Metrology
Metrology
Electronic Height Gages
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Electronic Height Gages
George Schuetz, Mahr Federal Inc

  In past columns we've looked at "basic" comparative height gages, which are used for layout tasks and other surface plate measurements. These consist of a comparator stand, plus a test indicator or an electronic gage head and amplifier. Related to these are instruments known as Electronic Height Gages. These offer a high degree of flexibility and functionality, so that, in addition to lab based work, they are useful as production gages. In quality departments, they are used for first part and incoming inspections, and layout work, while on the shop floor, machinists use them for checking features on one-off parts.

  The key features of the electronic height gage are: a probe that senses when the part is touched; a glass or capacitance scale that tracks the probe's height; and a readout/control unit. Many also incorporate a motor drive to position the probe. There is a base and a body, to maintain the components in a stable, rigid relationship, and to accurately position the scale perpendicular to the surface plate on which the gage rests. And often, there is an internal pump that generates a thin cushion of air beneath the base, allowing the gage to be moved around easily on the surface plate.
                                           


  Glass and capacitance scales have gotten so good that these gages are reliable enough for shop floor use and so accurate over a long range as to blur the lines between comparative and absolute gaging. Most height gages can measure in both modes, and even toggle between them on a single measurement. Resolution of .0001"/.001mm, with accuracy of .0005"/.013mm over a range of 24"/615mm is common, while high end instruments offer resolution down to 10µ"/.5µm and accuracy of .00012"/.0025mm.

  Two sensing technologies predominate: touch triggers and active probes. Both types can be set to trigger from both downward and upward touches. Once these points are collected, it's easy to calculate the difference between them, for either inside measurements (such as slot lengths and widths, and inside diameters) or outside measurements (such as ODs or thicknesses). One can also average the two readings to find hole centers or center lines. From there, it's an easy step to calculate distances between centers.

  The more common touch triggers send a signal to the scale only once per touch. Active probes, found on higher-end systems, constantly update their position, and record the position once they reach a stable reading on the part. In addition to single point measurements, gages with active probes can be used for "dynamic" measurements, to explore a feature for straightness, flatness, MIN, MAX, or TIR.

  Active probes have the potential to generate more accurate diameter measurements, because the user can tram the gage perpendicularly to the feature's axis, to capture the highest and lowest points on the top and bottom surfaces. (See figure.) To correctly measure a diameter with a touch trigger, a special contact is used, which is designed to seek the low or high point of the diameter.

  Even the relatively simple control units associated with touch triggers tend to be highly capable. These are usually programmable for multiple measurement routines, can accept presets, and calculate widths, thicknesses, and distances between centers.
 
  More powerful controllers, which usually accompany active probes, are required for dynamic measurements. These data processors are capable of generating SPC reports, and turning the single axis height gage into a virtual two-dimension measuring machine. One can measure bolt hole patterns and similar 2D relationships, by measuring the height of the holes, then reorienting the part 90° and re-measuring the hole heights again. The controller includes a one button "90° flip" function to calculate results as X-Y coordinates (e.g., a hole center is 6.000" in from one edge, and 2.000" from an adjacent edge) or as polar coordinates (e.g., a hole center is 16.342° from a reference point, on a hypotenuse of 4.500").

  The gage must be zeroed before measuring parts. This is usually done by touching the probe to the reference surface usually a surface plate. Gages can also be referenced against a gage block, or against a datum on the workpiece itself.

  Before measuring inside or outside dimensions, the diameter of the ball end of the probe must be compensated for. This involves touching the probe to the top and bottom of a special reference artifact. The controller calculates the diameter as the difference between the measured reading and the known distance between the two reference surfaces.

  While general purpose measurement devices like electronic height gages can't compete with some types of comparative gaging for measurements requiring very high resolutions or throughput, they are ideal for most surface plate layout work, and for inspection of parts produced in small quantities.