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Surface plates, , , , Objectives : At the end of this lesson you shall be able to, * state the constructional features of surface plates, , * state the application of different grades of surface plates, *_specify surface plates and state the uses of marking tables., , , , Surface plates - their necessity, , When accurate dimensional features are to be marked or, to be checked 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. (Fig 1) The most widely used datum, surfaces in machine shop work are the surface plates, and marking tables., , , , Fig, SPECIFIED CENTRE LINE, , RESULTING ERRORS WV, VERTICAL POSITION OF HOLES, , f WORK PIECE,, , , , [— SPECIFIED, , UU... p—-— mah tt HEIGHT, , , , , , , , , , , , FLATNESS ERRORS :, , EFFECT OF MARKING-OUT FROM NON-FLAT, SURFACE. NOTE THAT THE ERRORS IN THE HOLE, POSITIONS ARE EQUAL TO THE FLATNESS ERRORS, , , , , , , , , , 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 underside is, heavily ribbed to provide rigidity. (Fig 2), , , , Fig 2, , , , , HEAVILY, , RIBBED, UNDERSIDE, , (CAST RON SURFACE PLATE, , , , , , , , For the purpose of steadiness and convenience in leveling., 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, , 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, , 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, , Cast iron surface plates are designated by their length,, breadth, grade and the Indian Standard number., Example, , Cast iron surface plate 2000 x 1000 Gr1. I.S.2285., , Marking-off tables (Fig 3), , , , Fig, EOGE STRAIGHT AND, ‘SQUARE TO EACH OTHER, , , , , , , , , ‘ACCURATELY, ‘SCRAPED FLAT, SURFACE, , THREE POINT SUSPENSION, , , , aonra0%, , FOR STEADINESS