Three Heads Are Better than One

George Schuetz - Mahr Federal Inc.

  Electronic gages are the instrument of choice for many demanding inspection applications.  Digital electronic amplifiers offer high resolution, excellent stability, and the ability to output to data collectors, and can be integrated into feedback-controlled manufacturing systems.  Furthermore, they can be programmed to capture minimum or maximum readings, to calculate TIR and average ("nominal") measurements, and to combine readings from more than one gage head, in addition to numerous other options and features that vary with make and model.

  Users of electronic amplifiers can choose from a number of gage head types to generate the measurement signal.  Cartridge, pantograph, and lever-type gage heads are the three most common, differing 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 that can be integrated into electronic amplifier-based systems include capacitance gages and laser devices, and these can be useful in applications that require non-contact sensing.  But by far the greatest number of metalworking applications are satisfied with the three common "mechanical," or contact-type gage heads.  Each of these has particular advantages for different applications.

  The cartridge probe, or pencil-type gage head, is a compact cylindrical package, usually 3/8" or 8mm in diameter.  Not coincidentally, these are the same diameters as dial indicator stems, and the cartridge probe was designed for direct replacement of indicators.  Like dial indicators, the probe's spindle, or sensitive contact, has an axial motion.  Readily 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 6mm and lengths below 20mm.

  Numerous other options and variants are available to increase flexibility of application.  Measurement ranges vary from as short as ±0.010" (±0.250mm) to as long as ±0.100" (±2.5mm), with linearity ranging from ±0.05% to±0.5%.  Longer ranges are available, but they are usually not applied for tight-tolerance measurements.  (Linearity is typically a trade-off against longer range.)  The standard plain bushings that support the spindle tend to be quite durable, but in applications that subject the spindle to significant side-loading, ball-bearing bushings can provide longer life cycles.  The signal output cable is normally supplied straight and plastic-jacketed, but coiled cable is available for use on hand-held gages, and armored cable is available for harsh environments.  Cable may exit the probe from the back of the cartridge (axially), or at a right angle—a small detail that occasionally makes mounting the probe much more convenient.

  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 extremely harsh environments: for example, for in-process gaging during a grinding operation.

  Cartridge heads tend to have relatively heavy gaging pressure—about 3.5 oz. (99 g)—but here too, optional specifications are available from some manufacturers.  Another handy option 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.  (External springs or deadweights can be added if a specific gaging pressure is required.)  Pantograph gage heads offer a measurement range of ±0.010" (±0.250mm) with repeatability of <0.5 microinches (<0.01µm).  They are more accepting of side-loading than cartridge-type gage heads, and can be repaired more easily and economically if side-loading damage does occur.

  Where the cartridge gage head replicates the action of the dial indicator, lever-type gage heads are functional replacements for test indicators.  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, allowing the contact to be repositioned by as much as 20° without moving the body of the gage head.  Their ability to measure in both directions further enhances versatility.  In contrast, cartridge and pantograph gage heads are uni-directional, 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" (0.250mm) can be specified for use on really inaccessible workpiece surfaces.  Repeatability can be as good as <4µ" (<0.1µm), and gaging pressure, at <0.14 oz. (<4 g), is light, making these heads well 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.  All three can also be readily integrated with either digital or analog amplifiers.  These features help make electronic gaging a very flexible approach to high-accuracy inspection.