GMT Granite Surface Plates-Application and Accuracy

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 its inherent characteristics mentioned in an earlier blog. 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 with 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 work shop use.
• 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 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.

GMT PRECISION 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 which is is best suited for metrology application as it has  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
• High flatness accuracy
• Ideal for guideways for measuring instuments and air bearing surfaces
• High bending strength
• Insensitive to mechanical overloading
• All calibration equipment is to national and international laboratories standards
• Hand lapped for maximum accuracy

SPECIFICATIONS

FLATNESS TOLERANCE IN µm FOR ACCURACY GRADE 

SIZE OF PLATE             AS PER DIN 876:84                      AS PER IS 7327-2003                                        WEIGHT

   (in mm)            |  GRADE "00" | GRADE "0" | GRADE "1" | GRADE "2" | GRADE "3" | (Kgs) (Approx.)

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

TO MEET SPECIAL REQUIREMENTS WE OFFER :

1. Sizes larger than 2000 X 1000 mm. The thickness of the plate increases with size.
2. Granite Surface Plates with inserts with drilled and tapped inserts
3. Granite Straight Edges
4. Granite Master Squares
5. Granite Cubes
6. Master Cylinder made of hardened and ground steel