Switch From Mechanical To Laser Measuring Cuts Inspection Costs In Half

To anybody running a manufacturing operation, inspection productivity is certainly desirable. To the founder and CEO of an independent metrology service center, it is his lifeblood.

That is why Bob Kuehl is so delighted with his switch from mechanical to laser measuring in gage-pin calibration at Northeast Metrology, Inc.(NMI), in East Longmeadow, Massachusetts. Throughput is up 60 percent, with no trade-off in accuracy, and unit inspection costs are cut by half. And because the operation is now semi-automatic, two sources of human error—reading and transcription of results—have been eliminated.

Mr. Kuehl attributes the gain to the Mitutoyo LSM Series 544 Laser Scan micrometer (Mitutoyo Corp., Aurora, Illinois) and the Auto-Pin software he developed to optimize the calibration operation. "The new laser scanning mike speeds up the actual measurement, then our Auto-Pin software generates all that documentation," Mr. Keuhl says. "The end result is a pin-by-pin certification, right in the ISO format—at about a 60 percent time saving to us."

Since pin calibration represents about 20 percent of NMI's business, improving its efficiency has benefited both their market share and the bottom line. "We charge only 50 cents per pin for a blanket certification and 75 cents per pin for a certification showing actuals and yet remain profitable," Mr. Keuhl states. Because of their pricing advantage, NMI's pin calibration sales have doubled. The pin calibration business splits about 50-50 between new gage sets and recalibration of sets in use.

The standard protocol of calibrating "economy" pins (±0.0002 tolerance) is basically to record on reading on each pin. The standard procedure for calibrating class X, Y, Z gage pins is a bit more involved. The operator measures the diameter at three points along the length, rotates the pin 90 degrees, and measures at three points again (this is in the development stages). The average of the six results becomes the reported value. Measuring in two planes indicates out of roundness as well as taper conditions. In the past, each reading was made and recorded manually since the bench micrometer was an analog device.

Mr. Kuehl started looking for a better calibration method mainly for productivity reasons. "We were satisfied with the 15 millionths accuracy of the older Mitutoyo amplified mechanical bench mike," he says. "We simply wanted to speed up the operation and eliminate all of the manual recording." He jumped to the laser as soon as it became available.

After running the new Mitutoyo laser micrometer for several months, Mr. Kuehl noticed a couple of other advantages. Stability is 75 percent better than before, and repeatability is 40 percent better. "We see less than 5 millionths drift over the course of a day," he adds. Also, the micrometer anvil parallelism issue simply disappears.

The principle of operation is simple: a laser beam, spread out by a collimator lens, illuminates the sample, which blocks a portion of the beam energy. The amount of beam energy interfered with by the sample is directly proportional to its diameter. The remaining light is collected through a condensing lens and its intensity measured by the receiving element.

The process goes step-by-step like this: Receive the pin set from the customer, wipe all pins clean. Start up the laser mike, let it stabilize for about one hour, and calibrate it with their own set of masters. Calibrating the laser micrometer takes only 11/2 minutes. Next, the operator feeds in each pin, one at a time, beginning with the smallest diameter. If any dust is present, the operator sees it and simply relocates the pin to a clean spot. When ready to take a reading, the operator simply taps a foot switch. Then the software takes over. Since the readings are digital, manual reading errors are eliminated. The operator notes any wear on the printout;everything else is automatic.

Although Mitutoyo recommends re-calibrating the micrometer when switching size ranges, Mr. Kuehl has found it unnecessary because, "It locks on within the 15 millionths tolerance immediately." Although NMI re-calibrates as a matter of principle, they have found the readings identical.