Airy points and Bes­sel points

The Shed - - Levelling A Lathe -

Airy points are named af­ter Sir George Airy who de­vel­oped them. They rep­re­sent points on a beam where grav­ity will not cause the ends to de­flect if the beam sags from grav­ity. Or, in engi­neer­ing speak, they in­duce zero slope at the ends. They are of­ten used to sup­port stan­dard mea­sures like a stan­dard me­tre rule used for cal­i­bra­tion. Sur­face plates, which are large and very dense blocks of gran­ite or steel that are in­tended to be per­fectly flat, also need pre­cise sup­port, and Airy points are of­ten used. Airy points are in­tended to keep the ends per­pen­dic­u­lar and square. They will also keep the edges square and flat. Bes­sel points are named af­ter Friedrich Bes­sel and do some­thing sim­i­lar, but they will en­able a beam to main­tain its length. This is a better op­tion for sur­face plates in that Bes­sel points will en­sure that the sur­face stays rel­a­tively flat, which is the prefer­able con­di­tion. In prac­tice there is very lit­tle be­tween them and ei­ther will do. The di­a­grams show a very ex­ag­ger­ated ver­sion of the ef­fects. The change in flat­ness due to grav­ity is more pro­nounced the larger the plate, and the ef­fects are only mea­sur­able with some high-pre­ci­sion equip­ment. Pro­fes­sional sur­face plates are reg­u­larly checked and lapped if nec­es­sary to main­tain their flat­ness.

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