Patent ID: 7161451

Claim:
A magnetic chuck comprising: a first chuck layer and a second chuck layer, each of the chuck layers having a common center of rotation, a common periphery, and flat parallel upper and lower surfaces where one of the flat surfaces of one of the chuck layers is the working surface of the chuck for attracting and holding a workpiece; the first chuck layer comprising an even number of permanent magnet plates, each permanent magnet plate having an inner end at the center of rotation of the first chuck layer and extending radially outwardly to the periphery, each permanent magnet plate having opposed plate surfaces perpendicular to the upper and lower surfaces of the chuck layer, the plate being magnetized with its north magnetic pole (N) and its south magnetic pole at its opposing plate surfaces, and each plate having opposed plate edge surfaces extending from the inner end of the plate to the periphery; the first chuck layer further comprising a block of magnetically soft material interposed between each pair of magnetized permanent magnet plates around the center of rotation, with the magnetic poles of the plates being oriented so that the magnetically soft blocks are magnetized with alternating N and S polarities around the center of rotation, each block of magnetically soft material being larger than its adjacent permanent magnet plates and each block having opposing magnetically soft block surfaces lying, respectively, in the flat surfaces of the first chuck layer; the second chuck layer comprising permanent magnet plates and soft magnet blocks that are complementary in shape and position to the permanent magnet plates and soft magnet blocks of the first layer, the first and second chuck layers being assembled in the chuck so that magnetized soft magnet blocks of one chuck layer can overlie magnetized soft magnet blocks of the other chuck layer; the chuck being activated for magnetic attraction of a workpiece when the first and second chuck layers are positioned with respective overlying/underlying soft magnet blocks of the same magnetic polarity; and the chuck being inactivated for magnetic attraction of a workpiece when the first and second chuck layers are positioned with respective overlying/underlying soft magnet blocks of opposite magnetic polarity; whereby the working face of the chuck extends from the center of rotation to the periphery and comprises magnetically soft block surfaces that are each larger than the interposed permanent magnet plate edge surfaces.