EP0362449B1 - Machine tool for ultrasonic abrading - Google Patents

Description

The present invention relates to a machine for machining by ultrasonic abrasion, in particular for the machining of insulating, hard, brittle or brittle materials such as glass, quartz, silicon carbide, alumina, or when the machined parts to be obtained are quite complicated shapes. In such a machine a "sonotrode" is carried carrying at the end a tool which communicates to an abrasive suspended in a liquid ultrasonic vibrations. These have the effect of micro-tapping the workpiece and eroding it. The tool, whose shape is that of the impression you want to make, sinks into the room, reproducing in it its own form.

• un ensemble de support des pièces à usiner ;
• un ensemble vibrant terminé par un porte-outil et propre à animer l'outil d'un mouvement de va-et-vient à fréquence ultrasonore et à communiquer ces vibrations à un abrasif liquide d'usinage ;
• un système de descente contrôlée de l'outil dans la pièce à usiner ;
• la surface d'attaque de l'outil étant perpendiculaire à l'axe du porte-outil, de sorte que l'usinage par abrasion ultrasonore soit réalise en bout d'outil ;
• un dispositif de régulation de la descente de l'outil ; et
• entre l'ensemble vibrant et un bâti fixe, des filtres acousto-mécaniques disposés en des noeuds d'amplitude longitudinale des vibrations.

Generally, such a machine comprises:

• a support assembly for the workpieces;
• a vibrating assembly terminated by a tool holder and suitable for animating the tool with a reciprocating movement at ultrasonic frequency and for communicating these vibrations to a liquid abrasive for machining;
• a system of controlled descent of the tool into the workpiece;
• the tool attack surface being perpendicular to the axis of the tool holder, so that machining by ultrasonic abrasion is carried out at the end of the tool;
• a device for regulating the descent of the tool; and
• between the vibrating assembly and a fixed frame, acousto-mechanical filters arranged in nodes of longitudinal amplitude of the vibrations.

Because of this arrangement of acousto-mechanical filters, the vibrating assembly is perfectly immobilized in elongation, but it also makes it consider that in each sector the harmonic longitudinal vibration is accompanied by a radial vibration in phase quadrature. This radial vibration therefore presents a belly of amplitude to this fixation. It is therefore absolutely necessary to decouple the vibrating assembly perfectly from the frame so as not to transmit its radial vibrations to it.

The main object of the present invention is to develop an acousto-mechanical filter which satisfies this additional condition.

To this end, a machining machine of the type defined at the start will, in accordance with the present invention, be essentially characterized in that an acousto-mechanical filter consists essentially of two concentric rings connected by equidistant bridges, the inner ring being elastically deformable under the effect of radial vibrations.

Another object of the invention is to obtain perfect regulation of the machining conditions of the part to be machined, in particular as regards the depth of machining and the pressure exerted by the tool on the part.

To do this, the machine can also be characterized in that the device for regulating the descent of the tool essentially comprises, for maintaining the gap between the tool and the machining surface at a constant value. as the pressure exerted by the tool on the surface to be machined, a capacitive type force sensor capable of controlling, via an electronic regulation chain, a motor for controlling the vertical movement of a mobile assembly carrying the vibrating assembly.

Another disadvantage of the machines of the prior art resides in the fact that the production of tools of complex shapes - for obtaining machined parts of equally complex shapes - is expensive and requires significant manufacturing times. Furthermore, it turns out that the tool wears out quickly and that the achievable shapes are limited in finesse and dimensions.

Finally we understand that the price of the tool increases rapidly with the dimensions of the parts to be produced.

Another object of the present invention is to eliminate these other drawbacks of the prior art, and in particular to obtain a machine for machining by ultrasonic abrasion which allows the production of imprints of complex shapes, without being dependent on a tool. costly specific limited in precision and dimensions.

To this end, a machine for machining by ultrasonic abrasion in accordance with the present invention, capable of carrying out contour machining, can also be characterized in that it comprises a digital movement control device capable of moving the workpiece by relative to the tool in two directions perpendicular to each other and perpendicular to the axis of the tool holder.

Thanks to this arrangement and to those which precede, it is understood that the tool can be of very simple geometric shape and of particularly reduced dimensions, since it is by relative displacements between the attack surface of the tool and the part. to machine, in two directions perpendicular to each other, that one can obtain the desired complex shapes of machining, and no longer, as in the prior art, having to give the tool said complex shapes desired. The tool will therefore be inexpensive and can be developed very quickly. It is also important to note that the tool may be unique regardless of the shape to be obtained, since it is possible, thanks to the invention, to contour machining.

Thanks to the two-dimensional displacement device, the tool can for example perform grooving in a closed circuit on the perimeter to be cut, or by sweeping on the surface to be dug. It will also grooving in open circuit on the workpiece is possible by terminating the circuit loop outside the workpiece when the grooving extends to the edge of the workpiece. When the grooving is limited inside the room, the movement system can go back and forth.

When a circuit or a machining sweep has been carried out on the workpiece, another pass can be made, after lowering the tool, following the same circuit or the same sweeping path, and thus obtaining the machining to the desired depth, by as many successive passes as necessary.

A machine according to the present invention, namely established according to the general principles which have just been exposed, may have a certain number of additional characteristics and advantages, which will appear on reading the embodiment given below by way of in no way limitative.

• la figure 1 est une vue schématique générale en élévation d'une fraiseuse à ultrasons conforme à l'invention ;
• la figure 2 représente l'ensemble vibrant de la machine de la figure 1 ;
• la figure 2a est une vue en plan d'un filtre acousto-mécanique ;
• la figure 3 montre les moyens de liaison entre le porte-outil et l'outil, avec des coupes axiales partielles de ces pièces ;
• la figure 4 montre l'outil et le porte-outil assemblés, avec coupe axiale partielle ; et
• la figure 5 est un schéma de principe du système de descente contrôlée de l'outil.

The following description is made with reference to the figures of the appended drawing in which:

• Figure 1 is a general schematic view in elevation of an ultrasonic milling machine according to the invention;
• Figure 2 shows the vibrating assembly of the machine of Figure 1;
• Figure 2 a is a plan view of an acousto-mechanical filter;
• Figure 3 shows the connecting means between the tool holder and the tool, with partial axial sections of these parts;
• Figure 4 shows the tool and the tool holder assembled, with partial axial section; and
• Figure 5 is a block diagram of the tool lowering system.

In FIG. 1, reference is made in 1 to a support assembly intended to carry the parts p to be machined. he it is a set with displacements X, Y, namely that it consists of two tables 2 and 3, one of which (2) can move relative to a fixed frame 4 in a horizontal direction Y, and of which the other (3) can move relative to the previous one in a horizontal direction X perpendicular to the direction Y. Between the table 2 and the fixed frame 4, as well as between the table 2 and the table 3, a sliding high precision can be obtained by any suitable known means, for example by dovetail slides.

The position of tables 2 and 3 is identified at all times by optical reading displacement sensors (not shown). The displacement of these tables is obtained by means of electric motors M y and M x controlled by a computer 5.

The workpiece p is placed in a sprinkling tank 6 for recovering the abrasive liquid, which contains a metal plate for supporting the workpiece p . As shown in FIG. 1, the circulation of the abrasive liquid is ensured by a pump 7 which takes the liquid from the tank 6 and reinjects it there by means of the tool, which has been referenced at 8.

Figure 2, which is a part of Figure 1 shown on a larger scale and in more detail, shows the vibrating assembly, generally referenced at 9.

This vibrating assembly consists of a stack of three distinct cylindrical parts 10, 11 and 12, each of length λ / 2, λ designating the wavelength of the vibrations.

The first part 10 consists of a transducer which comprises two pellets 13 of piezoelectric ceramic (titanate, lead zirconate PZT), prestressed between two cylinders 14, 14 ′ of titanium with identical masses and lengths equal to λ / 4.

The second part 11 is a longitudinal vibration amplitude adapter. It is either made of titanium or duralumin, of bicylindrical shape and of length equal to twice λ / 4. The square of the diameter ratio of the two sections 15, 15 ′ which constitute it defines the adaptation ratio.

The third part 12 is a second adapter more commonly known as a "sonotrode", and terminated by the machining tool 8. Like the second part 11, it is generally of two-cylinder shape and of the same material. The ratio between the diameters of the two parts 16, 16 ′ which constitute it is this time a function of the shape and the mass of the tool 8, whether the latter is in one piece with the sonotrode, or attached.

In FIGS. 3 and 4, it has been shown how the connection between the tool 8 and the sonotrode 12 can be ensured. This connection is ensured by an interlocking with a Morse taper of conicity equal to 5%, a portion of conical outer surface 8 ′ of the tool 8 fitting into a hollow of corresponding shape 16˝ of the terminal portion 16 ′ of the sonotrode 12. This blocking is maintained by a bicylindrical stud 17 with two differential threads 18, 18 ′. The fine pitch thread 18 ′ of the upper large diameter part of the stud engages in a corresponding tapped hole 19 ′ of the sonotrode 12, while the coarse pitch thread 18 of the lower small diameter part of the stud s 'engages in a corresponding tapped hole 19 in part 8' of the tool.

This connection system has the advantage of increasing the contact surface between the sonotrode 12 and the tool 8, and therefore of allowing better transmission of vibrations to the tool, while ensuring their excellent axial coincidence.

In addition, it makes it easy to adjust the vibrating assembly 9 which has just been described on the frequency resonance, by slight displacement of the stud 17 in its housing, which avoids operating by successive touch-ups of the length of the sonotrode.

In FIG. 1, and in more detail in FIG. 2, there is also shown a decoupling system between the vibrating assembly 9 and the frame 4 of the machine, this system being intended to stop the ultrasonic vibrations at the level of the connection between the vibrating assembly and the frame, while maintaining this vibrating assembly perfectly perpendicular to the surface to be machined.

Figures 1 and 2 also show the amplitude of the longitudinal vibrations of the vibrating assembly (in solid lines), as well as the amplitude of the radial vibrations (in dashes).

As shown in particular in Figure 2, when the assembly oscillates in its own mode, the attachment to the frame is made in a node of longitudinal amplitude of vibration. The vibrating assembly is perfectly immobilized in elongation.

To obtain, as mentioned above, the desired radial decoupling, it is advantageous to use the acousto-mechanical filters 20 visible in profile in FIG. 2 and in plan in FIG. 2 a , and which are arranged, as indicated above. be explained above, in nodes (21 and 22) of longitudinal amplitude of the vibrations. Each electro-acoustic filter 20 consists of two concentric rings 23, 24 connected together by three equidistant bridges 25 and connected to the adapter by three other bridges 26 equidistant between them, offset from the first by 60 °. The inner ring 24 is elastically deformable according to the radial vibrations. The more massive outer ring 23 is connected to a movable assembly 27 of the frame by a flange 28 (Figure 1).

The machine also comprises, as already indicated at the beginning, a device for regulating the descent of the tool 8, intended to keep the distance between the attack surface of the tool and the surface to be machined constant.

This device is visible in part in Figure 1 but its general block diagram is provided in Figure 5. In these two figures we used as far as possible the same references to designate the same parts or organs of the device.

So that the interval between the tool and the machining surface remains constant, with a constant pressure on the surface to be machined, it is obviously necessary that the tool 8 descends into the part p as and when this- this is widening. We use the effect of gravity acting on a system of adjustable lever arms and counterweights shown diagrammatically at 29, supporting the mobile assembly 27 to which the vibrating assembly 9 is fixed.

This solution is valid when the machining surface is large; in this case, the counterweights are significant, of the order of a kilogram. But when the machining surfaces are small and, moreover, great finesse of execution is required, the masses involved are such that they do not compensate for the friction of the axes of articulation of the lever arm and those the torque of the return pulleys. The system is then stopped by the dry friction of the assembly, and it is then necessary to load excessively in mass, which introduces significant chipping and even a rupture of the part. In addition, it happens during the descent of the tool that the abrasive does not circulate regularly or that the concentration of abrasive changes. It follows a slowing down, even a descent stop. All the pressure is then transmitted to the part and there is an immediate rupture. To avoid these drawbacks, provides a force sensor which corrects the descent speed. The force sensor consists of a blade 30 embedded at one end and which forms an electrode of a capacitor. The entire weight of the mobile system 9-27 of the machine rests on this blade 30 when the machining is operating correctly. Opposite this electrode, a capacitive sensor is arranged which comprises a detection electrode 31, forming the other electrode of the capacitor associated with the deformation of the blade 30. An electronic chain, with bridge imbalance, corrects the rate of descent of stop.

When the machining speed slows down, for one of the reasons mentioned above, the sensor blade 30 is slightly relieved and the inter-electrode distance (blade-sensor) increases. The system responds by slowing the downhill Mz engine.

• 32 une butée mobile entraînée par le moteur Mz et supportant la lame encastrée 30 ;
• 33 un amplificateur de commande du moteur Mz ;
• 34 un condensateur à capacité de référence ;
• 35 un comparateur du type à déséquilibre de pont, commandant l'amplificateur 33 ; et
• 36 une consigne de vitesse moyenne.

In FIGS. 1 and 5, we have also referred to:

• 32 a movable stop driven by the motor Mz and supporting the embedded blade 30;
• 33 a motor control amplifier Mz;
• 34 a reference capacitor;
• 35 a comparator of the bridge imbalance type, controlling the amplifier 33; and
• 36 an average speed setpoint.

• La surface d'attaque de l'outil est perpendiculaire à l'axe de la sonotrode et reste à une distance constante de la pièce à usiner, cette distance dépendant de la dimension des grains d'abrasif utilisé et de l'amplitude des vibrations ultrasonores, ce qui confère à la pièce réalisée une excellente qualité de finition ;
• Le réglage de la distance entre la surface d'attaque de l'outil et la surface de la pièce à usiner est obtenu par l'intermédiaire d'un capteur de force donnant la pression minimale de contact de l'outil sur la pièce à travers la charge d'abrasif en mouvement ;
• L'outil peut être réalisé dans des matériaux très durs et résistants, par exemple du diamant polycristallin, compte tenu de sa forme simple. Il présente donc une usure bien moindre pour un travail équivalent, ce qui permet une précision d'usinage accrue ;
• Il n'y a pas d'adaptation d'impédance acoustique à chaque changement d'outil (sonotrode standard) ;
• L'ensemble vibrant, des pastilles piézoélectriques jusqu'à l'outil, peut être monobloc. Cet ensemble monobloc, sans solution de continuité par goujons, inévitable par la méthode d'usinage classique, permet la meilleure transmission possible des vibrations à l'outil.
• Il est possible de réaliser un "Kit Ultrasons" adaptable sur toute machine outil à commande numérique ;
• La dimension des pièces n'est plus limitée par la puissance de l'usineuse mais par la longueur et la précision de déplacement des tables X, Y. Cette limite est généralement moins contraignante que celle de la puissance ; et
• Dans l'usinage par contournage, objet de la présente demande, l'outil 8, généralement cylindrique, est parfaitement concentrique à la sonotrode 12.

That said, it can be seen that the invention has the following important advantages:

• The attack surface of the tool is perpendicular to the axis of the sonotrode and remains at a constant distance from the workpiece, this distance depending on the size of the abrasive grains used and the amplitude of the ultrasonic vibrations. , which gives the finished piece an excellent quality of finish;
• The setting of the distance between the leading surface of the tool and the surface of the workpiece is obtained via a force sensor giving the minimum contact pressure of the tool on the workpiece through the moving abrasive charge;
• The tool can be made from very hard and resistant materials, for example polycrystalline diamond, given its simple shape. It therefore has much less wear for equivalent work, which allows increased machining precision;
• There is no adaptation of acoustic impedance at each tool change (standard sonotrode);
• The vibrating assembly, from the piezoelectric pads to the tool, can be in one piece. This one-piece assembly, without solution of continuity by studs, inevitable by the conventional machining method, allows the best possible transmission of vibrations to the tool.
• It is possible to create an "Ultrasonic Kit" adaptable to any numerically controlled machine tool;
• The size of the parts is no longer limited by the power of the machine but by the length and the precision of movement of the X, Y tables. This limit is generally less restrictive than that of the power; and
• In contouring machining, which is the subject of this application, the tool 8, generally cylindrical, is perfectly concentric with the sonotrode 12.

Claims (5)

1. Machine for ultrasonic abrasion machining of the type comprising an assembly (1) supporting the parts (p) to be machined; a vibrating assembly (9) ending in a tool-holder (12) and adapted to drive the tool (8) with a reciprocal movement at ultrasonic frequency and to communicate these vibrations to an abrasive machining liquid; a system for the controlled descent of the tool towards the part to be machined, the attacking surface of the tool (8) being perpendicular to the axis of the tool-holder (12), so that the ultrasonic abrasion machining is carried out at the tool end; a device for regulating the downward movement of the tool; and, between the vibrating assembly (9) and a fixed frame (4), acousto-mechanical filters (20) disposed at longitudinal amplitude nodes (21, 22) of the vibrations, characterized in that an acousto-mechanical filter (20) is formed essentially of two concentric rings (23, 24) connected together by equidistant bridges (25), the inner ring (24) being deformable resiliently under the effect of the radial vibrations.

2. Machine according to claim 1, characterized in that the connection and relative centring between the tool-holder (12) and the tool (8) are provided by means of a stud (17) having two threaded portions (18, 18′) one of these threaded portions (18′) being engaged in a tapped hole (19′) in the tool-holder (12), whereas the other threaded portion (18) is engaged in a tapped hole (19) in the tool (8), the latter being engaged and centred in the tool-holder (12) by a tapered fit (8′, 16˝).

3. Machine according to claim 1 or 2, characterized in that the device for regulating the downward movement of the tool (8) comprises essentially, for maintaining the gap between the tool and the machining surface as well as the pressure exerted by the tool (8) on the surface to be machined at a constant value, a force sensor of capacitive type (30, 31) adapted for controlling, through an electronic regulation chain (33-36), a motor (Mz) controlling the vertical movement of a mobile assembly (27) supporting the vibrating assembly (9).

4. Machine according to claim 3, characterized in that said mobile assembly (27) bears on an embedded blade (30) which forms, with an electrode (31), said force sensor (30, 31).

5. Machine according to any one of the preceding claims, for machining parts by shaping, characterized in that it comprises a digital displacement control device for moving the part to be machined (p) with respect to the tool (8) in two directions (X, Y) perpendicular to each other and perpendicular to the axis of the tool-holder (12).

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