An engineering perspective on precision manufacture necessarily looks to a continuing refinement of machining and processing techniques for the overall improvement of products. The initial drive for high precision is firmly rooted in the industrial revolution. In the intervening century and a half, the precision of conventional machine tools has improved by orders of magnitude. Today, some types of components can be produced on a routine basis with a precision (ratio of dimension to tolerance) of one part in 100,000 or more. This is an improvement of perhaps as much as 1000x from those early beginnings.
It is nevertheless impossible to manufacture workpieces without deviations from the nominal shape. Workpieces always have deviations of size, form, orientation and location. When these deviations are too large, the usability of the workpiece for its purpose will be impaired. Control and measurement of these deviations is therefore critical.
Measurement is a procedure in which an unknown quantity is compared with a known standard, using an accepted and consistent system of units. A measurement provides a numerical value of the quantity of interest, within certain limits of accuracy and precision.
As people tend to generally consider accuracy and precision as having one and the same meaning, it is important to highlight the distinct difference between these two terms in order to comprehend the discussion on precision engineering.
A measurement procedure is accurate when it is absent of systematic errors, which are positive or negative deviations from the true value that are consistent from one measurement to the next. Precision is the degree of repeatability in the measurement process. Good precision means that random errors in the measurement procedure are minimized. Random errors are usually associated with human participation in the measurement process. Examples include variations in the setup, imprecise reading of the scale, round-off approximations, and so on. Nonhuman contributors to random error include temperature changes, gradual wear and/or misalignment in the working elements of the device, and other variations.
Closely related to measurement is gaging. Gaging (also spelled gauging) determines simply whether the part characteristic meets or does not meet the design specification. It is usually faster than measuring, but scant information is provided about the actual value of the characteristic of interest.
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