Gaging and Measuring

To All:
    Here is an excerpt of a short article in Modern Machine Shop. It's info that most probably know, but a few may not.

When Gaging Becomes Measuring That Becomes Gaging
By George Schuetz Marketing Manager, Tools and Gages Mahr Federal, Inc.
    In a previous column, we discussed the terms “gaging” and “measuring,” which are often used interchangeably. There are times when gaging is appropriate and other times when measuring is the best way to go. Measuring is a direct reading process in which the inspection instrument consists of (or incorporates ) a continuous series of linear measurement units, commonly known as a scale. These units usually start at zero and go up to the maximum range of the instrument. The workpiece is directly compared against this scale, and either the user counts the units or the measuring instrument displays the amount of movement indicated by the scale. Examples of measuring instruments include steel rules or scales, calipers, micrometers, height gages or CMMs.
Measuring instruments tend to be more versatile, though a little less accurate than gaging instruments. They are often used for incoming inspection or in a machine shop where volumes are low and high precision is not a requirement.
Gages, on the other hand, are indirect reading instruments. The measurement units live not on the scale but off site (for example, in a calibration room), and a master or other standard object acts as their substitute. The workpiece is compared against the master and only indirectly against the measurement units. The gage thus evaluates not the dimension itself, but the difference between the mastered dimension and the workpiece dimension. Examples of these comparative gages include an adjustable bore gage, mechanical snap gages or bench stands with dial indicators.
Gages tend to be faster and more accurate than measuring instruments. This is because they are dedicated to a particular size, and over their short range, they produce higher resolution and hence, greater accuracy. ====================================================================
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