QUALITY 101

The latest digital calipers provide numerous electronic features that make the device easier to use, yet add little in the way of cost. Source: Mahr Federal Inc.

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 The versatility of the caliper, along with its range and or comparative measurements. To ease of use, makes it well suited for a multitude of make a comparison, first measure the reference dimension and set the measuring applications. BY GEORGE SCHUETZ dial indicator to zero. Then measure the dimension to be compared. The indicator will show how much the alipers are extremely versatile tools • Vernier. The vernier caliper is the compared dimension varies from the Cfor making a wide range of distance original design and still the most original (plus or minus). measurements, including both outside rugged. Graduated much like a Another useful feature of the dial diameters (OD) and inside diameters micrometer, it requires the align- caliper are jaws that slide past each (ID). While micrometers are more ment of an etched scale on the other to allow contact points or depth accurate, they have a limited measure- vernier plate with an equally spaced rod extensions to fit into narrow ment range, typically several inches. scale running the length of the tool’s openings for small ID measurements. Calipers can span from 2 inches to 4 handle. Skillful alignment of the tool • Digital. In the past 25 years the feet, depending on the length of the and interpretation of the reading is digital caliper has made its way onto scale. External measurements are made necessary to achieve the measure- the shop floor. The latest designs by closing the jaws over the piece to be ment tool’s stated accuracy. provide numerous electronic fea- measured, while internal measurements • Dial. A dial caliper is a second- tures that make the device easier are made by opening up the inside generation caliper. Similar in con- to use, yet add little in the way of diameter contacts. Depth and other struction to the vernier caliper, this cost. These include: easy switching measurements can be made with a depth style replaces the vernier scale with a between inch and metric units on rod built into the instrument’s beam. dial indicator. The indicator is fixed the readout, tolerance indications, There are three basic types of caliper to the moveable jaw and engaged digital output to electronic data that may be found today in a machin- with a toothed rack on the body of collection systems, zero setting any- ist’s tool chest: the unit. The dial, which is typically where 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.

MEASUREMENT INFLUENCES The biggest problem with calipers is that measurements are subject to vari- ation from one operator to another. Two types of influences contribute to this variation: “feel” or inconsistent gaging force, and subjective, or psy- chological, factors. Figures a and b illustrate basic OD and ID measurements. Figures c and d illustrate Because the caliper is a contact more complex depth and step measurements. Source: Mahr Federal Inc. instrument, sufficient torque must be

Reprinted from Quality, Copyright August 2007 applied to the anvils to make good positive contact between the part and the instrument. The only torque calibration in the human hand is the operator’s “feel.” What feels like solid contact to one operator may not feel correct to another; therefore, the The vernier caliper is the original design and still the most rugged. Graduated much like a micrometer, it requires the alignment of an readings may be different. etched scale on the vernier plate with an equally spaced scale run- Subjective, or psychological, ning the length of the tool’s handle. Source: Mahr Federal Inc. influences are all in the operator’s head. Tell an inspector that the best machinist in the plant made this part and influence enters the picture. Or suppose the boss walks over and asks an inspector to measure a part and adds, “I just made it myself.” In these cases, measurements may tend to be better than the part deserves. There also are more subtle types of influences. For example, studies have shown that if an operator knows what size a selection of parts ought to be before he measures them, read- The water-resistant digital calipers are ings will tend to be closer to that designed for use in extreme production ideal than if the target dimension is environments in accordance with the IP67 unknown. standard. Source: Mahr Federal Inc.

USING CALIPERS The popularity of calipers is both based on their versatility and ease of use. They are used for basic OD and ID measurements, as well as Similar in construction to the more complex depth and step mea- vernier caliper, the dial caliper replaces the vernier scale with surements. Other common caliper a dial indicator. The indicator measurements include measuring the is fixed to the moveable jaw center distance between two holes, and engaged with a toothed comparing a shaft diameter to a hole rack on the body of the unit. ID and gaging remaining thickness Source: Mahr Federal Inc. when drilling a hole in a workpiece. Beyond these basics, however, there are a number of different anvils and contacts available that extend caliper use to many special measurement applications. able to measure threads, bores and more than a grad on the vernier scale is Ball contacts are used to measure serrations, recesses, recessed IDs and a sign that the parallelism of the anvil wall thickness of tubes and other cylin- ODs, grooves and distances between needs to be corrected. drical components. Calipers are avail- grooves. A depth measuring bridge Even the best and most basic hand able with one or two ball/radiused con- can extend the effective diameter of measuring tool can be made better tacts, and in some cases the ball con- the beam, allowing depth measure- by adapting it to special application tacts can be supplied as attachments ment of wider holes. Some models requirements. By choosing the most for use with standard flat-tipped anvils. even offer an attachable device for appropriate accessories for the applica- The attachments may be quickly and measuring force. tion, one will achieve faster and more easily applied, but when using this type However, because these accessories accurate measurement results. of attachment, the ball diameters must and attachments extend the measuring be taken into account by subtracting range, parallelism errors can creep into George Schuetz is director of precision gages them from the caliper reading. the measurement. Thus, it is important for Mahr Federal Inc. (Providence, RI). For more Similarly, other specialized anvil to check parallelism of the contacts information, e-mail [email protected], attachments and accessories are avail- using a precision ball. A discrepancy of call (401) 784-3100 or visit www.mahr.com.