The answer is 'yes' for almost every application. The advantages of granite include: No rust or corrosion, almost immune to warping, no compensating hump when nicked, longer wear life, smoother action, greater precision, virtually non-magnetic, low co-efficient of thermal expansion, and low maintenance cost.

The critical characteristics of granite, for use in metrology are as follows:

1. Hardness
2. Density
3. Modulus of Elasticity
4. Flexural strength
5. Compressive strength
6. Tensile breaking stress
7. Water absorption
8. Porosity co-efficient

Certain minerals present in the granite are the cause of its typical colour. The major problem is the presence of these minerals on the surface. If they are very hard and brittle (quartz for e.g.) they can wear quickly leaving pitted areas on the surface. It is an acknowledged fact that pure black granite from the Deccan plateau in the South of India is most resistant to wear.

This does not mean that other colours do not have metrology characteristics

As granite is a natural material and its properties can vary from vein to vain in the quarry, it is best to get the stone periodically tested for its properties. (See typical report).

It will not be possible to test the stone for each individual surface plate/other artefact supplied, but GMT gets the tests periodically whenever a new quarry or vein is located, to ensure that we supply the best possible stone.

Diabase is a variety of a stone type known as Gabbro. While it is closely related to granite, it is not the same. Diabase lacks several of the minerals (such as quartz and mica) which give granite its distinctive grain structure. The lack of quartz means that diabase is slightly less resistant to wear than the hardest granites. However, this is offset by the lack of mica, which is very soft, and tends to flake out of granite plates, leaving pits. Diabase is denser than granite, weighing about 3.15 gm/cm³, while most granites weigh between 2.6-2.7 gm/cm³,It is also less porous and more stable, which means that a smoother and more uniform surface finish can be achieved. In general, we quote diabase as the material for multi-dimensional, very high-accuracy applications that require a very smooth, stable surface like our angle plates, parallels, master squares, and straight edges.

Yes: DIN 876:1984, IS 7327:2003 (granite surface plate). Note that the Indian Standard does not have a classification for 00 accuracies.

The accuracy of a surface plate is its flatness.Flatness can be considered as all points on the surface being 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. This flatness measurement commonly carries a tolerance and may include a grade designation.

Fig 1: Typical graphical report of flatness taken with an electronic level

The flatness tolerances for four standard grades are defined in the DIN 876:1984, IS 7327:2003 specification as determined by the following formula:

1. Laboratory Grade 00  = 0.002 * L (Diagonal length) +1.25
2. Inspection Grade 0  = 0.003 * L (Diagonal length) +2.5
3. Tool Room Grade 1  = 0.006 * L (Diagonal length) +5.0
4. Shop floor for general inspection grade 2           = 0.012 * L (Diagonal length) +10.0

Where L is in mm.

For standard sized surface plates, we guarantee flatness tolerances which exceed the requirements of this specification. In addition to flatness, DIN 876:1984 & IS 7327:2003 specification address topics including: repeat measurement accuracy, material properties of surface plate granites, surface finish, support point location, stiffness, acceptable methods of inspection, installation of threaded inserts, etc.

GMT granite surface plates and granite inspection plates meet or exceed all of the requirements set forth in this specification. At present, there is no defining specification for granite angle plates, parallels, or master squares. Our standard tolerances (listed in our catalog) for these items are the tightest in the precision granite industry in India.