Measuring Tools Basics: Faster and Better
In a high-volume production environment, the cost of inspection is related to the speed with which measurements can be made and interpreted.
Let's look at some examples of a few common measuring instruments and consider the speed of measurement and the results that can be obtained.
First, however, let’s consider measurement as a direct-reading process in which the inspection instrument consists of (or incorporates) a scale that provides a continuous series of linear measurement units (inches or millimeters), usually ranging from zero to the maximum capacity of the instrument. The workpiece is compared directly against the scale, and the user counts complete units from zero and then fractions of units. The result generated by "measuring" is the actual dimension of the workpiece feature.
Inspection instruments include steel rules or scales, calipers, micrometers and height stands. Coordinate measuring machines (CMMs) may also be placed in this category.
The great thing about measurement instruments is their versatility. For example, although they have been around for a long time, calipers and micrometers are still extremely versatile and useful tools for making a wide range of length measurements (typically outer diameters and inner diameters). The caliper can span from 0 to 4 feet, depending on the length of the scale. External measurements are made by closing the jaws over the piece to be measured, while internal measurements are made by opening up the inside diameter contacts.
However, the problem with calipers is that despite the long measuring range and ease of moving from one size to the next, they are subject to a tremendous amount of user influence. While the caliper is a versatile tool, it is not one of the most precise. Skill is required for positioning the tool and interpreting the measurement result. As the user develops his or her "feel" for the tool, measurement results become more consistent.
While the digital caliper may take some of the guesswork out of reading the measured value, (as compared to a Vernier or dial caliper) it still requires skill on the part of the user to apply the tool properly to the dimension being measured. The jaws of the caliper must be square or perpendicular to the part and held firmly against it, but not to the point of deflecting the jaws. The part should also be kept as close as possible to the frame of the measurement tool. With a little skill, you can make a fast, direct measurement or comparison in seconds and move on quickly to the next important task.
It should be noted that calipers typically read in 0.0005-inch/0.01-mm units. If the tolerance is tighter than ±0.005 inch, a micrometer or some other higher-accuracy tool is the way to go.
Higher resolution is one of the reasons the micrometer is often the tool of choice for length/diameter measurements with tighter tolerances. With a resolution typically 10 times better than that of a caliper (digital micrometers read to 50 microinches/1 micron) the basic micrometer provides a higher degree of performance. Although a micrometer is still subject to some operator influence, ratchet or friction drives ensure repeatable gaging force in today’s micrometers, minimizing operator influence errors.
A limitation to the micrometer’s versatility is its size, measuring a range of only 1 inch/25 millimeters. Unlike a caliper, the sensitive contact needs to be rotated to the next measuring size rather than just sliding the jaw to the next measuring position. Thus, going from one size to the next within the range of the micrometer can be a turning experience. At 0.025 inch/0.5 millimeters per revolution, 40 rotations will be required to go the full 1-inch measuring range. When measuring multiple diameters on the same part, this can be a time-consuming process.
Fortunately, there are micrometers that can reduce this adjustment dilemma when used in a production environment, where both speed and the need to measure multiple diameters is required. Digital micrometers can be supplied with a quick drive spindle that does not rely on a rotational sensor but rather a linear sensor for position. With this capability, the measuring spindle can be moved with a greater distance per revolution: What was previously 40 rotations might now be only five rotations. Thus, measuring five diameters of different sizes on a shaft with this quick-adjust micrometer is a whole lot faster. This leads to more measurements in less time and a giant increase in measurement productivity.
Different instruments (and different operators) are prone to different errors.
Virtually every machine tool builder lists, as part of a machine's specification, accuracy and repeatability figures. What's generally not given is the method used to arrive at the figures. Though these methods are defined in linear positioning standards, not all builders use the same standards.
While countersunk and chamfered holes are similar in appearance, functionally they are quite different. Consequently, different gages exist to serve these different functional requirements.