METHOD OF MANUFACTURING MAGNETIC HEAD CORES

Abstract: A plurality of separate magnetic head cores (3) is cut from a one- piece ferrite core block. A winding aperture (6) is provided in each magnetic head core. A fracture (7) in the cross-section of the material of the head core is generated between the winding aperture and the future tape contact face during or after the winding aperture is provided. A non-magnetic material in a liquid form is provided between the fracture faces and is converted to the solid form.

Description: The invention relates to a method of manufac­ turing magnetic head cores in which a plurality of magnetic head cores is cut from a preprocessed ferrite core block. In the prior art methods of manufacturing ferrite magnetic heads the side faces of two ferrite sub-blocks, one of which accommodates a winding aperture, are first lapped, whereafter quartz is sputtered on the side faces thus preprocessed and the sputtering faces of the sub-blocks are cemented together by means of glass. A plurality of magnetic head cores is than cut from the core block so obtained, each divided into two parts by a glass gap. The required electrical turns are wound in the winding aper­ tures of the separate head cores in a later manufacturing stage. The lapping process is costly, because it requires great accuracy and is time-consuming, whilst a technical drawback of the lapping operation of the gap faces is that a damaged layer is formed on the gap faces. A further drawback is that the finished magnetic head cores consist of two halves cemented together by a thin glass layer, rendering them mechanically weak. It is an object of the invention to provide a bulk manufacturing method which is inexpensive, leaves the gap faces intact and leads to a mechanically strong magnetic head core. To this end the invention is characterized in that in a method of the type described in the opening paragraph the separate magnetic head cores are each provided with a gap after they have been cut from a one-­ piece core block by a method in which, first, a winding aperture is provided in the magnetic head core at a dis­ tance from the future tape contact face, which distance is at least substantially equal to the desired gap height, second, a fracture is generated in a cross-section of the magnetic head core located between the future tape contact face and the winding aperture, thus creating two fracture faces, third, a non-magnetic material in a liquid form is provided between the fracture faces and, finally, the non-­ magnetic material is converted to the solid form. By considering the gap in the magnetic head core to be a defined crack, a technology can be developed which is fast, continuous, inexpensive and accurate. It is found that under normal ambient conditions a defined crack can be created in a (monocrystalline) block of ferrite with the aid of simple and particularly thermal, that is, thermomechanical, means. When there is a brittle fracture, the fracture faces constitute a very accurate equidistant gap whose width can be adjusted in a manner to be further indicated. It will be evident that this technology is extremely suitable for inexpensive and accurate bulk manufacture. Within the scope of the invention the core block may be pre-formed in a special manner. The future tape contact face of the ferrite core block may be provided with a longitudinal v-shaped notch. Notching is effected over the areas where the weakest cross-section will be present in the magnetic head cores to be sawn from the core block and it facilitates the generation of a fracture in the magnetic head cores. The upper side of the core block should have such a surface quality that the notch to be provided is dominant and will thereby function as a fracture initiator. A tensile stress onthe tip of the notch brings about the desired crack growth. The desired local stress condition may be produced in different manners. In a first manner it is produced during the laser process of manufacturing the winding aperture. This is notably the case for relative small gap heights. A second manner is to provide a temperature gradient across the head core, for example, by unilaterally heating it after the winding aperture has been provided in the solid magnetic head core. This approach may be necessary for cores having larger gap heights. In both manners it may be advantageous to previously constrict the magnetic head core at the area of the desired gap. The fracture is then produced more easily in the case of cores having thicknesses of less than 200 µm, for example, a thickness of approximately 150 µm it has, however, been found that the fracture process can also be performed without the core being previously constricted. Constriction to the desired track width may then be effected afterwards, which involves different and entirely specific advantages. When the fracture is formed, it is opened as far as is required to realise the desired gap length of the future gap and the non-magnetic gap filler is provided. This may be, for example, an epoxy penetrant. It is to be noted that a method is known from United States Patent 2,836,881 by which a fracture is provided in a predetermined region of a sintered annular core of a transducing gap. The known method by which stres­ ses are generated using a plurality of screws, which stresses produce the fracture in a cross-section reduced by an internal V-groove, and by which a material foil is slid into the fracture thus formed is as such hardly related to the method according to the invention. An embodiment of the invention will be described hereinafter with reference to the accompanying drawings, in which : Fig. 1a shows a one-piece initial core block for manufacturing magnetic head cores, and a magnetic head core as cut off, Fig. 1b shows a magnetic head core during a manufacturing stage, Figs. 2a and 2b are plan views of two different magnetic head cores during a subsequent manufacturing stage, Fig. 3 is a side view of a magnetic head core having a winding aperture, Fig. 4 is a side view of a magnetic head core having a winding aperture over which a fracture is generated, Fig. 5 shows the path to be followed by a laser beam for providing a winding aperture, Fig. 6 shows a device for opening and checking the length of the gap in a head core, and Fig. 7 shows a head core having a gap obtained by means of a fracture mechanism. An initial core block 1 is formed from a mono­ lithic rod of monocrystalline Mn-Zn ferrite. The rod 1 has a future tape contact face 2. A plurality of magnetic head cores 3, having, for example, the dimensions 3 x 3 x 0.2 mm is sawn from this rod. The magnetic head cores 3 may either have a smooth tape contact face (Fig. 1a) or a tape contact face in which a notch 4 is provided (Fig. 1b). In the latter case the curved face 2 of the rod 1 is provided with a longitudinal scratch, for example, using a conical diamond stylus prior to the rod being sawn into pieces. To provide the said scratch, a stylus pressure of, for example, up to approximately 10 g and a relative rate of movement of 200 µm/sec. is sufficient. The magnetic head cores 3 may subsequently be locally constricted on the side of the tape contact face 5. Fig. 2a is a plan view of part of a locally constricted tape contact face 5. Fig. 2b shows an alternative form of a locally constricted tape contact face 5. In both cases the width W of the initial core is 200 µm; in this embodiment it is constricted to a width W' of 600 µm in a first stage and in a second stage it is constricted to a width of 23 µm which corresponds to the track width. When providing a winding aperture 6 using a focused laser in such locally constricted cores 3, a fracture 7 may be generated over the winding aperture 6 (Fig. 3, Fig. 4), creation of a fracture is simplified when the core 3 is provided with a scratch 4 on the side of the tape contact face 5 (Fig. 1b). SEM pictures have proved that the fracture faces are perfect mirror surfaces. A suitable laser both for constricting the head cores and for providing the winding aperture is, for example, an Nd-YAG laser having an effective spot diameter of 10 µm. Fig. 5 shows the path followed by the laser spot in a successful experiment of providing a head core cut from a one-piece initial block with a winding aperture. The point where the path of the laser spot begins is denoted by A and the end is denoted by B. When the gap height h is less than 40 to 50 µm, a fracture 7 is generally produced in the head core material during the laser process of providing the winding aperture 6 (Fig. 4). For gap heights h of more than 40 to 50 µm, there is generally no fracture during the laser process. By producing a temperature gradient across the head core 3, a fracture 7 may then be generated afterwards. To separate the fracture faces, the application of a temperature gradient across the head core is required. In order to realise a 0.3 µm distance between the fracture faces (a gap length of 0.3 µm is a require­ ment in present-day video recording systems ) it was necessary, for example, in a head core having the afore-­ mentioned dimensions to heat the side remote from the gap to a temperature of 70°C. To this end the head core 3 is sealed, for example, to a copper holder 8 which in turn is attached to a small heating plate 9 (Fig. 6). When the head core 3 is heated in this manner, a temperature gra­ dient is produced across the head core. The fracture 7 is inspected (for example, with the aid of a microscope 10) to check when it has opened to the desired width during the increase of the temperature. When this is the case, the tem­ perature is maintained constant and a small quantity of a bonding agent 11, for example, an epoxy adhesive is applied in the winding aperture 6 (Fig. 7). The bonding agent 10 penetrates the gap 7 formed between the fracture faces and is cured prior to or during subsequent cooling. Gaps having a gap length of 0.2 to 0.3 µm have been realised in this manner.

Claims: 1. A method of manufacturing magnetic head cores in which a plurality of magnetic head cores is cut from a pre-processed ferrite core block, characterized in that the separate magnetic head cores are each provided with a gap after they have been cut from a one-piece core block by a method in which, first, a winding aperture is provided in the magnetic head core at a distance from the future tape contact face, which distance is at least substantially equal to the desired gap height, second, a fracture is generated in a cross-section of the magnetic head core located between the future contact face and the winding aperture, thus creating two fracture faces, third, a non-magnetic material in a liquid form is provided be­ tween the fracture faces and, further, the non-magnetic material is converted to the solid form. 2. A method as claimed in Claim 1, characterized in that the future tape contact face of the ferrite core block is previously provided with a longitudinal V-shaped notch and that notching is effected over the areas where the weakest cross-section will be present in the head cores to be sawn from the core block. 3. A method as claimed in Claim 1 or 2, charac­ terized in that the fracture is generated while the winding aperture is provided by means of a laser. 4. A method as claimed in Claim 1 or 2, characterized in that, first, the winding aperture is provided and that subsequently the fracture is generated by applying a temperature gradient across the head core. 5. A method as claimed in Claim 3 or 4, characterized in that after the fracture is generated a temperature gradient is produced across the magnetic head core such as is required to separate the fracture faces to a pre­ determined distance from each other, whereafter a non-­ magnetic material is provided in a liquid form between the fracture faces. 6. A method as claimed in Claim 5, characterized in that the non-magnetic material is an epoxy resin. 7. A method as claimed in any of the preceding Claims, characterized in that after the non-magnetic material has been provided the fracture faces the tape contact face of the magnetic head core is constricted at the area of the gap formed.

Inventors: VERBUNT, JOHANNES PETRUS MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: G11B5
IPC codes: G11B5