GB2055011A

GB2055011A - Stylus assembly and process for making same

Info

Publication number: GB2055011A
Application number: GB8020351A
Authority: GB
Original assignee: ZAFIRA FRANCE
Current assignee: ZAFIRA FRANCE
Priority date: 1979-06-22
Filing date: 1980-06-20
Publication date: 1981-02-18
Also published as: GR68191B; FI801980A; OA06547A; NL8003577A; IL60313A; IT8022907A0; DD151523A5; JPS563407A; BR8003835A; GB2055011B; IL60313A0; YU160380A; CA1169781A; DK264280A; FR2460020B1; AU5952880A; ES492665A0; IT1150991B; SE8004202L; CH637781A5

Abstract

A stylus assembly for a magnetic cartridge comprises a stylus cantilever (2) formed from a thermoplastics material, a magnet (1) mounted at one end of the cantilever (2), and a stylus supported at the opposite end 3 of the cantilever (2). The portion of the magnet (1) adjacent the cantilever (2) is encased in a mass of thermoplastics material integral with the cantilever (2) whereas the portion of the magnet (1) remote from the cantilever (2) is left exposed. This exposed portion of the magnet is used to mount the stylus assembly in a damper of the cartridge. The stylus assembly is made using a mould which has a free space corresponding to the desired shape of the cantilever (2), and a chamber arranged to receive the magnet (1) and into which said free space opens. The zone of the chamber remote from the free space closely surrounds the portion of the magnet (1) received thereby whereas the chamber zone adjacent the free space provides a space around the corresponding magnet portion. This latter space and said free space are filled with a thermoplastics material to form the cantilever with the magnet attached. <IMAGE>

Description

SPECIFICATION Stylus assembly and process for making same The present invention relates to stylus assemblies for use in magnetic cartridges and to processes for making such stylus assemblies.

Magnetic cartridges are known which comprise a support structure or knob integral with a damper which is linked to a movable stylus assembly. This stylus assembly comprises a manget and a stylus cantilever with a diamond, sapphire or like stylus mounted on the cantilever.

it is well known to make the stylus cantilever out of a metal tube, the magnet being attached to one end of this tube while the diamond or the sapphire stylus is attached to the other end by glue. In stylus assemblies of this type it is necessary that the magnet be formed so as to facilitate its attachment to the cantilever. Thus, the magnet may be provided with a groove, and a flat area along its longitudinal dimension or with a hole extending along its axis. Such stylus assemblies suffer from serious drawbacks both with respect to the manufacture and the assembly of the magnet as well as to the formation of the tube itself and its assembly. A further drawback is the necessity of setting and gluing the diamond or sapphire stylus to the stylus cantilever.

Thus as regards the magnet, this must be given a predetermined shape while maintaining high machining consistency, which is known to be difficult. Furthermore, the assembly of the tube and magnet requires great care as it is very difficult to avoid deforming the tube which would destroy its rigid configuration. The fragility of the assembly also presents great difficulties both in mounting and gluing of the diamond or sapphire stylus to the cantilever tube.

According to one aspect of the present invention, there is provided a process for making a stylus assembly for a magnetic cartridge, said assembly comprising a stylus cantilever attached at one end to a magnet and carrying a stylus as its opposite end, said process including the steps of providing a mould both with a free space corresponding to the desired shape of said cantilever, and with a chamber adapted to receive said magnet, the chamber comprising a first zone adjacent to said free space and a second zone of smaller dimensions than said first zone, said second zone having dimensions closely corresponding to the portion of the magnet which it is arranged to receive; placing said magnet in said first and second zones; inserting thermoplastic material into said free space and said first chamber zone such that the portion of the magnet in said first zone becomes embedded in a mass of thermoplastics material integral with that forming the cantilever; removing the stylus cantilever and attached magnet from the mould.

Using this process the body of the stylus cantilever is formed from a mass of thermoplastic resin in which the magnet is partially mounted.

This resin is advantageously reinforced with mineral fibres, particularly glass or carbon, in a proportion of the order of about 20% to 30%.

The dimensioning of the second chamber zone of the mould is such that a sufficient portion of the magnet is left exposed as to allow the magnet to be inserted into the damper of a cartridge and thereby effect the connection between the fixed portion and the movable portion of the cartridge.

The exposed portion of the manget makes it possible to achieve the desired flexibility and optimal oscillation of the magnet.

It has been found in practice that flexibility and satisfactory oscillation of the magnet can be obtained when the proportion of the magnet embedded in the thermoplastics material is between about one quarter and one half of the total length of the magnet. Particularly advantageous results have been obtained when approximately one third of the magnet length is embedded.

Preferably, the mould is provided with means arranged to form an orifice in the end of the stylus cantilever remote from the magnet, this orifice serving for the direct insertion of a diamond or sapphire stylus into the cantilever without gluing.

According to another aspect of the invention, there is provided an assembly for use in a magnetic cartridge, said assembly comprising a stylus cantilever formed of a thermoplastics material, and a magnet mounted at one end of said stylus cantilever, the opposite end of said cantilever being adapted to mount a stylus, and the portion of the magnet adjacent the sty!us cantilever being encased in thermoplastic material integral with that of the cantilever whereas the portion of the magnet remote from the cantilever is left exposed.

The process according to the invention exhibits significant advantages when compared with previous techniques.

Specifically, by avoiding gluing and assembly of the magnet and cantilever, a stronger product is obtained, more rapidly than previously and with less cost. In addition difficult machining of the magnet can be avoided since the shape of the magnet is no longer as critical as was previously the case.

Using the process of the invention enables great precision to be achieved in the contour of the moulded portion. As a result, it becomes possible to assure a precise relative positioning of the damper and magnet, at the edge of the overmoulding, together with a very precise centering thereby making possible a perfect coincidence of the axis of the stylus cantilever and the axis of the magnet, and an exact coincidence of the vertical planes of symmetry of the magnet and of the stylus cantilever.

This great precision in the contour of the moulded portion also makes it possible to ensure a constant length of the stylus cantilever and a shape which is always perfectly defined across its transverse cross-section.

It is thus possible to create cantilever shapes which achieve both a careful distribution of the mass and increased resistance to the shocks and flexion; as a result, stylus assemblies can be manufactured which have particularly desirable characteristics, both with respect to the qualities of mechanical resistance and with respect to the improvement of the results obtained in sound reproduction.

Stylus cantilevers of the invention may have a cross-section which diminishes from the end situated adjacent to the magnet towards the end arranged to mount the diamond or the sapphire stylus.

In particular, the cantilever may be given a generally converging outline and, preferably, one which converges in a regular manner.

Advantageously, the cantilever has a conical shape (the term "conical" being used in its broadest sense to include all surfaces generated by the movement of straight line passing through a fixed vertex), particularly a cone shape (that is, its surface defined by the movement in a circle of a straight line passing through a vertex) whose vertex angle is preferably very small. Particularly favourable results are obtained for a vertex angle between about one and two degrees, the preferred angle being about one and one half degrees.

According to another embodiment of the invention, the cantilever may have the form of a truncated cone with an elliptical directrix. In a further embodiment of the invention, the stylus cantilever has a polygonal cross-section and an outline which converges (preferably in a regular manner) from the magnet end of the cantilever to the stylus end, the polygon faces of the cantilever correspondingly decreasing in width towards the stylus end of the magnet. As used herein the term polygonal is taken to mean those polygons having no internal reflex angles.

To facilitate the direct mounting, without gluing, of the stylus in the plastics material of the cantilever, the latter is preferably formed with a flat portion at its end remote from the magnet.

This flat portion is formed with the mounting orifici advantageously fashioned in the course of moulding by means of a point or other projection integral with the mould. Since this orifice can be formed with great precision, it can be imparted any desired shape with a view to assuring the precise positioning of the stylus whatever the shape of the latter.

Thus the orifice may have a slight conicity with the entrance cross-section being made slightly smaller than the cross-section of the stylus to be mounted.

Depending on the shape of the stylus, the orifice can be given a square, circular, polygonal or elliptical cross-section.

The choice of the thermoplastic resin used for the cantilever is of course a function of the mechanical qualities to be conferred to the stylus assembly and also a function of the qualities of sound reproduction to be obtained. Particularly advantageous results may be obtained by utilizing a resin comprising a polyphenylene oxide into which can be incorporated mineral fibres, particularly glass or carbon fibres, said fibres preferably comprising of the order of about 20 to 30% of the cantilever resin. Particularly advantageous results may also be obtained by utilizing a resin comprising a polyethylene tetraphthalate, this resin being able also to be reinforced with fibres of glass or carbon in the proportions indicated previously.

In all cases, besides its great acoustic qualities, the stylus assembly exhibits an increased mechanical resistance, particularly a high resistance to deformation, particularly those resulting from sticking in the course of manipulation.

Embodiments of the invention will now be particularly described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a longitudinal cross-sectional view of an assembly embodying the invention; Figure 2 is a cross-sectional view of a magnetic cartridge incorporating the assembly of Figure 1; Figures 3-5 illustrate preferred crosssectional forms of a stylus cantilever of the Figure 1 assembly; Figures 6-8 are plan view illustrating three different shapes of stylus-mounting orifice formed in one end portion of the stylus cantilever of the Figure 1 assembly; and Figure 9 is a vertical cross-section of the cantilever end portion shown in Figures 6 to 8 and illustrates a preferred form of stylus-mounting orifice.

The assembly shown in Figure 1 comprises a magnet 1 encased over one third of its length in a mass of thermoplastics material which is integral with one end of a stylus cantilever 2 made out of the same thermoplastic material. The thermoplastic material encasing the magnet 1 conforms exactly to the contours of the underlying magent surface.

The assembly has preferably been fashioned by the simultaneous moulding of the cantilever 2 and encasement of the magnet as indicated above.

The cantilever 2 has a flattened portion 3 at its free end. An orifice 4 is formed in the flattened portion 3 and serves for the direct mounting of a diamond or sapphire stylus without gluing.

Figure 2 illustrates a stylus assembly (constituted by the Figure 1 assembly with a diamond or sapphire stylus 5 attached) connected by means of a damper 6 to the fixed structure (knob 7) of a cartridge.

Figures 3, 4 and 5 show three transverse cross section views in perspective, each view illustrating a particular shape of the cantilever 2 integral with the magnet 1. The cantilever cross-sectional forms 8a, 8b, and 8c shown in Figures 3, 4 and 5 respectively, are triangular, polygonal and elliptical in shape.

Figures 6 to 8, illustrate three different shapes 4a, 4b and 4c of the orifice 4 adapted for mounting the diamond or sapphire stylus 5 to the end of the cantilever 3. The shapes 4a, 4b and 4v are respectively, circular, polygonal and elliptical.

Figure 9 is a longitudinal cross-section of the orifice showing a preferred conical shape 4d thereof.

Claims

1. A process for making a stylus assembly for a magnetic cartridge, said assembly comprising a stylus cantilever attached at one end to a magnet and carrying a stylus at its opposite end, said process including the steps of: a) providing a mould formed both with a free space corresponding to the desired shape of said cantilever, and with a chamber adapted to receive said magnet, the chamber comprising a first zone adjacent to said free space and second zone of smaller dimensions than said first zone, said second zone having dimensions closely corresponding to the portion of the magnet which it is arranged to receive; b) placing said magnet in said first and second zones;; c) inserting thermoplastic material into said free space and said first chamber zone such that the portion of the magnet in said first zone becomes embedded in a mass of thermoplastics material integral with that forming the cantilever; d) removing the stylus cantilever and attached magnet from the mould.

2. A process according to Claim 1, wherein the thermoplastic material is injected into the mould.

3. A process according to Claim 1 or Claim 2, further comprising the step of inserting the said stylus directly into the cantilever end portion remote from said magnet.

4. A process according to any one of the preceding claims, wherein said first and second chamber zones are so dimensioned that said manget is embedded within said thermoplastic material over between one quarter and one half of the length of said magnet.

5. A process according to Claim 4, wherein said magnet is embedded within said thermoplastic material over substantially one third of the length of the magnet.

6. A process according to any one of the preceding claims, wherein the cross-sectional area of said free space decreases from its end adjacent the first chamber zone to the other end thereof.

7. A process according to Claim 6, wherein the sides of the said free space converge from its end adjacent the first chamber zone towards the other end of the free space.

8. A process according to Claim 7, wherein the sides of the free space converge uniformly.

9. A process according to Claim 8, wherein the shape of said free space is such as to mould a generally conically shaped stylus cantilever.

10. A process according to Claim 9, wherein the shape of said free sapce is such as to mould a frusto-conically shaped stylus cantilver having a circular cross section and a vertex angle of between 1 and 2 degrees.

1 A process according to Claim 10, wherein said vertex angle is substantially equal to 1.5 degrees.

12. A process according to Claim 9, wherein said stylus cantilever has an elliptical directrix.

13. A process according to Claim 7 or Claim 8, wherein the shape of said free space is such as to provide a stylus cantilever of polygonal cross section.

14. A process according to any one of the preceding claims, wherein said mould includes means for forming an orificie in said stylus cantilever for receiving said stylus, said orifice being formed in the cantilever prior to its removal from the mould.

15. A process according to Claim 14, wherein the said means for forming said orifice is arranged to form an orifice which either is circular, elliptical or square.

16. A process according to claim 14, or 15, wherein the said means for forming said orifice comprises a projection integral with said mould.

17. A process according to any one of the preceding claims, wherein the end of said free space remote from said chamber is formed to provide a flattened portion on said stylus cantilever.

18. A process according to any one of the preceding claims, wherein the thermoplastic material inserted in the said free space and first chamber zone of the mould contains mineral fibres.

19. A process according to Claim 18, wherein said mineral fibres comprise glass fibres or carbon fibres.

20. A process according to Claim 18 or Claim 19, wherein said mineral fibres make up between 20% to 30% of said thermoplastic material.

21. A process according to any one of the preceding claims, wherein said thermoplastic material comprises polyphenylene oxide or polyethylene tetraphthalate.

22. A stylus assembly the production of which involves a process according to any of the preceding claims.

23. An assembly for use in a magnetic cartridge, said assembly comprising a stylus cantilever formed of a thermoplastics material, and a magnet mounted at one end of said stylus cantilever, the opposite end of said cantilever being adapted to mount a stylus, and the portion of the magnet adjacent the stylus cantilever being encased in thermoplastic material integral with that of the cantilever whereas the portion of the magnet remote from the cantilever is left exposed.

24. An assembly according to Claim 23, further comprising a stylus mounted on the said opposite end of the stylus cantilever.

25. An assembly according to Claim 23, wherein the said exposed portion of the magnet extends over between one quarter and one half of the length of the magnet.

26. An assembly according to Claim 25, wherein said exposed portion of the magnet extends over substantially one third of the length of the magnet.

27. An assembly according to any one of Claims 23 to 26, wherein said thermoplastic material incorporates mineral fibres.

28. An assembly according to Claim 27, wherein said mineral fibres make up between 20% and 30% of said thermoplastic material.

29. An assembly according to Claim 27 or Claim 28, wherein said mineral fibres comprise glass fibres or carbon fibres.

30. An assembly according to any one of Claims 23 to 29, wherein said thermoplastic material comprises polyphenylene oxide or polyethylene tetrapthalate.

31. An assembly according to any of Claims 23 to 30, wherein the cross sectional area of said stylus cantilever decreases from said one end to said opposite end.

32. An assembly according to any one of Claims 23 to 30, wherein the walls of said stylus cantilever are generally convergent from said one end to said opposite end.

33. An assembly according to Claim 32, wherein said stylus cantilever converges uniformly from said one end to said opposite end.

34. An assembly according to Claim 32 or 33, wherein said stylus cantilever has a generally conical shape.

35. An assembly according to Claim 34, wherein said stylus cantilever is frusto-conical in form.

36. An assembly according to Claim 35, wherein said frusto-conical stylus cantilever has a vertex angle of between 1 and 2 degrees.

37. An assembly according to Claim 36, wherein said vertex angle is substantially 1.5 degrees.

38. An assembly according to Claim 35, wherein said frusto-conical stylus cantilever has an elliptical directrix.

39. An assembly according to Claim 33, wherein said stylus cantilever is polygonal in cross section with each of the external facets of said stylus cantilever decreasing in width from said one end to said opposite end.

40. An assembly according to any one of Claims 23 to 39, wherein the said opposite end of the stylus cantilever has a flattened shape and is formed with an orifice arranged to receive a stylus therein.

41. An assembly according to Claim 40, wherein said orifice is slightly conically tapered to a cross section smaller than that of the stylus to be mounted therein.

42. An assembly according to Claim 40 or Claim 41 , wherein the cross sectional shape of said orifice is circular, elliptical, polygonal or square.

43. A magnetic cartridge comprising a support structure mounting a damper, and an assembly according to Claim 24, the said exposed portion of the magnet being mounted by said damper in a manner permitting movement and oscillation of said assembly.

44. A stylus assembly substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

45. A stylus assembly substantially as hereinbefore described with reference to Figure 1 in combination with any one of Figures 3 to 5 and/or any one of Figures 6 to 8 of the accompanying drawings.