The present invention relates to a magnetic apparatus for clamping ferromagnetic workpieces as defined in the preamble of claim 1. hi prior art a metal plate equipped with a work holding kit is known to be used for submitting a workpiece made of either ferromagnetic or non-ferromagnetic material to machining by a machine tool. The work holding kit secures the workpiece to the metal plate in a precisely determined position. For this purpose, a plurality of holes are formed in the plate on the side that supports the workpiece to be machined. This ensures exact positioning of the workpiece relative to the plate, for accurate and repeatable machining even on different workpieces. While such locking arrangement is somewhat advantageous for machining ferromagnetic or non-ferromagnetic workpieces, it still suffers from certain drawbacks especially when machining ferromagnetic workpieces only. Particularly, for ferromagnetic workpiece machining a magnetic plate provides advantages over a work holding kit as described above. Such magnetic plate allows the workpiece to have five unconstrained faces, and affords a more uniform clamping effect than the plate and kit combination.

Nevertheless, the use of such magnetic plate also has a few drawbacks. For instance, when the magnetic plate is placed on the bed of a machine tool having a tool arm, such arm may interfere with the bed. If this occurs, an operator is required to manually reposition the workpiece in a new precisely determined position of the magnetic plate. Such step of repositioning the workpiece on the magnetic plate is obviously very complex, as holes are formed in the pole pieces with poor accuracy. The magnetic place does not allow the workpiece to be clamped in a precisely determined position, i.e. with the precision usually required for machining, as the clamping surface cannot act as a reference. This is because the pole pieces that form the magnetic workpiece clamping surface are inserted in the frame of the magnetic plate.

This apparently involves time wastes, machine tool downtime and, as a result, increased costs for each workpiece produced. Furthermore, if the workpiece to be machined is made from soft ferromagnetic materials, such as ordinary steel, there is not problem in selecting the magnetic plate type (of electro-permanent type with single- or double-magnet configurations, or permanent magnet type or of electromagnetic type) however, when machining workpieces made of hard ferromagnetic materials, such as alloy steel or heat-treatment steel, single-magnet electro-permanent magnetic plates should be only used. The use of a single-magnet electro-permanent magnetic plate eliminates residual magnetism at the end of the demagnetization cycle. This result cannot be achieved using a double-magnet electro-permanent magnetic plate. Particularly, if a double-magnet electro-permanent magnetic plate is used, there may be such residual magnetism that the workpiece may be still clamped to the clamping surface even when the magnetic circuit is off. It will be understood that this characteristic will make it difficult, if not impossible, to remove the workpiece from the clamping surface. Nonetheless, it shall be noted that double-magnet electro-permanent magnetic plates provide better performances and are more commonly used than single-magnet plates. As a result, users are actually required to use two distinct magnetic plates for machining soft and hard ferromagnetic workpieces. Thus, if both hard ferromagnetic workpieces and soft ferromagnetic workpieces are to be machines, two different types of magnetic plates shall have to be provided.

Therefore, the present invention is intended to address the problem of providing a magnetic apparatus that has such structural and functional characteristics as to fulfill the above needs, while obviating the above prior art drawbacks. This problem is solved by a magnetic apparatus for clamping ferromagnetic workpieces as defined in claim 1.

This invention provides a magnetic apparatus that allows free exposure of the workpiece to be machined without any interference between the machine tool arm and the bed. Furthermore, both hard ferromagnetic workpieces and soft ferromagnetic workpieces may be alternately or simultaneously submitted to machining using a single magnetic apparatus. Particularly, a magnetic apparatus may be provided which is composed of two different magnetic plates, i.e. an electro-permanent magnetic plate having a single-magnet circuit and an electro-permanent magnetic plate having a double-magnet circuit so that a single machine tool may be employed for machining both soft ferromagnetic and hard ferromagnetic workpieces. Furthermore, the present invention also provides a magnetic apparatus having a control and monitoring unit preferably integrated in the apparatus, for supervising the operating conditions of the magnetic plates.

Also, a magnetic apparatus is obtained which allows determination of one or more reference points for setting measurements and performing workpiece machining with the desired precision. Particularly, these reference points are located outside the magnetically active portion of the clamping surface.

Further characteristics and advantages of the magnetic apparatus of the present invention will be apparent from the following description of one preferred embodiment thereof, which is given by way of illustration and without limitation with reference to the accompanying figures, in which:

- Figure 1 is an exploded view of a modular magnetic apparatus of the present invention; - Figures 2 A and 2B are top and bottom perspective views of the magnetic apparatus of Figure 1 respectively, when assembled;

- Figures 3 A and 3 B are perspective views of another embodiment of the magnetic apparatus of the present invention.

Referring to the accompanying figures, numeral 1 generally designates a magnetic apparatus having:

- a first magnetic clamping plate 2 with a first magnetic circuit therein for generating a first magnetic flux when the first magnetic circuit is on, to define a magnetically active portion 2D of the first magnetic plate 2, acting as a magnetic clamping surface,

- a spacer 3 extending in a predetermined direction of extension Y-Y, - a second magnetic clamping plate 4 with a second magnetic circuit therein for generating a second magnetic flux when said second magnetic circuit is on, to define a magnetically active portion 4D of the second magnetic plate 4, acting as a magnetic clamping surface. Particularly, the magnetic apparatus 1 has the spacer 3 interposed between the first magnetic plate 2 and the second magnetic plate 4, and the spacer 3, the first magnetic plate 2 and the second magnetic plate 4 are rigidly and integrally connected together into a mutually associated relationship.

Advantageously, in the magnetic apparatus 1 the first magnetic flux generated by the first circuit is higher than the second magnetic flux generated by the second magnetic circuit, with the clamping surfaces 2D, 4D defined in the first magnetic plate 2 and the second magnetic plate 4 having the same area.

In other words, when the clamping surfaces 2D, 4D defined in the first magnetic plate 2 and the second magnetic plate 4 have the same area, the first circuit is designed to generate a magnetic clamping force that is higher than the magnetic clamping force generated by the second magnetic circuit.

Particularly, the first magnetic circuit generates a magnetic clamping force that is at least 50% higher than the magnetic force generated by the second magnetic circuit, considering the same clamping surface area in the respective clamping plates 2, 4.

These values may be determined when identical clamping conditions are provided, i.e. the same gap and the same material.

Therefore, due to the characteristic that the first magnetic flux generated by the first circuit is higher than the second magnetic flux generated by the second magnetic circuit, with the clamping surfaces 2D, 4D having the same area, the magnetic apparatus 1 is highly advantageous in that it allows simultaneous and/or alternate machining of both soft and hard ferromagnetic workpieces.

For example, the first magnetic circuit may be implemented by a double-magnet electro- permanent circuit and the second magnetic circuit may be implemented by a single-magnet electro-permanent circuit.

The magnetic circuits within the frames of the first magnetic plate 2 and the second magnetic plate 4 are well-known to those skilled in the art and will not be described further hereinbelow. Alternatively, the first and second circuits might also be of permanent magnet or electromagnetic type, and/or result from various combinations of such circuits.

For a more detailed description of the elements that form the above mentioned magnetic circuit types, as well as their arrangement in the frames, reference shall be made to "L'arte del

Bloccaggio", in "Tecnologie Meccaniche", November 2003. More particularly, also referring to the accompanying figures, it may be seen that the first magnetic clamping plate 2 has a frame 2 A of given thickness Sl, width Ll and length 11 and defines a first side 2B opposite a second side 2B.

The second magnetic clamping plate 4 also has a frame 4A of given thickness S2, width L2 and length 12 and defines a first side 4 A opposite a second side 4B. The spacer 3 has in turn a first side 3 A opposite a second side 3B.

Particularly, the first magnetic plate 2 and the spacer 3 are rigidly and integrally connected at their respective second sides 2C, 3B, and the second magnetic plate 4 and the spacer 3 are rigidly and integrally connected at the second side 4B of the second magnetic plate 4 and the first side

3 A of the spacer 3, to be in a mutually associated relationship.

Therefore, the first side 2B of the magnetic plate 3 and the first side 4B of the magnetic plate 4 act as magnetic clamping surfaces for clamping a ferromagnetic workpiece to be submitted to machining.

It shall be noted that the width, length and thickness of the frames 2A and 4A may be coincident, but also differ from each other.

Referring to the accompanying figures, it may be seen that both the first magnetic plate 2 and the second magnetic plate 4 have a quadrangular, preferably rectangular shape, and the opposite sides 2B-2C and/o 4B-4C of the frames 2A and/or 4A are those having the greatest area in their plan projection.

Particularly, the opposite sides 2B-2C and/or 4B-4C define flat surfaces and are connected by a side whose height is equal to the thickness Sl or S2 respectively. Then, advantageously:

- the first magnetic plate 2 and the spacer 3 are rigidly and integrally connected at their respective second sides 2C and 3 B into a mutually associated relationship, and

- the second magnetic plate 4 and the spacer 3 are rigidly and integrally connected at the second side 4C of the second magnetic plate 4 and the first side 3 A of the spacer 3, also into a mutually associated relationship.

Particularly, the first magnetic plate 2 and the spacer 3 are rigidly and integrally connected at their respective second sides 2C and 3 B by first complementary connecting and locating means

5.

Referring now particularly to Figures 1 and 2A-2B, the spacer 3 is shown to extend in the direction of extension Y-Y and to have an extension H' greater than the sum of the thicknesses

S 1 and S2 of the first clamping plate 2 and the second clamping plate. Preferably, the extension H' is at least three times the sum of the thicknesses Sl and S2.

Conversely, the magnetic plates 2 and 4 extend transverse, preferably orthogonal to the direction of extension Y-Y of the spacer 3. The spacer 3 includes a shaft 3C, whose ends terminate with respective flanges, each acting as an interface surface with the sides 2C, 4C of the magnetic plates 2 and 4. Particularly, these D

flanges coincide with the above mentioned first side 3 A and second side 3B of the spacer 3. Preferably, the flanges extend transverse to the direction of extension Y-Y. These flanges define flat surfaces, which are adapted to receive the complementary connecting and locating means 5. Particularly, the complementary connecting and locating means 5 include a base 5A with a peg 5B projecting therefrom; the latter is screwed into a seat 5 C to create a nut and screw form-fit. Preferably, the base 5 A has a cylindrical shape like the seat 5 C, which also includes an additional seat (not shown) for receiving the portion of the peg 5B projecting from the base 5A. Preferably, the base 5 A and the peg 5B are disposed on the side 3B of the spacer 3, whereas the seat 5C is formed on the side 2C of the magnetic plate 2. In order to minimize any wrong centering of the magnetic plate 2 relative to the spacer 3, additional alignment means 6 are provided on said flanges, which means include a peg 6A mating with a corresponding seat 6B. Preferably, the peg 6A is disposed on the side 3B of the spacer 3, whereas the seat 6B is formed on the side 2C of the magnetic plate 2. In a preferred embodiment of the shaft 3C, the latter has a quadrangular, preferably rectangular plan shape.

Particularly, the shaft 3 C may be formed of a material different from that of the magnetic plates 2 and 4. For instance, the spacer 3 will be made of a non-ferromagnetic material, such as composite aluminum, steel or polymer. Furthermore, in order to limit the weight of the magnetic apparatus 1 upon the bed of a machine tool on which the apparatus 1 will be received, the spacer 3 has a hollow interior. Such hollow interior extends between the flange 3 A and the flange 3 B in the direction of extension Y-Y of the spacer 3.

In the same manner as described heretofore for the magnetic plate 2, the same connecting- locating means and centering means are also provided for the second magnetic plate 4 and the spacer 3. Thus, both the magnetic plate 4 and the spacer 3 are rigidly and integrally connected at the second side 4C of the second magnetic plate 4 and the first side 3 A of the spacer 3 by second complementary connecting and locating means 7 and additional alignment means 8. Once the first magnetic plate 2 and the second magnetic plate 4 have been associated with the spacer 3 to form the apparatus 1 , the latter may be placed onto the bed of a machine tool. For instance, the side 4B of the second magnetic plate 4 acts as a base designed to contact said bed, whereas the side 2B of the first magnetic plate 2 is the clamping surface against which the ferromagnetic workpiece to be machined by the machine tool is clamped. Advantageously, due to the extension H' of the shaft 3C, the workpiece may be freely exposed, as it is magnetically held against the side 2B at such a height that no interference occurs between the machine tool arm and the bed. Advantageously, the apparatus 1 may be turned 180°, as required by the particular steps to be carried out by the operator, for easier and faster operation. In other words, in one operating configuration, the side 2B of the first magnetic plate 2 acts as a base, whereas the side 4B of the second magnetic plate 4 is the clamping surface against which the ferromagnetic workpiece to be machined by the machine tool is clamped. Conversely, in another operating configuration, the side 2B of the first magnetic plate 2 acts is the clamping surface against which the ferromagnetic workpiece to be machined by the machine tool is magnetically clamped, whereas the side 4B of the second magnetic plate 4 acts as a base.

It shall be noted that the side of the first magnetic plate 2 or the second magnetic plate 4 that acts as a base may further be fixed to the bed by a fastening kit which, as is known, comprises brackets, bolts, and the like.

Referring now to the particular embodiment of Figures 3A-3B, an alternative embodiment of the spacer 3' is shown. Particularly, it comprises a shaft 3 A whose end is equipped with a plate-like element 3D, whereas the other end is free. Such plate-like element 3D extends transverse to the direction of extension Y-Y of the spacer 3, and acts as a base for supporting the apparatus 1. The plate-like element 3D may be connected to the bed of a machine tool in a per se known manner.

The shaft 3 E also has a hollow interior and, for instance, a quadrangular plan shape, the lateral surfaces of the shaft 3E defining the sides 3A and 3B.

In this configuration, the magnetic plates 2 and 4 extend parallel to the direction of extension Y- Y of the spacer 3'. This allows alternate and/or simultaneous machining of the ferromagnetic workpieces held against the magnetic portions 2D and 4B of the first magnetic plate 2 and the second magnetic plate 4 respectively.

Once again in this embodiment, the spacer 3' has an extension H" that is greater than the sum of the thicknesses Sl and S2 of the clamping plate 2 and the clamping plate 4. Preferably, the extension H' ' is at least three times the sum of the thicknesses S 1 and S2.

Particularly, the latter configuration of the apparatus 1 provides advantages if machining requires vertical positioning of the workpieces, i.e. in the direction of extension Y-Y of the spacer 3.

Furthermore, it shall be note that once again in this embodiment, due to the extension H" of the shaft 3E, the workpiece may be freely exposed at such a height that no interference occurs between the machine tool arm and the bed. Also, still referring to the accompanying figures, the magnetic plate 2 is shown to include a plurality of holes 9 arranged outside the portion designed for magnetic activation 2D.

In other words, the plurality of holes 9 is formed at the shoulders of the frame (the so-called ferromagnetic crown) and not at the pole pieces that identify the magnetic activation portion e.g. the portion 2D of the magnetic plate 2. This ensures the required precision for the machining processes using the machine tool, because these holes act as reference points.

Also, it shall be noted that the holes 9 have a matrix-like arrangement, i.e. are arranged along lines and columns of an ideal matrix.

Particularly, each hole of the plurality of holes 9 is a calibrated, preferably blind hole, which can define a center axis Z-Z. The center-to-center distance i _m between the center axes Z-Z of two holes of the plurality of holes 9 of the same line (or column) of the matrix, is designed to ensure a centesimal accuracy, so that the center-to-center distance between any two holes of said plurality of holes fulfills the following relation: i _m = ii + 10 ^"5 m where i _m is the actually measured center-to-center distance and ij is the ideal center-to-center distance between two holes of said first plurality of holes.

In other words, the center-to-center distance between any two holes of one line (or column) of the plurality of holes 9 shall be such that the holes are formed with an accuracy of one hundredth of a millimeter.

Preferably, the direction of extension of the axis Z-Z coincides with the direction of extension Y-Y of the spacer 3.

The magnetic plate 4 also includes a plurality of holes (not shown) which are arranged outside the magnetic activation portion 4D, in a matrix-like arrangement.

Advantageously, the magnetic apparatus 1 also comprises:

- a control unit 14 for simultaneously and/or selectively controlling the on/off states of the first magnetic circuit and the second magnetic circuit by powering the solenoids of the first and/or second magnetic circuits, and - the electric cables required for powering the solenoids of the first and second magnetic circuits. In one preferred aspect, there is a single control unit 14 for both magnetic circuits of the two magnetic plates 2 and 4, which is preferably disposed within the shaft 3C (or 3E) of the spacer 3 (or 3'). Particularly, the control unit 14 is disposed in the hollow interior of the shaft 3C (or 3E). This allows reduced use of power cables and affords more reliable temperature and/or magnetic force measurements than in prior art.

The control unit 14 consists of electronic circuitry designed for controlling the power to be delivered to the solenoids of the two magnetic circuits. It shall be noted that the control unit 14 is controlled by a control section (not shown), such as an electric panel or switchboard, disposed outside the apparatus 1 and in electric communication with the control unit.

A single terminal 12 is provided for supplying power to the control unit 14. Particularly, the terminal 12 is placed on the shaft 3C (or 3E) of the spacer 3 (or 3'). This advantageously allows the electric connector 12 to be placed in a position protected from machining residues that build up during ordinary workpiece machining.

Alternatively, each magnetic plate 2 and 4 is equipped with its own cables and its respective electric connector, hi this case, the control unit is disposed outside the apparatus 1. In another embodiment (not shown), in addition to the provision of the plurality of holes 9 arranged outside the magnetic activation areas 2D and 4D, each of the first magnetic plate 2 and the second magnetic plate 4 has a plurality of holes at the portions for magnetic activation 2D,

4D, in a matrix-like arrangement. Particularly, these pluralities of holes are placed within the area defined by the magnetic activation portions 2D, 4D.

Advantageously, each hole of the plurality of holes is a calibrated, preferably blind hole, which can define a center axis extending in the predetermined vertical direction of extension Z-Z. Particularly, the center-to-center distance between two contiguous holes of each plurality of holes of the same line (or column) of the matrix is measured with a centesimal accuracy, to fulfill the following relation: i _m = ii ± 10- ⁵ m where i _m is the actually measured center-to-center distance and i; is the ideal center-to-center distance between two contiguous holes of said first plurality of holes.

It shall be noted that, for each plurality of holes within the magnetic activation portions 2D, 4D to be formed with the above mentioned accuracy, the first magnetic plate 2 and the second magnetic plate 4 shall be so configured that the pole piece collector is formed of one piece with the frame 2A and 4A of the magnetic plates. Therefore, the pole piece collector shall be part of the frame of each magnetic plate 2 and 4. For this purpose, the pole piece collector shall be formed from a solid piece of ferromagnetic material by machining processes, such as material removal. This will provide a first magnetic plate 2 and a second magnetic plate 4 of monolithic type. Due to the characteristic that the pole piece collector is part of the frame of each magnetic plate 2 and 4, the plurality of holes formed within the magnetic activation portions 2D, 4D may be calibrated in such a manner that the center-to-center distance between two contiguous holes may be measured with centesimal accuracy.

For a more detailed description of a monolithic plate, reference may be made to Patent application PCT/IT2008/000278 by the applicant hereof, which is entirely incorporated herein by reference. Those skilled in the art will obviously appreciate that a number of changes and variants may be made to the magnetic apparatus as described hereinbefore, in response to specific requirements, without departure from the scope of the invention, as defined in the following claims.

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