Granite Surface Plate
This reference plate in granite is used to measure objects, mostly in metal work, for flatness and squareness. It is flat within 3 µm (1 µm "micron" = 0.001 mm) measured form a corner across the surface.
Size 457 x 152 x 51 mm
Weight 11 kg
Photo left: Example of use
Measuring the flatness of the sole of a plane, and the squareness of the angle between the sole and side of the plane.
To Measure the flatness of the sole of a plane
many use a steel straight edge placed against the sole. We don’t think this is the ideal method, as for various reasons, the straight edge is seldom very straight, and you can see, even with the naked eye, deviations of several thousandths of a inch along the edge, which is not good enough. Instead, place the plane on the granite plate and measure around the edges at various places with a feeler gauge. With high-priced metal planes, the deviation in flatness is normally around 0.05 mm, and with middle-end planes it could be about 0.1 mm
To Measure the squareness between the sole and the sides
the sole of the plane itself should not be used as a reference, as there are small differences in its flatness at different points on the surface. To best perform this task, one should place the plane on a granite surface plate in order to find a neutral reference. With a precision metal working square, placed on the plate against the side of the plane, you can see very precisely any deviation from square. The body of the square must be flat against the granite plate. With a feeler gauge you can measure any deviation between the blade of the square and the side of the plane. The thinnest feeler blade on your gauge that will not fit between the blade and the side is used at the deviation. With high-priced metal planes, the deviation is normally about 0.06 mm, with middle-priced planes it is about 0.15 mm.
Every manufacturer has, or should have, their own tolerances for these measurements. These can differ from the figures we give here. So if you are in doubt, it is best to contact the manufacturer directly.
Important: be careful that the Granite plate and the various measuring tools are dry and clean and free from dust or grit. They can break if dropped or heavy objects are placed upon them, and any use besides measuring can compromise their flatness.