When accurate dimensional features are to be marked, it is essential to have a datum plane with a perfectly flat surface. Marking using datum surfaces which are not perfectly flat will result in dimensional inaccuracies. The most widely used datum surfaces in machine shop work are the surface plates and marking tables.
Materials and construction of surface plate
Surface plates are generally made of good quality cast iron which are stress-relieved to prevent distortion. The work-surface is machined and scraped. The underside is heavily ribbed to provide rigidity.
For the purpose of steadiness and convenience in levelling, a three point suspension is given. Smaller surface plates are placed on benches while the larger surface plates are placed on stands.
Other materials used for surface plates
Granite is also used for manufacturing surface plates. Granite is a dense and stable material. Surface plates made of granite retain their accuracy, even if the surface is scratched. Burrs are not formed on these surfaces.
Classification and uses of Surface Plates
Surface plates used for machine shop work are available in three grades – Grades 1, 2 and 3. The grade 1 surface plate is more acceptable than the other two grades.
Specifications of Surface Plates
Cast iron surface plates are designated by their length, breadth, grade and the Indian Standard number.
Example for specification of surface plate
Cast iron surface plate 2000 x 1000 Gr1. I.S. 2285.
Care & maintenance of Surface Plates
- Clean before and after use.
- Do not keep job on the surface plate.
- Don’t keep any cutting tool on the table.