Calibration of Granite Surface Plates
Granite Surface Plates are a common tool used for precision gauging, inspection, and layout of machined parts for precise measurement and inspection. They are used by tool rooms, machine shops, and inspection and calibration laboratories because of certain advantages that they have over steel surface plates.
Granite is the most widely used material for the production of surface plates. Compared to steel, granite plates are rust and corrosion free, are non-magnetic, do not produce burrs when nicked, have a lower coefficient of thermal expansion, are more accurate, wears better and takes less daily care. Granite Surface plates offer the ideal gaging surface for making accurate measurements.
However, over time, granite surface plates can wear and the surface loses its flatness, which prevents them from being able to make accurate measurements. That’s why it is important to perform regular cleanings, maintenance, and calibrations to keep them operating properly.
Surface Plate Accuracy Specifications
In accordance with the ASME B89.3.7 standard, there are two separate flatness characteristics that are important – the overall flatness, which covers the entire work surface and the local variation in flatness, also called the repeat reading. The local variation in flatness applies to a smaller area of the surface plate and will therefore always have a smaller flatness tolerance than the overall flatness. For both characteristics, the same definition of flatness applies – it is the distance between two parallel planes that contain all the points on the surface of the plate. In practice, flatness is assessed by measuring points across the surface and determining the distance, or range, from the highest peak to the lowest value.
The ASME B89.3.7 standard defines the tolerance grades for surface plates. The highest accuracy plates are called Grade AA, and they are often found in calibration laboratories. Grade B plates are of the lowest accuracy, and they are typically found on general shop floors. The middle-grade plates, Grade A, are often used in quality control or inspection areas. Surface plates are typically manufactured in a variety of common sizes and in accordance with the standardized tolerance grades.
Surface Plate Calibration Method
Both the overall flatness and local variation in flatness are to be calibrated on granite surface plates by checking the flatness to the appropriate tolerance values. Granite surface plate calibration typically also includes cleaning and light polishing of the gaging surface.
The overall flatness is often checked with a laser interferometer, autocollimator, electronic levels, or master straightedge and indicator. For all of these methods, a series of measurement lines are made in a grid across the surface plate. Then the individual flatness values along the grid are combined into a three-dimensional analysis of the overall flatness.
The local variation in flatness is typically checked using a repeat reading gage. This measuring instrument has a small three-point base connected to a pivoting section with a fourth contact point. An indicator is used to measure the vertical movement of the fourth point relative to the base. The repeat reading gage is manually swept around the surface plate, and the measured local variation in flatness is the range of the indicator movement.
If necessary, adjustments to the surface plate flatness can be done by those with the required experience, skills and tools. Re-lapping or resurfacing is performed to bring the measurements back to within the appropriate grade guidelines. This procedure involves polishing the surface with a diamond-based abrasive paste and a large lapping plate is moved over the surface plate to remove the unwanted material. If the plate is not too badly worn it can be resurfaced on-site. If a plate is worn to the point where it is significantly out of tolerance, or if it is badly pitted or nicked, then it should be sent to the factory for a more thorough grinding prior to re-lapping.
Inspections and Calibration Frequency
By following regular surface plate inspections and calibrations a granite surface plate should last for many years. Proper cleaning and inspections are essential to ensure that the surface plate maintains the proper flatness to ensure measurement accuracy over time. Before a surface plate has worn beyond the specification for overall flatness, it may show worn spots. Monthly inspections for repeat measurement errors using a repeat reading gage will identify any wear spots.
Depending on the surface plate usage, the laboratory environment, and the required accuracy, the intervals between formal calibrations of the surface plate flatness will vary. A general rule of thumb is for a new plate to receive a full recalibration within one year of purchase and every year thereafter. If the plate is used frequently, or is used in a harsh environment, the calibration interval should be shortened to six months.
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