Measurement / Quality Exclusives

50 Years of Quality: Dimensional Gaging in Review

August 24, 2011
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This month, 50 Years of Quality takes another look at dimensional gaging.

Beyond the Basic Benchtop, November 2004

Benchtop gages sure have changed! The effects of the electronics revolution on them were on full display at the recent International Manufacturing Technology Show (IMTS) held in Chicago, Sept. 8 to 15, 2004. Twenty years ago, benchtop gages consisted of such things as mechanical micrometers, vernier calipers, 6-inch rulers and dial indicators in test stands. Today’s versions of those same devices contain digital readouts and can transmit their measurements to computer networks at the touch of a button.

The changes don’t stop there, however. The range of metrology equipment that fits on a benchtop has grown tremendously and has expanded the notion of what constitutes a benchtop gage. Miniaturization, information technology and applications engineering have allowed manufacturers of measurement devices to repackage technologies previously confined to large systems. Profilers, vision systems and coordinate measuring machines (CMMs) all come in benchtop models.

At the most basic level, electronics have given conventional hand gages features they did not have before. Consider measuring a bolt-hole pattern with a conventional height gage that displays its readings on an indicator. “You’d have to take one dimension, referencing from a gage block or a height standard to determine a number,” says Scott Robinson, technical support specialist, The L.S. Starrett Co. (Athol, MA). “Then you’d have to do the same to find the bottom of the hole, do the math to find out the dimensions of that hole and write the position on a piece of paper.”

The computers and trigger probes on high-end height gages today do all of that automatically. The touch-screen computers on these gages run Windows CE and have software that allows users to program measurement routines and run full statistical analyses. Most also have USB and printer ports on them for connecting to off-the-shelf printers, so users can print hard copies of the statistics or generate validation reports directly from the gage.

The electronics on these and other gages have communication ports for connecting to any PC and dumping the data directly to an Excel spreadsheet or other software. Although wires are still the most common means of establishing such a connection, wireless data transmission could become an option for replacing them soon, especially on hand tools like calipers and micrometers. “These products allow the operator to move freely and collect data on a timely basis, but not be tethered down by a wire,” explains Kurt Braun at Mahr Federal Inc. (Providence, RI). Wireless transmission avoids not only the worry of ensuring that the wires are long enough to accommodate every contingency, but also the bother of preventing the wires from becoming tangled in fixtures.

Height Gages Measure Up, February 2006

Height gages measure the height of components and product features by measuring the distance from a reference point, usually a granite surface plate, to the component or product. Height gages range from simple surface gages for measurement transfer and marking to motorized digital height gages. Height gages and their surface plates are versatile and relatively inexpensive, yet precise enough to be used as a rough incoming inspection tool or for final product approval.

“Low-end height gages can range from little more than a simple transfer stand with an indicator to a powerful data calculating tool with auto scanning functions,” says Walt Wardzala, major instruments product manager at Mitutoyo America Corp. (Aurora, IL). “Digital displays help to eliminate errors as well as keep track of reference points. Higher end instruments feature accuracy down to almost 1 micron and perform tedious operator measurements with a keystroke.”

Price and accuracy are key considerations when selecting height gages. “Pneumatic gliding systems require a granite plate for movement,” adds Wardzala. “The benefit is greater ease of use and increased speed. Digital mechanical models are lightweight and durable. Accuracy and repeatability are more important than resolution. Make sure the model you select meets your tolerances. Mechanical digital, dial and electronic models are so affordable that few people bother with Vernier height gages.”

Why Air Gaging Still Matters, April 2006

Air gages first started ensuring quality in 1919. One of the first air gage systems was regulated by “bubbling” air through a specific level of water within a cylinder in which the air was passed. The specific gravity of the water has a known constant value. However, “This process could not prevent the water from evaporating and thus changes to the readings by the air gage occurred over time,” says Jack A. Gaughan, sales manager of custom gaging at Edmunds Gages (Farmington, CT). “Other early air gages used bourdon tube technology to expand and contract based upon back pressure.”

Practical dimensional air gages began to appear in the late 1930s. “This was when flowmeter instruments with operating pressures of 10 psig were developed,” says Don Moors, president of Western Gage Corp. (Camarillo, CA). “About the same time dial-type back-pressure instruments with gage pressures at the midpoint of the scale ranging from 20 to 25 psig were developed. Both of these designs proved practical for many gaging applications, with many proponents of both systems.”

During the 1940s, there was increased demand for tighter tolerance from war-related needs. After this, aeronautical requirements forced tolerances to get even tighter. Air gaging and their displays took measurement to another level and for the first time operators were able to measure 0.00002 inch on the shop floor-well before electronic amplifiers and probes.

One thing that hasn’t changed considerably in the past 90 years is air gage’s tooling. While there have been upgrades in materials, coatings and processes, according to George Schuetz, director of precision gages at Mahr Federal Inc. (Providence, RI). “You could take a piece of tooling used in 1945, put it on a gage built in 2006 and it would still function. Also mechanical-style, analog displays have not changed too much over this period. Air gage displays today still use some type of pressure-sensing mechanical device with a magnifier to move a needle to display a length change. These are very similar to ones made 60 years ago. What really has changed are the options for displays that can be used. In the past 50 years, the electronic readout is what brought air gaging to a new level by providing more information in an easier-to-understand format to the operator.

“The electronic pressure sensor is what brought the air gage display into the 21st century. Combined with the power of microprocessors and computers virtually anything can be done with the air pressure change,” Schuetz says.

It’s true that one innovation today not present 90 years ago is that air gage systems can convert pressure readings to an electronic signal easily converted to a display on an electronic column or a microprocessor-based gaging system. “These displays are more of a one-size-fits all, where the resolution can be easily changed and the displays increased or decreased without changes to the min/max masters or the gage itself,” says Don Kumpula, sales manager at Air Gage Co. (Livonia, MI). “The gage results can now be stored and the data downloaded to a network system to be analyzed for statistical process control.”

Ins & Outs of ID/OD Comparative Gaging, February 2007

Circles are the most frequently produced machined form. Generated by many different processes-including turning, milling, centerless grinding, boring, reaming and drilling-there are, correspondingly, a wide variety of gaging methods used to measure inside and outside diameters. At the low end, a hole could be measured with a scale or a fixed go/no-go gage. At the other extreme, any number of precision measuring machines, including coordinate measuring machines (CMMs) and optical or vision machines are available. However, in production environments, most inside diameters and outside diameters (ID/ODs) can be accurately measured using one of several varieties of comparator gage.

ID/OD comparator gages come in two basic flavors-benchtop and portable-and are meant to be used in high-volume, high-performance applications by operators with shopfloor-level skills. Because they provide a comparative measurement, these gages require a master to set a zero reference point for determining part deviation. Comparator gages have relatively limited travel, but this allows the use of high-resolution dial or digital indicators or very high resolution electronic probes with amplifier readouts.

The choice between benchtop and portable styles depends mainly on the size of the part being measured, and whether the part will be brought to the gage or vice versa. Benchtop comparative gages are typically used on small parts and are generally restricted to measuring single dimensions or features that are less than 255 millimeters, or 9 inches, in diameter and not more than 25 millimeters, or 1 inch deep. Portable ID/OD gages can go as large as 2,500 millimeters, or 8 feet, and as deep as 125 millimeters, or 5 inches. If a manufacturer needs to go deeper than that, bore or plug gages are a better choice.

Levels of Precision: A Field Guide to Dimensional Gages, June 2010

Conceptually, the dimensional measurement process is quite simple. You get a drawing for a part-an electronic computer-aided design (CAD) file, blueprint, napkin doodle or whatever-which indicates certain critical part dimensions and tolerances: a particular diameter, for instance, must be 2.2370 inches ±0.0002 inch. All the manufacturer needs to do is to machine the parts to that dimension, then measure them to document that they are within the specified tolerance. What could be easier than that?

Probably a lot. Not only does the tolerance of the part need to be considered but also the number of parts to be measured, the time needed for measuring, the skill of the operator, the environment of the measurement and how much money is to be spent. All these questions go into finding the right dimensional gage for the process.

The subject is complex and there is no one-size-fits-all solution, even for the same dimension on the same type of part, measured under similar conditions but in different shops. There are, however, some broad distinctions that can be made in terms of levels of precision, speed and throughput required that can help make the gage selection and measurement process easier. Q

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Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
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