TABLE OF CONTENTS Mechanical measurements on machined parts are very precise, but not as precise as some other fields. Table 3.1 provides some perspective.
| Navigation | 1,000 feet |
| Agriculture | 6.0 inches |
| Construction | 0.1 inch (1/8 inch) |
| Machines | 0.001 |
| Semiconductors | 0.000 04 (1 micron) |
| Electron Microscopy | 0.000 000 1 |
Machine building and machine maintenance require measurements on the order of 0.001 inch, or one thousandth of an inch. This is commonly referred to as a mil. Measuring devices are commonly available to determine accuracy to one mil. These are dial or vernier calipers, micrometers, hole gauges, dial indicators, and feeler gauges. All of these instruments are portable and appropriate for field measurements on machines.
The field of metrology (the science of measurement) generally requires that instruments of measurement have an accuracy of 1/10 of the desired precision. This would require that dial indicators of .1-mil accuracy be used to determine .001 inch. This is an unnecessary and burdensome requirement because alignment measurements are relative and not absolute. To understand this, some terms need to be defined.
Measurement is a comparison process where an unknown quantity is compared to a standard. The comparison is done with an instrument that has known characteristics, or is calibrated. Calibration is the comparison of a measuring instrument's value to a known standard of higher order. This establishes the accuracy of the instrument.
Accuracy is being correct to a standard. The typical working standard...
