FR2636875A1 - PROCESS FOR MACHINING LATHES WITH A SURFACE THAT IS NOT A REVOLUTION - Google Patents

Abstract

The invention relates to a method for machining parts having a non circular cross-section. A blank is rotated about an axis, and is attacked with a cutting tool to which cyclic axial and/or radial motions are imparted synchroneously with the rotation of the blank. During at least one portion of the cycle during which the tool gets close to or goes away from a fixed reference plane or the rotation axis, the law of speeds of the tool motion is essentially formed of successive sections (101, 111, 121; 102, 112, 122) each corresponding to a constant acceleration, said accelerations bringing the axial or radial speed to the extremities of said portion to be equal with that of the remainder of the cycle at the transition time.

Description

 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 reading head of a VHS standard VCR, 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
VHS, 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 helix whose distance to any fixed reference plane perpendicular to the axis varies linearly with the center angle, and a connecting surface that connects the two ends of the functional surface. These two surfaces each extend over approximately 1800 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 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 diagram of the displacements of the tool with respect 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 on the horizontal which corresponds to the functional surface and which therefore extends over approximately a half-turn, and an S-shaped part, which corresponds to the connection surface, and comprises a substantially straight central portion and two large tangent roundings on one side to this central portion and on the other to one end of the rectilinear part which corresponds 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, because 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 this 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 method 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 fixed plane. of reference perpendicular to an axis, or of a fixed axis, and a second part, or connecting part, which connects the ends of the first part to each other by 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 compensate for the difference in distance of said ends of the first part from the reference plane or the axis,
this process having the particularity that the speed law used 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 clearly smaller 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 stresses of connection to the first part.

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.

 In FIG. 1, the 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 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 repeat after 3600.

 Curves B, B 'and C, C' represent the respective first and second derivatives, in the mathematical sense of the term, of the variation represented on 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 connecting part, comprises a rounded section 2, concave upwards, which tangentially connects at 3 to the lower end of the part 1, a straight ascending section 4, steeper slope, in absolute value, than part l, and tangentially connected at 5 to section 2, and a rounded section 6, has a downward concavity, 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 speeds, 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 the 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 the points 23, 25, 27 and 28, which correspond to points 13, 15, 17 and 18 of the 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 by considering the parts which correspond to the connection surface, and more especially, 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 D1 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 similarly less than the distance M2 at the same axis of the top of the hollow 26. It can however be provided that the distances D1 and D2 are, one and / or the other, equal respectively to the distances M1 and M2.

This means, in practical terms, that the tool undergoes, during the stage duration 121, a constant acceleration proportional to D1 and therefore less than or equal to the maximum M1 which it undergoes in the prior art. Likewise, the tool undergoes, during the duration of the bearing 122, a constant acceleration, in the opposite direction, proportional to
D2, and less than or equal to absolute values at most
M2.

 We will explain later why D1 and D2 are different.

 The curve B shows sections whose slopes correspond to the sections of the curve C: å the part 21 corresponds, naturally, the part 11 already examined, and with the stages 121 and 122 correspond rectilinear parts lli 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 to 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 the 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 curves CC '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 obvious interest in avoiding this phenonene.

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 connecting lines 123, 124, 125 on the curve C to be vertical, but the 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 106 has been inserted. , 116, 126, where the acceleration is in the opposite direction from the adjacent section 101, 111, 121. On curve A, we note that section 106 is tangentially connected to the compound part 1 on one side and, from l 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 the arrow 205. A cam 206 is carried by the 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 2Q3 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 perpendicular to the axis by the carriage 205, the tool 202 and the roller 208, the latter then having an axis parallel to the axis 204, and the slides 210 being turned 90 '".

 It should also be observed 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 speed law. , 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 speed of rotation, 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.

Claims (7)

 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 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 driving 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 from the reference plane or the axis,  characterized in that the law of speeds 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 (D1 , D2), the accelerations, in these sections, being in opposite directions, and the accelerations, outside these sections, having an absolute value lower than that which it 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 (l, 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 (106, 116 , 126) showing a constant acceleration and 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 sQnt obtained using programmable electromagnetic devices.  7. Application of the method according to one of claims 1 to 6 to the machining of fixed drums for the playback head of a video recorder.