Metrology
Metrology
Beyond the Height Gage and Surface Plate
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BEYOND THE HEIGHT GAGE AND SURFACE PLATE
What can take low-volume, precision inspection to the next level?
George Schuetz, Mahr Federal Inc.

 For many years now, the method of choice for low-volume, general purpose inspection has been surface plate work using test indicators and height gages.  Recently, electronic height gages have made the layout inspection process a little more accurate.  Speed of measurement has also improved, since electronic height gages give direct measurements and allow for storing data and programming repetitive measurement processes.

 Are there even more advanced electronic height gages on the horizon that will allow us to achieve greater precision while still maintaining our ability to do a wide variety of measurement tasks?  Probably not.  The problem is not the height gage, but the reference surface it rests on.

 No matter how precisely we build the height gage, the accuracy of the measurement is still dependent on the flatness of the surface plate.  Surface plates probably cannot measure up to the increasing demands for higher tolerance measurements.

 So where do we go from here?  Consider the Universal Measurement Machine or Universal Length Gage as a way to perform a wide variety of measurements and inspections with speed and improved accuracy.  While there may be no all-purpose machine that can do everything, a universal length gage can do quite a lot.  Applications include:

• Internal and external measurements of diameters and lengths (master rings and disc)
• Internal and external thread measurements
• Calibration inspection of mechanical indicators and gages (dial and digital indicators, LVDTs, testmasters)
• Location of points, lines, holes and surfaces
• Internal and external tapers
 

 Universal measurement machines were developed to speed up the inspection process and reduce the potential for measurement error.  They differ from typical comparative style gaging because they have a much larger measuring range, but still can obtain resolution and accuracy approaching some comparators.  In order to achieve high measurement performance, the machines have built-in reference standards either glass scales or an interferometer system.

 When equipped with various contact accessories, universal measurement machines can easily be used to check length, diameter, pitch diameter, roundness, straightness, parallelism and taper.  They will typically measure parts from 5 to 40 inches long, but machines are also available with even larger capacities.

 
 These rugged looking systems are frequently referred to as "machines" because they are built according to the same design criteria as machine tools.  Critically important measuring head and tail stock slide bearings are mounted on a strong and rigid base.  The reference system is mounted as close as possible to the machine's line of measurement to avoid abbe errors.  In addition, various computer techniques are used to map and correct slide errors; average multiple, lightning-fast measurements; and compensate for temperature variation. No, it's not a surface plate and height gage; but a machine that is extremely fast, versatile and very accurate.  However, just like anything else, measurements made with the machine are only as good as the measurement process.  Therefore, it is important to keep all the components of the process the same when setting (i.e., measuring the exact same location on the part, verifying gaging pressure, standardizing on a contact style, ensuring the utmost cleanliness of the part and contact, etc.).  Machines do this with extensive computer-aided systems that help you set up a measurement process and then lead subsequent users through it the same way every time and, of course, capture and report measurement results and analyses.

 The height gage and surface plate have been a mainstay for highly productive, low-part-volume general purpose measurements.  The universal measuring machine incorporates the spirit of this process, while relying on a highly accurate internal reference to significantly improve measurement precision and repeatability.  It is really an extension of the same line of thinking that has served manufacturers so well for over 100 years.