This invention refers to a permanent-magnet apparatus for anchoring or gripping metal pieces on machine tools, conveying and transporting systems, which makes use of a magnetic circuit capable of automatically adapting its characteristics in relation to the dimensions of the metal pieces to be anchored; for the purposes of this description, the term "permanent magnet anchoring apparatus" is understood as any device which makes use of permanently magnetized or magnetizable magnet core members, which can be activated and deactivated either by reversing the magnetization of some of the magnets of at least two pole units, or by magnetizing and demagnetizing the magnets themselves. The magnetic anchoring apparatus currently in use are mainly designed to be used for gripping workpieces of very variable dimensions, however it is highly unlikely for any apparatus to be able to maintain the same operating efficiency with all sizes and types of workpieces. There are in practice, two main categories of magnetic anchoring apparatus: the first category is designed to be very effectively used on workpieces of average or large dimensions, for which it is essential to have a considerable magnetic field depth in the workpiece to be anchored in order to utilize as much as possible the magnetomotive force necessary to grip the workpieces securely. The second category refers to the magnetic apparatus designed for use on small-sized workpieces, for which, more than the field depth or the magnetomotive force generated by the magnetic circuit, it is important to have a dense polarity on the anchoring or gripping surface, by bringing the individual pole as close as possible, so that each piece, however small it may be, can short-circuit at least two poles of opposite polarity.
In the case of the apparatus belonging to the first category, with which it is possible to magnetically grip workpieces to be subjected to heavy machining, such as milling for example, the need to have high magnetomotive tension and, therefore, high anchoring power involves keeping the poles sufficiently far apart from one another, as to reduce the losses of flux in the gap between adjacent pole units to a minimum; consequently, as a result of the high pole pitch that these devices must have, they prove to be totally unreliable in gripping relatively small workpieces. Conversely, in the case of the apparatus of the second category, the dense polarity, that is to say, a short distance between adjacent pole units, is obtained by using special pole plates for distributing the magnetic flux, which are characterized by structural complexity, high costs and very poor operating efficiency; they consequently prove to be wholly unreliable in gripping large-sized workpieces. In conclusion, each apparatus for both categories must necessarily be used in the field of work for which it was designed, with the need therefore to make use of different apparatus for differently sized workpieces to be magnetically gripped.
The scope of this invention is to provide a magnetic gripping apparatus, for example of the type described in U.S Pat. No. 4,356,467, U.S. Pat. No. 4,356,167, U.S. Pat. No. 4,507,635 and GB-A No. 758183, which is capable of automatically modifying the characteristics of its magnetic circuit, or the magnetomotive force, and the pitches or distances between poles, to adapt them automatically to the dimensions of the workpieces to be anchored, without impairing the high degree of efficiency of the apparatus and without changing any part of the latter.
A further scope of this invention is to provide an apparatus of the aforementioned type, which is capable of achieving all this by means of a comparatively inexpensive and mechanically sturdy magnetic structure.
A still further scope of this invention is to provide a magnetic gripping apparatus, as specified above, which, in addition to being capable of automatically modifying its pole pitch, since the actual dimensions of the workpieces to be gripped contribute to modifying the magnetic circuit of the apparatus which is thus able to provide two different pole pitches without altering the distances between the various pole units, can also be adapted for use as a permanent magnet device, both of the reversible flux type, as described for example in the three previously mentioned U.S. patents, and of the total demagnetization type, as described in GB-A No. 758183.