Granite Surface Plate Calibration

Granite is an igneous rock formed by volcanic eruptions. By comparison, marble is metamorphosed limestone. For metrology use, the granite selected should meet specific requirements outlined in Federal Specification GGG-P-463c, from now on called Fed Specs, and specifically, Part 3.1 3.1 Among the Fed Specs, granite should be a fine to medium-grained texture.

Granite is a hard material, but its hardness varies for several reasons. An experienced granite plate technician can estimate the hardness by its color which is an indication of its quartz content. Granite hardness is a property defined in part by the amount of quartz content and lack of mica. The red and pink granites tend to be the hardest, the grays are medium hardness, and the blacks are the softest.

Young’s Modulus of Elasticity is used to express the flexibility or indication of the hardness of the stone. Pink granite averages 3-5 points on the scale, grays 5-7 points and blacks 7-10 points. The smaller the number, the harder the granite tends to be. The larger the number, the softer and more flexible granite is. It is important to know the hardness of Granite when choosing a thickness required for tolerance grades and the weight of parts and gauges placed upon it.

In the old days when there were real machinists, known by their trig table booklets in their shirt pockets, black granite was considered to be “The Best.” The Best defined as the type that gave the most resistance to wear or is harder. One drawback is that the harder granites tend to chip or ding easier. Machinists were so convinced that black granite was the best that some manufacturers of pink granite dyed them black.

(d is the length of the diagonal)