Why granite? Is it better than steel or cast iron for precision surfaces?
The answer is ‘yes’ for almost every application. The advantages of granite include: No rust or corrosion, almost immune to warping, longer wear life, smoother surface to move components, greater precision, virtually non- magnetic, low co-efficient of thermal expansion, low porosity, low maintenance cost, vibration resistance and no compensating hump when nicked. If steel is scratched, the material on the edges of the scratch will be pushed out to form a hump. If large enough, the surface of the plate may have to be re-finished. This does not happen with granite. The composition of granite consists of larger particles, which simply chip and flake out. The flatness will not be affected by this (if sizes are small relative to the component).
What colour of granite is best for Metrology and what are the characteristics?
Generally black is considered best followed by a specific green called Hasan green.
The critical characteristics of granite, for use in metrology are as follows:
- a) Hardness
- b) Density
- c) Modulus of Elasticity
- d) Flexural strength
- e) Compressive strength
- f) Tensile breaking stress
- g) Water absorption
- h) Porosity co-efficient
Certain minerals present in the granite are the cause of its typical colour. A 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. It just means that a black grain, like that of Diabase, whose physical characteristics and surface are best for measurement references, is most consistent in mineral composition in the raw, rock form. So wastage is less. What we mean by waste is that often the supplier will send the raw stone, but flaws and inclusions will become visible only after machining. This granite will have to be discarded. Other granites can be, and are, equally good for measurement. In fact many others are used all over the world. South Africa has its famous pink granite and Sweden grey. Since Diabase is the most dense among all granites, the thickness of the product will be slightly less than other colours.
How to identify the granite best suited for metrology?
As granite is a natural material and its properties can vary from vein to vein in the quarry, it is best to get the stone periodically tested for its properties. (See typical report). Our experts have done this for several years. They go to the quarry to check and to coach the quarry staff on how to identify rocks best suited for us.
|3.||Modulus of Elasticity (Gpa)||55.8|
|4.||Flexural strength (Mpa)||28.31|
|5.||Compressive strength (Mpa)||286|
|6||Tensile breaking stress||17.71|
|7||Water absorption (%)||0.08|
|8||Porosity co-efficient (%)||0.26|
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.