Classic Gaging Tips

Accuracy is Spelled S-W-I-P-E

When a gaging system is not performing as expected, we often hear the same dialogue. The operator, who has only his gage to go by, says, “Don’t tell me the parts are no good- they measure okay on my gage.” The inspector replies, “Well, the parts don’t fit, so if your gage says they are okay, your gage is wrong.”

This is the natural reaction. People are quick to blame the instrument because it is easy to quantify. We can grab it, take it to the lab and test it. However, this approach will often fail to find the problem or find only part of it, because the instrument is only one-fifth of the total measuring system.

The five elements of a measuring system can be listed in an acronym: SWIPE. Rather than immediately blame the instrument when there is a problem, a better approach is to examine all five elements.

S represents the standard used when the system is set up or checked for error, such as the master in comparative gages of the lead-screw in a micrometer. Remember, master disks and rings should be handled as carefully as gage blocks, because nicks and scratches can be a significant contributor to error.

W is the work-piece being measured. Variations in geometry and surface finish of the measured part directly affect a system’s repeatability. These variations are difficult to detect, yet can sometimes manifest themselves as an apparent error in the measuring system. For example, when measuring a centerless ground part with a two-jet air ring, a three-point out-of-round condition will not show up because you are only seeing average size. Remember that as part tolerances get tighter and tighter, form and surface finish errors will eat up a larger part of the tolerance span.

I stands for the instrument itself. Select a gage based on the tolerance of the parts to be measured, the type of environment and the skill level of the operators. Remember what your customers will be measuring the parts with. Say, for example, you are checking bores with an air gage, but your customer inspects them with a mechanical gage. Since the surface is not a mirror finish, your air gage is giving you the average of the peaks and valleys, while the customer’s mechanical gage is saying the bores are too small because it only sees the peaks. Neither measurement is “wrong,” but you could end up blaming each other’s instruments.

P is for people. Failure to adequately train operating personnel will ensure poor performance. Even the operation of the simplest of gages, such as air gaging, requires some operator training for adequate results. Most important, the machine operator must assume responsibility for maintaining the instruments. Checking for looseness, parallelism, nicks and scratches, dirt, rust, etc., is absolutely necessary to ensure system performance.

E represents the environment. Believe it or not, with very tight tolerances, environmental errors are probably the biggest sources of gaging errors. Thermal factors such as radiant energy, conductive heating, drafts and room temperature differentials can significantly impact gage system performance. Again, dirt is the number one enemy of gaging. The issue could be as simple as your shop being a little warmer or a little dustier than your customer’s.

Before blaming your gage, take a SWIPE at it, and consider all the factors influencing its accuracy.

What Kind of Gage Do You Need? A Baker's Dozen Factors to Consider

Like every other function in modern manufacturing operations, inspection is subject to management's efforts at cost control or cost containment. Its good business sense to try to maximize the value of every dollar spent, but it means that hard choices must be made when selecting gaging equipment. Many diverse aspects may influence both the effectiveness and the cost of the inspection process.

For example, what's the ultimate cost of a bad part passing through the inspection process? It could be just a minor inconvenience to an OEM customer—maybe a two-second delay as an assembler tosses out a flawed two-cent fastener and selects another one. On the other hand, it could be a potentially disastrous equipment malfunction with expensive, even fatal, consequences. Even if the dimensional tolerance specifications for the parts are identical in both instances, management should certainly be willing to spend more on inspection in the second case to achieve a higher level of certainty—probably approaching 100 percent.

Many companies have achieved economies by moving inspection out of the lab and onto the shop floor. As this occurs, machinists and manufacturing engineers become more responsible for quality issues.

One could begin by comparing the hardware options. Let's take a "simple" OD measurement on a small part as an example. This inspection task could conceivably be performed with at least seven different gaging solutions:

1. Surface plate method, using V-blocks and test indicator
2. Micrometer
3. Purpose-built fixture gaging
4. Snap gage
5. Bench-type ID/OD gage with adjustable jaws
6. Hand-held air ring or air fork tooling
7. A fully automated system with parts handling

(Actually there are many more solutions available, but let's keep it "simple.") These options have a price range from about $150 to $150,000. There are also differences in gage accuracy, operator influence, throughput, data output, and on and on. It's confusing, to say the least.

A better approach is to first define the functional requirements of the inspection task, and let that steer one toward the hardware that is capable of performing the tasks as identified. In order to do this, the end-user should consider the following factors:

• Nature of the feature to be inspected. Is it flat, round or otherwise? ID or OD? Is it easily accessible, or is it next to a shoulder, inside a bore, or a narrow groove?
• Accuracy. There should be a reasonable relationship between job tolerance and gage accuracy resolution and repeatability—very often on the order of a 10:1 ratio. A requirement for statistical GR&R (gage repeatability and reproducibility) testing may require 20:1. But always remember:
•  Inspection costs. These increase sharply as gage accuracy improves. Before setting up a gaging operation for extremely close tolerance, verify that that particular level of accuracy is really necessary.
• Time and throughput. Fixed, purpose-built gaging may seem less economical than a more flexible, multi-purpose instrument, but if it saves a thousand hours of labor over the course of a production run, it may pay for itself many times over.
• Ease of use and training. Especially for shop-floor gaging, you want to reduce the need for operator skill and the possibility of operator influence.
• Cost of maintenance. Can the gage be maintained or is it a throw-away? How often is maintenance required, and who's going to perform it? Gages that can be reset to a master to compensate for wear are generally more economical over the long run than those that lose accuracy through extended use, but may require frequent mastering to ensure accuracy.
• Part cleanliness. Is the part dirty or clean at the stage of processing in which you want to measure it? That may affect labor requirements, maintenance, and the level of achievable accuracy, or it might steer you toward air gaging, which tends to be self-cleaning.
• Gaging environment. Will the gage be subject to vibration, dust, changes in temperature, etc.?
• "Mobility." Are you going to bring the gage to the part, or vice versa?
• Parts handling. What happens to the part after it's measured? Are bad parts discarded or reworked? Is there a sorting requirement?
• Workpiece material and finish. Is the part compressible? Is it easily scratched? Many standard gages can be modified to avoid such influences.
• Manufacturing process. Every machine tool imposes certain geometric and surface finish irregularities on workpieces. Do you need to measure them, or at least take them into consideration when performing a measurement?
• Budget. What do you have to work with?

All of these factors may be important when instituting an inspection program. Define as many as you can to help narrow the field, but remember that help is readily available from most manufacturers of gaging equipment—you just have to ask.

Nine Enemies of Precision Gaging

In some manufacturing plants, metal parts are made accurate to 0.01 inch. In other plants, there are products that cannot tolerate size differences of even a few millionths of an inch. Making parts to either tolerance range is impossible without accurate gaging. However, accurate gaging is impossible if liberties are taken with the design, handling and maintenance of precision measuring instruments.

Understanding the following nine major enemies of precision gaging will help defend your measurements against inaccuracy.

Wear.This is the enemy that is most often ignored. For example, linear measurements are usually made by contact between gaging and workpiece surfaces. The gage wears a little each time it's used, and inaccuracy grows by attrition. Wear also deforms gage contacts and flattens spherical contacts, producing discrepancies. The best therapy for gage wear is systematic checking and calibration against accurate masters.

Dirt.  Many measurement errors can be traced to someone's grubby hands. Those who measure in millionths of an inch should exceed even surgical standards of cleanliness. This applies especially to people who can't seem to wring gage blocks together without using what is known as wrist oil, a mixture of pore effluent, skin particles, grit, oil and coolant that coats gaging surfaces with a cement-like sludge ranging from 0.00005 to 0.0005 inch is height.

Looseness.The average user of gages tends to make sure the relevant screws, nuts and clamps are secure. However, internal looseness caused by wear may fool the user. For example, sometimes gage platens and bracket arms creep or a workpiece doesn't settle firmly into place. The key to diagnosing looseness is measurement repetition. If the same reading doesn't come up twice then looseness is the likely culprit.

Deflection. Ever present and active, deflection is never seen or felt except by special means. Isaac Newton described deflection in his third law of motion, which states that for every action, there is an equal and opposite reaction. Visualize pushing a cylinder into a gage. Although the contacts separate to accept it, the internal clamping force of the spindle acts equally against the frame, thus causing it to deflect slightly. What is being measured—the workpiece, the frame deflection or both?

Gaging Pressure.This force must be heavy enough to have unwavering authority, but not so heavy as to deform the workpiece. Pressure errors almost always stem from too much rather than too little force.

Temperature.Everyone agrees that a workpiece is bigger when it's hot. Any action taken to alleviate this usually involves cooling the part too much. There should be a big flashing sign in every precision gaging area that reads: "Keep the temperatures of the workpiece, gage and master the same."

Vibration.There are people who put a "millionth" comparator near an aisle used by fork trucks. Others sit them next to air compressors or thumping punch presses. The moral is: Do precision work where your comparator won't get the jitters.

Geometry.Measurement must be square to the axis. This is elementary, almost to absurdity. Nevertheless, it points out a major source of error. Whether the instrument is a hand "mike" or an interferometer, many operators persist in cocking the workpiece or cramping the gage just enough to get a wrong answer.

Approximation.A look at any mechanical micrometer reading shows where this enemy lurks. Perhaps it reads 0.494 inch—and a little more. What's your guess on the "little more"-0.4942, 0.4943 or 0.4944 inch? Do you use this as the true reading? The usual cure is to get an instrument with higher magnification, or one with an accurate scale subdivided more closely. Another solution is to switch to a digital readout.

There are other known causes of gaging error, and there are still more to be discovered. However, the firm that tackles this list will have taken a big step toward greater precision and accuracy.