Surface Plates

The surface plates are essential equipment in all metalworking operations and are a necessary quality control tool. They are used in most comparative measurements, where they are used in combination with other measuring instruments such as Johansson gauge blocks, dial indicators, height gauges, sine bars, V blocks, etc.

The foundation of accuracy is a flat and stable reference

History

During the Second World War, traditional cast iron plates were slowly replaced by granite because iron was needed elsewhere. As it turned out, this was a blessing in disguise, because granite plates are superior and demand for cast iron ones declined. Today, it is not unusual to find a large number of granite plates in a factory.

Manufacturing

Granite plates are generally made from fine-grained black granite, which is free of internal stresses for maximum stability during manufacturing. Granite is harder than steel, non-magnetic, and electrically non-conductive.

The top surface of the granite plate is the reference surface and has a finish (Grade) within flatness limits depending on quality and size. The highest accuracy, Grade 00, has the smallest flatness deviation and is sometimes referred to as laboratory grade.

Grade

Grade 1 could be called inspection grade, and Grade 2 and 3 workshop grades. Grade 0 is twice as accurate as Grade 1 and, similarly, Grade 1 is twice as accurate as Grade 2.

Surface

To avoid the possibility of the plate tipping over, which could happen if a heavy workpiece is placed on one side of the single leg, two safety stops are often fitted at each corner of large plates in the positions shown.

The legs are made from heavy-section L steel beams or square steel tube. Wheels can be added to smaller frames so they can be moved.

Granite surface plates (excluding small ones) are generally supported at three points, as shown, to provide minimal distortion under their own weight and eliminate the possibility of micro-vibrations.

Absolute flatness is an ideal that is not achieved in practice. A surface plate will generally be either slightly convex or concave in form, depending on the way it has been processed (or mishandled). Two examples are given above, one convex (a) and the other concave (b). For a granite plate 1000 x 630 mm, tolerances better than 5.5 pm are referred to as Grade 0, 11 pm as Grade 1, and 22 pm as Grade 2. Therefore, this plate is classified as Grade 1, only slightly outside the limits of Grade 0.

While the ideal surface plate is infinitesimally concave or convex, it is difficult to find a plate manufactured in concave form (b) directly from the factory. It is more likely that the plate will be slightly convex, as in (a), to allow for slight wear. The concave plate (b) is poorer in flatness tolerances than (a) even though, if it is larger, it is graded as a Grade 0 plate. A convex plate is always preferable to a concave one.

Example (a) is probably a plate that was manufactured recently, while (b) is likely to be a worn plate. The lowest point indicated, 7 μm, may be due to excessive use at that point. When it was new it would certainly have had a convex form rather than concave. Ideally, wear should be distributed over a flat surface by using a different area each time.

Flatness is defined according to DIN 876. It specifies that all variations of an area must lie within two imaginary planes, the distance between which determines the flatness tolerance.