BS 817:1988 pdf download.Surface plates.
7 General features
7.1 Supporting feet of surface plates (see Figure 1) Surface plates shall be provided with three primary supporting feet; safety feet to limit tipping shall be provided at the single foot end of surface plates of 400 mm × 250 mm or 18 in × 12 in and larger. NOTE Large plates for special purposes may be supported on more than three primary feet. The number of feet and the manner of setting should be as agreed between purchaser and supplier. Supporting feet shall be machined.
7.2 Stand The stand shall be constructed and braced such that it will rigidly support the surface plate. Support surfaces shall be machined and means of levelling the surface plate shall be provided together with means of resisting accidental displacement of the surface plate from the stand. NOTE Unless a special working height is needed the overall height of a surface plate on its stand, i.e. the height of the working surface above the floor, should be about 850 mm.
7.3 Projection of top Projections shall be provided on cast iron surface plates of 400 mm × 250 mm or 12 in × 12 in and larger. NOTE The provision of projections on cast iron surface plates smaller than these sizes or on granite surface plates is optional. Where provided, projections shall be at least 25 mm for surface plates of 400 mm × 250 mm or 12 in × 12 in and larger and at least 20 mm for surface plates smaller than these sizes. The underside of projections shall be finished such that they can accommodate clamps.
8 Finish
8.1 Cast iron surface plates The working surface of grades 0 and 1 surface plates shall be finished either by scraping or by other process that results in a surface such as that obtained by scraping. The working surface of grades 2 and 3 surface plates shall be finished either by one of the same processes or by machining. The proportion of bearing area shall be not less than 20 % for grades 0 and 1, and not less than 10 % for grades 2 and 3 surface plates (see note). High spots on scraped surfaces shall be uniformly distributed and the percentage of bearing area shall not be so high as to cause wringing. Unmachined surfaces of cast iron surface plates shall be finished by painting. NOTE A suitable method of test for assessing the bearing area is given in appendix C. Other proven methods may also be used.
8.2 Granite surface plates The working surface of grades 0 and 1 surface plates shall be finished by lapping. The working surface of grades 2 and 3 surface plates shall be finished by lapping or left as ground.
8.3 Finished working surface The finished working surface of new surface plates shall be free from such defects as will adversely affect the use of the surface plate for the intended purpose (see clause 3).BS 817 pdf download.