Advantages of granite in metrology is multi fold. Granite is the best material for providing an accurate surface reference in Metrology.
PROPERTIES OF GRANITE
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As a material for metrology aids granite, is a very hard natural igneous rock formation of visibly crystalline texture formed essentially of quartz and orthoclase / microcline. Due to its high quartz content provides greater resistance to wear and damage. Its anti-glare property results in less eyestrain for the person using the plates. Due to its hardness and wear qualities, non denting property, smoother action, greater precision, low maintenance cost, granite is used often for manufacturing of Surface plates, measuring prisms, guideways of precision instruments and other metrology aids.
BEHAVIOR OF GRANITE
When we go for the investigation of granite surface plates, it does not retain the surface topography with changes in atmospheric conditions like temperature. So, the behavior of the plate to be studied under varying conditions like:
- Change in humidity
- Water on the the surface plates
First the surface plate is divided into square of 100X100mm. An electronic level with a sensitivity of 0.0005/100mm placed on a bridge is used to measure the flatness of the surface. Readings are taken along and across the surface and the surface topography is computed to attain the contour. This bi-directional approach to every point gives accurate measurements of the surface.
Now, lets watch the behavior of the granite at various temperatures. We can see a significant change in the contour of the surface due to temperature rise. We cannot explain the magnitude of the change by the coefficient of thermal expansion of Dolerite rock, which is 2×10 -6 to 4X10 -6 per degree centigrade. At 26°C, we can notice the surface contour seems to be concave with overall flatness accuracy of 5.5 microns. When there is a rise in temperature to 40°C the surface contour changes to convex with overall flatness accuracy of 8.6 microns. This is the behavior of granite due to change in temperature.
When we study about the change in contour due to a change in atmospheric humidity is very insignificant. The change in contour of the surface with change in humidity of 20, 40 and 60% has been negligible.
When the surface was soaked with water and measurement taken within a short period the variation in surface contour is negligible. However a change was observed when the surface was allowed to be soaked with water for 12 hours. For a study, we take a granite plate of size 400X400X100. Before soaking in water the flatness was checked as 3.48 microns, when the top surface was soaked in water the flatness was recorded as 3.24 microns. Now the top surface was soaked in water for 12 hours, the reading of flatness was recorded as 2.34 microns and when all sides soaked in water for 3 hours, we got the flatness reading to be 1.91 micron. This was the effect of water on the accuracy of the surface plate.
However, we observe that for both, the moist condition and as well as for temperature change the surface returns to the original surface topography, once the surface plate returns to its original atmospheric condition. This is one of the advantages of granite in metrology. Studies on igneous rocks such as Granite and Dolerite showed that:
- The igneous rock has pores and capillaries
- The material consists of distinctly different crystals of different materials
As stated above granite has pores and these can have with them entrapped moisture. The moisture content is maximum at the surface and decreases in the deeper layers. When you soak the stone in water and when the surface dries, break it into two halves. You will notice the newly exposed upper layers have distinctly higher content of water whereas it is less wet as the depth increases.
We know that the igneous rock like Granite, Dolerite, etc., consists of various substances such as Quartz, Hornblende, Calcite, and Orthoclause. Due to this various ingredients of stone have uneven volumetric or linear expansion within them. For example quartz expands four times more than the feldspar and twice as much as hornblende. Due to rise in temperature the quartz exerts a pressure against its surroundings and causes an expansion of the surface, which even if small affects the surface accuracy.
The variation of the surface topography thus is an inherent phenomenon due to the properties of a stone. Its value changes from stone to stone. It is more in stones with higher porosity and quartz content.
On moving still deeper in studies about the advantages of granite in metrology, we find the fact that, granite expands only about 0.036% at 25ºC whereas water expands more than 0.32%, at the same temperature. If the walls of the pores prevents water from expanding it can exert about 70 atmospheric pressure against the pore walls. Thus the entrapped moisture or water in the pores comes to an equilibrium pressure due to capillary action and there will be a pressure on the walls of the pores, which act as small pressure vessels causing the surrounding portion of the rock to expand due to this internal pressure. However this pressure will be maximum at top layer of the surface and reduces in the deeper portion. Thus the upper layers of the stone expand more than the lower causing the surface to swell.
ADVANTAGES OF GRANITE IN METROLOGY
- High degree of hardness (High quartz- high hardness)
- Low heat conductance
- Low temp sensitivity
- Electrically Non-conductive
- Rust and acid resistance
- Matt surface, non-reflective surface
- Low water absorption
- High modulus of elasticity- Torsionally rigid, sag free, good slid’s prop
- Dolerite – a better scratch resistance
- Modulus of elasticity – 26-86 GPa
- Compressive strength – 107 Mpa / 15600 PSI
- Water absorption – 0.07 – 0.31
- Density – 3g/cc
- Hardness – 6-7 Mohs scale
- Tensile strength 5 Mpa
- Co-efficient of thermal expansion – 6.1X10 -6/°C
The above points provide us required data regarding the advantages of granite in metrology. It also proves 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. Although the coefficient of thermal expansion is low when you compare to other engineering materials like cast iron and steel. And if any small impact or scratch made over the granite surface plate, it results in crushing of localized zone. The 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. This affects the surface accuracy and often these are hard to notice by the naked eye.
Just the 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. Which 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.
The ageing qualities also adds to the advantages of granite in metrology. Measurement of a 1.6m x 1.0m surface plate kept in a metrology room immediately after its lapping over a period of four years, confirms this aspect. The variation in shape and the accuracy of this surface is negligible. Thus, the advantages of granite in metrology are many and provides an accurate surface for measurement.
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