FR2714629A1 - Method and device for deburring mechanical parts. - Google Patents

Abstract

The invention relates to a deburring method in which the part to be deburred (50) is placed in an essentially closed enclosure (10) containing a large number of small balls (30), then it is created by means of a sonotrode (20 ) a strong ultrasonic field for the balls to strike the part to be deburred over its entire exposed surface, and this ultrasonic field is maintained for a predetermined period sufficient to eliminate burrs without affecting the integrity and the state of the surface of the part . The device (100) for implementation comprises an essentially closed enclosure (10) receiving a predetermined quantity of calibrated spherical balls (30), a closure cover (22) to which the part to be deburred (50) is attached, a sonotrode ( 20) arranged to create a powerful ultrasonic field by means of a generator (11) and a transducer (12), and means for varying the amplitude of the vibrations of the sonotrode (20) between approximately 20 and 100 micrometers.

Description

 The invention relates to the field of deburring mechanical parts.

 The term "mechanical parts" must be understood in a very broad sense, as will emerge from the numerous examples of application of the invention which will be explained in the description which follows.

 When the parts to be deburred are hard parts, in particular metal parts, for example stamped or forged parts, parts coming from foundry or cutting, one currently uses either a mechanical-chemical technique, or a purely mechanical technique, or a manual technique.

 The generally used mechanical-chemical technique involves a tonnelage step (the part is struck by logs or pyramidal pawns, metallic or ceramic, of suitable hardness), followed by electrolytic hardening. Such a technique is long, imprecise, and soaking has the drawbacks inherent in the presence of solvents. In addition, this technique is limited as to the types of parts to be treated, since hammering risks deforming sensitive areas (holes, bores for example).

 Purely mechanical techniques are therefore often preferred to the previous technique, and brushing or grinding devices are generally used. Deburring by brushing is currently the subject of extensive research, in particular for carrying out a stepped brushing with special metal brushes (reference may for example be made to document EP-A-0386 303). However, this technique remains long and of limited precision. In addition, brushing or grinding can only concern directly accessible parts of the part to be deburred, which is relatively restrictive as to the types of parts to be treated.

 Manual techniques are also essential when it comes to parts requiring very precise deburring and / or fragile parts (for example sand cores used in foundries). Various tools are then used, such as files, brushes or ball chains. Hand work nevertheless remains long and tedious, and involves costs which are often incompatible with deburring on series of parts.

 The invention specifically aims to solve this problem, by designing a new deburring technique that does not have the drawbacks and limitations of the above techniques.

The object of the invention is therefore to design a technique for deburring mechanical parts which is both efficient in terms of precision and working time, simple to implement, and applicable to very varied types of parts to be deburred. , i.e. both hard parts (metal parts) and fragile parts (sand cores for example), or flexible parts (rubber for example)
I1 is more particularly a method of deburring a mechanical part, characterized in that the part to be deburred is placed in an essentially closed enclosure in which a large number of small balls have been placed, then one creates by means of a sonotrode, a powerful ultrasonic field capable of animating the balls with an intense movement which thus strikes the part to be deburred over its entire exposed surface, and this ultrasonic field is maintained for a predetermined duration sufficient to eliminate the burrs of the part without affecting the integrity and the surface condition of said part.

 According to a particular embodiment, the ultrasonic field is created by setting an acoustic bowl constituting the sonotrode, which bowl forms the bottom and the side walls of the enclosure, said enclosure being closed by a cover to which can be hung the part to be deburred, said cover being held above the upper edge of the bowl, at a distance less than the size of the balls, for the predetermined duration of maintenance of the ultrasonic field.

 According to another embodiment, the ultrasonic field is created by setting an acoustic transmitter constituting the sonotrode, which transmitter has an active face forming at least part of the bottom of the enclosure, the rest of the enclosure being held around the transmitter at a lateral distance less than the size of the balls, with a closure cover to which the part to be deburred can be hung, for the predetermined duration of maintenance of the ultrasonic field. In this case, the closure cover can be kept fixed on the enclosure for the predetermined duration of maintenance of the ultrasonic field, or as a variant be moved horizontally during the maintenance of the ultrasonic field, in order to carry out a continuous deburring of a series of parts, the movement of the cover being chosen such that each individual part remains in the enclosure for the predetermined duration of maintenance of the ultrasonic field.

 If it is a relatively fragile part (sand core for example), it is advantageous that the trajectory of the balls leaving the active face of the emitter is deflected by an intermediate deflector disposed between this active face and the part to be deburred, in order to avoid a too brutal direct strike of said part.

 Advantageously also, the burrs and fractions of burrs are sucked through the bottom of the enclosure, through the gap surrounding the acoustic emitter, while maintaining the ultrasonic field.

 If the part to be deburred is a flexible part (rubber part for example), the process of the invention is then adapted by providing that before placing the part to be deburred in the essentially closed enclosure, this part is subjected to a low temperature heat treatment to be cured, for example by immersion in liquid nitrogen.

 Preferably, the predetermined duration of maintenance of the ultrasonic field is determined by first determining the size and the material of the balls as a function of the nature and the desired roughness of the part to be deburred, then the number of balls. necessary, and finally by calculating the theoretical limits lower TA and upper TOM between which the duration of maintenance of the ultrasonic field must lie.

où ó 2 est la limite d'élasticité du matériau constitutif de la pièce, p est la masse volumique des billes, km est l'amplitude des vibrations de la sonotrode, et fo la fréquence d'excitation de ladite sonotrode, et leur nombre n est déterminé en choisissant leur masse totale M entre les deux valeurs K [(Vo-V1)Ro/t] x K et [(Vo-Vl)Ro/(m] x K, où VO et V1 sont respectivement le volume de l'enceinte et le volume de la pièce à ébavurer, et K un coefficient fixé empiriquement à 1 kg/m3.In particular, the balls are spherical, their radius Ro is chosen as a function of the maximum roughness Ra of the part to be deburred, being between 2tpRa and 3çRa, in which

where ó 2 is the elastic limit of the material constituting the part, p is the density of the balls, km is the amplitude of the vibrations of the sonotrode, and fo the excitation frequency of said sonotrode, and their number n is determined by choosing their total mass M between the two values K [(Vo-V1) Ro / t] x K and [(Vo-Vl) Ro / (m] x K, where VO and V1 are respectively the volume of l 'enclosure and the volume of the part to be deburred, and K a coefficient empirically fixed at 1 kg / m3.

Advantageously then, the theoretical limits lower Tom and upper TOM are respectively given by the formulas

où n est le nombre de billes utilisé, S0 et S, S1 sont respec- tivement la surface de l'enceinte et la surface exposée de la pièce à ébavurer, L est la distance maximale entre la paroi de ladite enceinte et ladite pièce, et N est la partie entière du rapport H/h augmentée de 1, où H est l'épaisseur maximale des bavures et h la profondeur de la cuvette créée dans la pièce par le choc d'une bille individuelle, c'est-à-dire sensiblement 47rRo/tp.

where n is the number of balls used, S0 and S, S1 are respectively the surface of the enclosure and the exposed surface of the part to be deburred, L is the maximum distance between the wall of said enclosure and said part, and N is the whole part of the H / h ratio increased by 1, where H is the maximum thickness of the burrs and h the depth of the bowl created in the part by the impact of an individual ball, i.e. substantially 47rRo / tp.

 Preferably, the amplitude of the vibrations of the sonotrode will be chosen so as to have a theoretical limit of upper duration TOM not exceeding one to two minutes.

 The invention also relates to a device for implementing the aforementioned deburring process, said device being characterized in that it comprises an essentially closed enclosure, a predetermined quantity of calibrated spherical balls placed in said enclosure, a closure cover. to which the part to be deburred can be attached, a sonotrode arranged to create in said enclosure a powerful ultrasonic field thanks to an ultrasonic generator and an associated ultrasonic transducer, as well as means for varying the amplitude of the vibrations of the sonotrode between approximately 20 and 100 micrometers so that the ultrasonic field is just sufficient to eliminate the burrs of the part without affecting the integrity and the surface condition of said part.

 According to a particular embodiment, the sonotrode is an acoustic bowl, and the closure cover is supported above the upper edge of said bowl at a distance less than the diameter of the balls, and preferably at half of this diameter, but nevertheless sufficient to allow the evacuation of burrs under the action of the acoustic wind. In particular, the acoustic bowl is a body of revolution whose geometry is chosen for a specific action of the balls on the part to be deburred.

 According to another embodiment, the sonotrode is a mushroom-shaped acoustic transmitter whose active face forms at least part of the bottom of the enclosure, the remainder of the enclosure being supported around the transmitter at a distance lateral lower than the diameter of the balls, and preferably at half of this diameter, but nevertheless sufficient to allow the evacuation of burrs by the bottom of the enclosure, said enclosure being hermetically closed by a cover to which the part to be attached deburr.

 Advantageously then, the device further comprises means for sucking burrs, arranged below the enclosure. In particular, the suction means comprise a hermetic suction chamber and at least one suction pipe opening into said chamber.

 Furthermore, the closure cover can be wedged, without the possibility of lateral displacement, on the upper edge of the enclosure, or alternatively be arranged in the form of a strip, on the lower face of which are hung parts to deburring in series, said strip being mounted to pass horizontally over the upper edge of the enclosure.

 For deburring a hard part (for example a metal part) or hardened part (for example a rubber part previously soaked in liquid nitrogen), provision may be made for the balls to be made of steel whose HRC hardness is preferably between 58 and 63.

 For deburring a fragile part (for example a core of sand), plastic balls may be provided, the enclosure being preferably also equipped with a protective deflector arranged between the active face of the transmitter and the part to be deburred.

Other characteristics and advantages of the invention will appear more clearly in the light of the description which follows and of the appended drawings, relating to particular embodiments, with reference to the figures where
- Figure 1 illustrates in section a device according to the invention, for implementing the deburring process according to the invention, the mechanical part to be deburred here being a metal part having machining burrs on its external surface and its internal surface, according to an embodiment in which the sonotrode used is an acoustic bowl
- Figure 2 is a detail in section, on a very enlarged scale, illustrating the striking of an individual burr by the balls which are animated by a powerful ultrasonic field
- Figure 3 illustrates in section a device similar to that of Figure 1, for deburring a mechanical part of flexible material (for example rubber) said part having been previously hardened, preferably by immersion in liquid nitrogen
- Figure 4 illustrates in section another device according to the invention, the sonotrode of which is an acoustic transmitter, here arranged to deburr a fragile mechanical part (for example a core of sand), said device being further equipped with a system evacuation of burrs or portions of burrs detached from the part
- Figure 5 illustrates in section yet another variant of the device according to the invention, the workpiece cover is arranged to scroll horizontally while closing the enclosure, which allows for deburring in series and continuously small rooms.

 FIG. 1 illustrates a deburring device 100, comprising an enclosure 10 which is essentially closed, that is to say the closure of which is not completely sealed, for reasons which will be explained later. A mechanical part 50, which here is for example a metallic part having an axial recess on the inside, is placed in this enclosure 10 to undergo a deburring process. The part 50 is here hooked to a closure cover 22 which is held, by a support 23, just above the upper opening of the enclosure 10.

 In accordance with an essential characteristic of the invention, a powerful ultrasonic field capable of animating the balls 30 with an intense movement is created by means of a sonotrode (term traditionally used by specialists to designate an ultrasonic electrode). that these balls strike the part to be deburred 50 over its entire exposed surface (the term "exposed surface" meaning the surface which can be struck by the balls set in motion), therefore here both the external surface and the internal surface of said part, and this ultrasonic field is maintained for a predetermined duration sufficient to eliminate the burrs of the part without affecting the integrity and the surface condition of said part.

 In the embodiment illustrated in FIG. 1, the sonotrode consists of an acoustic bowl 20, made of steel or titanium, said bowl comprising a base 25 and a side wall 26 of revolution, which can be of straight profile or , like here, shifted. The geometry of the acoustic bowl can be chosen for a specific action of the balls on the part to be deburred thus, for a uniform action, we will choose a cylindrical geomoetry with a straight profile, and for a specific action, we will choose a geometry with a suitable profile, straight inclined or curvilinear. With the profile illustrated here for example, the action of the balls will be more energetic on the lower part of the part than on the upper part.

The essentially closed enclosure 10 is therefore here constituted by the acoustic bowl 20 which is the sonotrode, and by the closure cover 22 supporting the part to be deburred 50. Means for generating a powerful ultrasonic field are further provided, and these means are for example constituted by an ultrasonic generator 11 connected to an ultrasonic magnetostrictive transducer 12 (one could also use a piezoelectric transducer). A main winding system 14 associated with the ultrasonic transducer is connected to the generator 11, and a complementary return winding 15 is here provided, which makes it possible to keep the amplitude of the vibrations on the transducer at a predetermined fixed level. Indeed, as will be explained later, the amplitude of the vibrations of the sonotrode constitutes one of the parameters which it is advisable to regulate to obtain an optimal action for the deburring of the mechanical part. Also provided here is a cooling system associated with the ultrasonic transducer 12, for example by circulation of water. The cooling means 16 will preferably be composed of a pump, a water tank and a heating radiator. For the schematic representation given here, a cooling circuit has simply been illustrated, a coil 17 of which is distinguished, an outgoing pipe 18 and a return pipe 19. The sonotrode 20 may be placed directly on the ultrasonic transducer 12, but we preferred here to use an ultrasonic conical concentrator 21 made of steel, which makes it possible to increase the amplitude of the vibrations very significantly. For operator comfort, an insulating enclosure 13 has also been provided covering the acoustic bowl and the ultrasonic transducer. Moreover, the ceiling 13.1 of this enclosure is used here to hang the support 23 serving to maintain the cover 22 at the desired height, thanks to a vertical adjustment system not shown here.

Indeed, the vibrations of the sonotrode must not be communicated to the part to be deburred 50, and therefore to the support cover 22. In practice, the cover 22 will be held above the upper edge 27 of the acoustic bowl 20, at a distance less than the size of the balls 30, and preferably less than half the diameter of the balls which are chosen to be spherical and calibrated. This small distance defines a peripheral passage 24 which is sufficient to allow the evacuation of burrs under the action of the acoustic wind, during the operation of the device. Thus, when the workpiece cover 22 is in the position illustrated in FIG. 1, the interior volume 40 of the enclosure is essentially closed, so that no ball can escape during the disordered movement imparted to the balls by the powerful ultrasonic field generated.

 The ultrasonic generator 11 also comprises means for varying the amplitude of the vibrations of the sonotrode 20 between approximately 20 and 100 micrometers, so that the ultrasonic field is just sufficient to eliminate the burrs of the part, without affecting the integrity. and the surface condition of said part. These means comprise for example digital display keys and / or adjustment knobs arranged on the front face of the cabinet of the ultrasonic generator.

The value of the amplitude of the vibrations of the sonotrode is very important, because it is imperative to avoid an excessively violent action of the balls on the part to be deburred indeed, this action would excessively modify the surface condition of said part. This is how the process of the invention is radically different from the ultrasonic hardening processes of metal parts, which are also called shot peening processes. As such, we can refer to the document
WO-A-93/20247 of the applicant. In fact, in the context of shot peening, on the contrary, it is sought to achieve very energetic bombardment by balls precisely to prestress the surface layer of the mechanical part, which implies an amplitude of vibrations generally greater than 100 micrometers. On the contrary, in the context of the invention, it is sought to achieve a "soft" action just sufficient to deburr the part, which in particular allows the deburring of fragile parts, such as sand cores, which was completely unthinkable with the operating conditions of shot peening. In addition, for shot peening, the acoustic bowl must be made of titanium to have a "hard" action of the balls, while for deburring, we can generally be satisfied with a steel bowl.

 There was therefore a prejudice to overcome in order to imagine using an ultrasound technique to deburr mechanical parts.

 In practice, we can use an ultrasonic generator with a power of the order of 4 KW operating on ultrasonic frequencies from 18,000 to 22,000 hertz. The ultrasonic transducer may be a magnetostrictive transducer with a power of 1 to 1.5 kilowatts, cooled by water, as illustrated in FIG. 1.

 During the vibrations of the walls of the sonotrode 20 corresponding to an ultrasonic frequency of the order of 20,000 hertz, it is possible to create in the working enclosure a powerful ultrasonic field, which gives rise to strong acoustic currents and a radiation pressure. high. The balls 30 are animated by an intense and disordered movement, and, upon contact with the walls of the sonotrode, these balls receive a significant speed and kinetic energy which cause them to bombard the exposed surface of the part to be deburred. The principle of an elastic shock of the balls against the internal wall of the bowl and against the part to be deburred must be absolutely maintained in order to have the desired deburring action. It is important to note that the ultrasonic field is present throughout the available volume of the working enclosure, which in particular allows balls 30 to penetrate into any interior cavities of the parts to be deburred.

After its contact with the wall of the sonotrode, each individual ball is driven by a speed Vo which is substantially equal to 4wf (m, where fo is the ultrasonic frequency, and km the amplitude of the vibrations of the sonotrode.

 To better understand the process of progressive destruction of a burr, reference should be made to the detail in FIG. 2. In this detail, there is a burr 31 projecting from the surface (external or internal) of the part to be deburred 50. The maximum thickness of the burr 31 has been noted H. Each ball striking the burr 31 deforms this burr by printing a spherical bowl 32, the depth of which is noted h, and the surface of which is noted S, The size of this bowl , i.e. the value of the parameters h and Sl, naturally depends on the plastic properties of the material of the part to be deburred the more elastic the material, for example aluminum or an aluminum alloy, the more the bowl will be deep, and on the contrary, the harder the material, for example steel, the smaller the bowl. The size of the bowl also depends on the speed of the ball at the time of its impact on the burr.

où Ro est le rayon de la bille,p la masse volumique de son matériau, et ao 2 la limite élastique du matériau de la pièce à ébavurer. Si l'on note l'expression

la profondeur de la cuvette 32 s'écrit alors h = 4rRo/
Pour éliminer toutes les bavures, il est indispensable que toute la surface de la pièce à ébavurer soit couverte de cuvettes, c'est-à-dire que, sur chaque partie exposée de cette surface, soit porté au moins un coup. La demanderesse a déterminé que, pour couvrir toute la surface de la pièce à ébavurer et aussi les surfaces des bavures avec au moins une frappe de bille, il faut maintenir le champ ultrasonore pendant une durée proche de la valeur
# = (S0+S1) .L.#
4 # .R0.#m.f0.n où n est le nombre de billes utilisé, S0 et S, 1 sont respectivement la surface de l'enceinte et la surface exposée de la pièce à ébavurer, et L est la distance maximale entre la paroi de ladite enceinte et ladite pièce. Cette formule a été obtenue en examinant le mouvement chaotique de n billes ayant une vitesse moyenne de 4ltkm.fo dans un volume fermé qui a une surface intérieure égale à SO+S1. Following theoretical and experimental research carried out by the applicant, it could be established that the depth of the bowl h is defined by the equation

where Ro is the radius of the ball, p the density of its material, and ao 2 the elastic limit of the material of the part to be deburred. If we note the expression

the depth of the bowl 32 is then written h = 4rRo /
To eliminate all burrs, it is essential that the entire surface of the part to be deburred is covered with bowls, that is to say that, on each exposed part of this surface, be struck at least once. The Applicant has determined that, to cover the entire surface of the part to be deburred and also the surfaces of the burrs with at least one ball strike, the ultrasonic field must be maintained for a period close to the value
# = (S0 + S1) .L. #
4 # .R0. # M.f0.n where n is the number of balls used, S0 and S, 1 are respectively the surface of the enclosure and the exposed surface of the part to be deburred, and L is the maximum distance between the wall of said enclosure and said part. This formula was obtained by examining the chaotic movement of n balls having an average speed of 4ltkm.fo in a closed volume which has an interior surface equal to SO + S1.

To completely eliminate a thick burr
H, it is theoretically necessary to carry a number of strokes at least equal to the whole number immediately greater than the H / h ratio, that is to say the whole part of the H / h ratio increased by 1. However, it is advisable to correct this approach insofar as, after each impact, the material constituting the burr hardens. A good approximation leads to consider that after each impact of the balls, the elastic limit of the material increases by 20 to 30 t. Therefore, the depth of the bowl decreases each time, not by an amount h, but by a lesser amount, so that the treatment time increases correspondingly with each impact.

This makes it possible to calculate the theoretical limits lower Tom and upper TOM between which the duration of maintenance of the ultrasonic field must lie. If we denote by N the integer part of the H / h ratio increased by 1, we can write the following two equations giving the theoretical limits lower Tom and upper TOM according to the above parameters

 The summations of the coefficients 1,2 and 1,3 correspond to the increase values from 20 to 30 a estimated for the increase in the elastic limit of the material of the part after each impact of the ball, and this for N impacts.

 As shown by the experimental research carried out by the applicant, the number n of the balls to be used must be chosen in a precise proportion according to the parameters of the working chamber, the amplitude of the vibrations and the diameter of the balls. In practice, it is convenient to use not the number of balls (this number can reach a few hundred thousand), but their mass M which corresponds to M = 4.n.itRo3.p.

3
If we note V0 the volume of the enclosure and Vl the volume of the part to be deburred, we come to choose the total mass
M of the balls between the allowed values of
M [(VO - V1) RO / km] x K and [(V0 - Vl) RO / kmi x K, in which K is a coefficient which can be empirically fixed at one kg / m3.

 Thus, in practice, we will begin by measuring the area SO and the volume VO of the working chamber, then we create an amplitude of vibrations of the walls of the sonotrode, chosen at an average level fixed by the power concerned of the ultrasonic generator and by the structure of the sonotrode used. Then comes the choice of the size and the material of the balls. As regards the material, the elasticity of the shocks against the wall of the enclosure and against the part must be preserved.

In general, steel will be used, in particular steel whose HRC hardness is preferably between 58 and 63, when the part to be deburred is a hard or hardened part (the term "hardened" will be explained below in reference to Figure 3). When it is a question of deburring a fragile part, for example a core of sand, the material of the balls will rather be a less dense material, in particular a plastic material, to avoid that the striking of the part does not alter the integrity of it. As for the radius of the balls used, it will be advantageous to choose this radius according to the maximum roughness of the part to be deburred. Indeed, the size of the balls defines the roughness of the surface after treatment if the diameter of the balls is large, a high roughness will be obtained, while with a small diameter of balls, a lower surface roughness will be obtained. The Applicant has established by theoretical and experimental research that the roughness Ra of the surface after ultrasonic treatment by means of balls with a radius Ro verifies the following inequality
Ro / 29 s Ra 5 Ro / 3 <p, inequality in which the parameter tp is that which has been defined above where the elastic limit of the material of the part to be deburred, the density of the balls, the amplitude of the vibrations are involved , and the ultrasonic frequency. As a result, the radius Ro of the balls will preferably be chosen between 2gRa and 3Ra. After having chosen the radius of the balls, the total mass M of these balls is defined by choosing a value between
M [(Vo - V1 Ro / (m] x K and [(V0 - V,) RO / (m] x K.

This makes it possible to deduce therefrom the number n of the balls which enters the above-mentioned equations giving the theoretical limits lower Tom and upper TOM indicated above.

 The Applicant has experimentally verified the validity of the lower and upper theoretical limits between which the duration of maintenance of the ultrasonic field must lie, as will emerge from two examples given below. Experience has thus been able to demonstrate that with a duration below the lower theoretical limit Tom the burrs are not completely eliminated, while with a duration greater than the upper theoretical limit TOM the burrs are certainly eliminated, but the roughness of the surface increases excessively, so that the deburred part may no longer be suitable for the application intended for it.

 The foregoing formulas make it possible to establish abacuses giving the time intervals in which the duration of maintenance of the ultrasonic field must be situated for a determined amplitude of the vibrations of the sonotrode. We will thus have, in most cases, several produced in the form of an acoustic bowl, and the value of 60 micrometers to a sonotrode of the acoustic emitter mushroom type).

 Figure 3 illustrates in section a device 100 similar to that of Figure 1, for deburring a mechanical part 51 of flexible material, for example rubber.

 In such an application, the deburring method according to the invention can still be applied, provided that a preliminary step is implemented during which the part to be deburred 51 has been previously hardened (in order to recover the aforementioned principle of elastic shock of the balls) for example by being subjected to a heat treatment at low temperature, before being placed in the enclosure 10. For example, it is possible to soak in liquid nitrogen just before placing the part to deburr in enclosure 10 to perform deburring in accordance with the process described above. The part should be cooled to a temperature not higher than that of liquid nitrogen. Indeed, if the cooling is insufficient, the burrs will only be imperfectly removed during the bombardment by the steel balls. The piece of flexible material 51 can be hooked as before under the closure cover 22 which is arranged just above the sonotrode 20.

In FIG. 4, a device 101 has been illustrated which differs from the previous device by the arrangement of its sonotrode and of its essentially closed enclosure.
This figure shows the same means for generating the ultrasonic field, up to the conical ultrasonic steel concentrator 21, so that it is unnecessary to describe these means again. In this variant, the sonotrode is no longer constituted by an acoustic bowl, but by an acoustic transmitter 20 ′ in the form of a mushroom, which transmitter is mounted on the conical concentrator 21.

The essentially closed enclosure 10 is then constituted by a housing 60, the bottom 61 of which has been hollowed out in its center to allow the passage of the upper part of the transmitter 20. The upper face 28, which is the active face of the sonotrode, is then at the level of the face of the bottom of the housing 60 This active face 28 thus forms part of the bottom of the enclosure 10, the remainder of the enclosure being held around the transmitter at a lateral distance less than the size of the balls (circular interval noted 29). The housing 60 is in fact held by the ceiling 13.1 of the acoustic protection enclosure 13, and there is the closing cover 22 to which the part to be deburred 52 is attached, said cover resting here on the upper edge 65 of the housing 60. The interval 29 thus defined around the upper part of the transmitter 20 must have a width which is necessarily less than the diameter of the balls 30, and preferably less than half this diameter, while being sufficient to allow the evacuation of burrs through the bottom of the enclosure 10.

 Thus, unlike the previous device, the side walls of the housing are not vibrated.

The only vibrating part is the active face 28 of the sonotrode 20 '. So that the balls 30 can move easily reaching all the areas of the working space 40 inside the housing 60, a conical deflector 62 is advantageously provided arranged at the bottom 61 of the housing. Furthermore, when the part to be deburred 52 is a fragile part, as may be the case with a sand core, it is advantageous to provide that the trajectory of the balls 30 starting from the active face 28 of the emitter 20 is bent by an intermediate deflector 63 which is disposed between this active face and the part to be deburred 52, in order to avoid too brutal direct striking of said part. The presence of this protective deflector 63, here in the form of a biconvex meniscus (for the deflecting action and to avoid retaining balls on its upper face) makes it possible to provide a powerful regime for bombardment by the balls, without risk of damage the lower edge of the part to be deburred. This lower edge will naturally be touched by balls, but after these have rebounded at least once on the walls of the housing, which walls are not vibrated, so that these balls have a lower kinetic energy. Furthermore, in the context of such an application, preferably plastic balls will be chosen, in order to avoid any risk of altering the integrity of the part, as could have been the case with steel balls. In practice, we will preferably choose a plastic material constituting the balls such that we have a coefficient of restitution at least equal to 0.8 (the coefficient of restitution being the ratio between the speed of a ball after the impact on the surface of the steel or titanium alloy constituting the walls of the enclosure, and the speed of the ball before the shock) in order to preserve the principle stated above of the elastic shock of the balls against the walls of the enclosure and against the part to be deburred. Indeed, if the coefficient of restitution of the balls is lower than the aforementioned value, there is a risk that the balls have insufficiently rapid movement in the ultrasonic field, and that consequently the burrs are not completely eliminated. As for the protective deflector 63, it will be advantageous to provide a support (not shown here) accessible from the outside of the housing 60, making it possible to adjust the height position of said deflector. Indeed, it is advantageous to provide that this protective deflector is located at a distance of the order of half a wavelength from the active face of the sonotrode (the wavelength corresponding to the vibration frequency fo). Indeed, if the deflector is placed too close to the active face of the sonotrode, there is a risk of disturbing the resonance phenomena in the ultrasonic field, and the treatment will then be carried out with an insufficient movement of the balls, so that the burrs will not be completely eliminated This would also unjustifiably increase the processing time. If, on the other hand, the protective deflector is placed at an excessive distance, then the distance of travel of the balls from the active face of the sonotrode to the part to be treated will increase considerably, and the speed of the balls will not be sufficient, so that the burrs will not be completely removed.

 In addition, FIG. 4 distinguishes means 70 for burr suction, arranged below the enclosure 10. It is in fact advantageous to provide, especially if it is a piece of agglomerated powder material , like a core or a sand mold, that burrs and fractions of burrs can be sucked through the bottom of the enclosure 10, through the gap 29 surrounding the acoustic emitter 20 ', while maintaining the ultrasonic field.

Burrs and fractions of burrs already have a natural tendency to exit through the interval 29 under the effect of the acoustic wind, but the efficiency of the evacuation is considerably enhanced if active means of suction are provided, such as those that have been schematized here. These means comprise for example a hermetic suction chamber 71, and at least one suction pipe 72 opening into said chamber.

 In the three applications which have just been described, the cover 22 disposed above the working enclosure 10 was wedged, without possibility of lateral displacement, on the upper edge of the enclosure 10, that is to say just above the upper edge 27 of the acoustic bowl 20 for the device 100 illustrated in FIGS. 1 and 3, and on the upper edge 65 of the housing 60 for the device 101 illustrated in FIG. 4. It is however possible to provide a closure cover movable in an essentially horizontal direction, in order to perform a continuous deburring of several parts supported in series by this movable cover. Such an embodiment is illustrated in FIG. 5.

 In this figure, we find the same means as before for the generation of the powerful ultrasonic field, as well as a sonotrode 20 in the form of a mushroom. However, the enclosure 10 of the device denoted 102 is here produced by a thick plate, so that the active face of the sonotrode 20 forms the entire bottom of the enclosure. The working chamber 40 is then of small size, and has a relatively small height, just sufficient to be able to obtain the desired deburring action by direct projection of the balls onto the small parts 53, mounted on the underside 43 d 'A cover arranged in the form of a continuous strip 42 moving horizontally as shown schematically by the arrow 200. The strip 42 here bears on the upper face 41 of the enclosure, in the manner of a cover, which ensures the closing of the working chamber 40. It is thus a "short" arrangement, with a direct strike by the balls with few rebounds on the side walls of the enclosure. The parts to be deburred 53 are small parts, and there may be mentioned by way of example small stamped metal parts. These parts being small, the burrs are generally short, so that one can afford to concentrate the flow of the balls for their bombardment. We can for example arrange the parts 53 by group of several parts (here three parts) so as to achieve a continuous deburring and scrolling of a series of parts. The speed of movement of the strip 42 will be chosen such that each individual part 53 remains in the enclosure 10 for the predetermined duration of maintenance of the ultrasonic field. In fact, if the movement of the strip 42 was too rapid, the parts to be deburred would remain for an insufficient period of time in the working enclosure, and their deburring would be de facto imperfect. There is provided here a strip 42 passing right through the protective enclosure 13, by the side walls thereof, but it goes without saying that we can provide any other type of arrangement for continuous scrolling of the strip supporting the parts to be deburred.

 There are also the suction means 70 which make it possible to evacuate, by the residual interval 29 surrounding the active face 28 of the sonotrode 20 ', the burrs and fractions of burrs resulting from the action of the balls striking the parts. to deburr.

 According to another variant (not illustrated here), provision may be made for a slightly different arrangement for deburring metal sheets whose cutting edge is to be rounded. The sheet can then be advanced step by step, with or without being carried by the enclosure cover, to obtain its deburring.

 We will now cite two concrete examples of treatments carried out experimentally by the applicant, and which make it possible to verify the validity of the aforementioned theoretical approach.

 In a first example, it is a matter of removing burrs from an aluminum alloy automobile part (jazz = 400 MPa) carried out with the device 100 illustrated in FIG. 1. For greater ease in calculations, we chose cylindrical parts with a multitude of interior holes. An upper roughness limit was imposed after treatment of the order of a micrometer. The maximum thickness H of the burrs was 200 micrometers. The diameter of the steel balls, determined by the theoretical formula exposed above, gives a diameter of 1 millimeter, and, the free volume being 2.5 x 104cl3, it has been deduced that a mass of about 400 was required. grams for all of the beads used. This corresponds to a number of approximately 100,000 balls.

 The time intervals for maintaining the ultrasonic field determined by calculating the theoretical lower 7X and upper TOM limits using the above formulas give 1 to 2 minutes for a vibration amplitude of 20 micrometers, and 18 to 38 seconds for a vibration amplitude of 30 micrometers. The experimental treatment of each part was carried out with an amplitude of 30 micrometers for a period of 30 seconds, and it was possible to observe a perfect deburring of the part. We could also verify that below 18 seconds the deburring was imperfect (we simply observed a hardening of the burrs) and that with a treatment lasting more than 38 seconds the roughness of the surface exceeded 2 micrometers, the quality parts no longer corresponding to the required requirements, not to mention the processing time which becomes excessive and incompatible with industrial rates. Thus, the experiment made it possible to verify the validity of the theory for the deburring technique according to the invention.

 In a second example, we proceeded to eliminate the burrs for small metal parts stamped in steel (at 2 = 800 MPa), with a device 102 conforming to that which is shown diagrammatically in FIG. 5. In the frame from this application, a vibration amplitude of 60 micrometers and a roughness after treatment were chosen which should not exceed one micrometer. The maximum burr thickness was 200 micrometers. From there, it was determined that the radius of the balls should not exceed 0.4 millimeters. As a result, the mass of beads needed should be between 9 and 18 grams. The theoretical lower Tom and upper TOM limits of the duration of the ultrasonic field hold were then determined by calculation, and an interval ranging from 3 to 8 seconds was found. Experimentally, the Applicant has treated four parts simultaneously for 5 seconds and has found that the deburring of these parts is of very high quality. The Applicant has also verified that the ultrasonic treatment carried out for a duration of less than 3 seconds leaves burrs remaining, and that a treatment carried out for a duration greater than 8 seconds induces a transformation of the geometric dimensions of the parts, so that the quality parts no longer met the technical requirements. Again, the experimental results fully corroborate the theory developed.

 We have thus succeeded in devising a technique for deburring mechanical parts which is both efficient in terms of precision and work rates, simple to implement, and applicable to very varied types of parts to be deburred. that is to say both hard parts (metal parts of simple or complex shapes, with bores and / or in the form of sheets with sharp edges) as fragile parts (sand cores for example), or flexible parts (in rubber for example). In addition, the beads are indestructible (to preserve the elastic shock) and can be used again with each new treatment. This avoids having to check the quality of the balls each time.

 The invention is not limited to the embodiments which have just been described, but on the contrary encompasses any variant incorporating, with equivalent means, the essential characteristics set out above.

Claims (22)

 1. Method for deburring a mechanical part, characterized in that the part to be deburred (50, 51, 52, 53) is placed in an essentially closed enclosure (10) in which a large number of small have been placed balls (30), then a powerful ultrasonic field is created by means of a sonotrode (20, 20 ') capable of animating the balls (30) with an intense movement which thus strike the part to be deburred over its entire exposed surface , and this ultrasonic field is maintained for a predetermined duration sufficient to eliminate the burrs of the part without affecting the integrity and the state of the surface of said part.  2. Method according to claim 1, characterized in that the ultrasonic field is created by a vibration of an acoustic bowl (20) constituting the sonotrode, which bowl forms the bottom and the side walls of the enclosure (10 ), said enclosure being closed by a cover (22) to which the part to be deburred can be hung, said cover being held above the upper edge of the bowl, at a distance less than the size of the balls, for the predetermined holding time of the ultrasonic field.  3. Method according to claim 1, characterized in that the ultrasonic field is created by a vibration of an acoustic transmitter (20 ') constituting the sonotrode, which transmitter has an active face (28) forming at least a part from the bottom of the enclosure (10), the remainder of the enclosure (10) being held around the transmitter at a lateral distance less than the size of the balls, with a closure cover (22; 42) to which can be attached to the part to be deburred, for the predetermined duration of maintenance of the ultrasonic field.  4. Method according to claim 3, characterized in that the closure cover (22) is kept fixed on the enclosure (10) for the predetermined duration of maintenance of the ultrasonic field.  5. Method according to claim 3, characterized in that the closure cover (42) is moved horizontally while maintaining the ultrasonic field, in order to carry out a continuous deburring of a series of parts, the movement of the cover being chosen so that each individual part remains in the enclosure (10) for the predetermined duration of maintenance of the ultrasonic field.  6. Method according to one of claims 3 to 5, characterized in that the trajectory of the balls starting from the active face (28) of the transmitter (20 ') is deflected by an intermediate deflector (63) disposed between this active face and the part to be deburred, in order to avoid a too brutal direct striking of said part.  7. Method according to one of claims 3 to 6, characterized in that the burrs and fractions of burrs are sucked through the bottom of the enclosure (10), through the gap (29) surrounding the acoustic transmitter (20 '), while maintaining the ultrasonic field.  8. Method according to one of claims 1 to 7, for deburring a flexible part, characterized in that before placing the part to be deburred (51) in the essentially closed enclosure (10), this part is subjected to a treatment thermal at low temperature to be hardened, for example by immersion in liquid nitrogen.  9. Method according to one of claims 1 to 8, characterized in that the determination of the predetermined duration of maintenance of the ultrasonic field is carried out by first determining the size and the material of the balls (30) as a function of the nature and the desired roughness of the part to be deburred (50, 51, 52, 53), then the number of balls required, and finally by calculating the theoretical limits lower Tom and upper TOM between which the holding time must lie of the ultrasonic field.  10. Method according to claim 9, characterized in that the balls (30) are spherical, their radius Ro is chosen as a function of the maximum roughness Ra of the part to be deburred, being between 2 <Ra and 3tira, in which:

 where a02 is the elastic limit of the material constituting the part, p is the density of the balls, km is the amplitude of the vibrations of the sonotrode (20, 20 '), and fo the excitation frequency of said sonotrode , and their number n is determined by choosing their total mass M between the values K [(VO - V1vRo / (m] x K and [(VO - Vl) RO / (m] x K, where VO and V1 are respectively the volume of the enclosure (10) and the volume of the part to be deburred, and K a coefficient empirically fixed at 1 kg / m3.  11. Method according to claims 9 and 10, characterized in that the theoretical limits lower Tom and upper TOM are respectively given by the formulas:

 where n is the number of balls used, Sa and S1 are respectively the surface of the enclosure and the exposed surface of the part to be deburred, L is the maximum distance between the wall of said enclosure and said part, and N is the part whole of the H / h ratio increased by 1, where H is the maximum thickness of the burrs (31) and h the depth of the bowl (32) created in the part by the impact of an individual ball, i.e. - say substantially 4rRo / ç.  12. Method according to one of claims 9 to 11, characterized in that one chooses the amplitude of the vibrations of the sonotrode (20, 20 ') to have a theoretical limit of upper duration TOM not exceeding one to two minutes .  13. Device for implementing the deburring method according to one of claims 1 to 12, characterized in that it comprises an essentially closed enclosure (10), a predetermined quantity of calibrated spherical balls (30) arranged in said enclosure, a closing cover (22; 42) to which the part to be deburred can be hung (50, 51, 52, 53), a sonotrode (20; 20 ') arranged to create in said enclosure a powerful ultrasonic field thanks to an ultrasonic generator (11) and an associated ultrasonic transducer (12), as well as means for varying the amplitude of the vibrations of the sonotrode (20; 20 ') between about 20 and 100 micrometers so that the ultrasonic field is just sufficient to remove burrs from the part without affecting the integrity and the surface condition of said part.  14. Device according to claim 13, characterized in that the sonotrode is an acoustic bowl (20), and the closure cover (22) is supported above the upper edge (27) of said bowl at a distance less than the diameter of the balls , and preferably at half of this diameter, but nevertheless sufficient to allow the evacuation of burrs under the action of the acoustic wind.  15. Device according to claim 14, characterized in that the acoustic bowl (20) is a body of revolution whose geometry is chosen for a specific action of the balls on the part to be deburred.  16. Device according to claim 13, characterized in that the sonotrode is an acoustic transmitter (20 ') in the form of a mushroom, the active face (28) of which forms at least part of the bottom of the enclosure (10), the remaining of the enclosure being supported around the transmitter (20 ') at a lateral distance less than the diameter of the balls, and preferably at half of this diameter, but nevertheless sufficient to allow the evacuation of burrs by the bottom of the enclosure, said enclosure being hermetically closed by a cover (22; 42) to which the part to be deburred is optionally attached.  17. Device according to claim 16, characterized in that it further comprises means (70) for suction of the burrs, arranged below the enclosure (in).  18. Device according to claim 17, characterized in that the suction means (70) comprise a hermetic suction chamber (71) and at least one suction line (72) opening into said chamber.  19. Device according to one of claims 16 to 18, characterized in that the closure cover (22) is wedged, without the possibility of lateral displacement, on the upper edge (65) of the enclosure (10).  20. Device according to one of claims 16 to 18, characterized in that the closure cover is arranged in the form of a strip (42), on the lower face (43) from which are hung parts to be deburred in series, said strip being mounted to pass horizontally over the upper edge (41) of the enclosure (in).  21. Device according to one of claims 13 to 20, for deburring a hard or hardened part, characterized in that the balls (30) are made of steel whose HRC hardness is preferably between 58 and 63.  22. Device according to one of claims 16 to 20, for deburring a fragile part, characterized in that the balls (30) are made of plastic, and the enclosure (10) is equipped with a protective deflector (63) arranged between the active face (28) of the emitter (20 ') and the part to be deburred.