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The Plane Truth About Flatness

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The plane is also the basis or reference for most dimensional and geometric measurements; including height, location of features, squareness, and datum’s. A reference plane may be a feature on the part itself, or it may be part of the measuring instrument; but in either case, the measurement can only be as accurate as the reference.  So, whether you’re making parts or measuring them, you may have to measure flatness.

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There are many tools and methods available, depending upon the nature of the part and the degree of accuracy required. Surface plates serve as a general-purpose reference for many flatness measurements. If the flat surface of the work-piece can be put in direct contact with the plate, it is possible to measure flatness using feeler stock. Although this is a low-resolution method, and only the perimeter of the part is accessible.  An air or electronic gauging probe installed flush in the surface plate can provide much higher resolution, if the part is small enough to move around on the plate.  Each type of probe has its benefits. Air jets are self-cleaning and non-contact, while electronic transducers can be connected with gauging amplifiers or remote indicators with dynamic measuring capabilities, to automatically capture the maximum deviation, or to output data for SPC.

If the part is too big to slide around, or if its configuration is such that the flat surface can’t be put in direct contact with the surface plate, then it must be staged.  A test stand with a mechanical indicator or an electronic gauge head is slid around on the surface plate to explore the part.

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This however may fail to distinguish between errors of flatness and errors of parallelism. To break out flatness, measurements are taken at equally spaced points on the surface, then the data is plotted on a graph and a best-fit line calculated.  Deviations from the best-fit line represent errors of flatness. If the measurements are taken on a vertical surface (using, for example, a "smart" height gage with the gage head turned 90 degrees), one would duplicate the procedure to break flatness out from possible squareness errors.

To measure really large areas like machine beds or surface plates, electronic levels are often the appropriate tool.  Levels may be connected to gauging amplifiers that will automatically convert angular readings into dimensional error.  Large areas can also be measured with electronic probes, using a precision straightedge as the reference.

With the proper software, the data obtained from large-area flatness measurements can be converted into a 3D plot.  This information can be used in at least three ways: the user can do his setups on the flattest areas and avoid the worst sections of the surface plate or machine tool; he can use the data to compensate mathematically for out-of-flatness; and he can use it as a guide to correct the out-of-flat condition.

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Optical flats are references for measuring small, high-precision parts, such as gauge blocks. Usually made from fused quartz or high quality glass, the puck-shaped optical flat is certified to within 1, 2, 4, or 8 micro-inches. It is wrung to the part and viewed under a monochromatic (helium) light source. A perfectly flat part will reflect straight, regularly spaced, easily visible interference bands, each representing an interval of 11.6 micro-inches (the half-wavelength of helium light).  Air gaps (i.e., low spots) between the part and the flat will distort the interference bands proportionally to the flatness error: a band that is "bent" by one half its thickness indicates out-of-flatness of 5.3 micro-inches (1/2 x 11.6).  The location of low spots can be identified by the direction of the distortion.

Regardless of the method, before a part can be measured for flatness it is important to know the level of uncertainty in the reference. Flatness may be transferred from certified standards to masters, then from masters to gauges, and then to work-pieces, but be aware that the level of uncertainty increases at each step.


– Tech Tip courtesy of Mahr Federal Inc.

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