Patent GB-1401802-A (GB)
Title: Permanent magnet devices
IPC: H01F7/02
Filed: 1972-02-02
Abstract: 1401802 Magnetic holding devices JAMES NEILL HOLDINGS Ltd 23 Jan 1973 [2 Feb 1972] 4784/72 Heading H1P A permanent magnet flux switching device has several pairs of ferromagnetic polypieces 1 within which lie upper and lower rows of magnets 2 with opposed faces of opposed polarities and adjacent magnets in each row presenting opposed polarities at corresponding polar faces. Thus if all magnets lie between respective polepiece pairs and corresponding faces of upper and lower magnets are of similar polarities an external field is produced above and below the polepieces (Fig. 1). If magnets 2 are moved half a pole pitch in each row in opposite directions to bridge the gaps between polepieces so that each pair is contacted by an adjacent north and south pole of the upper and lower rows (Fig. 2) the external field is reduced to zero by flux diversion between magnets through the polepieces (Figs. 3, 4, 5, not shown). By movement of the magnets a further half pole pitch an external field of maximum strength may be produced in the same or opposite sense. Alternatively, (Figs. 6, 7, 8, not shown) one row of magnets may remain stationary and the other moved a full pole pitch to reduce a full strength external field to zero, and a further full pole pitch in either direction to restore a full strength field. A magnetic chuck (Figs. 9, 10, 11) comprises a non-magnetic baseplate 3 carrying ferromagnetic end plates 4 and sideplates 5 and formed with longitudinal recesses 6 into which plural polepieces 1 are bolted in longitudinal and transverse directions. The longitudinal gaps between adjacent polepiece rows are maintained by transverse rods 5A between the sideplates and distance pieces 5B; the gaps being occupied by upper and lower rows of slab shaped ferrite permanent magnets 2 to provide alternatively opposed polar faces, each row being located by recessed support bar 8. The gaps between polepieces of the upper face are filled with, e.g. a resinous non-magnetic material and machined to a plane surface. After assembly further endplates 10 are secured to the sideplates, inwardly of which is provided mechanism to simultaneously traverse the upper and lower support bars and their magnets in opposed directions. Corresponding ends of the upper and lower bars are vertically slotted at 11 (Fig. 10) to engage pins 12 extending from rotatable locking plates 13 related by handle 14 to move each bar to an ON position (Fig. 10) in which external field of maximum strength is produced or to an OFF position in which the magnetic flux is internally diverted to zeroize the external field. Thus a ferromagnetic workpiece is retainable or releasable, and slots 11 are proportioned to move the magnets beyond a position of zero external field in OFF position to produce a low strength external reverse field to demagnetize the workpiece.  In a modification (Figs. 12, 13, 14) wherein only one row of magnets is moved to full pole width relatively to a fixed row of magnets, a non- magnetic baseplate 15 has ferromagnetic sideplates 16, 17 and locating pegs 19, and longitudinal rows of polepieces 1. Lower row fixed ferrite magnets are formed alternately as compressed packs of polepieces and magnets secured by adhesive; which packs are located by vertical slots 20 engaging pegs 19 and secured by bolts. Lubricated spacers are inserted in the gaps between adjacent pairs of polepieces, resin is applied to fill gaps between the polepieces and the upper surface machined flat; after which the spacers are removed and recessed support bars for the movable array of ferrite magnets of opposed polarity faces are inserted in the gaps. Thus adjacent faces of the linear fixed magnet rows alternate in polarity longitudinally and transversely, and the support bars are movable between positions in which a maximum and a zero external field is produced by providing the lower edge of each support bar 21 with a rack 22 engaging a full width pinion 23 rotated by an external gear-wheel engaging an internally toothed gear-wheel 25 driven by handle 27, where movement is limited by stops 29 of cover-plate 28 between ON and OFF positions, so as to move all the support bars simultaneously. Alternatively the stops may be positioned so that at OFF a low strength reverse field is applied to demagnetize the workpiece.