Bes­sel points

The Shed - - Levelling A Lathe -

It’s not strictly nec­es­sary for a small plate like mine but sur­face plates are usu­ally bal­anced on ei­ther Bes­sel or Airy points. Both serve to pro­vide the best pos­si­ble sup­port against grav­ity. Sur­face plates have one im­por­tant char­ac­ter­is­tic — their flat­ness. To main­tain that pre­ci­sion flat­ness they must be bal­anced on points that will min­i­mize the de­flec­tion caused by grav­ity. Even 100mm thick gran­ite blocks will suf­fer from some sag over time due to grav­ity.

It might be barely no­tice­able without high-pre­ci­sion equip­ment but it will be mea­sur­able. Airy points en­sure that there will be no slope on the ends and Bes­sel points en­sure that a beam will not change in length. That will also min­i­mize any dis­tor­tion to the sur­face, so they are ideal for sur­face plates, although both types are used. Bes­sel points are cal­cu­lated by mul­ti­ply­ing the length of the block by .557 and this num­ber is sub­tracted from the length and the re­sult di­vided by 2 to ar­rive at the lo­ca­tion of the points from

I haven’t used the level since I first set up this lathe a cou­ple of years ago

Bor­ing the holes for the Bes­sel points that will sup­port the block

Weld­ing the an­gle iron to con­tain the gran­ite block

Tap­ping the Bes­sel point holes

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