GMT METROLOGY DIVISION - GRANITE SURFACE PLATES

Why GMT Granite Products?

GMT's metrology division is located at Hosur, Tamil Nadu. This region is home to large deposits of world famous Indian black diabase granite. Indian black granite is best suited for metrology application as this granite as a very fine-grained structure. The properties, which help in selecting a rock for metrology application are density, water absorption & porosity, flexural and compressive strength.

Advantages of Indian Black Granite:

	Low coefficient of expansion and thermal stability
	Water absorption and porosity is low and hence long term dimensional stability is good
	It is fine-grained and the density & hardness is suited for dimensional metrology
	Flexural strength, Compressive strength, Modules of rupture and Tensile stress
	Totally corrosion free, granite does not rust nor corrode or react chemically with other substances
	Non Magnetic and non-conductive
	Non-reflective matt finish; Hand lapped for maximum accuracy
	High flatness accuracy
	Ideal for guideways for measuring instruments and air bearing surfaces
	High bending strength
	Insensitive to mechanical overloading
	All calibration equipment is to national and international laboratories standards
	PRECISION GRANITE SURFACE PLATES SPECIFICATIONS FLATNESS TOLERANCE IN µm FOR ACCURACY GRADE SIZE OF PLATE (in mm) AS PER DIN 876:84 AS PER IS 7327-2003 WEIGHT in Kgs. (Approx.) GRADE "00" GRADE "0" GRADE "1" GRADE "2" GRADE "3" 250 x 250 3 3.5 7 15 30 10 300 x 300 3 3.5 7 15 30 22 400 x 250 3 4.0 8 16 32 20 400 x 400 3 4.5 9 17 34 40 500 x 300 3 4.5 9 17 34 35 630x 400 4 5.0 10 20 39 65 630 x 630 4 5.0 10 21 42 100 800 x 500 4 5.0 10 21 42 120 1000 x 630 4 6.0 12 24 49 320 1000 x 1000 4 7.0 14 28 56 500 1200 x 800 4 7.0 14 28 56 475 1600 x 1000 5 8.0 16 33 66 1100 2000 x 1000 6 9.5 19 38 75 1650 Application Surface plates are used as reference planes for dimensional inspection and calibration of mechanical artifacts. This plate acts as a datum line or surface. So, the flatness of the plate is very important. Dolerite is the perfect material for metrology, because of the inherent characteristics mentioned above. The definition of flatness is, ‘all points on the surface being contained within two parallel planes, the base plane and the roof plane’. This flatness measurement commonly carries a tolerance and may include a grade designation. Flatness tolerance of the surface plate is provided in the standards with different grades based on flatness. GMT Metrology Granite Surface Plates meet the DIN and IS Standards specified in the table above. Granite Surface Plates are manufactured to precision standards using the highest quality granite. Accuracy Rigorous quality control inspection is carried out at all stages. Final inspection is carried out at a High Precision Computerised Electronic Level calibrated by an NABL accredited lab at regular intervals.. Accuracy under Load Granite Surface Plates are capable of supporting a total normal load of 25 kgs. per 0.1 sq.mtr. Certificate A certificate of accuracy is supplied with each plate as per standards specified. Technical Data Granite Surface Plates are manufactured in five grades. The typical application of these five grades of plates is as under: GRADE 00 as per DIN : 876 : 84 is of laboratory grade. It is specified for precision measurement in gauge rooms and metrology laboratories for calibration masters. GRADE 0 is of inspection grade. It is specified for general work in quality control. GRADE 1 is of tool room grade. It is specified for production checking work through the shop. GRADE 2 & 3 are intended for use in shops for general inspection. The choice of Grade 2 & 3 will depend on the application needs of the customer. MATERIAL Granite Surface Plates are made from select granite with uniform distribution of light and dark constituents. In such granite the minerals are evenly distributed resulting in a homogenous appearance. The following are the representative properties of our granites : Texture Fine Specific Gravity 3.08 g/cm3 Hardness Scleroscope over 90 Modules of Elasticity 9.0 to 12.0 x 105kg/cm2 Moisture of absorption Nil FLATNESS TOLERANCE The flatness deviation of a local area of 250 mm x 250 mm of the working surface shall not exceed : 1 3.0 µm for plates of Grade 00 2 3.5 µm for plates of Grade 0 3 7.0 µm for plates of Grade 1 4 15.0 µm for plates of Grade 2 5 30.0 µm for plates of Grade 3 GRANITE SURFACE PLATE STANDS These are fabricated of heavy angle iron and reinforced to ensure rigidity. The stands are designed to withstand the weight of the granite surface plates and are supplied with 3 leveling screws for leveling the granite surface plates. OPTIONAL ACCESSORIES Plate will be supplied with a suitable Rexine Cover to protect both the working surface and side faces of the plate at extra cost.

ADVANTAGES OF GRANITE SURFACE PLATES

ADVANTAGES OF GRANITE

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

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.