EP0556102A1 - Process for roll-burnishing metallic parts, particularly of light alloy wheels, and apparatus for the application of this process - Google Patents

Abstract

The invention relates to a method for roll-burnishing intended to introduce superficial compressive stresses into metallic parts. In accordance with the invention, the surface finish of the zone of the part (P) which is to undergo the roll-burnishing is first measured, and the width of the roller (G) to be used for the roll-burnishing is deduced therefrom; next, the roller thus selected is positioned on the tool holder (PO), at the end of the associated flexion bar; this roller thus put in place is then applied against the part (P) by exerting, on the said roller, a predetermined load which is a function of the desired superficial compressive stresses; the aforementioned roller is finally used to proceed with the actual roll-burnishing of the part (P). The roll-burnishing process according to the invention is particularly advantageous in the case of wheels made from lightweight alloy, for example made from aluminium or from magnesium. <IMAGE>

Description

The invention relates to a burnishing process for placing compressive stresses on metal parts. These metal parts can in particular be of revolution, although this in no way constitutes a limitation to the field of application of the invention.

It should be remembered that burnishing is a technique using a plastic surface deformation obtained by the pressure of a rolling or sliding tool on the surface of an already roughed part. While moving, the tool compresses the microscopic peaks of the surfaces concerned in the adjacent hollows, and thus allows a densification of these surfaces.

Rolling therefore has both a role of smoothing surfaces and a role of compression of these surfaces. The resulting mechanical effects, on the surface and over a certain depth, allow a significant increase in the service life of equipment and structures subject to cyclic changes (fatigue) or contact corrosion. This technique appears even more effective than shot peening for surface compression stress, and very significantly increases the endurance in fatigue, in stress corrosion, and the resistance to the effect of friction corrosion.

Therefore, burnishing is a very interesting technique for particularly stressed metal parts, as is the case for the wheels of aircraft landing gear, made of light alloy, for example aluminum alloy or magnesium.

The burnishing must therefore relate to the areas of the part subjected to large loads, and also to areas where stress concentrations are to be feared (circular grooves, radii, connecting leaves for example). This operation is carried out by applying a force to one or more rotating rollers, said one or more pebbles are also often driven in advance. The application of this force can be implemented advantageously by using a roller connected to a mobile tool holder by a bending bar.

We have already proposed (EP-A-0330 734) roller assemblies using a pair of parallel elastic blades, the interchangeability of which makes it possible to vary the bearing force, by choosing the blades whose stiffness best suits the room.

Modern roller burnishing techniques use a tool holder which can be combined with a numerically controlled machine tool. In particular, when it comes to parts of revolution, the machine is a lathe which rotates the part to be rolled, and whose spindle (on which the tool holder of the roller is mounted) is movable along two orthogonal axes , including an axis (perpendicular to the axis of revolution of the part) allowing the roller to be applied to the surface to be worked, and an axis (parallel to the aforementioned axis of revolution) allowing the roller to follow the profile of the room.

Specialists are well aware that the application force of the roller must be adjusted according to the type of part concerned, and also to the material of which said part is made.

This adjustment is however delicate and essentially empirical, and the optimal burnishing conditions are only found after multiple checks of the parts after burnishing. Such checks are most of the time of the destructive type, which constitutes a non-negligible drawback in the case of parts whose structure is sophisticated and whose cost is relatively high, as is the case for example for landing gear wheels of light alloy, for example aluminum or magnesium alloy.

For the burnishing of light alloy aircraft wheels, US-A-4 835 826 teaches to use a roller support connected by an omega spring to the spindle of a numerically controlled machine, spindle whose movement is programmed (depending on the geometry of the wheel and the thickness of the zones concerned), the pressure exerted by the roller being then given by the programmed displacement of this spindle. GB-A-881 229 teaches to use a roller support which is on the one hand connected by elastic blades to a tool holder positioned manually, and on the other hand to the rod of a pneumatic actuator with diaphragm exerting the support pressure required on the roller: it is indicated in this document that such a floating elastic mounting of the roller makes it possible to avoid variations in the support force which result from the circularity defects of the roller wheel, and that '' a radius of curvature of 3 mm is suitable for burnishing aircraft wheels.

None of these techniques makes it possible to avoid the aforementioned checks of the parts after burnishing in order to seek to optimize the burnishing conditions.

In addition, the local compression deformation resulting from burnishing is plastic, and therefore irreversible, so that excessively exceeding the desired values of surface compressive stresses results in a scrap of the rolled part.

Therefore, it is necessary to multiply the number of checks and preliminary adjustments to achieve the best roller burnishing conditions, and program the machine tool that will perform the burnishing process accordingly.

The object of the invention is to design a burnishing process and a device for implementing said process, which are more efficient with regard to the aforementioned drawbacks and / or limitations, by enabling burnishing to be carried out under optimal conditions for different types. pieces to be burnished and different constituent materials.

• a) on mesure l'état de surface de la zone de la pièce devant subir le galetage, et on en déduit la largeur du galet à utiliser pour le galetage de ladite pièce ;
• b) on met en place le galet ainsi sélectionné sur le porte-outil, en extrémité de la barre de flexion associée ;
• c) on applique le galet ainsi mis en place contre la pièce concernée en exerçant sur ledit galet un effort prédéterminé qui est fonction des contraintes superficielles de compression désirées ;
• d) on procède au galetage proprement dit de la pièce avec le galet précité.

It is more particularly a method of roller burnishing intended for compressing the surface stresses of metal parts, in particular light alloy wheels, using a roller connected to a mobile tool holder by a bending bar, characterized in that it comprises the successive stages following:

• a) the surface state of the part area to be burnished is measured, and the width of the roller to be used for the burnishing of said part is deduced therefrom;
• b) the roller thus selected is put in place on the tool holder, at the end of the associated bending bar;
• c) the roller thus applied is applied against the part concerned by exerting on said roller a predetermined force which is a function of the desired surface compressive stresses;
• d) the actual rolling of the part is carried out with the aforementioned roller.

Preferably, for step (a) of the aforementioned method, a predetermined curve is used for the type of part concerned and the material of said part, said curve giving the optimal values of the roller widths for a given surface condition.

Advantageously also, one chooses, during step (b), a bending bar whose deflection is determined as a function of the force to be exerted during step (c).

As a variant, when it is a process using a movable angularly adjustable tool holder, during step (c), the inclination of the roller is chosen as a function of the force to be exerted.

It is also advantageous to ensure that, during step (d), the flexion and the compression of the flexion bar are measured in order to verify that the burnishing forces remain within a predetermined range, said process being stopped if the forces come out of said range.

The invention also relates to a device for implementing the above-mentioned burnishing method, comprising a roller connected to a movable tool holder by a bending bar, said device then being characterized in that the roller is removably mounted on an axis extending the bending bar, so as to be able to mount on said axis a roller of predetermined width.

According to a particular characteristic, the rollers which can be used have a peripheral edge of rounded shape. It goes without saying that an elliptical or even angular shape may be provided as a variant.

It may be advantageous for the bending bar to be detachably connected to the tool holder, so as to be able to mount on said tool holder a bending bar of predetermined elasticity.

Another way of adjusting the "compliance" of the tool consists in using a movable tool holder which is angularly adjustable, so that the inclination of the roller relative to the part to be subjected to burnishing is variable.

Advantageously also, the bending bar is equipped with strain gauges making it possible to measure the bending and the compression of said bar, and consequently to control the forces exerted during burnishing.

• la figure 1 illustre schématiquement les différentes étapes successives du procédé de galetage selon l'invention ;
• la figure 2 illustre en coupe un outillage de galetage, à galet amovible, spécialement destiné à la mise en oeuvre du procédé selon l'invention ;
• la figure 3 illustre un dispositif de mise en oeuvre du procédé précité, ici appliqué au galetage d'une jante de roue, qui pourra être en alliage léger tel qu'un alliage d'aluminium ou de magnésium, avec un porte-outil mobile associé à une machine à commande numérique (non représentée).

Other characteristics and advantages of the invention will appear more clearly in the light of the description which follows and of the appended drawing, relating to a particular embodiment, with reference to the figures in which:

• Figure 1 schematically illustrates the different successive stages of the burnishing process according to the invention;
• FIG. 2 illustrates in section a burnishing tool, with a removable roller, specially intended for implementing the method according to the invention;
• Figure 3 illustrates a setting device work of the aforementioned method, here applied to the burnishing of a wheel rim, which may be of a light alloy such as an aluminum or magnesium alloy, with a mobile tool holder associated with a numerically controlled machine (not shown ).

We will first of all endeavor to describe the different successive stages of the burnishing process according to the invention, the burnishing being intended for the surface compression stresses of metal parts, which may or may not be of revolution.

The first step of the method, denoted (a), first of all involves measuring the surface condition of the area of the part to be burnished, as shown on block 10 illustrating a part P which is measured the surface condition (parameter Ra). This measurement of the surface condition is fundamental, because it will make it possible to choose the functional width of the roller suitable for optimal conditions when burnishing the part concerned. Such a measurement of the surface condition before burnishing constitutes an essential parameter in determining the burnishing conditions, in accordance with a characteristic aspect of the invention. The surface condition is traditionally identified by a parameter Ra which is homogeneous at one length.

The width of the roller to be used is determined from preset curves for each type of workpiece to be rolled and for each type of material concerned. Block 21 thus illustrates a curve giving the optimal values of the widths of rollers L for a determined surface condition Ra. This curve, denoted 25, relates to a specific part, and to a particular material. The operator thus has a series of pre-established curves, preferably using scrap parts or raw parts with an extra thickness corresponding to the parts concerned to be burnished. It is interesting to note that the curve 25 is established for a predetermined range of values of the parameter Ra, between two limits Ra1 and Ra2. When the parameter Ra is lower than the limit Ra1, we can no longer carry out burnishing because we cannot plasticize the material with a roller (in reality, the response of the material is then practically elastic, the compressed material being pushed back, so that it there is no work hardening). When the parameter Ra is greater than the value Ra2, this means that it is no longer possible to properly harden the entire surface to be rolled, that is to say that we only succeed in collapsing the peaks but without actually compress sufficiently.

Thus, the measurement of the parameter Ra in accordance with the schematic block 10 makes it possible to deduce the optimal functional width L of the roller to be used in accordance with the schematic block 21, which roller is then selected from the range of the operator, in accordance with the schematic block 20 in which there is a roller G having the functional width L thus chosen. These steps for measuring the state of the surface and for selecting the width of the roller to be used for burnishing, form the first stage (a) of the burnishing process according to the invention.

In a following step, noted (b), the roller G thus selected is put in place on a tool holder PO connected to a movable spindle BM, at the end of an associated bending bar BF fixed to said tool holder. This installation step is only shown diagrammatically in block 30, but the corresponding tool will then be the subject of a more detailed description with reference to FIGS. 2 and 3.

In a step (c), the roller G thus applied is applied against the part concerned P by exerting on said roller a predetermined force which is a function of the desired surface compressive stresses. This operation is shown diagrammatically by the block 40, and a distinction is made between the roller G applied against the workpiece P, forming an angle A, the bending bar BF being bent as a result of the movement of the tool holder PO connected to the movable spindle BM which moves in an essentially "normal" direction X at the plane of the surface P. As has been said above, it is necessary to avoid exerting on the roller an excessive force which would generate surface compressive stresses exceeding the desired value, without the possibility of recovering the part thus incorrectly rolled.

FIG. 1 illustrates a preferred method of obtaining for the choice of the predetermined force to be exerted on the roller G as a function of the desired surface compressive stresses.

The operator has two curves for each part to be rolled and for each material concerned, including a curve 45 associated with the parameter CSC corresponding to the values of surface compressive stresses, and with the parameter EG corresponding to the values of the forces exerted on the roller. , this curve being shown diagrammatically in block 41. In practice, such a curve 45 will be preset for stress values ranging from 0 to the plasticization limit beyond which one would begin to crack the material (for an alloy of aluminum, this will correspond to a value of the order of 400 MPa). Thus, the curve 45 immediately makes it possible to determine the force to be exerted on the roller EG for a given value of surface compressive stresses. The operator then uses a second curve 46, illustrated in the schematic block 42, which corresponds to the aforementioned EG parameter, and to the FBF parameter relating to the deflection of the bending bar at the level of the roller. In this case, the curve 46 is essentially rectilinear, because the bending bar behaves like a spring, and has an arrow proportional to the force exerted. Thus, after having determined the force to be exerted on the roller EG for the desired value of the surface compressive stresses, the operator can easily determine the corresponding bending of the bending bar, and program the machine tool so that the movement of the tool holder in direction X (which is orthogonal to the plane of the surface to be rolled) is performed until a desired bending value for the BF bending bar is obtained. In practice, there will be at least one strain gauge on the bending bar, in order to obtain an instantaneous measurement of the bending of said bar, so that the aforementioned programming of the movement of the tool holder can be done very easily as soon as the operator enters the desired value of superficial compression constraints in the machine. The step which is shown diagrammatically in block 40, may also include the choice of the inclination A of the roller G as a function of the force EG to be exerted. There are of course several means for angularly adjusting the position of the roller G: this angular adjustment may result from an additional degree of freedom of the movable spindle BM, or else from an articulation at the level of the tool holder PO, or even from the choice of tools to be mounted on the movable spindle from a range with variable opening angles.

As a variant, it will be possible to adjust "compliance", by selecting a flexion bar BF whose characteristics are such that this bar has a determined deflection conforming to the value FBF corresponding to the force to be exerted on the roller G. In this case , the BF bending bar must be removably mounted on the PO tool holder, in order to be able to easily disassemble and reassemble.

Once the preliminary steps (a), (b), (c) have been carried out, in accordance with the procedure described above, we can then proceed to the actual burnishing of the part P with the aforementioned roller G, in accordance with the step final (d), as shown in block 50. Note that the schematic representation of the movable spindle BM has degrees of freedom along two orthogonal axes X and Y, so that the roller can follow exactly the profile of the part P to be burnished.

During this burnishing step proper, it it is always possible to modify the displacement of the movable spindle BM in the direction X in order to vary the force exerted on the roller G: it is then advantageous to measure the bending and the compression of the bending bar BF in order to verify that the EG burnishing forces remain well within a predetermined range. By using, for example, strain gauges glued to the bending bar, it will be possible to follow the variations of the parameter EG as a function of time T (this parameter EG depending directly on the parameter FBF representative of the bending of the bending bar). This control is shown diagrammatically by block 51, on which there is a curve 55 illustrating the variations of the parameter EG as a function of time, the values of this parameter having to remain within a predetermined range, between predetermined limit values EG1 and EG2. If the burnishing force goes outside this range, a stop order is automatically sent to the machine tool, which avoids any risk of drift in the surface compressive stresses actually exerted on the part when it is burnished. .

We will now describe the burnishing tool used, with reference to FIG. 2.

The tool comprises a tool holder PO in which is embedded a flexion bar BF, preferably by being blocked by removable means, in order to be able to possibly change the flexion bar. In Figure 3, there are shown bolts 103 for fixing the flexion bar BF on the tool holder PO. It may be a rectangular blade or not, the cross section of which will be chosen according to the desired inertia. The flexion bar BF is extended by an end axis 104 on which the roller G is mounted by means of a bearing 109.

According to an essential characteristic of the invention, the roller G is removably mounted on the axis 104 so as to be able to mount on this axis a roller of predetermined width L. The roller thus comprises a central hub 106 engaged around the associated bearing 109, the axial retention being provided on the one hand by a shoulder 110 of the hub 106, and by a flange 113 fixed with removable means such as bolts 105 at the hub of the roller. The axial maintenance of the bearing 109, and consequently of the roller G, is itself provided by a bearing stop 111 secured to the axis 104, and, at the end of said axis, by a fixing nut 112 or the like. The roller G can thus freely rotate about its axis 150, and it has a disc 107 whose active edge is noted 108. The peripheral edge 108 is here rounded in a semicircle, but it goes without saying that we may choose other shapes, for example an elliptical shape or even an angular shape. More generally, this shape can be optimized case by case depending on the parts to be burnished and the materials concerned.

Also shown, bonded to the flexion bar BF, two strain gauges JE1 and JE2 for measuring the flexion and compression of the flexion bar BF, and therefore to control the forces exerted during burnishing.

In FIG. 3, there is a piece to be rolled P which is here a wheel rim, and PR has been noted the profile of the area to be rolled. We recognize the constituent members already mentioned of the roller burnishing device 100, with a movable spindle BM supporting the tool holder PO by a member 101 here shown schematically in the form of a bar. The link 102 between the bar 101 and the tool holder PO may possibly be articulated, in order to vary the angular inclination of the roller G relative to the part to be rolled, that is to say the angle A that makes the disk 107 of said roller with the "normal" to the surface to be rolled, that is to say in this case perpendicular to the axis YY of rotation of the part P which is here of revolution. Once the preliminary adjustments have been made, in accordance with the process previously described in with reference to FIG. 1, the part P is driven in rotation about its axis YY, which causes the rotation of the roller G applied against it around its axis 150. The displacements of the movable spindle BM in the directions X and Y are programmed in such a way that the roller G follows the profile PR of the part to be rolled, while maintaining the application pressure of said roller at the desired value.

We also distinguish in FIG. 3 the fixing bolts 103 associated with the flexion bar BF, these bolts constituting naturally only an example aimed at making it clear that the mounting of this flexion bar on the tool holder PO can be removable , so that a predetermined elasticity bending bar can be mounted on said tool holder.

The choice of the angle A will depend in practice on the type of compliance that one wishes to have: indeed, the angle A will be chosen small if one wishes to favor the bending of the bending bar BF, or on the contrary large, that is to say close to 90 °, if one wishes to promote the compression of said flexion bar (in the latter case, one practically loses the effects of compliance).

It has thus been possible to design a burnishing process and a device for implementing said process making it possible to burnish under optimum conditions for different types of work pieces and different constituent materials. The preliminary adjustments are now considerably simplified, and any risk of excessive overshoot of the surface compressive stress values is also eliminated. Once the burnishing is finished, there is no need to check the final surface condition which is in any case better than the surface condition measured before burnishing for the choice of the effective width of the roller to be used.

In addition, it is possible to modify the operating conditions during burnishing, for example the force exerted on the roller or the inclination of said roller, or even to change the tooling. In any case, the burnishing process according to the invention makes it possible to produce burnishing for all the possible inclinations of the surfaces to be burnished (horizontal, vertical, or inclined). The control of the bending and compression of the bending bar makes it possible to ensure that the roll-up forces remain within the predetermined range which is suitable. In the event of excessive drift compared to the nominal value chosen, a signal is automatically sent to the machine tool to control its stopping, which constitutes an advantageous security when it comes to parts of sophisticated shape and expensive to manufacture.

Thus, the burnishing method of the invention always makes it possible to choose a roller which makes it possible to obtain with great precision the desired value of surface compressive stresses, while improving the surface condition compared to what it was before burnishing.

The invention is not limited to the embodiment which has just been described, but on the contrary encompasses all variants incorporating, with equivalent means, the essential characteristics set out above.

Claims (10)

Roller burnishing method intended for compressing the surface stresses of metal parts, in particular light alloy wheels, using a roller connected to a movable tool holder by a bending bar, characterized in that it comprises the following successive steps: a) the surface condition of the part area (P) to be subjected to burnishing is measured, and the width (L) of the roller (G) to be used for the burnishing of said part is deduced therefrom; b) the roller (G) thus selected is put in place on the tool holder (PO), at the end of the associated bending bar (BF); c) applying the roller (G) thus placed against the part concerned (P) by exerting on said roller a predetermined force (EG) which is a function of the desired surface compressive stresses; d) the actual rolling of the part (P) is carried out with the aforementioned roller (G). Method according to claim 1, characterized in that, for step (a), a predetermined curve (25) is used for the type of part concerned and the material of said part, said curve giving the optimal values of the widths of roller (L) for a given surface condition. Method according to claim 1 or 2, characterized in that, during step (b), a bending bar (BF) is chosen whose deflection (FBF) is determined as a function of the force (EG) to be exercised during step (c). Method according to claim 1 or 2, in which the mobile tool holder (PO) is also angularly adjustable, characterized in that, during step (c), the inclination (A) of the roller is chosen (G) depending on the effort (EG) to exercise. Method according to one of claims 1 to 4, characterized in that, during step (d), the flexion and compression of the flexion bar (BF) are measured in order to verify that the burnishing forces (EG) remain within a predetermined range, said method being stopped if the efforts go beyond said range. Device for implementing the method according to one of claims 1 to 5, comprising a roller (G) connected to a tool holder (PO) movable by a bending bar (BF), characterized in that the roller ( G) is removably mounted on an axis (104) extending the bending bar (BF), so that a roller of predetermined width can be mounted on said axis. Device according to claim 6, characterized in that the usable rollers (G) have a peripheral edge (108) of rounded shape. Device according to claim 6 or 7, characterized in that the bending bar (BF) is detachably connected to the tool holder (PO), so that an elastic bending bar can be mounted on said tool holder predetermined. Device according to one of claims 6 to 8, in which the movable tool holder (PO) is also angularly adjustable, so that the inclination (A) of the roller (G) relative to the workpiece to be burnished be variable. Device according to one of claims 6 to 9, characterized in that the flexion bar (BF) is equipped with strain gauges (JE1, JE2) making it possible to measure the flexion and the compression of said bar, and consequently to control the forces exerted during burnishing.

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