About Us

About Granite Surface Plate Calibration and Resurfacing

Owners Darren and Gerald Smith bring over 30 years of field experience to Precision Granite.  Their technical expertise is the primary reason so many of their customers choose Precision Granite and continue to rely on them for their precision calibrations.

Two precision instruments are used in conjunction with one another to determine the accuracy of a given surface plate. The Repeat-O-Meter is the instrument that is used to check the repeatability, or small wear spots that occur from normal everyday use. These small wear spots are often the result of the constant sliding of your parts and/or measuring gauges along the surface of your granite plate. The Autocollimator is the instrument used to check the overall flatness or corner to corner flatness of a surface plate. Using these particular instruments the technicians at Precision Granite are able to determine the profile of a given surface plate. According to Federal Specifications GGG-P-463c, each surface plate must pass both the repeatability and overall flatness test in order to be legally certified. The allowed tolerance of a surface plate is defined by its size and grade.

Grade AA – Laboratory Grade
Grade A – Inspection Grade
Grade B – Tool Room Grade

The Surface Plate Calibration Process
The field team at Precision Granite provides on-site granite surface plate calibration and resurfacing. During surface plate calibration, our technicians will measure both surface flatness and repeat-ability by taking measurements in locations across the surface. These reading will be used to determine deviation from the guidelines set for the grade of surface plate. Once low areas are identified, we will resurface the plate to restore flatness.

Both flatness and repeat-ability readings are critical steps in granite surface plate calibration to insure a precision surface that provides accurate measurement.

Flatness is measured using points on the surface contained within two parallel planes, the base plane and the roof plane. The measurement of distance between the planes is the overall flatness of the surface.

Repeat-ability is verified by measuring various local flatness areas across the surface to help guarantee that a measurement taken anywhere on the surface of a plate will repeat within the stated tolerance.