| 摘要 |
758,183. Magnets. NEILL & CO. (SHEFFIELD), Ltd., J., and BOWER, W. L. Sept. 18, 1953 [June 18, 1952], No. 15371/52. Class 35. [Also in Group XXIII] A magnetic work-holder comprises a magnetic circuit which extends between magnetically separated pole-pieces 7, 9, Fig. 1, and includes a permanent magnet 1, 2, and an electromagnetic coil 6 encircling part of the circuit whereby a member of magnetic material applied to the pole-pieces will be held by flux derived from the permanent magnet, but can be released by passing electric current through the coil. The permanent magnet is formed by a pair of bars of aluminium-cobalt-iron alloy which are magnetized, after the workpiece has been placed on the pole-pieces, by passing a direct current through the coil 6 for a short time. The workpiece may be released by passing a direct current through the coil in the reverse direction, but demagnetization is preferably effected by alternating current of progressively decreasing strength. A mild steel plate 4 and non-magnetic bolts 5 secure the permanent magnet to a mild steel base-plate 3 carrying the pole-pieces 7 which are connected to a central pole-piece 9 by non-magnetic bolts 13. Bars 10, consisting of alternate strips of magnetic and non-magnetic material arranged to provide a series of gaps of graduated reluctance, are mounted on non-magnetic plates 12 between the pole-pieces. Figs. 4 and 5 show an alternative construction in which the workholding surface is composed of pole-pieces 29 in contact with the upper faces of magnet bars 21, 22 and pole-pieces 27, 28 in contact with the lower faces through a base-plate 23. When the coil 26 is energized, flux passes, through the workpiece 25, from pole-pieces 28 and 29 to pole-pieces 27. After magnetization the flux passes from pole-pieces 29 to pole-pieces 27 and 28. The magnetizing current is provided from a single or polyphase alternating current supply A-A, Fig. 6, by a full-wave rectifier B while the electromagnetic coil C is centretapped so that its two ends C1, C3 may be connected alternately with the negative pole of the rectifier while the centre tap C2 remains permanently connected to the positive pole in order to demagnetize the work-holder. These connections are effected automatically by switches S2, S3 actuated by a cam-plate on an electric motor M which also adjusts a variable resistance VR in the alternating current supply to reduce gradually the strength of the current and finally opens a switch S1 to switch-off the motor and a pilot light L2. The demagnetizing current may alternatively be produced by grid controlled rectifiers or thyratron tubes and the power supply by a rotary converter with adjustable excitation. Final demagnetization may be effected by moving the permanent magnets in relation to the pole-pieces. Specification 429,199, 633,223 and 633,608 are referred to. |
