ADVANTAGES OF GRANITE
It is observed that
fine-grained rocks have relatively higher density and hardness. They
have higher resistance to scratch and are less porous. However stability
of granite under varying temperature and humidity conditions is a major
limitation even though the coefficient of thermal expansion is low
compared to other engineering materials like cast iron and steel. If
there is any small impact or scratch made over the granite surface plate, it
results in crushing of localized zone and a broken piece or powder comes
out without harming the surface accuracy. However in the case of cast
iron or steel surfaces, under similar circumstances a bulge would result
and the surface accuracy is affected and often these remain unnoticed
by the naked eye.
The granite color alone is not an indication of the physical qualities of the stone. In general, the presence or absence of minerals determines the color of the granite and this may have no bearing on the qualities that make good surface plate material. There are pink, gray and black granites that are excellent for surface plates. However the consistency of several made from the same vein in a quarry can be unpredictable.this inconsistency is much less in pure black granite.
The aging qualities of these rocks are also excellent. This is conformed by a measurement carried out on a 1.6m X 1m surface plate kept in a metrology room immediately after its lapping and sparingly used over a period of four years. The variation in shape and the accuracy of this surface is negligible. Thus granite has been found to be a useful material for providing an accurate surface.
1.
|
High degree of hardness (High quartz- high hardness) |
2.
|
Low heat conductance |
3.
|
Low temp sensitivity |
4.
|
Non-magnetic |
5.
|
Electrically Non-conductive |
6.
|
Rust and acid resistance |
7.
|
Matt surface, non-reflective surface |
8.
|
Low water absorption |
9.
|
High modulus of elasticity- Torsionally rigid, sag free, good slid's prop |
10.
|
Dolerite - a better scratch resistance |
11.
|
Modulus of elasticity - 26-86 GPa |
12.
|
Compressive strength - 107 Mpa / 15600 PSI |
13.
|
Water absorption - 0.07 - 0.31 |
14.
|
Density - 3g/cc |
15.
|
Hardness - 6-7 Mohs scale |
16.
|
Tensile strength 5 Mpa |
17.
|
Co-efficient of thermal expansion - 6.1X10 -6/°C |
The granite color alone is not an indication of the physical qualities of the stone. In general, the presence or absence of minerals determines the color of the granite and this may have no bearing on the qualities that make good surface plate material. There are pink, gray and black granites that are excellent for surface plates. However the consistency of several made from the same vein in a quarry can be unpredictable.this inconsistency is much less in pure black granite.
The aging qualities of these rocks are also excellent. This is conformed by a measurement carried out on a 1.6m X 1m surface plate kept in a metrology room immediately after its lapping and sparingly used over a period of four years. The variation in shape and the accuracy of this surface is negligible. Thus granite has been found to be a useful material for providing an accurate surface.
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