US3840984A

US3840984A - Method of making a magnetic recording head

Info

Publication number: US3840984A
Application number: US00406871A
Authority: US
Inventors: E Lipps
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-10-16
Filing date: 1973-10-16
Publication date: 1974-10-15
Anticipated expiration: 1991-10-15

Abstract

A method for making the core of a magnetic recording or playback head that comprises bending a flat strip of core material into an intermediate product, a W-form, i.e., having at its outer or remote ends two vertical legs, which join, at the bottom, two bottom segments which extend straight toward one another, and which eventually form the sides of the core. These bottom segments join two inclined segments which rise angularly, like an inverted V, to an apex. This apex and the tops of the two end legs are then simultaneously lap ground in a common plane, until the ''''apex'''' is almost ground away, leaving an extremely thin amount of connecting material between the inclined segments. This bit of remaining connecting material forms an easily bendable hinge between the two inclined segments. The two halves of this intermediate product are then bent through 180* on this hinge until the two ground ends of the first mentioned legs abut to form a so-called back gap. The two core ends on opposite sides of the hinge are then closed and grip between them a thin, nonmagnetic spacer or shim, of gap thickness. The core in that form is encapsulated in a metal case and potted therein with polymerized resins. The hinge, when in this position, is located exterior to the gap, and forms a projection through an aperture in the case. By grinding operation, the hinge, a portion of the core just under it and of the top wall of the case, are ground away, leaving the gap exposed. In a modification, the core is composed of laminations, stamped initially to the shape of the intermediate product. These are assembled, and the assembly then treated as in the first procedure. In still another modification, the core and a holder are first assembled, and it is the holder rather than initial part of core that forms the hinge.

Description

United States Patent 1 Lipps 1 1 Oct. 15, 1974 METHOD OF MAKING A MAGNETIC RECORDING HEAD [76] Inventor: Edwin Alvin Lipps, 205

Chautauqua, Pacific Palisades, Calif. 90272 22 Filed: 0ct.16,1l973 21 App1.No.:406,8711

[52] US. Cl. 29/603, 29/418, 179/1002 C [51] int. Cl. Gllb 5/42 [58] Field of Search 29/603, 418; 179/1002 C;

340/l74.l F; 346/74 MC Primary ExaminerCharles W. Lanham Assistant ExaminerCarl E. Hall Attorney, Agent, or Firm-Forrest .1. Lilly [5 7 ABSTRACT A method for making the core of a magnetic recording or playback head that comprises bending a flat strip of core material into an intermediate product, a W-form, i.e., having at its outer or remote ends two vertical legs, which join, at the bottom, two bottom segments which extend straight toward one another, and which eventually form the sides of the core. These bottom segments join two inclined segments which rise angularly, like an inverted V, to an apex. This apex and the tops of the two end legs are then simultaneously lap ground in a common plane, until the apex is almost ground away, leaving an extremely thin amount of connecting material between the inclined segments. This bit of remaining connecting material forms an easily bendable hinge between the two inclined segments. The two halves of this intermediate product are then bent through 180 on this hinge until the two ground ends of the first mentioned legs abut to form a so-called back gap. The two core ends on opposite sides of the hinge are then closed and grip between them a thin, non-magnetic spacer or shim, of gap thickness. The core in that form is encapsulated in a metal case and potted therein with polymerized resins. The hinge, when in this position, is located exterior to the gap, and forms a projection through an aperture in the case. By grinding operation, the hinge, a portion of the core just under it and of the top wall of the case, are ground away, leaving the gap exposed. In a modification, the core is composed of laminations, stamped initially to the shape of the intermediate product. These are assembled, and the assembly then treated as in the first procedure.

In still another modification, the core and a holder are first assembled, and it is the holder rather than initial part of core that forms the hinge.

16 Claims, 18 Drawing Figures PAIENIED B 1 51914 8. 840. 984

sum 2 or 2 E m'r'" METHOD OF MAKING A MAGNETIC RECORDING HEAD FIELD OF THE INVENTION This invention relates generally to magnetic tape heads, i.e., electromagnetic heads for recording information on magnetic media, such as a tape, or reproducing or erasing it therefrom.

BACKGROUND OF THE INVENTION The critical magnetic iron core parts of such heads are manufactured in two C-like core halves. The ends of these C-cores are lapped to virtually perfect flatness. They are then assembled to the coil with a spacer to size the gap length, and are then encapsulated with epoxy resins to mechanically fix the assembly within the case. This operation is performed visually and requires great operational skill. As with all such highly skilled operations, there are numerous rejects due to operator error. The only alternative heretofore has been the use of highly precise and very expensive tools and jigs.

The general object of the invention is to provide a relatively simply executed and inexpensive process for making cores for these electromagnetic heads, and which results in a superior, dimensionally precise core gap, yielding improved translation efficiency, and uniform frequency response along the gap.

BRIEF DESCRIPTION OF THE INVENTION The invention involves, in a present preferred embodiment, the forming of the core from a strip of fiat magnetic core material by bending it initially into a W- form. The intermediate apex and the two outside legs of the W-form are then lap ground, preferably in a common plane, until the core thickness at the apex is, typically, 1 to mils. Thus, a thin ductile hinge is formed. The two halves of the W-forrn are then bent towards one another on this hinge through an angle of 180, until the aforementioned outside legs abut one another, at a so-called back gap, a coil being first placed around one or both of the core halves. On opposite sides of the hinge, the core ends approach one another during this bending operation. The gap filler, in the form of a very thin non-magnetic shim, plate or chip, e.g., is inserted and finally pinched between the closing core ends. Theback gap is squeezed together by some means such, e.g., as a spring clip. The assembly is now inserted into a case for encapsulation with polymerizing resins. Afterthe resins are cured, the head face is ground to size. The external hinge, which has been contiguous to the core throughout the operation, is now ground away, exposing the gap in its normal configuration.

The hinge, until ground away, preserves the gap dimensionally and geometrically, a result never, to my knowledge, heretofore accomplished.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a side elevational view of a core bent into its W-form;

FIG. 2 is a side elevational view of the core of FIG. 1, after lap grinding;

FIG. 3 is a view of the core form of FIG. 2, but showing the two halves thereof bent part way toward final form;

FIG. 3a is an enlarged detail of a portion of FIG. 3;

FIG. 4 is an exploded perspective view of the core form approximately in the position of FIG. 3, but showing a coil placed on one leg thereof, and showing a spring clip used for holding the back gap in the core closed, and also the non-magnetic gap spacer in position to be inserted into the still wide-open gap space;

FIG. 4a is a detail showing the core bent nearly closed, and a gap filler in the pocket;

FIG. 5 is a longitudinal medial sectional view of the magnetic head, showing the core and coil bent to their final position, assembled with terminals, and inserted inside the case;

FIG. 6 is an enlarged view of the upper portion FIG. 7, showing the epoxy resin filler material within the case, and showing also the upper portion of the case and the gap hinge ground away to final configuration;

FIG. 7 is a side elevational view of the W-form core in a present successfully used manufacturing design;

FIG. 8 shows the upper portion of the head of FIG. 7, with the core bent to its final position, and the core and case ground to their final contour;

FIG. 9 is an enlarged detail of a modification of FIG.

FIG. 10 shows an enlarged detail of a modification of FIG. 2;

FIG. 11 shows the hinge and gap portion of the core of FIG. 10 bent to a position in which the dimple is closed;

FIG. 11a shows the hinge and gap portions of the core of FIG. 10 in a finally closed position;

FIGS. 12 and 13 show two successive positions of a laminated core; and

FIGS. 14 and 15 show two successive positions of another modification.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT A strip 10 of magnetic iron alloy core material, Permalloy, i.e., Mu metal, of standard cross sectional dimensions, is bent, forexample, in a bending press, or a punch press containing the properly shaped dies, into an intermediate product 11, as shown in FIG. 1, which is sometimes hereinafter referred to (because it resembles the letter W) as a W-form. This intermediate product is characterized by two angularly related segments 12, converging asan inverted letter V, to an apex 13. The lower spaced-apart ends of the segments 12 merge with horizontal, oppositely extending side segments 12', which merge, in turn, at their remote ends, with vertical legs 14. The legs 14 preferably initially rise to, or preferably slightly above, a horizontal reference plane H that contacts the apex 13. They may initially rise above the plane H, or may not quite reach it; but they should, in the illustrative case, rise at least slightly above the level to which the apex will presently be ground down, as to be explained hereinafter.

A second plane of reference C is vertical and bisects the geometric center. of thew-form, and thus bisects the apex 13. The directions horizontal and vertical as used herein have no meaning, of course, beyond the establishment of a convenient frame of reference.

The apex 13 maybe somewhat rounded at the top, as at 18, with an interior angle underneath, which will generally be relieved or have a rounded fillet 19. The exteriorly rounded top surface 18 will be seen to point or face generally in the direction of convergence of the segments 16. Thus, in other language, the apex points in parallelism with, i.e., in the same direction as, the two legs 14.

The core is then annealed to relieve any work hardening or magnetic damage caused by the stamping and- /or forming operation.

The upper ends of the legs 14, and the apex 13, are then simultaneously lap ground in a common plane using generally and preferably a single lap, down to a lap limit line 21 which is located a distance of the order of l mil to 5 mils above the underneath formation or fillet 19 between the convergent segments 12 leading to the apex. See FIG. 2. For some special case, this grinding may not be done simultaneously, at both leg ends and the apex, but in general is an important feature of advantage of the invention. Thus the apex 13 is modified to have precisely flat lap ground upper surface 15, bisected by the vertical reference plane C. Thus a bit of remaining core metal, a few mils in thickness, is left to join the two halves 13a and 13b of the ground surface 15 at the center plane C, to function, by bending, as the aforementioned hinge, hereinafter designated by reference numeral 20. The core material, being annealed, has low elasticity and is very ductile, so that it bends easily as a hinge, and has low spring-back. The two halves of the W-form may be bent on this hinge (see FIG. 3) a number of times before the hinge will fatigue and break. Ordinarily, however, only one bend is required. I

While the core magnet coil may be wound or placed on one of the core legs 14 with the core still in the W- form of FIG. 2, or a coil might be wound on each of the sides 13 with the core still in the U-form, the coil is generally assembled with the core bent about to the form of FIG. 3. Thus, a wire coil 22, wound on a plastic bobbin 24, with a rectangular core hole 25 through it, is

slipped onto one leg 14 thereof (see FIG. 4). A thin,

flat non-magnetic filler or shim 26 is provided (FIGS. 4 and 4a) to go into the pocket or angle between the two flat core faces above the hinge. The two halves of the initial W-form are then further bent on the hinge 20 until the legs 14 abut one another, thus forming the so called back gap 27. A metal U-shaped spring clip 28 is slipped on over the sides 12 of the core, and holds the back gap closed.

The plastic bobbin flange 30 at each end of the coil mounts a conductive terminal strip 31, and the two upper ends of these terminal strips will be understood to be connected to opposite ends of the coil, while their lower ends project below the open bottom metal case 33 into which the core assembly as described is inserted. j

Attention is directed to the fact that the core metal at the hinge 20 has, byreason of the bending operation by which the W-form is changed to the shape shown in FIG. 5, been subjected to tension, in the region at and just above the surface of the fillet 19, and thereby stretched and forced outwardly from between the core faces 13a and 13b to produce exterior ridge or loop 29. This automatic rise or extrusion of the hinge 20 into this exterior ridge or loop facilitates its removal in a subsequent step as the gap is completed. Further advantages of this feature will be stressed hereinafter.

A metal case 33 receives the core and coil assembly with a close fit, and the hinge ridge 29 protrudes upwardly through a rectangular aperture 34 in a rounded top wall 35 of this case. With the core. and coil assembly in position in the case (FIG. 5), it is permanently encapsulated therein with polymerizing resins, according to standard practice. This potting material is indicated in FIG. 6 at 36.

The head face of the case and the protruding hinge ridge are next ground to final configuration along the slightly curved grind line 37 (FIGS. 5 and 6). In this operation, the top wall 35 of the case is ground down somewhat, and the entirety of the exterior hinge ridge 29 is removed. The outline of the parts before this grinding operation is shown in phantom lines at 39 in FIG. 6. The grinding operation removes the hinge ridge 20 entirely and may remove some of the core and nonmagnetic filler 26 at the gap. It also thins the top wall of the case, as indicated. The top of the head is thus defined by a smooth, slightly convex curve, with the tops of the two core segments exposed and spaced by the gap filler shim 26. v i

It will now be evident that the dimensions of the gap are positively fixed against geometrical distortions so long as the hinge 20 remains intact, so that nonparallelism, i.e., a V-gap, a step or offset gap, an open gap, and all other conceivable distortions, are prevented.

The hinge is not removed until after the core assembly has been fixed in the polymerizing resins in the case. The precise gap dimensions are thus permanently established. The removal of the hinge has also the incidental but important advantage that all material magnetically damaged by the bending operation is removed from the core. A further noteworthy feature of the invention 'is that, in general, all four gap surfaces are lapped simultaneously in a common plane, thus assuring uniformity and alignment of gap surfaces. For any special exceptions to this rule, the invention still preserves the hinge intact until the gap is fixed, thus assuring this dimensional control. FIG. 6 shows the gap region of the ultimate product. The connecting hinge 20 has been removed, and the final magnetic gap is filled by the non-magnetic filler shim 26.

In FIGS. 7 and 8 I have shown, to scale, a present successful manufacturing design of the head. Parts of the head in these figures corresponding to parts of FIGS. 1-6 are identified by the same reference numerals, and only the modifications will be described. FIG. 7 corresponds to FIG. 1 in that it shows the initial core strip just after forming to its initial W-form. The core in this instance is fabricated from a strip of 0.022 inch thick Mu metal. The apex 13 of the angular segments 12 is formed on curves of radii substantially increased over those at 18 and 19 in FIG. 1, as will be evident. The inclined segments 12 of the core form comprise in this case two lower portions 42 and 43, inclined at an inclined angle of about 54, and two

upper portions

44 and 45, respectively, which are offset towards one another, and steepened so that each is at about 7 to vertical. The top of the apex is thus in the shape of an arch 46 in this design, facilitating manufacture.

The second step is again the lap grinding operation, which in this case grinds the two legs 14 and the arch at the top of the modif ed apex down to the lap line 47, which is located a distance of the order of 1 mil from the high point of the underside of the arch 46. The

bend axis then falls above the center of what remains of the arch 46.

The upper portion of the lapped W-form is shown in FIG. 8, after bending in the manner of the earlier embodiment, with an upper portion of the case also shown. The figure shows the case and core after grinding, the ground-away parts being indicated in phantom lines.

FIG. 9 shows, to an enlarged scale, a modification of the embodiment of FIGS. 7 and 8. Here the underside arch of the apex is formed with an inverted V-notch 49 to locate accurately the point where the hinge action will occur. The distance from the bottom of the notch to the top side of the arch is of the order of a mil. This modification is useful when a wide arched apex is employed.

Returning now to the embodiment of FIGS. 1-6, a refinement of importance will be described. As the core halves are bent on their hinge to the form of FIGS. 3 and 4, and then especially as they close to the position of FIG. 5, there is, as earlier described, a tension in the hinge region between the two hinge connected segments 12 that cause a stretching and extrusion of the hinge from between the two closing halves 13a and 13b of the ground upper surface 15. It is desirable that this hinge material be extruded as much as possible from between these closing core faces 13a and 13b. As the two core halves are bent on the hinge, the thin layer of core metal that constitutes the hinge 20 turns or fulcrums about a neutral axis n located about one third of the way down from the upper to the lower surfaces of the hinge. As viewed in FIGS. 3 and3a, the hinge material above this axis is in compression, and the hinge material below the axis is under tension, and stretches. The stretched material bows outwardly, finally forming an extruded externally located hinge ridge or loop. However, the operation tends to compress a small amount of the hinge material above the axis, and leave it as a small bead of metal between the surfaces 13a and 1312.

I have found it possible to extrude the entirety of this hinge metal, so the faces 113a and 13b will close perfectly flush on one another. This can be done, with the core form still in the position of FIG. 2, by forming a wedge-shaped indentation or dimple 48 in and across the very center of the ground face 15 (see FIG. 10). This can be done with a dimple die. This dimple has a preferred depth of about'one third the thickness of the remaining core metal at the hinge. A corresponding slight bulge 46 may occur at the bottom of the hinge. The hinge action then takes place, at the beginning, about a new neutral axis spaced a little below the bottom of this dimple. When the core form is then turned further on the dimpled hinge, the faces 13a and 13b rotate or fulcrum on the new, slightly lowered axis, until the opposite faces of the wedge shaped dimple have closed on one another. (FIG. 11). The two halves of the core then fulcrum on the line d of the closed dimple (FIG. 11), with no part of the hinge region thereafter in compression. Instead, the hinge region is now all in tension, and stretches more and more as the bending action is continued. With all the hinge metal in tension, maximum stretching, and maximum extrusion are accomplished; and the entirety of the hinge is displaced from between the core end faces 13a and 13b. A perfect closure is thus obtained (FIG. 11a). All of the hinge material magnetically damaged by bending is thus out of the final gap, and will be ground away. A precise parallel faced gap is assured.

Description of a Laminated Core Embodiment FIGS. 12 and 13 show a modification using an assembly of stamped laminations 54 to make up the core body 55, which is in the stage of construction corresponding to that represented in FIG. 2 of the earlier embodiment.

The laminations are glued face to face to form a core body such as shown in FIG. 12, capable of being bent as an assembled body. The core form, made up of these assembled laminations, has an apex 56 at the top of convergent legs or segments 57, from which extend horizontal core sides 58 and erect end legs 59. The top surface of the apex and the top surfaces of the legs 59 are simultaneously lap ground to a precisely fiat, common level, as in the earlier embodiments. Coils 60 are wound on the core sides 58. As before, the grinding of the apex takes the core material down to a thickness of the order of a few mils at center. A hinge is thus provided as in the earlier embodiment, and the core is bent on this hinge to the form illustrated in FIG. 13, and the hinge later ground away.

Selected numbers of the laminations may be wrapped with coils in individual bundles, which may be used sin gly or in groups for single or multi-track heads. The core is thus assembled from laminations stamped initially into the shape of FIG. 12, rather than bent initially from core ribbon material.

Description of a Further Embodiment Reference is next directed to FIG. 14, showing another embodiment of the invention. This embodiment is shown as using a lamination type core, in general respects like that of FIGS. 12 and 13; but in this case, the apex of the W-form is comprised partly of core metal and partly of aformation on a plastic or metal core housing or holder, conveniently a die casting. The hinge on which the two core halves are bent is actually a part of the apex formation of the housing member.

The core housing member or holder 66 is shown as a generally rectangular hollow body or box, open on one side,'and composed of a material, metal or plastic, which is relatively ductile and bendable, without spring back, when reduced to a thickness of the order of a few mils, and will thus have the characteristics to serve as a hinge of the type described herein above.

The core, composed of laminations of the same general character as those of FIGS. 12 and 13, excepting for a discontinuity at the center, has angularly related, upwardly convergent segments 67, sides 68, and end legs 69, much as in FIG. 12. The convergent segments 67 are fabricated to comprise outer legs 67a of core metal, integral with the side legs 68, and inner legs 67b integral with and rising from the housing bottom, and lying contiguously with the outer legs 67a. Segments 67 are thus formed with a compound apex 74. The housing, the end legs 69, and the apex 74 are constructed to rise initially to the level indicated by the phantom lines, and are then simultaneously ground down to the level shown in full lines in FIG. 14. The inside surfaces of the angular convergent housing legs 67b merge into a concave curveor fillet 73. The lap grinding step grinds the apex, made up of the coplanar uppermost surfaces of the legs 67a and 67b, precisely flat. The ground upper ends of the legs 67a thus have a gap or discontinuity precisely filled by the ground upper ends of the legs 67b. The lap grinding is continued until the ground apex region at the top of the legs is reduced to a slight thickness, so as to give the hinge formed thereby the hinge bending quality desired, which may vary with dimensions as well as the material of the housing. The formations described provide the housing with a deep indentation rising from the bottom to concavity 73, thus adapting the housing to bend on the hinge. Coils 75 are wound in the side legs 68.

The housing and the two core metal parts are then bent through 180 on the hinge, turning about an axis just above the underside of the arch at the top of the two housing legs 67b. The core and housing are then in the position shown in FIG. 15, a gap filler having been inserted between the two closing core ends, as before. The holder is then filled in around the core with potting material 90. The extruded hinge loop 80 and portions of the housing thereadjacent are then ground away on the convex grind line 91, affording a finished convex curvature for the gap end of the housing and also for the core tips adjacent the gap.

The present process for making a core is of course subject to variation within the scope of the invention, and of course, the design of the core can be modified in various ways to accommodate it to the process, or to enhance its own characteristics, all without departing from the scope of the invention.

What is claimed is: 1. The process of making a core for the electromagnet of a magnetic recorder or playback head, that comprises:

forming a strip of core material to have two angularly related central segments converging to an apex, and to have two portions extending generally oppositely to. one another from the ends of said segments remote from said apex, with said last mentioned portions merging into ends which extend toward and substantially to a reference plane perpendicular to a center plane disposed between said angularly related segments and bisecting said apex, said reference plane contacting said apex; lap grinding said core ends and said apex to parallel surfaces of precise flatness, until the remaining core metal of the apex is of only slight thickness at said center plane, so as to provide a thin, bendable, core metal hinge, I

bending the core form so made through an angle of 180 on said hinge until the ground ends of said strip abut one another to form a closed back gap, and so that the portions of the precisely ground flat surface on opposite sides of said hinge come into substantial face to face opposition, with the metal of the hinge rising substantially out of the gap to form an exterior ridge owing to said 180 bending of said form, and

grinding away said hinge ridge and a portion of the core metal thereunder to form a final head surface configuration having a recording and/or playback gap therein.

2. The process according to claim 1, including lap grinding said core ends and said apex simultaneously with a single lap to achieve'lapped surfaces which are both precisely flat and precisely rectilinearly aligned.

3. The process according to claim 1, including assembling the original strip of core material from a plurality of stamped out laminations.

4. The process of claim 1 including, following closing of the back gap, introducing the resulting assembly into a case having an apertured head wall, so that the hinge ridge projects therethrough,

filling the case with a hard setting potting compound that, when set, will preserve dimensional accuracy of and spacial relationships of parts, and

in the final grinding operation, grinding the apertured head wall to the same contour.

5. The process according to claim 1, including further forming a tapered, wedge-shaped indentation transversely across the center of the flat ground surface of the apex to promote extrusion of the hinge forming metal of the core from between the two hinging halves of the flat ground surface of the apex.

6. The subject matter of claim 1, including also inserting a non-magnetic gap filler strip in the pocket between the two hinging halves of the flat ground surface of the apex.

7. The process of making a core for the electromagnet of a magnetic recorder or playback head, that comprises:

bending a flat strip-of magnetic core metal to form two angularly'related central strip segments converging to an apex; bending the strip at the ends of said segments remote from said apex to form two strip portions which extend generally oppositely from the segment ends and'then toward a reference plane perpendicular to a center plane disposed between said angularly related segments and bisecting said apex, said refer- I ence plane contacting said apex;

grinding said 'apex' down to a precisely Hat and horizontal apex surface parallel to said reference plane, and substantially bisected by said center plane, leaving a very slight remaining thickness of metal at the intersection of said center plane with the remainder of the apex, and thereby affording a thin, easily bendable core metal hinge,

grinding the terminal ends of said strip portions to precise flatness and at locations and angles which will cause them to precisely abut when the core strip is bent through on said bendable hinge,

bending the intermediate core form so made through an angle of 180 on said hinge until the ground ends of said strip are in such an abutting relationship, and so that the portions of the precisely ground flat surface on opposite sides of said hinge come. into substantial face to face opposition, with the metal of the hinge rising substantially out of the gap to form an exterior ridge owing to said 180 bending of said form, and v inserting and pinching a non-magnetic gap filler shim between said surface portions of said gap during the immediately preceding step.

8. The process according to claim 7, wherein the strip is so bent that the apical juction between the convergent segments has a convex exterior curvature.

9. The process of claim 7, including assembling an electromagnet coil would on a bobbin onto said strip, over one end thereof, before said ends are closed to form said back gap.

10. The process of claim 4 including, following closing of the back gap, introducing the resulting assembly into a case having an apertured head wall, so that the hinge ridge projects therethrough,

11. The process according to claim 7, including:

also bending the convergent strip segments away from one another to form the core with two longitudinally aligned segments which extend oppositely from one another, and

bending the last mentioned segments through 90 i into end segments terminating in said ends of said strip.

12. The process according to claim 1, including:

lap grinding said strip ends and said apex simultaneously to a precisely common level.

13. The process according to claim 2, including:

forming the underside of the apical junction of the convergent segments with a concave contour.

14. The process of making a core for the electromagnet of a magnetic recorder or playback head, that comprises as steps:

providing an elongated core form having two angularly related segments converging to an apex, said segments and apex comprised at least in part of core metal, and having two core metal side segments extending oppositely from the ends of said angularly related segments remote from said apex, and thence extending toward, and with its ends in proximity to, a reference plane which is perpendicular to a'center plane between saidangularly related segments and substantially bisects said apex, said reference plane lying in relatively close proximity to said apex;

lap grinding said core metal ends and said apex to provide parallel ground top surfaces of precise flatness, and until the remainder of the apex is of only slight thickness at said center plane, so as to provide a thin, interconnecting hinge on which bending of said angular segments can occur;

bending the two angular segments of said core form so made on said interconnecting hinge through an angle of so that the ground ends of said form abut one another to form a closed back gap, and so that the portions of the precisely ground flat sur face portions on opposite sides of said hinge come into substantial face to face opposition, with a front gap therebetween, and with the material of the hinge displaced substantially out of the gap and looped to form an exterior hinge ridge;

inserting a non-magnetic gap filler into said front gap during said bending step;

embedding at least portions of said hinge-connected angular segments, after being bent through said 180, in hardening potting compound, and

grinding away said hinge ridge along a smooth surface contour which exposes said front gap and gap filler.

15. The process according to claim 14, including:

tightly fixing the core form, prior to bending on its hinge, inside an open-topped housing, with its apex and core form ends facingupward, fabricating said angularly related segments each to comprise an outside leg integrally joined to the corresponding side segment, and an inside leg contiguous to the corresponding outside leg and integrally joined to said housing, forming the upper ends of said outside and inside legs to constitute the lap ground top surface of the apex, with the lap ground surface on the inside legs in the middle, and the lap ground surfaces on the outside legs contiguous thereto and on the outside thereof, forming in the bottom of the housing a deep indentation defined by the inner surfaces of said inside legs and completed at the top by a concave surface that is only slightly spaced from the ground top surface on said inside legs, whereby to form said hinge integrally with the housing;

all in such manner that the angular core segments are bent on said hinge by closing said housing parts.

16. The process according to claim 1, including preparing said strip of core material by assembling it from a plurality of adhesively joined laminations.

Claims

1. The process of making a core for the electromagnet of a magnetic recorder or playback head, that comprises: forming a strip of core material to have two angularly related central segments converging to an apex, and to have two portions extending generally oppositely to one another from the ends of said segments remote from said apex, with said last mentioned portions merging into ends which extend toward and substantially to a reference plane perpendicular to a center plane disposed between said angularly related segments and bisecting said apex, said reference plane contacting said apex; lap grinding said core ends and said apex to parallel surfaces of precise flatness, until the remaining core metal of the apex is of only slight thickness at said center plane, so as to provide a thin, bendable, core metal hinge, bending the core form so made through an angle of 180* on said hinge until the ground ends of said strip abut one another to form a closed back gap, and so that the portions of the precisely ground flat surface on opposite sides of said hinge come into substantial face to face opposition, with the metal of the hinge rising substantially out of the gap to form an exterior ridge owing to said 180* bending of said form, and grinding away said hinge ridge and a portion of the core metal thereunder to form a final head surface configuration having a recording and/or playback gap therein.

2. The process according to claim 1, including lap grinding said core ends and said apex simultaneously with a single lap to achieve lapped surfaces which are both precisely flat and precisely rectilinearly aligned.

4. The process of claim 1 including, following closing of the back gap, introducing the resulting assembly into a case having an apertured head wall, so that the hinge ridge projects therethrough, filling the case with a hard setting potting compound that, when set, will preserve dimensional accuracy of and spacial relationships of parts, and in the final grinding operation, grinding the apertured head wall to the same contour.

7. The process of making a core for the electromagnet of a magnetic recorder or playback head, that comprises: bending a flat strip of magnetic core metal to form two angularly related central strip segments converging to an apex; bending the strip at the ends of said segments remote from said apex to form two strip portions which extend generally oppositely from the segment ends and then toward a reference plane perpendicular to a center plane disposed between said angularly related segments and bisecting said apex, said reference plane contacting said apex; grinding said apex down to a precisely flat and horizontal apex surface parallel to said reference plane, and substantially bisected by said center plane, leaving a very slight remaining thickness of metal at the intersection of said center plane with the remainder of the apex, and thereby affording a thin, easily bendable core metal hinge, grinding the terminal ends of said strip portions to precise flatness and at locations and angles which will cause them to precisely abut when the core strip is bent through 180* on said bendable hinge, bending the intermediate coRe form so made through an angle of 180* on said hinge until the ground ends of said strip are in such an abutting relationship, and so that the portions of the precisely ground flat surface on opposite sides of said hinge come into substantial face to face opposition, with the metal of the hinge rising substantially out of the gap to form an exterior ridge owing to said 180* bending of said form, and inserting and pinching a non-magnetic gap filler shim between said surface portions of said gap during the immediately preceding step.

10. The process of claim 4 including, following closing of the back gap, introducing the resulting assembly into a case having an apertured head wall, so that the hinge ridge projects therethrough, filling the case with a hard setting potting compound that, when set, will preserve dimensional accuracy of and spacial relationships of parts, and in the final grinding operation, grinding the apertured head wall to the same contour.

11. The process according to claim 7, including: also bending the convergent strip segments away from one another to form the core with two longitudinally aligned segments which extend oppositely from one another, and bending the last mentioned segments through 90* into end segments terminating in said ends of said strip.

12. The process according to claim 1, including: lap grinding said strip ends and said apex simultaneously to a precisely common level.

13. The process according to claim 2, including: forming the underside of the apical junction of the convergent segments with a concave contour.

14. The process of making a core for the electromagnet of a magnetic recorder or playback head, that comprises as steps: providing an elongated core form having two angularly related segments converging to an apex, said segments and apex comprised at least in part of core metal, and having two core metal side segments extending oppositely from the ends of said angularly related segments remote from said apex, and thence extending toward, and with its ends in proximity to, a reference plane which is perpendicular to a center plane between said angularly related segments and substantially bisects said apex, said reference plane lying in relatively close proximity to said apex; lap grinding said core metal ends and said apex to provide parallel ground top surfaces of precise flatness, and until the remainder of the apex is of only slight thickness at said center plane, so as to provide a thin, interconnecting hinge on which bending of said angular segments can occur; bending the two angular segments of said core form so made on said interconnecting hinge through an angle of 180* so that the ground ends of said form abut one another to form a closed back gap, and so that the portions of the precisely ground flat surface portions on opposite sides of said hinge come into substantial face to face opposition, with a front gap therebetween, and with the material of the hinge displaced substantially out of the gap and looped to form an exterior hinge ridge; inserting a non-magnetic gap filler into said front gap during said bending step; embedding at least portions of said hinge-connected angular segments, after being bent through said 180*, in hardening potting compound, and grinding away said hinge ridge along a smooth surface contour which exposes said front gap and gap filler.

15. The process according to claim 14, including: tightly fixing the core form, prior to bending on its hinge, inside an open-topped housing, with its apex and core form ends facing upward, fabricating said angularly related segments each to comprise an outside leg integrally joined to the corresponding side segment, and an inside leg contiguous to the corresponding outside leg and integrally joined to said housing, forming the upper ends of said outside and inside legs to constitute the lap ground top surface of the apex, with the lap ground surface on the inside legs in the middle, and the lap ground surfaces on the outside legs contiguous thereto and on the outside thereof, forming in the bottom of the housing a deep indentation defined by the inner surfaces of said inside legs and completed at the top by a concave surface that is only slightly spaced from the ground top surface on said inside legs, whereby to form said hinge integrally with the housing; all in such manner that the angular core segments are bent on said hinge by closing said housing parts.