Electronic Gage Heads

Using probes to reduce uncertainty during the electronic gaging process.

Columns From: 6/16/2010 Modern Machine Shop,

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George Schuetz

Users of electronic amplifiers can choose from a number of gage head types to generate the measurement signal. The three most common—cartridge, pantograph and lever-type gage heads—differ from each other mainly in the orientation of their sensitive contacts and the mechanisms by which contact movement actuates the transducer. Other types of dimensional sensing devices, such as capacitance gages and lasers, can also be integrated into electronic amplifier-based systems. Each offers particular advantages for different applications.
 
The cartridge probe, or pencil-type gage head, is a compact cylindrical device, usually 0.375 inch or 8 mm in diameter and less than 3 inches (75 mm) long. Similar to dial indicators, the probe’s spindle, or sensitive contact, has an axial motion. Incorporated into fixture gages and in-process gages, several cartridge probes can be positioned within close proximity to measure closely spaced part features. For even tighter spacing requirements, some manufacturers offer special miniature probes, with diameters as small as 6 mm and lengths ranging to less than 20 mm.
 
Most cartridge gage heads operate on a linear variable differential transducer (LVDT) principle. The LVDT is an electromechanical device consisting of a primary coil, flanked by two secondary coils connected in series. All coils surround a movable, magnetic core—the spindle—which provides a path for magnetic flux linking the coils. When the primary coil is energized by a sinusoidal signal from the amplifier, voltage of opposite polarity is induced in the secondary coils. The difference between the two secondary coil voltages is the device’s net output. When the core is centered, net output is zero.
 
The null, or zero position, is very stable, making LVDTs ideal for high repeatability comparative measurements. Also, because the LVDT works on an inductive principle, its resolution is, in theory, virtually infinite. In practice, it is limited by the amplifier’s ability to display the results. Ranges vary from ±0.010 to ±0.100 inch (±0.250 to ±2.500 mm), with linearity ranging from 0.5 to 0.05 percent over the nominal range.
Some LVDT probes are signal-conditioned to produce a digital format. Others have a digital scale built in for extended range. Both produce similar results. Moreover, even standard-duty LVDTs are rugged, and heavy-duty versions are capable of extended use in harsh environments.
 
Options and variants are available to increase application flexibility. The standard plain bushings that support the spindle tend to be durable, but in applications that subject the spindle to significant side loading, ball-bearing bushings can provide longer life cycles. The signal output cable normally features a straight, plastic jacket, but coiled cable is available for use on handheld gages, and armored cable is available for harsh environments. Cable may exit the probe from the back of the cartridge or at a right angle.
 
Most cartridge heads are splash-proof, with a protective rubber boot surrounding the probe’s stem. Hermetically sealed versions are also available for use in harsh environments.
 
Cartridge heads tend to have relatively heavy gaging pressure—about 3.5 ounces (99 grams)—but some manufacturers supply customer-specified options. Also available is a pneumatic-retraction accessory to minimize side loading on the spindle when inserting a workpiece into a gage fixture.
 
Pantograph, or reed-spring gage heads, are most often used in benchtop height comparators, where both ruggedness and extremely high accuracy are required. The gage’s contact is suspended by a pair of reed springs, which provide virtually force-free and friction-free measurement. Pantograph gage heads offer a measurement range of ±0.010 inch (±0.250 mm) with repeatability of less than 0.5 microinch (0.01 micron). They are more accepting of side loading than cartridge-type gage heads, and they can be repaired more easily and economically if side-loading damage occurs.
 
Lever-type gage heads are functional replacements for test indicators when the cartridge gage head replicates the action of a dial indicator. Electronic lever-type gage heads are typically used in connection with a height stand, often for surface-plate work. When mounted on a tiltable, extendable cross-bar, they can be positioned with a great deal of latitude relative to the workpiece. A clutch on the swivel further assists in positioning convenience, enabling the contact to be repositioned by as much as 20 degrees without moving the body of the gage head. Lever-type gage heads’ ability to measure in both directions further enhances versatility. In contrast, cartridge and pantograph gage heads are unidirectional, and must be positioned perfectly in-line with the dimension being measured.
 
The extended, pivoting contact of the lever-type gage head provides good access to working surfaces that may be hard to reach with other contact styles. Contacts with special shapes, and diameters as small as 0.010 inch (0.250 mm), can be specified for use on inaccessible workpiece surfaces. Repeatability can range to less than 4 microinches (0.1 micron). Gaging pressure of less than 0.14 ounce (4 grams) makes these heads suited for high-resolution measurements on delicate surfaces or compressible materials.
 
All three types of electronic gage heads can be combined in a single fixture gage or application, and many amplifiers will accept all three interchangeably. They can also be integrated with either digital or analog amplifiers. These features help make electronic gaging a flexible approach to high-accuracy inspection. 

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