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.
With many of today’s electronic gages, the line between gage and measuring instrument is becoming blurred. It comes down to the definition of what constitutes a long measuring range as opposed to a comparative measurement. Is a 1-inch digital indicator on a bench stand, referenced on the measurement surface, a gage or a measuring instrument? These are the kinds of discussions that can go on for hours in a pub full of gage engineers on a Friday night.
However, there are situations where you can make a measuring instrument into a gaging instrument to improve its performance. For example, most calipers and their big brothers, height gages, use electronic scales as internal masters. The device compares the size of the part to the electronic scale. With the addition of a little computing power, these scales can be “mapped” :that is, their imperfections can be determined and correction factors can be added to improve performance.
Often, you will see the performance of such an instrument defined as “2 + L/600,” where the basic units are in microns, and the L is in millimeters. What this means is that there is a basic error in the system of 2 microns, plus an additional error based on the length of travel. Thus, if the scale is moved to its full length—which is when L is 600 mm—then the error becomes 2 + 1 or 3 microns.
If you want to get rid of this additional 1 micron, then you can use the height gage as a comparative gage. Height gages are usually referenced at “0” on the granite surface plate on which they sit, and all measurements are made with regard to this surface. The way to improve performance is to limit the amount of travel seen by the scale by using a gage block or other master to set a new reference point above the granite surface. For example, if you want more accuracy at 200 mm above the granite surface, then use a 200-mm gage block to set a new reference point and make your measurements in comparison to the 200-mm set point.
So now we have another bar argument beginning to brew. Is my height gage a measuring instrument or a gaging instrument?
Whatever it is, we have improved the performance of the height gage by getting rid of the travel error. This technique is not just limited to length performance of a measuring instrument: squareness error can also be improved by mastering on a reference square that is better than the inherent spec built into the gage.
Regardless of whether you are measuring or gaging, performance is almost always improved by shortening the measuring range.