DIN – 876
Scope:
This standard applies to natural hard rock surface plates, which are used as a plane, or datum surface in dimensional metrology, in particular in testing for deviations of form or position.

Material and finish:
The surface plate should be made from natural hard rock, sound, free of flaws, close grained and of uniform texture, grade subject to agreement. In the finishing process, the stone should be ground and the tolerance surfaces lapped. So that to attain a non-wringing surface. It shall, however, be noted that the two most frequently used sorts of natural hard rock, granite and diabase, differ significantly in their respective modulus of elasticity, Egranite=39200 to 64700 N/mm2, Ediabase=94200 to 96700 N/mm2.

In consequence, given identical dimensions, modes of installation and loading, a granite surface plate will exhibit approximately twice as great a deflection as a corresponding surface plate made of diabase.

Dimensions – As per DIN standard, the dimension of surface plate has to be:

The depth e is an important factor in respect of deflection and is much easier to test. No fixed dimensions have been specified for the sizes of the surface plates, only ranges for the length L. ISO/TC3/SC3 is at present engaged in the preparation of standards dealing with surface plate. This standard has incorporated some of the interim results of that work, but the competent subcommittee of the Normenausschuss Langeund Gestalt (Length and Shapes Standards Committee) has decided not to adopt the proposed international standards in their entirety, not least because closer tolerances are considered desirable in Germany.

Support:
The requirements relating to the flatness of the working surface shall only apply to surface plates resting on three supporting feet, to be specified and identified by the manufacturer. No supporting feet need be provided for surface plates of length L not exceeding 400mm. Such surface plates may be laid flat on the entire undersurface. The floor area on which the surface plate stands shall be solid, rigid, flat and level. Special foundations may be required to support large surfaces which have a high unloaded mass and which are subject to heavy loading. As a result of their poor thermal conductivity, natural hard rock surface plates are not suitable for the tampering of test objects.

Flatness tolerance:
The flatness can be considered as all points on the surface being contained within two parallel planes, the base plane and the roof plane. The flatness tolerances for some values of length (L) are drawn up in the below table,

For local areas of the working surface, testing is limited to checking the deviation from straightness of profile lines at randomly selected points on the working surface. The length of the profile line under examination shall not be less than 100mm and not exceed 354mm (length of the diagonals of the area of the working surface). A straightedge conforming to DIN 874 part 1 may be used as straightness standard.

Marking:
Surface plates conforming to the present standard shall display the following marking on one of the side faces:

DIN 876
Accuracy grade
Manufacturer’s trade mark or name.