will not change even if the sliding part moves. Electronic calipers can alternate between metric and English measurements, can be zeroed anywhere so "difference" measurements can be read directly rather than subtracting or adding, and can be set to flash their numbers on and off when an object is out of maximum or minimum tolerance. A tape readout of measurements can be made with extra equipment.

2.2.4 The Micrometer

Another common tool for length measurement is the micrometer. Like the vernier caliper, in the United States, it is typically made with English measurements. Whereas the caliper can provide precision measurement (to ±0.002 in.), you need a micrometer for greater precision (±0.0001 in.).

The micrometer is a very accurately machined machine bolt and nut. If you rotate a machine bolt into a nut a specific number of times, you'll notice that the bolt will have traveled a specific distance (see Fig. 2.11)

On the micrometer, the rotating handle is the nut, and the plunger is the machine bolt. Figure 2.12 demonstrates how to read a micrometer.

A good-quality micrometer is not as expensive as a caliper of equal quality. However, each design of a micrometer is capable of only one type of measurement and can measure only a limited range of that measurement (typically one inch). By comparison, the caliper can read inside, outside, and depth measurements. The caliper, as well, can read from zero to (typically) 6 inches. Therefore, to have an equivalent set of micrometers to do what one caliper can do would require up to 18 micrometers. Despite this shortcoming, a good machinist would not consider using a caliper for anything but a rough measurement and would insist upon using a micrometer for all final measurements.

To make a micrometer measurement, place the item to be measured against the anvil of the micrometer. Then, rotate the handle or ratchet until contact is made. All final contact of the spindle against the measured item should be made with the ratchet. The ratchet provides limited control over the handle. Once it encounters

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Fig. 2.10 The electronic caliper.

If the threads are cut at 50 revolutions per inch, 10 revolutions will move the nut one-fifth of an inch, and one-half a revolution will move the nut one-hundredth of an inch.

Fig. 2.11 The micrometer is a highly machined nut and bolt.

resistance, it will spin with a clicking noise but will offer no more pressure against the handle. This feature guarantees consistent spindle pressure against the anvil for all readings.

A micrometer can also be used as a go/no-go gauge. The lock can be tightened, preventing the handle from rotating. Thus, by setting a measurement and by tightening the lock, the measurement is set. Then anything that fits (or does not fit) between the spindle and anvil is accepted or not accepted depending on your needs. For accuracy greater than 1/10,000 in., an air gauge is used. These gauges have accuracy to 1/1,000,000 in., and are used in the most demanding situations. The air gauge is specialized beyond the scope of this book, and no further explanation is included.

Rotating Handle

On the barrel you can read 1.5 units. The measurement process is continued on the thimble.

On the thimble, you can read 0.23 units (butvno more). If the micrometer you are using has a

Vernier, you could read to the 1000th place.

1.5

+0.23

Fig. 2.12 How to read a micrometer.