GRANITE SURFACE PLATES – SPECIFICATION – INDIAN STANDARD -IS7327: 2003
The Bureau of Indian Standards adopted this Indian Standard, after the draft finalized by the Engineering Metrology Sectional committee had been approved by the Light Mechanical Engineering Council.
This standard was originally published in 1974 and in its preparation considerable assistance had been derived from the Canadian Government Specification Board Specification 39-GP-31. It reveals the four grades 0,1,2,3 to be maintained based on its accuracy regarding the flatness tolerances for each grade . In the preparation of this standard considerable assistance has been derived from ISO/DIS 8512/2-1988.
It covers the requirements for rectangular or square, granite surface plates ranging from size 160mm X 100mm to size 2500mm X 1600mm, as preferred sizes in four grades of accuracy. That is 0, 1, 2 and 3 grade. It applies to new granite surface plates, granite surface plates in use and those reconditioned according to their grade.
The Indian standards that are necessary adjuncts to this standard is,
IS NO. TITLE
1076 (Part 1 to 3) Preferred numbers. 2285 : 1990 Cast iron surface plate (second revision) 12937 : 1990 Engineering metrology methods of testing Straightness, flatness and perpendicularity
The surface plates shall be made from stable, workable rock presenting a wear resistant surface, which does not tend to scratch steel surfaces. The surface plates made of the stable igneous rocks such as diabase, gabbro that holds uniform texture, sound, close grained and free from flaws and fissures shall be used under this granite surface plate category for the purpose of this standard. Repair of defects in the top surface is not permitted. The color of granite is no importance, but the color of any individual plate shall be uniform.
When we consider the preferred sizes, it is shown in the table below. The lengths of sides of plates shall be within 5 percent of the nominal size mentioned. All dimensions in millimeters and tolerances in micrometers. However, if plates of other sizes are required, the relevant requirements of this standard shall apply.
Two flatness tolerances are applied, one for the working surface overall and one for any local area of 250mm X 250mm of the working surface. The flatness deviation of the working surface overall shall not exceed the appropriate tolerance for size and grade of accuracy as specified in the table given below,
Size of Plate (mm) Diagonal length (mm) Tolerance on deviation from flatness Overall for plates of Grade (µm) In the case of sizes which differ from those in the preferred range, the tolerance on deviation from flatness overall shall be calculated by using the formula, Given, t = c1l + c2 Where, t – tolerance on deviation from flatness overall, in micrometers l – nominal length of the diagonal of the plate rounded up to the nearest 100mm c1– constants for the grade of plate c2– values given in the below table.
A surface plate should be supported at 3 points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered. Only 3 points can rest solidly on anything but a precision surface. Plates larger than size 1000mm X 630mm shall have safety feet in addition to it.
If the plate is properly supported, precise leveling is only necessary. Leveling is not necessary to maintain the accuracy of a properly supported plate.
The rigidity of the plate is considered by the plate thickness, which should be good enough to hold a concentrated load about its center without showing a deflection below the remaining area of the plate by more than 1 µm/200N. The limiting value for deflection applies to surface plates of size 400mm X 250mm and larger.
Each plate shall bear a designation plate attached to one side, the following information in characters not less than 3mm high, shall be included 1.The indication of the source of manufacture 2.The number of this standard 3.The grade of accuracy.