US3149518A

US3149518A - Apparatus for cutting magnetic cores

Info

Publication number: US3149518A
Application number: US757711A
Authority: US
Inventors: Theodore L Winestock
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1958-08-28
Filing date: 1958-08-28
Publication date: 1964-09-22
Anticipated expiration: 1981-09-22

Description

p 1964 T. L. WINESTOCK APPARATUS FOR CUTTING MAGNETIC CORES 4 Sheets-Sheet 1 Filed Aug. 28, 1958 P 1954 T. L. WINESTOCK 3,149,518

APPARATUS FOR CUTTING MAGNETIC CORES Filed Aug. 28, 1958 4 Sheets-Sheet 2 p 1954 T 1.. WINESTOCK 3,149,518

APPARATUS FOR CUTTING MAGNETIC CORES Filed Aug. 28, 1958 4 Sheets-Sheet 3 F-\ "1 l l .J

P 1964 'T. 1.. WINESTOCK APPARATUS FOR CUTTING MAGNETIC CORES 4 Sheets-Sheet 4 Filed Aug. 28, 1958 6 ,3 [1/3 H aria e3.

United States Patent 3,149,518 APPARATUS F612 CUTTEIG MAGNETTC CORES Theodore L. Winestocir, Lenox, Mass, asriguor to General Electric Company, a corporation of New York Filed Aug. 28, 1958, Ser. No. 757,711 5 61451115. (Cl. S3176) This invention relates to an improved apparatus for cutting magnetic cores for stationary electrical induction apparatus such as transformers, and also to an improved cutting tool.

It is an object of this invention to reduce the cost of manufacturing magnetic cores.

It is a further object of this invention. to provide an improved magnetic core cutting apparatus which will result in finished magietic cores which have lower core losses and exciting currents than obtainable wim prior art core cutting apparatus.

It is a further object of the invention to provide an improved magnetic core cutting mechanism which will be smaller, more compact, safer and lower n cost than prior art core cutting mechanisms.

It is an additional object of the invention to provide an improved core cutting tool which will not deform the core material or introduce flaws therein such as edge burrs.

It is an additional object of the invention to provide an improved cutting tool whose cutting edges will not get out of alignment or become loose with respect to each other due to wear or stresses imposed thereon but which will be self-correcting.

it is an additional object of the invention to provide an improved core cutting tool which can be used to cut a wound magnetic core from the inside to its outside so that the cut segments can be laced with a preformed electrical coil assembly without Waiting for the Whole core to be cut. The concept of lacing the core by cutting it into segments from the inside to the outside is the invention of Beardsley et al. and is claimed in their patent application Serial No. 724,679, new Patent No. 3,122,821, which was filed on March 28, 195 8, and which is assigned to the same assignee as the instant application.

The invention is useful in cutting cores of the curved or bent strip type and plate cores. In plate cores the laminations are flat and are stacked one upon the other. In curved or bent cores the laminations are radially nested and they are curved or bent. Both types of cores have Window openings and their laminadons are arranged around the window openings into closed or substantially closed magnetic flux circuits.

The invention probably provides more advantages over prior art core cutting mechanisms for manufacturing curved cores than plate cores. Therefore, the preferred form of the invention will be illustrated with reference to the manufacture of curved or bent type magnetic cores.

There are several ways of manufacturing curved or bent type cores. The invention may provide more advantages in some methods than in other methods. However, the invention is not necessarily limited to any single method.

One Well known method of manufacturing curved cores comprises spirally winding a strip of magnetic core material intoa plurality of radially superposed turns about a generally rectangular shaped mandrel. After the desired 3,149,518 Patented Sept. 22, 1964 build is obtained in the core its turns are clamped, banded, braced or otherwise held closed and the core along with itsmandrel is placed in an annealing furnace. The core is annealed to give its generally rectangular shaped turns a permanent set and also to relieve or remove the deformations and strains which were introduced in the core strip during the process of winding it on the mandrel. The core does not necessarily have to be annealed on its mandrel but the mandrel can be removed prior to putting the core in the furnace and the core turns can be otherwise braced in their generally rectangular shaped Wound form. However, annealing the core While it is on its mmdrel is widely used inasmuch as this is a convenient way to retain the rectangular form of the core turns. Additionally, the mandrel does not have to be rectangular shaped but it can have a circular or other form. After the required number of turns are wound then the mandrel can be removed and a rectangular shape can be imparted to the Wound turns by forming dies before putting the core in the annealing furnace. However, if rectangular shaped turns are required in the final core then a rectangular shaped mandrel is preferred due to. ease of manufacture.

After the core is annealed the clamps and mandrel are removed. When the core comes out of theannealing furnace theoretically it is in its ideal condition inasmuch as all the deformations and. strains in the core strip have been relieved or removed by the annealing process. Therefore, at this point the problem is to link the core with an electrical coil without changing its ideal strain and deformation free condition. The electrical coil can be a preformed electrical winding cylinder having a window opening and it is possible to link the core with the coil without cutting the core. This can be done by several methods, one. of which can comprise lacing the continuous core strip through the electrical coil and expanding the core into a plurality of larger turns and then collapsing the expanded turns into compact position with respect to the electrical coil. It is not necessary to unwind or expand the annealed core. For instance, it is possible to keep the core in its. annealed closed position and link the core and coils by winding the coils on the core. However, these methods of linking the core and coils have several disadvantages. Therefore, the practice of cutting the core into segments in order to link it with the electrical coil is widely used.

In one prior art method of cutting the core into segments the annealed core is unwound from its outside and cut into a plurality of segments which can have any desired length. For instance, the segments have lengths of /2, 1, 1 /2 or 2 turns of the core. Typically the core is placed on a turntable and as the core is rotated the outer end of the core strip is fed into a core strip shear cutting tool. If the core is fairly large and it is being cut into segments of say about two. turns of the core the free end of the strip or the cut segment will be quite long and unwieldy and the cutting operation will require a considerable amount of manufacturing space. If the free end of the core strip or the segment is not properly supported it may sag, buckle or otherwise be deformed beyond its elastic limit which will result in increased core loss and exciting current in the finished core. It will be app-reoiated that in the larger size cores the cut segments might run to as much as 30' or so feet which is. too long for a single operator to support throughout its whole length. In this prior art method of cutting the core into segments from the outside to the inside the core has to be completely out before the segments can be laced with the electrical coil. This is because obviously the inner turns of the core have to be laced first with the electrical coil. This slows down the manufacturing process and increases the amount of manufacturing space required or introduces other disadvantages. For instance, the amount of additional manufacturing space required could be kept to a minimum by radially nesting the cut segments into one or several packets. However, when it comes to lacing the segments with the electrical coil this is not accom plished conveniently inasmuch as the nested segments tend to stick together and typically the assembler works with heavy gloves which make it difficult for him to separate the segments.

Besides the foregoing disadvantages, the prior art core cutting mechanisms do not successfully maintain the theoretically ideal strain and deformation free condition of the annealed core strip. The prior art core cutting mechanisms typically comprise large, heavy and expensive devices. They have a generally rectangular shaped frame. If the opening between the two blades is rather small when they are in their fully opened position the curved corners of the magnetic strip might have to be straightened out and deformed beyond their elastic limit in order to quickly and conveniently draw the strip through the cutting mechanism. This increases the core loss and exciting current in the core. Therefore, large openings between the separated cutting blades are typically used. However, with a larger opening the movable blade has to be moved correspondingly faster in order not to prolong the time required to make a single cut. Typically a high speed closure of the blades in order to make a fast cut is obtained by kicking the movable blade into closed position with a quick acting and short stroked piston. However, this means that considerable inertia is im parted to the movable blade which correspondingly increases the amount of reinforcing required in the frame. This in turn increases the size and cost of the cutting mechanism. However, even with proper reinforcement and support for the blades it is not unusual for the blades to become misaligned after a relatively short period of use. The misalignment between the blades might comprise too much spacing between the planes of the blades. With too much spacing between the planes of the blades the cut edges of the core strip have burrs formed thereon. That is, instead of having a clean shear cut the strip is torn apart by the cutting mechanism along edges which are jagged. Burrs on the cut edges of the core strip resalt in increased core losses and exciting currents in the finished core inasmuch as they introduce short circuit loops in the core. If the shear blades are widely misaligned besides introducing burrs at the cut edges the strip is also severely bent or deformed adjacent to the cut edges. A further disadvantage of a large opening between the blades when they are in their fully opened position is that the strip might be positioned about halfway between the two blades which means that when they are closed the strip may be bent. Additionally, a large separation between the blades is a safety hazard.

The improved core cutting apparatus of my invention overcomes the above discussed disadvantages of the prior art. In the preferred form of the invention the improved cutting tool comprises two sleeves which are telescoped with respect to each other. Each of the telescoped sleeves are slotted on opposite sides thereof. The slots on one side of the telescoped sleeves are aligned with the slots on the other side of the telescoped sleeves. The aligned slots on one side of the telescoped sleeves provide the cutting edges and the aligned slots on the other side of the telescoped sleeves provide clearance between the strip and the sleeves so that the strip is not deformed or otherwise damaged when the two sleeves are rotated with respect to each other to cut the strip. The telescoped sleeves have an extremely close fit with respect to each other. That is to say, the clearance between the two sleeves is of the order of zero thickness. This cldse fit between the two sleeves provides burr free cut edges in the magnetic core strip. This close fit is self-maintaining or restoring inasmuch as the two sleeves are continuously urged toward each other. This can be accomplished by having the inner slotted sleeve under compression and the outer slotted sleeve under tension.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the acompanying drawings in which:

FIG. 1 is a perspective view of one form of apparatus for cutting a magnetic core;

FIG. 2 is a central sectional view of the core and upper platform 3 of FIG. 1;

FIG. 3 is a longitudinal sectional view through the shear head 6 of FIG. 1;

FIG. 4 is a transverse sectional view through the shear head in its open position; and

FIG. 5 is a transverse sectional view through the shear head in its closed position.

FIGS. 6 to 9 illustrate progressive stages of the inner sleeve of the shear head during its manufacture.

In FIG. 6 the inner sleeve is illustrated in its initial cylindrical slotted condition. In FIG. 7 the slotted end of the sleeve has been compressed. In FIG. 8 the compressed sleeve has been ground down to a slightly smaller cylindrical shape, and in FIG. 9 the compressive forces on the ground sleeve have been relieved.

Like reference numerals will be used throughout the various figures to indicate like parts.

Referring now particularly to FIG. 1 of the drawings, illustrated therein is a table 1 which has

platforms

2 and 3 movably mounted thereon. The lower platform 2 is movable in opposite directions along the arrow 4 and the upper platform 3 is movable in opposite directions along the arrow 5. The upper platform 3 moves back and forth along arrow 5 so that the cuts in the core will be offset or staggered with respect to each other. The lower plat form 2 moves towards the right along arrow 4 so as to move the core into position with respect to a shear head 6. The shear head 6 is moved up and down by an actuating cylinder 7 or other equivalent means. The shear head 6 comprises two telescoped slotted sleeves 8 and 9 which are rotatable with respect to each other. See FIGS. 3 to 5. The sleeves 8 and 9 are rotated withrespect to each other by a torque cylinder 10 of FIG. 1 or other equivalent means.

A magnetic core 11 is positioned on the upper platform 3. The core 11 represents one which has just come from the annealing furnace. It comprises a continuous strip of magnetic core material which has been wound into a plurality of radially superposed and generally rectangular shaped turns. The platform 3 has an opening 12 therein which is positioned within the window opening of the core 11. See also FIG. 2. A pair of'vacuum cups 13 or other equivalent means come up out of the opening 12 and grip and separate the first turn 14 of the core from the lefthand side of the core. After this a locating in 15 comes up out of the opening 12 to retain the turn 14 in its separated position and then the vacuum cups 13 will retract or release. Then another pair of vacuum cups 16 come up out of opening 12 to grip and separate the second turn 17 from the left-hand side of the core. Then another locating pin 18 comes up out of opening 12 to retain the turn 17 separated from the left-hand side of the core after which the vacuum cups 16 will retract or release. As illustrated in FIG. 1, the remaining sides of the separated turns stay in contact with their respective core sides. Instead of having the grippers move to the left and then to the right to separate the turns the grippers can be moved just up and. down and separation of the turns can be accomplished by having the core itself move toward and away from the grippers in the direction of arrow 4. If the grippers are to move to the left and right to separate the turns then the inner pair of grippers should be offset from the outer pair of grippers as illustrated in FIG. 1 so that the former will not be in the way of the latter.

The disclosed apparatus is for cutting the core into segments of two turn lengths. However, the invention can be use to cut segments having a length of /z or a single turn of the core or any other desired length. A segment of the core which is not shown has been cut and removed from the core at point 19 which is located below the locating pin 18. Since it is desirable to offset or stagger adjacent butt joints in a magnetic core with respect to each other the next cut will be made in the turn 17 above the locating pin 18. In order to do this the platform 3 will move or index over to the right slightly before the shear head 6 comes down to straddle the turn 17 and its locating pin 18 in order to out turn 17. The inner sleeve 8 is large enough on its inside to receive the locating pin 18 therein. Thus the platform 3 successively indexes to the left and then to the right so as to offset successive cuts. As additional turns are cut the platform 2 moves to the right to position the lefthand side of the core closer to the shear head. It will be appreciated that if the turns are separated by too great a distance from the left-hand side of the core deformations may be introduced therein. Therefore, the amount of separation which is required between the separated turns and the left side of the core to properly position the separated turns with respect to the shear head is kept at a minimum by moving the core 11 to the right as the inner turns are cut.

Referring now to FIGS. 3 to 5, the outer sleeve 9 is made stationary by connecting it to the casing of the torque cylinder 1% and the inner sleeve 3 is made rotatable by connecting it to the rotary piston rod of the torque cylinder. The internal mechanism of the torque cylinder has not been illustrated inasmuch as this is an article of commerce which can be readily obtained on the market. For example, the torque cylinder used could be a Carter rotary actuator, No. 4RAS, manufactured by Carter Controls, Inc. of Lansing, Illinois. Each of sleeves 8 and 9 are cylindrical shaped and are slotted on opposite sides thereof as illustrated in FIG. 4. The slots on one side of the cutting tool are aligned with those on the opposite side so that the cutting tool can straddle the core strip 29 in order to cut the same. The opening 21 is the area in which the strip is cut and the opening 22 is provided to give clearance between the strip 20 and the two sleeves so that when they are rotated with respect to each other they do not damage the strip 29. The corner edge 23 of member 9 and the corner edge 24 of the member 8 are the cutting edges. These edges may be specially hardened to prolong their life. The surface 25 of member 8 which intersects with the outer cylindrical surface of member 8 to define the cutting edge 24 is beveled slightly. FIG. 5 shows the cutting tool in closed position. en the member 8 is rotated to closed position the opening 22 is still open so that the adjacent portion of strip 26 is not damaged by relative rotation between the two members 8 and 9. This is accomplished by making the slot in member 8 at opening 22 large enough so that opening 22 is not closed when member 8 is rotated counterclockwise.

In the invention the members 8 and 9 are telescoped with respect to each other with an extremely close fit. That is to say, because of the force fit or high pressure surface contact between members 8 and 9 there is almost zero clearance between the cylindrical outer surface of mem er 8 and the cylindrical inner surface of member 9. The negligible clearance which is present is due to the fact that members 8 and 9 are separate members, but it is sufiicient to permit an oil film therebetween if so desired which has a thickness which can be measured in terms of molecular thicknesses. However, this oil film is not absolutely necessary since the sleeves move so little that there is very little wear or heat generated. Also, this close fit between the two members 8 and 9 is always maintained. Therefore, the cutting edges 23 and 24 will always out the core strip cleanly without damaging it in any respect and without forming any edge burrs there on. That is to say, when the cutting edges cross each other there will be practically no clearance therebetween but these edges will move across each other in a single plane.

One method of obtaining the above described zero clearance between the members. 8 and 9 which is self-maintaining will be described now with respect to FIGS. 6 to 9 which illustrates one way of manufacturing the inner member 8. In FIG. 6 is shown some cylindrical stock 26 which has been slotted on its opposite sides as indicated by the slot 27. FIG. 7 illustrates the configuration of the slotted cylindrical stock after its slotted end has been compressed slightly. It will be appreciated that the deformation in stock 26 after its compression probably would not be visible to the naked human eye and that the distortion has been exaggerated in the drawings for purposes of illustration. After the member 26 is compressed its slotted end will have a slightly conical outer surface 28. This surface is ground down into a cylindrical surface 29 while the member is still under compression as illustrated in FIG. 8. After the member 26 is ground down into the cylindrical surface 29 if its compressive forces are then relieved it will have an outer surface 36 which is slightly conical in the opposite direction relative to conical surface 28 as illustrated in FIG. 9. The ground member of FIG. 9 is force fitted into the outer sleeve 9 which has an inner cylindrical surface. This, of course, will again compress the slotted end of the ground member of FIG. 9 whereby the inner sleeve will be under compression and the outer sleeve will be under tension. These forces continuously urge the outer cylindrical surface of the member 8 and the inner cylindrical surface of the member 9 towards each other which means that the members 8 and 9 will inherently take up any slack therebetween whereby the cutting corner edges 23 and 24 will always meet to provide a burr free shear cut. A similar effect could be obtained by giving the inner and outer sleeves slight tapers and then continuously forcing them longitudinally towards each other after they are telescoped to take up any slack therebetween. In such an arrangement preloading before grinding of the sleeves could be omitted.

In the operation of the apparatus of FIG. 1 after an inner segment is cut it is lifted away from the core and can then be immediately laced with a preformed electrical winding cylinder. While the operator is lacing the cut segment the machine can be energized to cut the next segment. The details of the means for operating and controlling the operational sequence of parts such as the

platforms

2 and 3,v the cylinders 7' and 1t and the turn separators 13, 1'5, 16 and 18' are housed in a control casing 31 and are not shown since such details are not part of the invention and are matters of machine design which can take different forms as will be obvious to those skilled in the art. Furthermore, it will be obvious that these functions could be performed manually. For instance, the core turns could be separated manually and the shear head 6 could be raised and lowered and actuated manually, although a fully automatic machine is preferred. Also, the shear head 6 per se lends itself to use as a portable hand tool by virtue of its light weight and compactness. In such use of the shear head relative rotation between the sleeves S and 9 could be obtained through the medium of two hand operated lever arms connected to the sleeves 8 and 9.

Additionally, the self-maintaining feature of the telescoped sleeves is not intended to be restricted to the particular forms disclosed since other ways of accomplishing this are possible. For instance, a clamping band could be placed about the lower end of the shear head to urge the surfaces and cutting edges of both sleeves towards each other to continuously take up any slack therebetween. In such event the special manufacturing steps for the inner sleeve outlined in FIGS. 6 to 8 could be omitted. The clamping band would close. the lower slotted end of the shear head which means the strip would have to be pulled through the cutter. Nevertheless, the cutter would still provide many advantages over prior art cutters. That is, the invention is not restricted to a shear head which is open at its slotted end. One of the sleeves could be integrally closed at its slotted end and the other sleeve could be continuously urged toward the closed sleeve by machining techniques similar to those disclosed or other means such as springs. However, a shear head which is open at its slotted end so that it can move into straddling relationship with the strip to cut the same and then raise out of the Way is preferred since then the strip does not have to be threaded through the shear head.

While there have been shown and described particular embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and therefore, it is intended by the appended claims to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus for cutting awound magnetic core which has a window opening and a plurality of radially nested spiral turns of magnetic core material strip, said apparatus comprising means for supporting such a core, means for separating one side of the inner turns of such a core from their corresponding core side while the remaining sides of said separated turns still contact their corresponding core sides, a cutting tool for cutting said separated turns by straddling the same, said cutting tool comprising a pair of telescoped sleeves which have longitudinal openings in opposite sides thereof at one of their ends, means for moving said cutting tool into and out of said separated turns, and means for shifting said core and cutting tool with respect to each other to offset successive cuts in said core from each other.

'2. An apparatus for cutting a wound magnetic core which has a window opening and a plurality of radially nested spiral turns of magnetic core material strip, said apparatus comprising means for supporting such a core in a generally horizontal'position, means for separating one side of the inner turns of such a core from their corresponding core side while the remaining sides of said separated turns still contact their corresponding core sides, a cutting tool above said separated turns for cutting the same by straddling them, said cutting tool comprising a pair of telescoped sleeves which have longitudinal openings in opposite sides thereof at one of their ends, a pair of cutting edges on said sleeves adjacent one of said openings, said cutting edges moving across each other when said sleeves are rotated with respect to 'each other to close said one opening, means for 'moving said cutting tool into and out of said separated turns, means for rotating said sleeves with respect to each other, means for shifting said core and cutting tool with respect to each other to offset successive cuts' in said core from each other, and means for moving said core one side and cutting tool toward each other.

V 3. A cutting tool comprising two generally cylindrical sleeves, said sleeves being telescoped wiht respect to each other, each of said sleeves having a longitudinal slot formed therein at one of their corresponding ends, said slots being aligned with each other into an opening through said sleeves, the outer surface of the inner one of said sleeves and the inner surface of the outer one of said sleeves being in high pressure surface engagement, two cutting edges formed on said sleeves adjacent said opening, said cutting edges moving across each other when said sleeves are rotated with respect to each other to close said opening, one of said cutting edges being defined by said outer surface and one side of the slot in said inner sleeve and the other cutting edge being defined by said inner surface and one side of the slot in said outer sleeve, means for rotating said sleeves with respect to each other to close said opening, and said sleeves being force fitted with respect to each other to place said inner sleeve under compression and said outer sleeve under tension whereby said surfaces are continuously urged toward each other to maintain said high pressure surface engagement.

4. A cutting tool comprising two sleeves which are telescoped with respect to each other, each of said sleeves being slottedon two opposite sides thereof at one of their corresponding ends, all of said slots being aligned with respect to each other into two aligned openings through said sleeves, the outer surface of the inner one of said sleeves and the inner surface of the outer one of said sleeves being in high pressure surface engagement, two cutting edges formed on said sleeves adjacent one of said openings, said cutting edges moving across each other when said sleeves are moved with respect to each other to close said one opening, one of said cutting edges being defined by said outer surface and one side of the slot in said inner sleeve which is at said one opening and the other cutting edge being defined by said inner surface and one side of the slot in said outer sleeve which is at said one opening, means for moving said sleeves with respect to each other to close said one opening, the slots in the other opening being arranged with respect to each other and having dimensions whereby said other opening will still be open when said one opening is closed, and said sleeves being force fitted with respect to each other to place said inner sleeve under compression and said outer sleeve under tension whereby said surfaces are continuously urged toward each other to continuously maintain said high pressure surface engagement.

5. A cutting tool comprising two generally cylindrical sleeves, said sleeves being telescoped with respect to each other, each of said sleeves being longitudinally slotted on two opposite sides thereof at one of their corresponding ends, all of said slots being aligned with each other into two aligned openings through said sleeves, the outer surface of the inner one of said sleeves and the inner surface of the outer one of said sleeves being in high pressure surface engagement and being separated from each other only by a molecular film of lubricating material, two cutting edges formed on said sleeves adjacent one of said openings, said cutting edges moving across each other when said sleeves are rotated with respect to each other to close said one opening, one of said cutting edges being defined by said outer surface and one side of the slot in said inner sleeve which is at said one opening and the other cutting edge being defined by' said inner surface and one side of the slot in said outer sleeve which is at said one opening, means for rotating said sleeves with respect to' each other to close said one opening, the slots at the other opening being arranged with respect to each other and having dimensions whereby said other opening will still be open when said one opening is closed, and said sleeves being force fitted with respect to each other to place said inner sleeve under compression and said outer sleeve under tension whereby said surfaces are continuously urged toward'each other to continuously maintain said high pressure surface engagement.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Foote Dec. 12, 1905 Becker Apr. 13, 1920 Eaton Mar. 15, 1927 Barry ]u1y 30, 1929 Woods July 8, 1930 Flink May 31, 1932 Muros Mar. 18, 1941 1.0 Vienneau Dec. 22, 1942 Rose June 3, 1947 Brunner Oct. 7, 1947 Havely Feb. 5, 1952 Spangler Apr. 7, 1959 Mann Aug. 29, 1961 FOREIGN PATENTS Great Britain Nov. 12, 1931

Claims

1. AN APPARATUS FOR CUTTING A WOUND MAGNETIC CORE WHICH HAS A WINDOW OPENING AND A PLURALITY OF RADIALLY NESTED SPIRAL TURNS OF MAGNETIC CORE MATERIAL STRIP, SAID APPARATUS COMPRISING MEANS FOR SUPPORTING SUCH A CORE, MEANS FOR SEPARATING ONE SIDE OF THE INNER TURNS OF SUCH A CORE FROM THEIR CORRESPONDING CORE SIDE WHILE THE REMAINING SIDES OF SAID SEPARATED TURNS STILL CONTACT THEIR CORRESPONDING CORE SIDES, A CUTTING TOOL FOR CUTTING SAID SEPARATED TURNS BY STRADDLING THE SAME, SAID CUTTING TOOL COMPRISING A PAIR OF TELESCOPED SLEEVES WHICH HAVE LONGITUDINAL OPENINGS IN OPPOSITE SIDES THEREOF AT ONE OF THEIR ENDS, MEANS FOR MOVING SAID CUTTING TOOL INTO SAID OUT OF SAID SEPARATED TURNS, AND MEANS FOR SHIFTING SAID CORE AND CUTTING TOOL WITH RESPECT TO EACH OTHER TO OFFSET SUCCESSIVE CUTS IN SAID CORE FROM EACH OTHER.