Every shop has a variety of handheld gages hanging around. You may use them every day and not even give them a second thought. This month,Qualitytakes a look back at the industry’s workhorse.

Desire for Accuracy Drives Digital Tools, March 2002

Electronic, digital handheld tools have been around for more than three decades, but they were somewhat slow to be accepted. Today, many suppliers say that digital tool sales are equal to or greater than nondigital tool sales.

“Early on, they were still somewhat mistrusted by the measuring and manufacturing world,” says Steve Pike, group manager, precision measuring tools, Mitutoyo America Corp. (Aurora, IL). “The first ones were just okay. Not just Mitutoyo’s, but all manufacturers’ early versions. We struggled with trying to make a tough, reliable and dependable digital tool with a battery life that was pretty good and a display that was readable in low-light conditions,” he says. “Now, we have built six or seven generations of digital calipers and it is our most popular tool.”

The digital micrometer took a little longer to catch on, probably because of the tool’s extreme tolerances. Dave DiBiasio, national sales manager for Brown & Sharpe (North Kingstown, RI), says the jump from the analog micrometer to the digital micrometer was a profound advancement because of the extremely small measurements that the digital version can take. “With a standard micrometer, there is a main barrel scale that breaks the inch down to a 1⁄₁₀ inch, another scale on the barrel that breaks it down to thousandths, and a vernier scale on the barrel that breaks it down to 10,000ths of an inch,” he says. “Picture trying to use this in a standard shop environment, which may not be well lit, and trying to look at one scale, then turn the micrometer over and look at another one and go to a third to see which lines are lining up. All to try and determine that reading. Without skill, there is a huge potential for error.”

The electronic nature of new micrometers has allowed manufacturers to go beyond even these miniscule measurements. Today’s digital micrometers can read down to 50 millionths of an inch, DiBiasio says.

Digital is not just confined to calipers and micrometers. A slew of handheld measuring tools such as ultrasonic thickness gages, hardness testers, surface analysis tools, height gages, torque wrenches, gap and flush transducers and others have all added electronic capabilities.

Ease of use is a big reason; it is much easier to read a number than to count numbers on a vernier scale or lines on the barrel of a micrometer. The growing use of statistical process control (SPC) is another reason. In the past, results would be noted with paper and pencil, a time-consuming process that can lead to transposition errors, papers getting smudged by contaminants and, most importantly, out-of-tolerance conditions that are allowed to continue because some data analysis is not immediately accessible. Digital tools of all stripes usually are available with RS232 serial ports to connect via cable to a personal data assistant, portable data collector, or laptop or PC computer. This eliminates errors by cutting out the paper step, and also makes SPC analysis of measurement data available more quickly, sometimes in real time.

Quality 101: Caliper Basics, August 2007

Calipers are extremely versatile tools for making a wide range of distance measurements, including both outside diameters (OD) and inside diameters (ID). While micrometers are more accurate, they have a limited measurement range, typically several inches. Calipers can span from 2 inches to 4 feet, depending on the length of the scale. External measurements are made by closing the jaws over the piece to be measured, while internal measurements are made by opening up the inside diameter contacts. Depth and other measurements can be made with a depth rod built into the instrument’s beam.

There are three basic types of caliper that may be found today in a machinist’s tool chest:

Vernier. The vernier caliper is the original design and still the most rugged. Graduated much like a micrometer, it requires the alignment of an etched scale on the vernier plate with an equally spaced scale running the length of the tool’s handle. Skillful alignment of the tool and interpretation of the reading is necessary to achieve the measurement tool’s stated accuracy.

Dial. A dial caliper is a second-generation caliper. Similar in construction to the vernier caliper, this style replaces the vernier scale with a dial indicator. The indicator is fixed to the moveable jaw and engaged with a toothed rack on the body of the unit. The dial, which is typically balanced, meaning it can move in either plus or minus directions from zero, may be graduated in either inch or metric units.

The dial caliper is a dual-purpose tool and can make either direct or comparative measurements. To make a comparison, first measure the reference dimension and set the dial indicator to zero. Then measure the dimension to be compared. The indicator will show how much the compared dimension varies from the original (plus or minus).

Another useful feature of the dial caliper are jaws that slide past each other to allow contact points or depth rod extensions to fit into narrow openings for small ID measurements.

Digital. In the past 25 years the digital caliper has made its way onto the shop floor. The latest designs provide numerous electronic features that make the device easier to use, yet add little in the way of cost. These include: easy switching between inch and metric units on the readout, tolerance indications, digital output to electronic data collection systems, zero setting anywhere along the caliper’s range and retention of the zero setting even when the caliper is turned off. With no moving parts in the readout, the digital caliper is durable, and newer units are even waterproof.

Data Acquisition for Handheld Gaging, February 2008

When looking for a system to handle data collected by handheld gages, there are two choices: wireless and wired. Both systems have their place in industry, and carefully considering the attributes of each will allow one to make the best decision.

Paradoxically, one could make the case that a wireless system’s strength also is its weakness, and that at wired system’s strength, too, is its weakness. A wireless system has no wires-so it has the feature of increased mobility, with gages that are not tethered to a multiplexer or computer; but this also means that EMI can potentially creep up and ruin or impede an operator’s work. A wired system has wires-so it offers an inherent robustness, because EMI cannot compromise an operator’s work; but cabling also means reduced mobility and potential safety issues.

As Starrett’s Jeff Wilkinson explains, “Imagine having a tool with sharp jaws, like a slide caliper; now imagine having a tool like that with 20 feet of cabling running from it. You’re holding the tool delicately and someone walks by and trips on the cable, yanking it out of your hands with a lot of force. It could lacerate your hands very easily.”

Another problem with cabling is entanglement. “If you have five tools in a production area and they’re all wired to the same multiplexer, they will invariable get tangled,” explains Wilkinson. “At the end of the day, when they’re all tangled together, someone moves one and they all come down, which can ruin the tool or injure someone.”

There is the issue of cost. First, while the owner of a wired system has to pay for cabling, the owner of a wireless system has to replace transmitter batteries.

Secondly-and more importantly-are the disparities in initial system cost. A wireless system will typically be four to five times as expensive as its wired counterpart. Wireless systems, then, are only advantageous for specific applications. As Mahr’s George Schuetz points out, “Wireless transmitters are relatively expensive and are often used in applications where cabling is difficult, usually where the gage has to be brought to the part-say in a machine-and the cables become a liability to the operator.” A scenario with few gages and easily handled parts, for example, would be better off with a wired system.

However, using a large number of gages in a wired system could offset its initial value. “Wireless systems are not inexpensive. But at a certain point, most of the multiplexers used in wired gaging become pretty expensive,” explains Wilkinson. “So once you get more than eight gages, it makes sense to go with wireless. In a large deployment, wireless is cheaper.”

While both wireless and wired data acquisition systems for handheld gaging are viable options in industry, the appropriateness of each is application specific. In terms of component versatility, both systems are equal, providing compatibility with virtually any third-party software and gage. Therefore, it is the environment in which the system is to be used as well as what it is to inspect that should guide one’s decision-making process.

Quality 101: Measure Precisely with Hand Tools, September 2008

The sense of touch becomes important when using contact measuring tools. A skilled machinist with a highly developed sense of “feel” can readily detect a difference in contact made by changes in a dimension as small as 0.00025 inch. While the acuteness of the sense of touch varies with individuals, it can be developed with practice and proper handling of tools. In the human hand, the sense of touch is prominent in the fingertips. Therefore, a contact measuring tool is correctly balanced in the hand when held lightly and delicately in such a way that uses the fingers to handle or move the tool. If the tool is clumsily or harshly grasped, the sense of touch or “feel” is greatly reduced.

Sight and touch are frequently combined by the skilled worker to estimate measurements finer than the graduated limits of a tool. For example, on the average micrometer graduated to read in thousandths of an inch, the space between the smallest graduations of the thimble is approximately ¹⁄₁₆ inch. Variations in size much smaller than a thousandth of an inch can readily be felt and judged by the eye with reasonable accuracy.

Quality 101: Proper Care of Handheld Measuring Tools, January 2010

The nature of quality control measurement is continually changing in response to developments in coordinate measuring machines (CMMs) and other technology-laden metrology instruments. Nevertheless, the precision and repeatability of handheld dimensional measuring tools-calipers, micrometers and gages-are still heavily relied on throughout most of manufacturing. And as tolerances of manufactured parts become ever tighter, it is even more important that the accuracy of handheld measuring tools be maintained, requiring the tools themselves to be cared for properly.

There are two main categories of maintenance for hand measuring tools. The first is in response to everyday use and handling. This assumes that the correct tool is selected in the first place, for example, making sure the IP or Ingress Protection Rating is suitable.

The second type of maintenance is specified by formal, periodic and documented inspection and calibration routines. Calibration is most commonly performed in-house, but many quality programs specify additional calibration at accredited labs. These labs provide calibration traceable to final standards such as NIST (National Institute of Standards and Technology). Related to calibration is use of gage blocks-extremely precise artifacts with a care regimen all their own.Q