BS 817:1988 pdf free download.pecification for Surface plates.
1 Scope
BS 817 specifies requirements for rectangular or square, cast iron or granite, surface plates ranging from 160 mm X 100 mm to2500mmxl600mmandfroml2inxl2in to 96 in x 48 in in four grades of accuracy. It applies to new surface plates, to surface plates in use and to those which, after wear, have been resurfaced as appropriate to their grade and size.
NOTE 1 The inch sizes are given as this information is still required by industry.
NOTE 2 General information on the care and use of surface plates is given in appendix A.
NOTE 3 The titles of the publications referred to in BS 817 are listed on the inside back cover.
2 Nomenclature and definitions
For the purposes of BS 817 the nomenclature shown in Figure 1 and the following definitions apply.
2.1
surface plate
an iron casting or block of granite with a worked surface which is used as a working plane and a datum
NOTE Surface plates of larger sizes which are mounted on a stand are usually known as surface tables but for the purposes of simplicity in this standard the term surface plate” has been used throughout.
2.2
deviation from flatness overall of the working surface
the minimum distance separating the two parallel planes between which the working surface can just be contained
2.3
variation of the working surface
the variation in the indicator reading when the working surface is explored with the variation gauge shown in Figure 2
2.4
variation gauge
a freely moving base with three co-planar pads for bearing on the surface being tested, and carrying an indicator measuring variations of that surface through a fourth pad, as shown in Figure 2
3 Classification
Surface plates are classified as grade 0, 1, 2 or 3 according to their accuracy as specified in clause 9.
NOTE Grade 0 is necessary for measurements to a high accuracy. Grade I is intended for general inspection purposes. Grade 2 is intended for marking out purposes. Grade 3 is suitable for lower grade marking out or as a general support plate.
4 Materials
4.1 Cast iron
Close-grained cast iron not inferior to grade 180 of BS 1452 shall be used. The castings shall be free from blowholes or porous patches. The plugging of minor defects in the working surface with material of a similar composition is permitted provided that such a repair does not affect stability or accuracy.
4.2 Granite
The granite 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 working surface is not permitted.
6.2 When a load is applied at the central area of a surface plate of 400 mm x 250 mm or 12 in x 12 in, or larger, the loaded area shall not deflect below the remaining area of the surface plate by more than 1 im per 200 N.
NOTE A simple method of test is given in appendix B.
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 X 250mm or 18 in X 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 he about 850 mm.
7.3 Projection of top
Projections shall be provided on cast iron surface plates of 400mm x 250mm or 12 in x 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 x 250 mm
or 12 in x 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.
7.4 Sides and edges
The four sides of the surface plate shall be machined and their lengths shall each be within ± 2 % of nominal size. Their edges shall be rounded.
NOTE If the purchaser requires the sides to be finished straight, parallel and mutually square the tolerances should be agreed with the supplier.
7.5 Handles
Granite surface plates shall not he provided with
handles.
NOTE Cast iron surface plates may be provided with handles.
7.6 Tapped holes or inserts
Where tapped holes or inserts are required the necessary work shall be carried out before the working surface is finished (see note 1). The holes or inserts shall be counterbored.
NOTE 1 Tapped holes or inserts are only provided if requested by the purchaser.
NOTE 2 The preferred size thread is M6 but smaller sizes are acceptable (see BS 3643-1).
NOTE 3 The use, for clamping purposes. of tapped holes in cast iron surface plates or tapped inserts in granite surface plates can give rise to distortion of the working surface if high clamping pressures are applied. Metal inserts in granite surface plates can cause distortion due to differential expansion.
8 Finish
8.1 Cast iron surface plates
The working surface of grades 0 and I 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).
9 Accuracy
9.1 The working surface
The working surface, which excludes a border zone, shall comply with the requirements of 9.2 and 9.3.
The width of the border zone shall not exceed 2% of the length of the shorter side, or 10 mm whichever is the smaller, and no point on the border shall project above the working surface.