Nine Enemies Of Precision Gaging

In some plants, metal parts are made accurate to 0. 01 inch.

Columns From: 1/1/2006 Modern Machine Shop,

In some 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 in height. Abrasion from dirt also speeds wear and causes internal looseness.

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, or if an indicating meter hand flutters irresolutely, 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 at the nearest drinking fountain. 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.”

There is also a tendency to put measurement equipment in locations that cripple their effectiveness. Favorite spots are next to radiators, in the direct rays of the afternoon sun or near a door that’s opened and shut 50 times on subzero days.

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, 0.4944, or 0.4945 inch? Do you use this as if it were the true reading? Approximation probably causes more wasted time and money in rejections, salvaging, disputes and correspondence than all other measurement errors combined. 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.

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