black granite surface plate

Scope This part of ISO 8512 specifies requirements for rectangular or square granite surface plates ranging from 160 mm x 100 mm to 2500 mm x 1600 mm, as preferred sizes, in four grades of accuracy O, 1, 2 and 3. NOTE 1 “Granite” is a commonly used description; a more specific description is given in clause 4. This part of ISO 8512 applies to new granite surface plates, granite surface plates in use, and those reconditioned according to their grade. 2 Definition For the purposes of this part of ISO 8512, the following definition applies. deviation from flatness of the working surface: The minimum distance separating two parallel planes between which the working surface can just be contained. 3 Nomenclature For the purposes of this part of ISO 8512, the nomenclature shown in figure 1 applies. 4 Material Surface plates complying with this part of ISO 8512 shall be made from stable, workable rock presenting a wear-resistant surface which does not tend to scratch steel surfaces. Igneous rocks such as diabase, gabbro and various other granites have been found suitable; they are commonly referred to as “granite” for the purposes of this part of ISO 8512. Rock used for surface plates shall be close-grained and of uniform texture, sound and free from flaws and fissures and from inclusions of softer materials. Repair of defects in the top surface is not permitted. The colour of the granite, which is dependent on the mineral composition, is of no importance, but the colour of any individual plate shall be uniform. NOTE 2 Some of the physical properties of granite used for surface plates are given in annex E. 5 Preferred sizes The preferred sizes for granite plates are given in table 1. If plates of other sizes are required, the requirements of this part of ISO 8512 shall nevertheless apply. The lengths of faces of plates shall be within +- 5 O/. of the nominal size. NOTE 3 It will be seen that the nominal lengths of faces, with one exception, are taken from the R5 series of preferred numbers; one plate, size 2000 mm x 1000 mm, is included because it is an established and widely used size, although 2000 is not an R5 preferred number. 6 Rigidity The thickness of granite surface plates shall be such that when a concentrated load is applied about the centre of the plate, the loaded area shall not deflect below the remaining area of the plate by more than 1 pm/200 N. The limiting value for deflection applies to surface plates of size 400 mm x 250 mm and larger.

Finish of the working surface The top (working surface) of grades O and 1 plates shall be finished by lapping, whereas grades 2 and 3 plates may be finished by lapping or left as ground. However, an item placed on it shall not wring to the surface of the plate. 8 General features 8.1 Supporting feet of surface plates All surface plates shall be supported on three feet. Plates larger than size 1000 mm x 630 mm shall have safety feet. Feet shall be positioned within the boundary of the working surface to minimize deflection. NOTE 5 The setting of adjustable feet may affect the deviation from flatness of the working surface; specific instructions for adjusting the feet are given in footnote 3 to table 1. Edge The feet shall be smoothly machined, but machining of the supporting feet into a plane parallel to the working surface is optional. 8.2 Projection of top If the purchaser so requires, the top of each plate may extend beyond the faces by at least 20 mm on plates of diagonal length up to 400 mm and by at least 25 mm for larger plates. The underside of the projecting surface shall be reasonably flat for accommodating clamps. 8.3 Surfaces other than the working surface If the purchaser requires the faces to be finished straight, mutually parallel and square, the tolerances shall be specified in the purchasing order. Side and end faces shall be finished smooth. All edges and corners shall be rounded with of at least 2 mm.