WO2000030129A2 - Electromagnetic lifting system - Google Patents

Abstract

An electromagnet (10) having a core with a plurality of elongated channels (16), a bottom opening (34) to each channel, and a coil (14) made up of a plurality of lengths of electrically conductive wire extending through each channel. The core has a top wall (24) and a plurality of spaced parallel vertical side walls (26, 28) which define the channels. The bottom end of each vertical side has horizontal flanges (30) which form the poles of the magnet (10) and also define the openings (34) to the channels.

Description

ELECTROMAGNETIC LIFTING SYSTEM

FIELD OF THE INVENTION This invention is an electromagnet structure, particularly adapted to move ferrous metal workpieces onto and off of a supporting surface.

BACKGROUND OF THE INVENTION

One method of manufacturing shaped ferrous metal workpieces from a large metal plate of ferrous metal utilizes a cutting table. A large ferrous metal plate is placed on a heat resistant cutting table and workpieces of various shapes are cut from the plate using laser beams, plasma beams, or flame cutting torches. The cutting process may be manual or automated. The result of the cutting operation is, typically, a large number of parts or workpieces of various sizes and shapes positioned within a skeleton like structure of the scrap remaining from the original plate. In order to make this cutting table process most efficient, it is desirable to provide a system for lifting the stock plate onto the table and positioning it, and then a system for very quickly picking up all of the workpieces, scrap, and the skeleton plate once the cutting is complete. It has been found to be very desirable to employ an electromagnetic system for not only initially positioning the stock plate on the cutting table, but also for picking up the workpieces which have been cut from the plate once the cutting has been completed. A number of problems, however, can result from the use of conventionally designed electromagnets for this cutting process. First of all, any electromagnetic system which is designed to cover a large surface area is typically formed of very thick core materials which are typically very heavy. In order to support the series of electromagnets typically required to cover large surface areas, and particularly when those electromagnets themselves are very heavy, often requires a very heavy structural framework for supporting the electromagnetics. Thus, the entire electromagnetic system and its supports are often very heavy and require a substantial amount of work to lift them, particularly if they are carrying the workpieces. This high weight for the electromagnetic system not only increases the energy requirements necessary to carry out the lifting function, but also increases the potential danger associated with heavy equipment suspended in the air.

Another problem with using conventional electromagnetic systems for the lifting system is that the conventional electromagnetics are typically constructed with circular coils which wrap around an oval track which is formed in the body of the core. In general, it is necessary to have numerous, closely-spaced coils, in order to assure that small workpieces will be effectively attracted to the magnet. At the ends of these tracks are inhomogeneous magnetic field structures which tend to form "weak spots" at each end of each series of electromagnetic coils. These "weak spots" are often ineffective at carrying out the lifting function, particularly for small pieces which are positioned at the outer edges of the cutting table. As a result, it is often necessary to build the magnet system far larger than what would otherwise be necessary and larger than the cutting table surface. The "weak spot" structure is shown in Figure 21 which is a bottom view of a conventional lifting system magnetic structure.

These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, a principal object of the present invention to provide an electromagnet structure which is low in weight, and, more specifically, has a low weight-to-magnetic working area ratio, thereby producing a magnet which is efficient to use as a lifting magnet.

Another object of this invention is to provide an electromagnet structure that has a large magnetic working area which area has relatively uniform magnetic fields throughout and is free of magnetic "weak spots", so that the magnet is capable of attaching small ferromagnetic objects at any point in its working area.

It is a further object of the invention to provide an electromagnet structure which is capable of being manufactured of high quality and at a low cost, and which is capable of providing a long and useful life with a minimum of maintenance.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

BRIEF SUMMARY OF THE INVENTION An electromagnet having a core with a plurality of elongated channels, a bottom opening to each channel, and a coil made up of a plurality of lengths of electrically conductive wire extending through each channel. The core has a top wall and a plurality of spaced parallel vertical walls which define the channels. The bottom end of each vertical side wall has horizontal flanges which form the poles of the magnet and also define the openings to the channels. Each pole has a different polarity from adjacent poles.

This invention may be viewed as electromagnet system, including an electromagnet structure which has a core and a coil, and a supply of electric current for energizing the coil and magnetically exciting the core.

This invention may also be viewed as electromagnetic lifting system including an electromagnet structure which has a core and a coil, a supply of electric current for energizing the coil and magnetically exciting the core, and a movement system adapted to provide controlled movement of the structure.

This invention can also be viewed as a cutting table lifting system, including a cutting table, a workpiece of ferrous material, an electromagnet structure which has a core and a coil, a supply of electric current for energizing the coil and magnetically exciting the core, said supply adapted to control the attraction of the structure to the workpiece, and a movement system adapted to provide controlled movement of the structure with respect to the table, said lifting system being adapted to place the workpiece on the table and to remove the workpiece from the table.

BRIEF DESCRIPTION OF THE DRAWINGS

The character of the invention, however, may best be understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a front elevational view of an electromagnetic lifting system embodying the principles of the present invention;

FIG. 2 is a fragmentary bottom plan view of the lifting system; FIG. 3 is a fragmentary top plan view of the lifting system; FIG. 4 is a fragmentary side elevational view of the lifting system;

FIG. 5 is a front elevational view of a shield of non-magnetic material for application to the lifting system of the present invention;

FIG. 6 is a front elevational view of the electromagnetic lifting system of the present invention which includes the shield of FIG. 5; FIG. 7 is a fragmentary side elevational view of the core portion of the lifting system; FIG. 8 is a fragmentary bottom plan view of the core;

FIGS. 9-18 are operational views illustrating how the electromagnetic lifting system of the present invention is used for separating parts and scrap from a cutting table;

FIG. 19 is a schematic end view of a prior art electromagnet; FIG. 20 is a schematic end view of the electromagnet of the present invention;

FIG. 21 is a schematic bottom plan view of a prior art electromagnet;

FIG. 22 is a schematic bottom plan view of the electromagnet of the present invention; and

FIGS. 23 and 24 are schematic views of two possible winding strategies for the coil portion of the electromagnet of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1-6, the electromagnetic lifting apparatus of the present invention is generally indicated by the reference numeral 10 and comprises a core, generally indicated by the reference numeral 12 and a coil, generally indicated by the reference numeral 14. Referring also to FIGS. 7 and 8, the core 12 comprises a plurality of elongated channel beams, generally indicated by the reference numeral 16. Channel beams 16 are arranged side-by-side in a row and are fixed to a pair of structural beams 18 by nuts 17 and bolts 19. The structural beams 18 extend transversely of the channel beams 16 and are located at opposite ends of the channel beams. A pair of central structural beams 20 are located in the center of the channel beam 16 transversely of the channel beam. A cross beam 22 extends between the central structural beams 20 at a mid point of the structural beams 20. The beams 20 are fixed to the channel beams 16 by nuts and bolts. A lifting ring 24 is fixed to the cross beam 22 for receiving a hoisting chain 26.

Referring again to FIGS. 7 and 8, each channel beam 16 has a top wall 24 and a pair of vertical side walls 26 and 28. The lower end of each side wall 26 and 28 has an inwardly extending horizontal flange 30. The top and side walls of each channel beam 16 defines a generally rectangular channel 32 and the inner ends of the flanges 30 define an opening 34 to the channel 32. Each pair of abutting oppositely extending flanges forms one of the poles of the core, generally indicated by the reference numeral 36. As shown in FIGS. 3 and 4, each coil 14 is made up of electrically conductive insulated wire located within the channels 32. Beginning with the first two channels 32, the electrically conductive wire is wound in several loops around the side wall pair 26 and 28 which separate the first two channels. , Each side wall pair 26 and 28 function as a single side wall. The plurality of loops form a loop bundle 37. Each successive pair of channels 32 contain a loop bundle 37. All of the loop bundles 37 can be formed from a single continuous electrically conductive wire 39 as shown in FIG. 23 so that the loop bundles 37 are in series. Only one loop per loop bundle is illustrated in FIG. 23 for clarity. In the preferred embodiment, three successive loop bundles 37 are formed from a single wire. Additional groups of three loop bundles 37 are formed from additional wires. Current flow is indicated by the arrows. In another embodiment as illustrated in FIG. 24, each loop bundle, generally indicated by the reference numeral 37, is formed from a first wire 41 and a second wire 43. Current flow for each of the wires 41 and 43 indicated by the arrows. Current flow through wire 43 is counter to that of the current flow through wire 41. Each loop bundle in the embodiment of FIG. 24 comprises a plurality of loops from each of the wires 41 and 43. A partial loop for each loop bundle is shown in FIG. 24 for clarity of illustration. Loop bundles from wire 41 alternate with loop bundles from wire 43. This provides an option of reducing the magnetic force of the magnet by 50% by deenergizing one of the wires 41 and 43. A plurality of vertical stops 38 of non magnetic material are fixed to the flanges 30 for propping the outer ends of each loop bundle 37 at an upward angle at the end of each channel beam. Each end of each channel beam 16 tapers downwardly and outwardly from the top wall 24, as shown in FIG. 1, so that the end of each loop bundle 37 is located above the tapered portion of the channel beam.

Referring to FIGS. 5 and 6, the electromagnet 10 is covered with a shield, generally indicated by the reference numeral 40. The shield 40 is made of a non-magnetic material, i.e. stainless steel and comprises a bottom wall 42, a pair of vertical side walls 44 located at opposite ends of the shield, and a pair of supporting walls 46 fixed to the upper ends of the vertical walls 44. Each supporting wall 46 has a horizontal inwardly extending component 48 and a vertical downwardly extending component 50. The lower end of each vertical component 50 is spaced from the bottom wall 42 and has a horizontal flange or foot 52. The shield 40 is applied to the electromagnet 10 so that the bottom wall 42 is located below the poles 36 and the openings 34. The vertical end walls 44 are located outside of the stops 38 and the supporting wall 46 extends above the structural beams 18 so that the feet 52 rest on the top walls 24 of the channel beams 16. Although the shield 40 is shown as a single integral piece, it can be fabricated from a plurality of pieces.

In addition to the overall effectiveness of the magnet system in lifting both the frame and workpieces from a cutting table on which the workpieces had been laser cut from a starting material plate, there is a separation aspect of the present invention. The "frame- workpiece separation feature" aspect of the present invention is shown in FIGS. 9-18 and essentially functions as follows. After a metal plate has been laser cut into a frame and a large number of workpieces, the entire resulting structure (both frame and workpieces) are engaged by the electromagnet and lifted off of the cutting table. The electromagnet is then positioned over a scrap table. The voltage to the magnet is then reduced to zero and the electromagnet slowly begins to lose its magnetism. After a period of time, but generally before the magnetism reaches zero, the frame or scrap falls from the magnet, leaving only the workpieces magnetically connected to the magnet. At this point, the voltage is immediately reapplied to the magnet and the workpieces remain attached to the magnet. The magnet is then positioned over a workpiece collection area. The voltage is again reduced to zero and, eventually, the magnetism reaches zero and the workpieces fall from the electromagnet into a workpiece collection area. In this way, the workpieces are easily separated from the frame or scrap. The theoretical reason for the scrap falling off of the magnet first is not entirely understood.

Considering the "frame-workpiece separation process" more specifically, as shown in FIG. 9, the process begins with the electromagnet 10 suspended over a work table 45. A workpiece plate, generally indicated by the reference numeral 56, is shown cut into individual workpieces 58 and 60 surrounded by a frame 62. FIG. 9 shows the workpiece as it has just been cut into the frame 62 and the individual workpieces 58 and 60. As shown in FIG. 10, the voltage is at zero and the magnetic force is at zero.

In FIG. 11 , the electromagnet 10 has been lowered to contact the workpiece plate 56. As shown in FIG. 12, the voltage has been raised to voltage VI and the magnetism increases to magnetic force Ml. As shown in FIG. 13 and 14, the electromagnet 10 is lifted and, because the workpiece plate 56 is attracted to it, the workpiece plate 56 is lifted along with the electromagnet 10. The voltage remains at voltage V and the magnetism remains at magnetic force Ml.

In FIG. 15 and 16, the electromagnet 10 is moved over to a scrap table, and the voltage is dropped to zero. Slowly, the magnetic force drops from magnetic force Ml to lower values. Because it has been found that a greater magnetic force is required to hold the frame against the magnet than is required to hold the workpieces against the magnet, as the magnetic force lessens, the frame 62 drops off first. Once that happens, the voltage is returned to voltage VI and the magnetic force returns to magnetic force Ml.

As shown in FIG. 17 and 18, the electromagnet 10, carrying the workpieces 58 and 60, is then moved to a workpiece collection table 64. At that point, the voltage is dropped again to zero. The magnetic force decays to zero and the workpieces fall off onto the workpiece collection table 64. The electromagnetic 10 can thereafter be returned to the cutting table 45 to pick up another cut up workpiece plate.

The voltage to the electromagnet 10 can be controlled by the operator through a manually actuated voltage regulator., The voltage to the electromagnet 10 can also be controlled automatically by a voltage regulator operatively connected to control switches at the scrap table and the workpiece collection table. The control switches can be of a type which are mechanically actuated or electronic proximity switches which are actuated in the presence of metal such as that of the electromagnet.

A less sophisticated and simpler method of separating the workpieces from the frame makes use of flat permanent bar magnets. Prior to the deployment of the electromagnet, several spacer bars are placed on the frame at spaced locations. The spacer bars are of a non- ferrous material such as aluminum. However, bars of a ferrous material can be used as well as permanent bar magnets which will stay in place on the frame. The electromagnet is lowered toward the frame until it rests on the spacer bars which create spaces between the frame and the electromagnet. The frame and workpieces are lifted by the electromagnet and positioned over the scrap table. A pry bar is inserted in one of the spaces between the frame and electromagnet. The frame is forced away from the electromagnet by the pry bar so that the frame drops to the scrap table. The workpieces continue to be held by the electromagnet. The workpieces are then carried to the workpiece collection table. The electromagnet is then deenergized which allows the workpieces to fall to the workpiece collection table.

The electromagnet 10 of the present invention is capable of loading and unloading a laser-, plasma-, or flame cutting table fully loaded with cut products in one cycle, without micro joints being necessary. In comparison to electromagnets currently in use such as multiple by-polar or three pole magnets and permanent pelletizing magnets, the weight of the electromagnet 10 of the present invention is very low, preferably below the product's weight. As a result, the initial cost of the magnet 10 is lower than existing magnets and the operating cost is also lower since less energy is needed to operate the magnet. There is a full magnetic area capable of picking up small cut products of typically 50 mm, or less in diameter.

Traditional magnet construction for cutting table lifting systems normally yields high weights. This is because, as shown in FIGS. 19 and 21, traditionally, electromagnet construction for lifting systems employ thick and heavy soft iron or steel cores 66. The core 66 has channels 67 which contain coils 65. The north and south poles are indicated by the reference numerals 71 and 70, respectively. Since the cutting table lifting system application typically does not require high flux densities, core cross sections can be reduced drastically, resulting in extreme low weights. Secondly, the need of magnetically "weak" zones at both outer edges of the magnetic area is eliminated. The weak zones of the prior art electromagnets are indicated by the reference numeral 68 in FIG. 22. This loss of magnetic area is the result of the space required for the "coil turn" or bend. In the present invention, the coil turns or ends of the loop bundles 37 are positioned on top of the tapered end of the core allowing the poles 36 to run all the way to the edge of the magnet.

With five U-channels of 50 mm x 25 mm x 8 mm x 2 mm cross-section (width x height x edge x thickness) and length 250 mm, giving a total area of 250 mm x 250 mm, the following results were measured: Plate size: 150 mm wide, covering three channels (outer channels covered by extra plate), 250 long and 15 mm thick Air gaps: 0.8, 2, 4 mm (stainless plate in between channel and plate).

Actual air-gaps are bigger because of channel unflatness, edges not perpendicular and edge radius (3 mm). The total effect is estimated between 0.5 and 1 mm. All forces mentioned for these examples are for a plate under test of 250 mm 150 mm x 5 mm unless stated otherwise.

Forces in kgf (kilogram (force)) (gm-gf-cm-sec system) jnp turns Air Gap= =0.8 mm 2 mm 4 mm

131 1.2 0.2 0

264 5.2 1.2 0.6

399 10.2 3.2 1.2

536 14.8 5.7 2.2

675 21.2 8.2 3.7

A plate of 250 mm x 150 mm x 15 mm weighs 4.4 kgf.

In order to hold a plate of 15 mm thickness with a safety factor of two at 0.8 mm gap, approximately 365 ampere turns are needed based on above measurement results.

Both finite elements and a simple circuit analysis give about the same values (with +/- 10%). These values are much higher than measured. The difference can be explained by the fact that actual air-gaps are bigger than just the thickness of the inserted stainless plate as used in the calculations.

For example, for a channel as described above.

Ampere turns: 399

ir-gap Measured forces Calculated at air gap as Calculated with an used in measurements. additional air-gap of

(circuit analysis) 0.5 mm

0.8 10.2 15.8 9.4

2 3.2 5 3.5

4 1.2 1.55 1.3

The extra internal gap needed to make the calculations fit is well within the range estimated from the observed unflatness of the test piece.

The conclusion is that the circuit approach can be used to estimate the performance of other cross-sections. In evaluation of designs, we need to use at least 0.5 mm extra internal gap is required. An extra internal gap of 1 mm is best.

Channel thickness

Reducing channel thickness to 1 mm results in some 45% force loss. Reducing to 1.5 mm results in 15% force loss.

A choice of 2 mm is appropriate.

Edge size

There is an optimum choice for edge size. In general, the bigger the edge size, the less ampere turns are needed to drive sufficient magnetic flux over the gap into the target plate. On the other hand, the leakages will increase with larger edges, resulting in more flux through the steel top of the channel and consequently, also more saturation on that spot. An extreme case of no force at all is found if the edge is so big that the channel is closed. The optimum depends on ampere turns available (399), total air-gap (say 1.5 mm), channel thickness (say 2 mm) and channel width (50 mm assumed).

Edge (mm) Force (kgf)

8 6

12 7.4

16 8.1

20 8.1

22.5 7.8

Choice of channel sizes.

A standard U-channel beam can be employed.

Some standard channels are: A 40mm x 20mm x 10mm x 2mm, a bit small B 80mm x 40mm x 16mm x 2mm, too big for small pieces.

Channel size A could be to small for many applications and channel size B could be to big for applications having small pieces. Manufactured channel.

Manufacturing by bending is not very practical because of the large edge required. An alternative is to use standard rectangular thin walled tubing and to mill an opening or slot lengthwise. With this method, cores can be made having any desired channel size.

A testing was made beginning with small standard channel irons or beams. The reason for this, besides it being readily available, is that the performance on small work pieces is expected to be better than with the use of larger channel irons or beams. If the edges are big, then the flux does not need the surface of a small piece as a conductor: all flux can pass through the rest of the steel edge alongside the piece. There may be a limit to the edge size is half of the smallest piece (say 12 mm). This, however, could not be verified. For a proper best laser cut, parts are needed since the air gaps between the cut parts play an important part in holding performance.

Expected performance.

Given a coil with 450 Amp. turn and a channel of 40 mm x 20 mm x 10 mm x 2 mm, holding forces are calculated at: Air gaps Forces at 250 x 150 x 15 (mm) plate Maximum plate thickness mm kgf mm

1.5 13.6 46

3 4.4 15

Expected peak performance at 1000 Ampturns:

The magnet is expected to pick up 15 mm material at a distance of: 7 mm. This is acceptable Coil design.

The experimental channels were observed to run hot (not touchable) at 117 Watt heat input over a total channel length of 1.25 meters. With half the best input (45 Watt/meter channel) temperatures were acceptable. Based on this, a coil was designed for a 2.5 x 1.25 meter magnet with the following parameters: Voltage: 246 VDC

Wire: 40 turns of 2.5 mm squared cross-section (litze wire) Ampere-turns (continuously): 450 Power " continuous"": 2750 Watt or 35 Watt per meter channel.

Peak Values: 1000 Amp turns (4 x bigger forces) 550 Volts peak, 13.75 kW Conclusion with respect to target weight.

In the above description, the general design of a cutting table lifting magnet is described

So far this approach gives a magnet with a weight of: Steel channels + 0.8 wear plate: 145 kgf Coil copper wire: 65 kgf

Structural parts: variable The target (equal to plate weight) seems feasible. The preferred cross-section of the channels, disregarding the air gap, is square. However, the channel cross-section could be rectangular, with the air gap on a long or a short side. Also, but substantially less desirable, the cross-section could be triangular or round. In fact, the cross-section can be any shape, but a shape that allows the air gap side to be flat and of maximum width, for example, a semi-circle with the air gap on the diameter side, is much preferred.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desire to secure by Letters Patent is:

Claims

What is claimed:

1. An electromagnet comprising:

(a) a core having a top wall and a plurality of spaced parallel vertical side walls extending downwardly from said top wall to a free bottom edge, said side walls and said top wall defining a plurality of parallel open ended channels, each of said channels extending along a longitudinal axis, said core having a horizontal flange extending transversely of said longitudinal axes from each of said bottom edges toward each adjacent one of said side walls, each of said flanges having a horizontal dimension transverse to said axes which is less than half of the transverse distance between two adjacent ones of said side walls, so that each of said channels has a pair of oppositely facing flanges, which are horizontally spaced and define a bottom opening to the channel; and

(b) a coil having a plurality of lengths of electrically conductive wire within each of said channels, said lengths of electrically conductive wire being arranged so that the flanges at the lower free end of each of said side walls functions as a magnetic pole having a polarity which is opposite from that of the magnetic poles formed by the flanges at the lower free ends of the adjacent ones of said side walls.

2. An electromagnet as recited in claim 1. wherein each of said side walls has a pair of opposite longitudinal end edges which extend downwardly and outwardly longitudinally from said top wall and said flanges extend to the outer ends of said side walls and beyond said top wall.

3. An electromagnet as recited in claim 2, wherein the outer end edges of each of said side walls extends from said top wall at an angle of 30 degrees to 60 degrees relative to said top wall.

4. An electromagnet as recited in claim 1, further comprising at least one structural beam located above said top wall and fixed to said top wall, said structural beam extending transversely of said side walls.

5. An electromagnet as recited in claim 4, further comprising a lifting ring fixed to said structural beam.

6. An electromagnet as recited in claim 2, wherein the lengths of electrically conductive wire in each of said channels are connected to the lengths of electrically conductive wire in an adjacent channel and form a plurality of loops of electrically conductive wire.

7. An electromagnet as recited in claim 6, further comprising stops of non magnetic material are located at the outer ends of said channels for propping said loops at an upward angle.

8. An electromagnet as recited in claim 7, further comprising a shield of non magnetic material a horizontal bottom wall below said side walls, a vertical end wall extending upwardly from each end of said bottom wall to an upper end outside of the corresponding tapered ends of said side walls and a supporting wall extending inwardly and downwardly from the upper end of each of said end walls to said top wall.

9. An electromagnet as recited in claim 8, wherein each of said supporting walls has a horizontal component extending inwardly from the corresponding end wall and a vertical component extending downwardly from the vertical component the top wall.

10. An electromagnet as recited in claim 1, wherein the lengths of electrically conductive wire in each of said channels are connected to the lengths of electrically conductive wire in an adjacent channel in the form of loops at each end of the channel.

11. An electromagnet as recited in claim 1, wherein said core comprises:

(a) a plurality of channel beams, each channel beam having a top wall, a pair of spaced vertical side walls and a bottom wall which has a central longitudinal opening, said channel beams being arranged so that the top walls of said channel beams lie in the same horizontal plane to constitute the top wall of said core and the side walls of said channel beams abut to constitute the side walls of said core;

(b) a pair of structural beams located at opposite ends of said channel beams, each of said structural beams extending transversely of said channel beams and resting on top of the top walls of said channel beams; and

(c) means for fastening said structural beams to each of said channel beams.

12. An electromagnet as recited in claim 11 , further comprising a central structural beam located between said pair of structural beams transversely of said channel beams and fixed to said channel beams.

13. An electromagnet as recited in claim 12, further comprising a lifting ring fixed to said central structural beam.

14. An electromagnet as recited in claim 13. wherein the outer end of each side wall tapers downwardly and outwardly from said top wall and said flanges extend to the outer ends of said side walls.

15. An electromagnet as recited in claim 14, wherein the outer end of each of said side walls tapers at an angle of 30 degrees to 60 degrees.

16. An electromagnet as recited in claim 15, wherein the lengths of electrically conductive wire in each of said channels are connected to the lengths of electrically conductive wire in an adjacent channel in the form of loops at each tapered end of the channel.

17. An electromagnet as recited in claim 1 , wherein the width of each of said poles transversely of said channels is substantially greater than the thickness of each of said side walls.

18. An electromagnet as recited in claim 17, wherein the width of each of said poles is more than twice that of the thickness of each of said side walls.

19. A method of separating ferrous workpieces from a frame of ferrous material from which said workpieces have been cut, said workpieces and said frame being supported on a cutting table of non magnetic material, said method comprising the following steps:

(a) positioning an electromagnet above said workpieces on said cutting table the magnetic attractive force of said electromagnet being controllable;

(b) lowering said electromagnet into contact with said workpieces and said frame;

(c) increasing the magnetic attractive force of said electromagnet to a first value until said workpieces and said frame are full supported by said electromagnet;

(d) raising said electromagnet while said electronic is at said first value until said workpieces and said frame are spaced above said cutting table;

(e) moving said electromagnet and the workpieces and frame carried by said electromagnet to a position above a scrap table wherein said workpieces and said frame are spaced from said scrap table at a distance at which said electromagnet would be incapable of lifting said frame and said workpieces to said electromagnet while said electromagnet is at said first value and said workpieces and said frame are supported on said scrap table;

(f) reducing the magnetic attractive force of said electromagnet to a second value which is substantially less than said first value, said second value being sufficient to enable said workpieces to be held by said electromagnet and to release said frame to said scrap table;

(g) increasing the magnetic attractive force of said electromagnet to said first value;

(h) moving said electromagnet to a position above a workpiece collection area; and (I) reducing said electromagnet force to a third value which is less than said second value, said third value being insufficient to hold said workpieces on said electromagnet so that said workpieces fall to said workpiece collection area.

20. A method as recited in claim 19, wherein said magnetic attractive force is selectively increased and decreased by selectively increasing and decreasing the voltage to said electromagnet.

21. A method of separating ferrous workpieces from a frame of ferrous material from which said workpieces have been cut, said workpieces and said frame being supported on a cutting table of non magnetic material, said method comprising the following steps:

(a) positioning an electromagnet above said frame and workpieces which are supported on said cutting table, the magnetic attractive force of said electromagnet being controllable, said electromagnet comprising a core having a top wall and a plurality of spaced parallel vertical side walls extending downwardly from said top wall to a free bottom edge, said side walls and said top wall defining a plurality of parallel open ended channels, each of said channels extending along a longitudinal axis, said core having a horizontal flange extending transversely of said longitudinal axes from each of said bottom edges toward each adjacent one of said side walls, each of said flanges having a horizontal dimension transverse to said axes which is less than half of the transverse distance between two adjacent ones of said side walls, so that each of said channels has a pair of oppositely facing flanges, which are horizontally spaced and define a bottom opening to the channel; and

(b) lowering said electromagnet into contact with said workpieces and said frame;

(c) increasing the magnetic attractive force of said electromagnet to a first value until said workpieces and said frame are full supported by said electromagnet;

(d) raising said electromagnet while said electronic is at said first value until said workpieces and said frame are spaced above said cutting table;

(e) moving said electromagnet and the workpieces and frame carried by said electromagnet to a position above a scrap table wherein said workpieces and said frame are spaced from said scrap table at a distance at which said electromagnet would be incapable of lifting said frame and said workpieces to said electromagnet while said electromagnet is at said first value and said workpieces and said frame are supported on said scrap table;

(f) reducing the magnetic attractive force of said electromagnet to a second value which is substantially less than said first value, said second value being sufficient to enable said workpieces to be held by said electromagnet and to release said frame to said scrap table;

(g) increasing the magnetic attractive force of said electromagnet to said first value;

(h) moving said electromagnet to a position above a workpiece collection area; and (I) reducing said electromagnet force to a third value which is less than said second value, said third value being insufficient to hold said workpieces on said electromagnet so that said workpieces fall to said workpiece collection area.

22. A method of separating ferrous workpieces from a frame of ferrous material from which said workpieces have been cut, said frame having a top surface, said workpieces and said frame being supported on a cutting table of non magnetic material, said method comprising the following steps:

(a) placing a plurality of flat spacers on said top surface so that each of said spacers is spaced from adjacent ones of said spacers;

(b) positioning an electromagnet above said frame and workpieces which are supported on said cutting table;

(c) lowering said electromagnet into contact with said spacers on said frame so that said workpieces and said frame are attracted to said electromagnet sufficiently to be supported by said electromagnet and there are a plurality of spaces between said electromagnet and said frame;

(d) raising said electromagnet so that said workpieces and said frame are spaced above said cutting table;

(e) moving said electromagnet and the workpieces and frame carried by said electromagnet to a position above a scrap table wherein said workpieces and said frame are spaced from said scrap table at a distance at which said electromagnet would be incapable of attracting said frame sufficiently to lift said frame while said frame is supported on said scrap table;

(f) inserting a pry bar in the spaces between said electromagnet and said frame and forcing said frame away from electromagnet so that said frame falls to said scrap table while said workpieces continue to be supported by said electromagnet;

(g) moving said electromagnet to a position above a workpiece collection area; and

(h) reducing electromagnet force in said electromagnet sufficiently to enable said workpieces to fall free from said electromagnet to said workpiece collection area.

23. A method as recited in claim 22, wherein each of said spacers is a permanent bar magnet.

24. A method as recited in claim 23, wherein each of said spacers is a bar of non ferrous material.

25. A method of separating ferrous workpieces from a frame of ferrous material from which said workpieces have been cut, said frame having a top surface, said workpieces and said frame being supported on a cutting table of non magnetic material, said method comprising the following steps:

(a) placing a plurality of flat spacers on said top surface so that each of said spacers is spaced from adjacent ones of said spacers;

(b) positioning an electromagnet above said frame and workpieces which are supported on said cutting table, said electromagnet comprising a core having a top wall and a plurality of spaced parallel vertical side walls extending downwardly from said top wall to a free bottom edge, said side walls and said top wall defining a plurality of parallel open ended channels, each of said channels extending along a longitudinal axis, said core having a horizontal flange extending transversely of said longitudinal axes from each of said bottom edges toward each adjacent one of said side walls, each of said flanges having a horizontal dimension transverse to said axes which is less than half of the transverse distance between two adjacent ones of said side walls, so that each of said channels has a pair of oppositely facing flanges, which are horizontally spaced and define a bottom opening to the channel;

(c) lowering said electromagnet into contact with said spacers on said frame so that said workpieces and said frame are attracted to said electromagnet sufficiently to be supported by said electromagnet and there are a plurality of spaces between said electromagnet and said frame;

(d) raising said electromagnet so that said workpieces and said frame are spaced above said cutting table;

(e) moving said electromagnet and the workpieces and frame carried by said electromagnet to a position above a scrap table wherein said workpieces and said frame are spaced from said scrap table at a distance at which said electromagnet would be incapable of attracting said frame sufficiently to lift said frame while said frame is supported on said scrap table;

(f) inserting a pry bar in the spaces between said electromagnet and said frame and forcing said frame away from electromagnet so that said frame falls to said scrap table while said workpieces continue to be supported by said electromagnet;

(g) moving said electromagnet to a position above a workpiece collection area; and

(h) reducing electromagnet force in said electromagnet sufficiently to enable said workpieces to fall free from said electromagnet to said workpiece collection area.