BS-817:2008 pdf free download – Specification for surface plates.
7 Construction
7.1 Size of plate
Unless otherwise specified by the purchaser [see 4.la)], the plate shall be of one of the nominal sizes specified in Table 1.
7.2 Border zone
If the working surface is surrounded by a border zone, the width of the border zone shall not exceed the value specified in Table 1.
For sizes of plates not specified in Table 1 [see 4.la)] the width of the border zone shall not exceed 2% of the length of the shorter side of the plate, or 10 mm, whichever is the smaller.
7.3 Supporting feet
Unless otherwise agreed between the contracting parties [see 4.2a)], surface plates shall be provided with three primary supporting feet; one at the centre of one short side and one at each end of the other short side. To limit tipping, two safety feet shall be provided at the single foot end of surface plates (toward the ends of the short side) of 400mm x 250mm or 18 in x 12 in and larger.
Supporting feet shall be machined.
7.4 Stand
The surface plate shall be supplied with a stand which shall be constructed and braced in such a way that it rigidly supports the surface plate. The support surfaces on the stand shall be machined and means of leveling the surface plate shall be provided together with means of resisting accidental displacement of the surface plate from the stand.
NOTE Unless a particular working height is specified by the purchaser [see 4. lb)], the overall height of a surface plate on its stand, i.e. the height of the working surface above the floor, should be approximately 850 mm.
7.5 Projection of top
Projections shall be provided on cast iron surface plates of 400mm x 250mm or 12 in x 12 in and larger.
If specified by the purchaser, projections shall be provided on cast iron surface plates smaller than 400mm x 250mm or 12 in x 12 in and on granite surface plates [see 4.lc)].
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 400mm x 250mm or 12 in x 12 in and larger and at least 20mm for surface plates smaller than these sizes. The undersides of projections shall be finished such that they can accommodate clamps.
7.6 Sides and edges
The four sides of the surface plate shall be machined and, unless otherwise agreed between the contracting parties [see 4.2b)], the lengths of the sides shall each be within ±2% of the nominal size specified in Table 1 and the edges shall be rounded.
7.7 Handles
If specified by the purchaser, cast iron surface plates shall be provided with handles [see 4.ld)].
Granite surface plates shall not be provided with handles.
7.8 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.