EP0419582A1

EP0419582A1 - Method for lathe machining of parts presenting a surface which is not a revolution surface

Info

Publication number: EP0419582A1
Application number: EP89910869A
Authority: EP
Inventors: Etienne Gancel
Original assignee: J2T VIDEO (TONNERRE) SA
Current assignee: J2T VIDEO (TONNERRE) SA
Priority date: 1988-09-23
Filing date: 1989-09-22
Publication date: 1991-04-03
Also published as: FR2636875B1; WO1990003249A1; KR900701468A; FR2636875A1; JPH03501469A

Abstract

L'invention est relative à un procédé pour usiner des pièces à section droite non circulaire. On fait tourner une ébauche autour d'un axe, et on l'attaque avec un outil de coupe auquel on imprime des mouvements axiaux et/ou radiaux cycliques de façon synchronisée avec la rotation de l'ébauche. Sur au moins une partie du cycle au cours de laquelle l'outil s'approche ou s'écarte d'un plan fixe de référence ou de l'axe de rotation, la loi de vitesses du mouvement de l'outil est formée essentiellement de sections successives (101, 111, 121; 102, 112, 122) correspondant chacune à une accélération constante, ces accélérations amenant la vitesse axiale ou radiale aux extrémités de cette partie, à être égale à celle du reste du cycle au moment de la transition.The invention relates to a method for machining parts with a non-circular cross section. A blank is rotated around an axis, and it is attacked with a cutting tool to which cyclic axial and / or radial movements are imparted in a manner synchronized with the rotation of the blank. Over at least part of the cycle during which the tool approaches or deviates from a fixed reference plane or from the axis of rotation, the speed law of the movement of the tool is formed essentially of successive sections (101, 111, 121; 102, 112, 122) each corresponding to a constant acceleration, these accelerations causing the axial or radial speed at the ends of this part to be equal to that of the rest of the cycle at the time of the transition .

Description

PROCESS FOR MACHINING LATCHES WITH A SURFACE THAT IS NOT

NO REVOLUTION

The present invention relates to a process for the lathe machining of parts comprising a surface which is not of revolution.

The origin of the invention lies in a problem of machining a part of the read head of a device for recording or reading simple magnetic tapes or with diagonal tracks, namely a fixed drum called " lower drum ", but the invention can find many other applications, as will be understood below.

The lower drum of a read head which is used to guide the support strip in front of a rotary upper drum carrying the read heads, has a functional surface which has the shape of a propeller commonly called "lead", the distance to any fixed reference plane perpendicular to the axis varies linearly as a function of the angle at the center, and a connecting surface which connects the two ends of the functional surface. These two surfaces each extend about 180 ° around the center.

The machining of this drum is usually done on a lathe: a blank is mounted on a lathe chuck, and one rotates this while alternately moving a cutting tool parallel to the axis according to a synchronized speed law with the rotation of the lathe.

During the machining of the functional surface, the shape of the propeller results in the cutting tool moving at a constant speed, approaching or moving away, depending on the case of the fixed reference plane. The shape of the compensation surface is not dictated by considerations relating to the operation of the VHS read head. It is understood, however, that, to avoid jolts during machining, it is good that there are no sudden variations in the speed of movement of the tool.

The tool movement diagram by relation to the fixed reference plane, as a function of time, or, which amounts to the same in practice, as a function of the angle at the apex, consequently comprises a rectilinear part, inclined to the horizontal which corresponds to the functional surface and which consequently extends over approximately a half-turn, and an S-shaped part, formed, in detail, of a succession of broad rounded edges, and, possibly, rectilinear portions, all tangent to each other at their respective connection points, the ends of this S-shaped part being tangent to the ends of the straight portion corresponding to the functional surface.

A common procedure is to operate the cutting tool using a cam, which rotates at the same speed as the mandrel. A follower roller, carried by the tool support, is pressed by a spring against the cam.

It has been found that this system does not allow satisfactory machining at high speed, because unacceptable vibrations appear as soon as a certain angular speed is exceeded, and this limits the production rates.

The studies which are at the origin of the present invention have shown that the vibrations were due to detachments of the roller which deviates at times from the cam at high rotational speeds. An increase in the spring force which applies the roller against the cam is hardly possible, since it results in faster deterioration of the surfaces in contact, as well as an increase in the forces in the machine, which also affects precision. . Known camless systems also have less precision at such speeds.

Quite similar problems can arise in the case of machining of parts comprising other surfaces which are not of revolution, for example a cylindrical surface with non-circular section. In that case. The cutting tool no longer moves parallel to the axis, but perpendicular to it.

The object of the present invention is to provide a machining process which allows significantly improved machining speeds and rates compared to those which are currently obtained.

To achieve this result, the invention provides a method of machining a part having a surface which is not of revolution and consists of a first part, the ends of which are at different distances from a plane. fixed reference perpendicular to an axis, or a fixed axis, and a second part, or connecting part, which connects the ends of the first part to each other while being tangent to said ends, this method comprising the steps of:

mounting a chamber on a rotary support such as a lathe chuck and driving it in rotation on said axis, and

- attack the blank with a cutting tool, by moving it parallel and / or perpendicular to the axis, to generate said cross section, this tool moving radially according to a first law of speeds to generate the first part of the straight section, and a second law of speeds to generate the second part of the straight section, and this second law of speeds providing that, at the ends of the second part, the axial speed and / or the radial speed is equal to the axial speed and / or the radial speed observed at the adjacent ends of the first part, and that, in the central part, said axial and / or radial speed is in the opposite direction to the average axial and / or radial speed corresponding to the first part in order to compensate for the difference in distance of said ends of the first part to the reference plane or to the axis, this method having the particularity that the speed law used p to generate the second part consists essentially of successive sections, each of them showing a constant acceleration, the accelerations being in opposite directions in two successive sections. According to a simple method, the speed law used to constitute the second part consists essentially of two of said successive sections.

According to another modality, the law of speeds used to constitute the second part comprises two of said successive sections of which at least one is separated from the first part by a complementary section showing a constant acceleration and of opposite direction to that of the adjacent section, this complementary section having an angular extension markedly less than that of the adjacent section.

Preferably, there is essentially no transition between the sections, nor between each section adjacent to the first part of the straight section and the speed law corresponding to this part. According to a preferred embodiment, the axial movement of the cutting tool is controlled by a cam rotating in synchronism with the rotary support, and on which a roller integral with the cutting tool is pushed by a spring, and the acceleration, in a section where it generates a force which is subtracted from that of the spring, is determined so that the tool reaches the maximum speed admissible under the operating conditions, and the acceleration in the other section is determined by the constraints of connection to the first part.

The present invention also relates to a form of surface for connecting a workpiece to the lathe, and intended to connect the ends of a functional surface, these ends being at different distances from a reference surface. This connection surface must be able to be machined on a cam or digital lathe, faster than the surfaces of known connections, without its machining giving rise to more vibrations or errors of precision than in the case of the machining of known surfaces.

According to the invention, such a connection surface is characterized by a generator whose distance to a reference surface varies according to a law consisting of at least two substantially parabolic parts, when this generator moves.

The invention and its advantages will now be explained more precisely with the aid of a practical example, illustrated with the drawings among which:

Figure 1 is a diagram as a function of time of the positions, speeds and accelerations of the tool during a fraction of a turn of the blank, in solid lines according to the invention, in dashed lines according to the prior art.

Figure 2 is a similar diagram showing a variant of the invention.

Figure 3 is a diagram, in perspective, of the assembly. Figures 4 and 5 are perspective views of connection surfaces according to the invention.

In FIG. 1, curves A and A ′ show the distance from the tool to a fixed plane of reference to the axis respectively according to the invention and according to an example of the prior art. Curves B and B ', the speeds of the tool in the axial or radial direction and curves C and C, the axial or radial accelerations to which the tool is subjected. The origin of the times is arbitrary, as are the scales. It will be understood that time can be measured in angle units, if the blank rotates at constant speed, and that the initial conditions are repeated after 360 °. Curves B, B 'and C, C represent the respective first and second derivatives, in the mathematical sense of the term, of the variation represented in curves A and A '.

Curve A ′ (prior art) comprises a straight downward part 1 (it would be upward if the blank rotated in the opposite direction), which extends over approximately a half-turn and which corresponds to the functional surface.

The rest of the curve A ′, which corresponds to the connection part, comprises a rounded section 2, with an upward concavity, which connects tangentially at 3 to the lower end of the part 1, an upward straight section 4, steeper slope, in absolute value, than part 1, and tangentially connected at 5 to section 2, and a rounded section 6, with concavity downwards, which tangentially connects at section 5 and 8 at upper end of part 1.

The curve B ', which gives the slopes of the tangents to the curve A', comprises a horizontal part 11, located below the axis of zero velocities, which corresponds to the rectilinear part 1 of the curve A ', a section ascending 12, located above the axis of zero speeds, a bearing 14 and a descending section 16 which correspond respectively to sections 2, 4 and 6 of the curve A '. The connection points 13, 15, 17 and 18 correspond to points 3, 5, 7 and 8 of the curve A '. Curve C in turn gives the slopes of tangents to curve B '.

It includes a part 21, coincident with the axis of zero accelerations, a peak 22, a bearing 34 also coincident with the axis of zero accelerations, and a recess 26, which correspond to the sections 12, 14 and

16 and are attached to points 23, 25, 27 and 28, which correspond to points 13, 15, 17 and 18 of curve B '.

If we now consider the curves A, B, C which correspond to the invention, we find, of course, the parts 1, 11, 21 which are imposed since it is the functional law and are external to the subject of the invention. The differences appear in considering the parts which correspond to the connection surface, and more particularly, by comparing the curves C and C '.

Curve C, in fact, has two horizontal bearings 121, 122 which are connected together by a vertical line 123, and at the ends 23 and 28 of the part 21 by other vertical connecting lines 124, 125. The distance Dl from the first level 121 to the axis of zero accelerations is less than the distance M1 to the same axis from the apex of the neighboring peak 22, and the distance D2 from the second level '122 to the axis of zero accelerations is likewise less than the distance M2 at the same axis of the top of the hollow 26. It can however be provided that the distances Dl and D2 are, one and / or the other, equal respectively to the distances Ml and M2.

This means, in practical terms, that the tool undergoes, during the stage duration 121, a constant acceleration proportional to Dl and therefore less than or equal to the maximum Ml which it undergoes in the prior art. Similarly, the tool undergoes, during the duration of the step 122, a constant acceleration, in the opposite direction, proportional to D2, and less than or equal in absolute values to the maximum M2.

We will explain later why Dl and D2 are different.

The curve B shows sections whose slopes correspond to the sections of the curve C: to the part 21 corresponds, naturally, the part 11 already examined, and to the bearings 121 and 122 correspond to the straight parts 111 and 112, the ascending one, the other downward, connecting at a point 113, which corresponds to line 123.

Curve A, in turn, shows, apart from part 1, two sections 101 and 102 which connect tangentially to each other at point 103, which corresponds to point 113 of curve B, and which connect to points 3 and 18 in Part 1. Sections 101 and 102 are parts of well-defined geometric curves known as parabolas _*

As can be seen, the curves A and A 'have a fairly similar shape. However, a mathematical treatment of double derivation reveals, at the level of curves C and C, fundamental differences, and which have important practical consequences, namely a significant reduction in the accelerations undergone by the tool at equal speed of rotation, or a possibility of greater speed for the same acceleration.

In the general case, the displacements which correspond to the curves described in the figure are obtained using a rotating cam on which a roller is pushed by a spring. The shape of the cam corresponds to the curve A, or A 'in the prior art.

When the contact point of the roller on the cam moves away from the reference plane or the axis of rotation of the latter, the roller is pressed on the cam with a force which is the addition of the force of the spring and of that which results from the movement of the cam and which is proportional to the acceleration visible on the CC curves and to the inertia of the moving parts. When the contact point of the roller on the cam approaches the center, the forces below subtract, and the roller may take off from the cam.

There is a clear interest in avoiding this phenomenon. The acceleration of the movement towards the center of the cam is therefore calculated accordingly. The angular extension of the corresponding section is determined according to the maximum speed reached at the end of this section. The acceleration in the opposite direction, in the other section, is calculated according to the connection constraints. There is therefore no reason why the accelerations of parts 121 and 122 should be the same. In practical embodiments, it is not absolutely necessary for the connection lines 123, 124, 125 on the curve C to be vertical, but, more they deviate from the vertical, the more the length of the bearings 121 is reduced, and to reach the necessary speed, it will be necessary to increase the values D1, D2 of the accelerations, which is not to be desired. Connection lines must therefore be as vertical as possible.

Those skilled in the art can easily make the necessary adaptations in the event that the imposed parts 1, 11, 21 of the curves A, B, C have a shape or a length different from that which is described.

FIG. 2 shows a variant of the curves of FIG. 1. Between the imposed parts 1, 11, 21 of the curves A, B, C, and the sections 101, 111, 121, of the same curves, an additional section was inserted 106 , 116, 126, where the acceleration is in the opposite direction to the adjacent s-action 101, 111, 121. On curve A, we note that section 106 is tangentially connected to the compound part 1 on one side and, on the other side, is connected by an inflection 107 to section 101. This inflection 107 corresponds, of course, to an angular point 117 on the curve B of the speeds, and to a step 127 of the curve C of the accelerations. The angular extension of the additional section 106, 116, 126 is in the example chosen between 5 and 15% of that of the adjacent section 101, 111, 121.

The lower drum of the VHS VCR read heads is an elongated cylindrical part so that, for its machining, the cutting tool must be moved parallel to the axis, these displacements being synchronized with the rotation of the part being d 'machining.

FIG. 3 illustrates this assembly: a blank 200 is mounted on a lathe chuck 201. A cutting tool 202 is mounted on a support 203 comprising a hydraulic system capable of sliding the tool parallel to the axis 204 of the chuck according to arrow 205. A cam 206 is carried by spindle 207 and rotates in a plane perpendicular to the axis 204, at the same speed as the mandrel 201. A roller 208 is integral with the support 203 and it is pressed against the cam 206 by a spring 209. The support 203 is mounted on slides 210 parallel to the axis to move according to the double arrow 211.

Those skilled in the art will appreciate that a similar device can be used to machine a cylindrical surface with a non-circular cross section. The cam 206 will be shaped in this case to move the assembly formed by the carriage 205, the tool 202 and the roller 208 perpendicular to the axis, the latter then having an axis parallel to the axis 204, and the slides 210 being turned 90 ^e .

It should also be noted that it is possible to machine, according to the method of the invention, more complicated surfaces, the tool 202 moving axially according to a speed law, and moving radially according to another law of speeds, and at least one of these laws, and preferably both, meeting the requirements of the invention. The tool should then move in both directions. The design of suitable cam systems is simple and within the reach of those skilled in the art. According to a variant of the device, the cam or cams are eliminated, and the movements of the tool are obtained using programmable electromagnetic devices, known in themselves. There are, then, no problems of detachment on the cam, but the invention makes it possible to reduce the effects of inertia of the moving parts for a given rotation speed, and therefore to improve the precision and / or d '' increase the machining speed. This variant is particularly advantageous in the case of frequent variations in the shape of the surface to be machined.

Although we have spoken of lathe in the present text, any machine capable of imparting to the blank a rotational movement around an axis can, of course, be used without departing from the invention.

FIG. 4 shows a drum 401 a tape recorder, such as a VCR. The drum 401, which is the abovementioned "lower drum", essentially comprises two parts: a body 402 cylindrical of revolution, and a shoulder 403 whose diameter is slightly greater than that of the body 402. The front face 404 of the shoulder 403 has a complex surface of which a part referenced 405 constitutes said functional surface or guide surface of the magnetic strip. This surface 405 extends over approximately 180 ° and has a helical profile (the trace of its developed is a straight line). The ends of the surface 305 are referenced 406, 407. These ends are at different levels, that is to say that the planes passing through these ends and perpendicular to the axis AA 'of the drum 401 are distinct. The connection surface 408, forming part of the face 404, connects the ends 406 and 407, to form, with the surface 405, a continuous guide surface of constant width (width equal to the difference between the diameters of the parts 402 and 403 ). The projection of the surface 408, obtained by unwinding on a plane the lateral face 409 of the part 403, has a trace, in this unwinding plane, formed, according to the invention, of a succession of sections of parabolas.

In other words, the surface 408 can be defined as the succession of surfaces generated by a straight line (cutting edge of the lathe machining tool), each of these surfaces being such that:

- the straight line 410 carrying the generator straight line segment SS 'passes through the axis AA' of the drum 401, or in the immediate vicinity of this axis (the segment SS 'is located in a plane passing through AA ¹ or in the immediate vicinity of AA '),

- the generator segment SS 'moves so that its medium C is at a constant distance D from the axis AA',

- for a first elementary angular displacement (of the generator segment SS 'around the axis AA', the height H of the middle C of this segment with respect to a reference plane P perpendicular to the axis AA '(we have only shown the intersection R of this plane P with the axis AA') varies according to a parabolic law H such that : H "a ω ² + bGJ + c the coefficients a, b, c being determined so that each elementary portion of the surface 408 is tangent to the adjacent sections at its two ends, and that the general shape of the surface 408 is compatible with the characteristics of the recording device using the drum 401 and of the machine for machining this drum. In addition, the coefficients a, b, c are optimally defined when, while respecting the above-mentioned constraint, the value H "" 2a is minimal. The surface 408 is formed of a succession of several elementary surfaces such as the surface elementary defined above, for example three or four elementary surfaces, which extend over different arcs In the case described above, the generator SS ¹ is a line segment perpendicular to the axis AA ′. In other embodiments, the functional surface can have a curved generator, each point of which is situated in a plane perpendicular to AA ′. The connection surface then also has a curved generator. This generator can be broken down into several very straight line segments shorts, each of which meets a definition similar to that of the segment SS '. Of course, these generators are located in planes passing through AA'.

FIG. 5 shows another embodiment of the connection surface according to the invention, in the case where the generator of the functional surface and of the connection surface is practically parallel to the axis of the part comprising these surfaces (this is also the axis around which the part rotates during its machining). The part 500 shown in FIG. 5 is a cam. The cam 500 has a functional surface 501 the ends of which are referenced 502, 503. These ends are at different distances from the axis of the part 500. A connection surface 504 connects these ends. The generator GG 'of the surface 504 is parallel to the axis 505 of the part 500. The distance D * of GG' to a reference cylinder 506 with circular section enveloping the part 500 follows a law similar to that followed by the distance H of the device of FIG. 4.

Claims

1. A lathe machining process for a part having a surface which is not of revolution and consists of a first part, the ends of which are at different distances from a reference plane perpendicular to an axis, or of a fixed axis, and a second part, or connection part, which connects the ends of the first part to each other while being tangent to said ends, this method comprising the steps of: - mounting a blank on a rotary support such as a lathe chuck (20) and drive it in rotation on said axis, and

- attack the blank with a cutting tool (202), by moving it parallel and / or perpendicular to the axis, to generate said cross section, this tool moving radially according to a first law of speeds (1, 11 , 21) to generate the first part of the cross section, and a second law of speeds to generate the second part of the cross section, and this second law of speeds providing that, at the ends of the second part, the axial speed or the radial speed are equal to the axial speed and / or the radial speed observed at the adjacent ends of the first part, and that, in the central part, said axial and / or radial speed is of opposite direction to the axial and / or radial speed mean corresponding to the first part in order to compensate for the difference in distance of said ends of the first part to the reference plane or to the axis, characterized in that the speed law used to generate The second part consists essentially of successive sections (101, 111, 121; 102, 112, 122; 106, 116, 126), each of them showing a constant acceleration (Dl, D2), the accelerations, in these sections, being in opposite directions, and the accelerations, outside these sections, having an absolute value lower than that that she has in these sections.

2. Method according to claim 1, characterized in that there is essentially no transition between the sections (101, 111, 121; 102, 112, 122; 106, 116, 126) and between each section and the law of speeds (1, 11, 21) corresponding to the first part of the cross section.

3. Method according to one of claims 1 or 2, characterized in that the speed law used to constitute the second part consists essentially of two of said successive sections (101, 111, 121; 102, 112, 122).

4. Method according to one of claims 1 or 2, characterized in that the speed law used to constitute the second part comprises two of said successive sections of which at least one is separated from the first part by a complementary section (136, 116 , 126) showing a constant acceleration and in the opposite direction to that of the adjacent section, this complementary section having an angular extension significantly less than that of the adjacent section.

5. Method according to one of claims 1 to 4, and wherein the movement of the cutting tool is controlled by a cam rotating in synchronism with the rotary support, and on which a roller integral with the cutting tool is pushed by a spring, characterized in that the acceleration (D2), in the section (122) where it generates a force which is subtracted from that of the spring, is determined so that the tool reaches the maximum admissible speed under the operating conditions, and the acceleration in the other section is determined by the constraints of connection to the first part.

6. Method according to one of claims 1 to 4, characterized in that the movements of the tool are obtained using programmable electromagnetic devices.

7. Application of the method according to one of Claims 1 to 6 for the machining of stationary drums for the playback head of a video recorder.

8. Connection surface (408, 501) of a lathe machined part, intended to connect the ends (406, 407 or 502, 503) of a functional surface of this part, these ends being at different distances from a reference surface (R, 506), characterized in that the distance (H, D ') from the generator (SS', GG ¹ ) from this connection surface to the reference surface varies according to a law consisting of 'at least two substantially parabolic parts, when this generator moves.

9. Surface according to claim 8, characterized in that its generator is located in a plane passing through the axis of the workpiece (401), the reference surface being a plane perpendicular to this axis.

10. Surface according to claim 8, characterized in that its generator (GG ') is parallel to the axis (505) of the part (500), the reference surface being a cylinder (506) with circular section.

11. Surface according to claim 8 or 9, characterized in that it connects the ends of a functional tape guide surface of a drum of a tape recording apparatus.