Different grades of surface plates

 Surface plates

. the constructional features of surface pkates

. application of different grades of surface plates

. specify surface plates and the constructional features and uses of marking tables

Surface plates-their necessity

When accurate dimensional features are to be marked, it is essential to have a datum 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

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 under side is heavily ribbed to provide rigidity.

For the purpose of steadiness and convenience in levelling, a thress point suspension.

Smaller surface plates are placed on benches while the larger surface plates are placed on stands.

Other materials used

Granite is also used for manufacturing surface plates. Granite is a dense and stable material. Surface plates made of granite retain thier accuracy, even if the surface is scratched. Burrs are not formed on these surfaces.

Classification and uses

Surface plates used for machine shop work are available in three grades grades1,2and 3.The grade 1 surface plate is more acceptable than the other two grades.

Specifiction

Cast iron surface plates are designated by their length, breadth, grade and the indian standard number.

Example

Cast iron surface plates 2000×1000gr1.I. S 2285.

Marking off tables

These are heavily ribbed cast iron tables fitted with strong rigid legs. The top surface is accurately machined flat, and the sides  square.

These are used for carring out marking on heavy components. On certain types parallel lines are engraved in both direction at  a set distance.

These line serve as guides for positioning components while setting and marking.