Manual Measurement Equipment

Manufacturing Engineering, Mar 2007 by Tolinski, Michael

Small dimensional metrology tools are becoming more usable and durable

The precision and accuracy of calipers, micrometers, gages, and other small measuring devices may have not improved much over the last couple decades, but these tools are better in other ways. "The biggest improvement has been in ease of use," says Tom Bress, a researcher and coordinator of laboratories for the Department of Mechanical Engineering at the University of Michigan in Ann Arbor.

The introduction of digital/electronic measuring tools provided immediate advantages for users. Obviously, digital readouts eliminate the difficulty of deciphering a vernier scale for thousandths of an inch precision. Plus, electronic tools can be zeroed at different set points, and can be toggled between English and metric units with the press of a button.

Summing up these improvements, Bress offers the historical analogy of the move from slide rules to electronic calculators. "Slide rules were analog; calculators are digital. They're both accurate enough to be used to design bridges, cars, and spacecraft, but calculators are much easier to use."

Manual measurement devices continue to be fortified with features-making them easier to use and more rugged. This points to their continuing importance in manufacturing, despite the availability of cheaper, smaller, and more portable CMMs and other table-top tools.

The dimensions of many machined parts still have to be checked frequently, quickly, and by hand, so small measuring tools are certainly not going away. "The use of precision tools or gages is obviously necessary and important. However, machined parts may not have to be checked by hand nearly as frequently today," says Scott Robinson of L.S. Starrett Co. (Athol, MA). "Today, modern high-speed machines are so accurate they can make repetitive parts much more consistently than even five years ago. So you might not have the same interval of inspection periods, but at some point you have take a sample."

"Of course since the widespread adoption of automated metrology systems, hand tools account for a smaller share of 'total measurement activity' than before," explains George Mullen of Mitutoyo America Corp. (Aurora, IL). "That said, most machinists and metrology professionals keep their micrometers and calipers close at hand. Sort of like a doctor who has easy access to an EKG but who, nevertheless, uses a stethoscope as step one in a heart exam." Along with their utility, small measuring devices and manual gages continue to be relatively inexpensive, compared with larger, more expensive metrology equipment like CMMs.

The most high-tech improvement to small measurement tools has been the use of wireless technology to transmit measurement values to a display/storage unit. This allows electronic data transfer without cables dangling from each measuring tool, reducing both measurement and maintenance time.

Different companies have established wireless technology for small metrology instruments. Going wireless last year were two manual electronic gages from Marposs Corp. (Auburn Hills, MI): the M1 Wave bore gage and M2 Wave snap gage, for checking inner and outer diameters, respectively.

To transmit measurement data reliably, wireless devicemakers typically equip tools with small radio transmitters that send discrete values to a display or storage unit. The Marposs gages use Bluetooth technology, which can provide a continuous stream of data from the wireless gage, says Bob Harman, the company's Testar Division gaging products manager. He says the Ml and M2 gages are unlike wireless measurement devices that have a LCD that shows the output value and a button that transmits the reading to computer storage.

He offers the analogy of "snapshot versus video." Like a video camera, the Bluetooth connection allows a continuous live feed of data, allowing the probing of a bore or diameter. "So the user can look and see what the actual condition of the bore is in order to measure such characteristics as taper or ovality."

He says the wireless gages are particularly helpful for checking large machined engine or transmission castings, for example. For checking multiple bore diameters in a casting, wireless bore gages make life much easier for the inspector. Traditional electronic gages, each wired to an amplifier, invite downtime from twisting, turning, and knotting cables. "You can imagine a bench 6 or 7' (1.8 or 2.1-m) long holding the component, with a bunch of these gages lying around, and the operator using these in sequential order time after time." Even with pulleys or hangars to separate the cables, "in these kinds of applications, the wireless rids the user of the problem of having to manage the wires."

Similar benefits apply when comparing wireless gages to air gages, he adds. The air gage requires a hose for compressed air, plus a chain of hardware connections for communicating inspection data; it's connected to a converter and then connected to a box that is interfaced with an amplifier. "With the wireless system, we actually take away that chain of components."