Magnetic chuck

Claim 1

A machine tool, comprising a. a housing having an annular electro-magnetic coil mounted therein, b. a rotatable spindle mounted in the housing concentrically of the coil, c. a first backing plate element mounted on the spindle and rotatable with it, the first element being provided with a central hub from which integral arms extend outwardly to provide a first pole, there being segmental inserts located between the arms and being held fixedly in place and insulated from the arms to form second poles, d. a second backing plate element mounted on the first backing plate element and having an axially-extending lip for contact with a workpiece, the second element being provided with an axial bore concentric with the said lip and being divided into a plurality of segments by radial layers of insulation, the segments being alternately first poles and second poles to match the arms and inserts, respectively, of the first element, the first element being provided with an axially-extending boss, while the second element is provided with a recess to receive the boss, the elements being held in face-to-face relationship, consisting of an annular element inserted in a groove on the lip that is concentric with the bore and consisting of a plurality of hardened inserts separated by narrow gaps which are filled with material that acts as an insulation to electro-magnetic lines of force.