The Surface Plate
As the name suggests a surface plate refers to a plate having a perfectly flat surface which is manufactured out of cast iron normally having a rectangular shape. The bottom side of the plate has several stiffening ribs and there are two handles on either side which are used to lift and shift the surface plate. Apart from cast iron other materials such as Granite and glass have also been employed in the manufacture of surface plates
You might be wondering as to what is the purpose of making such a fine flat surface? This surface acts as a tool to measure and gauge whether another surface is flat, concave or convex. Just keep in mind that when I am referring to curves surfaces they do not normally mean surfaces which can just be detected by the eye but to those surfaces which seemingly seem flat but may not be actually so.
When the plate is not in use for a certain period of time, just remember to smear some amount of oil on the surface so that it goes not catch rust. Also do keep in mind that the surface plate is not meant to carry out your hammering or other operations for it will get dented and will not serve its intended purpose.
Surface plates come in various sizes ranging from one foot to two feet in length with a proportionate corresponding breadth. Work pieces which are exceptionally large are measured using other methods such as marking-off table which will be discussed elsewhere.
Surface plates used in different situations require different levels of accuracy and hence they are graded according to their level of refined surface and accuracy that they provide and some of the grades include AA, A etc. Normally the grade used in workshops is of relatively lower precision but sufficient enough for the types of work done in a typical workshop. A technician normally uses surface plates in conjunction with other similar equipment such as dial indicator, height gage and so forth.
The surface to be tested can be placed against the surface plate and checked for light leakage from the other side. For more accurate reading you can smear the surface of the surface plate with red lead and oil and then press the work piece against it. On lifting up the work piece the red colour would indicate areas of higher height while clean areas would signify depressed regions.