FR3124749A1 - MACHINE AND METHOD FOR MACHINING CELL(S) IN A METALLIC PART - Google Patents

Abstract

Machine (20) for machining cell(s) (34) in a metal part (24), this machine comprising: - a support frame (22) for the metal part (24) to be machined, - a tool for machining (26) which is carried by a support (28) and which has a generally elongated shape along a machining axis (LL), and - moving members (30) configured to move the workpiece (24 ) relative to the tool (26), or vice versa, characterized in that it further comprises: - a tank (42) filled with a dielectric liquid (44) and in which said part (24) is intended to be immersed ), and in that the machining tool (26) comprises an upstream longitudinal portion (26a) which comprises an electroerosion machining electrode (46) and which is intended to be immersed with the part (24) in the dielectric liquid (44), and a downstream longitudinal portion (26b) which includes a spindle (48) for machining by broaching. Figure for abstract: Figure 8

Description

MACHINE AND METHOD FOR MACHINING CELL(S) IN A METALLIC PART

Technical field of the invention

The present invention relates to a machine and a method for machining cell(s) in a metal part, such as a rotor disk for an aircraft turbine engine.

Technical background

A rotor disc of an aircraft turbomachine comprises at its periphery cells which are intended to receive, by male-female fitting, rotor blade roots. The feet and the cells therefore have complementary shapes which are, for example, dovetail or fir.

This type of cavity can be machined by broaching or by sinking spark erosion.

Broaching is a material subtractive machining technique belonging to the family of conventional machining by chip removal. This operation makes it possible to obtain more or less complex shapes using a single rectilinear translation of a broaching tool, approaching the orthogonal cut in many aspects. The translational speed is called cutting speed. These shapes can be made inside a room as well as outside a room. Examples of shapes that can be broached are shown in .

The kinematics of the broaching operation is simple and allows obtaining a shape in a single linear movement with dimensional and geometric characteristics within repeatably reduced tolerance intervals. Likewise, this type of machining offers good productivity and a correct cost per part despite the high cost of spindles compared to other types of machining and the production of complex shapes.

To obtain the desired shape, a succession of tools called pin elements, and constituting a set of pins, is used. Each element of a set of spindles has a different shape and makes it possible to machine part of the desired shape. A spindle element is shown in .

As with any machining operation, there are different broaching strategies. The major difference compared to other conventional machining processes is that this strategy is not defined by the machining path, but by the choices of evolution of the shape of the successive spindle elements leading to material removal in different areas to achieve the desired shape. For example, the presents the different steps for obtaining a pine-type cell. Each number noted in this figure corresponds to the profile machined by each element of the spindle set in chronological order. In the example given, the elements noted from 1 to 4 are simple elements with straight rectilinear edges making it possible to produce the rough shape of the shape to be machined (creation of a groove) whereas the elements 5 to 7 are elements of more complex shapes to obtain the desired final fir tree shape. For each of these elements, a progression strategy is selected in order to best guarantee the smooth running of the broaching operation.

A broach element is made up of a succession of cutting edges separated from each other by a certain distance called pitch (P) and staggered between them by a value called tooth progression (h) as illustrated in . The quantity of material removed (chip 8) in part 9, from one tooth 10 to the other, is dependent on these two parameters ( ). Thus, the number of teeth 10 and the total length of a spindle element, and by extension the set of spindles, is influenced by these parameters.

Depending on the materials to be machined, the parameters P and h are limited to guarantee the mechanical strength of the tool and the precision of the shape obtained. Thus, the production of a turbine or fan disk cell requires a set of spindles with a length varying from 2 meters to more than 20 meters for large cells.

A broaching operation is carried out on a machine dedicated to this type of operation. As a general rule, the machines are said to be "horizontal", allowing the movement of the tool, or the part, in a horizontal direction. Due to the total length of a spindle set, the machines used are very large. There are so-called "vertical" broaching machines, where the set of broaches is moved from top to bottom, making it possible to reduce the footprint on the ground, but which require a substantial vertical height linked to the number of elements of pin to use.

Electroerosion or spark machining is a material removal technique caused by a thermal erosion phenomenon generated by sparks from controlled electrical discharges between an electrode (tool) and the part immersed in a dielectric liquid. A polarized voltage higher than the breakdown voltage of the dielectric is applied between the tool and the workpiece allowing electrical discharges initiated at the point where the workpiece/electrode distance is the smallest. Thus the successive sparks cause the fusion then the vaporization of the material in order to create a cratering of the machined surfaces.

There shows a representation of a die-sinking EDM machine, with its main components:

- moving parts of the electrode 11 vis-à-vis the workpiece 12 to be machined,

- a digital control system,

- a direct current generator 13 (polarized), connected on the one hand to part 12 and on the other hand to electrode 11,

- a work tank 14 or a sealed cabin, containing a dielectric liquid 15 in which the part 12 and the electrode 11 are immersed, and

- a dielectric liquid filtration system 16 equipped with a pump.

The generator 13 must perform several functions:

- initiating the electric discharges 17,

- maintain voltage and current during discharge,

- ensure a pause time between two successive discharges, and

- provide a repetition of discharges.

The numerical control system ensures the control of the displacement members during machining by endeavoring to maintain constant the distance between the tool (electrode 11) and the part 12 which mainly depends on the programmed Gap voltage.

Making a shape by sinking consists of using a tool of complementary shape and moving it in one direction. The material is gradually removed, and the tool electrode gradually reproduces its shape in the part, in negative, with a slight difference in dimensions linked to the clearance J between the part 12 and the tool. This technique requires the prior manufacture of one or more electrodes. It was the first used in EDM ( Electrical Discharge Machining ), and its main interest is to transfer the difficulty of direct machining of the part, related to the material or the shape to be produced, to the machining of the electrode, which is easier. On the one hand, the material of the electrode (e.g. copper or graphite) is chosen so that it is easier to machine than that of a cutting tool, and on the other hand, its shape is simpler than that of a cutting tool. such a cutting tool.

However, broaching and die-sinking EDM operations have drawbacks.

In the case of broaching, the size of the shape to be broached directly influences the number of pin elements and the total length of the pin set. In order to obtain the desired shape, it is essential to adjust and align several pin elements with each other (typically 9 to 15 elements). During its life, a tool that can be re-sharpened, the relative positioning of each element in relation to the others varies between each assembly. Thus, the procedure for mounting and adjusting the spindle elements is long and tedious.

For the largest broached shapes, it is sometimes impossible to fit all the elements of the broach set on the same line because the total length of the broach set exceeds the capacity of the machine. This therefore involves carrying out the broaching in several stages, sometimes with steps for assembling and dismantling spindle elements depending on the type of machine used. This has the effect of increasing the duration of the operation and the risk of final non-compliance.

Finally, in general, broaching machines have very large dimensions, occupy a large area within a workshop and are not easily moved, which reduces the flexibility of the workshops in which they are located.

In conclusion, the size of the machines, as well as the complexity and cost of the tools used and their adjustment constitute the main difficulties in implementing a broaching operation.

With regard to electroerosion, this machining process, by its principle of melting and vaporization of the material during the discharge as well as its sudden cooling during the cutoff of the discharge, will generate a thermally affected layer on the associated surface. to residual tensile stresses and possibly to the generation of micro-cracks. All of these events inevitably lead to a reduction in the fatigue properties of the material proportionate to the thickness of the disturbed layer at the surface and under the surface, which can reach 50% (in stress).

EDM performance is also limited. The material flow is limited by the energy brought into play by each discharge and by the very principle (fusion-vaporization-ejection). It can reach 1 cm3/min in roughing (energy of the order of 1 Joule). It is a few mm ³ /min in finishing (energy from 10 ^-5 to 10 ^-4 Joule). It mainly depends on the intensity of the current and the duration of the pulse. The material flow is also limited by the evacuation of eroded particles, which is often difficult.

With regard to consumables during an electroerosion operation, the machining phenomenon taking place at both electrodes, the so-called tool electrode also undergoes a removal of material, reflected in production by relative wear. : volume of material lost on the tool, compared to the volume of material removed on the part. Wear is minimized thanks to the use of high-performance materials for the electrodes: materials with a high melting point or vaporization (graphite or tungsten), materials capable of evacuating energy by conduction (copper or copper alloys), or a combination of both (graphite with flakes or copper powder). However, the wear is not zero: it is less than 0.5% in roughing and it can reach 30 to 50% in finishing.

The sinking speed of the spark erosion electrode depends on the section of material to be removed and the operating conditions. Aggressive operating conditions reduce the time required but have a significant impact on the part and cause significant wear of the electrode, while less aggressive conditions reduce the impact on the part and the wear of the electrode, but greatly increase the duration of the operation.

For example in the case of machining steel with a graphite electrode:

- for a target final surface condition Ra < 6.3 µm, the operating conditions to be applied (Duration of pulse = 100 µs; Intensity = 12 A) allow a removal rate of 35 mm ³ /min for volumetric wear of the electrode of the order of 2.5%,

- for a final surface state Ra = 11 µm, the operating conditions to be applied (Duration of pulse = 50 µs; Intensity = 64 A) allow a removal rate of 400 mm ³ /min for volumetric wear of the electrode of the order of 1%.

In conclusion, the spark erosion operation induces on and under the surface of the part a heat affected zone (HAZ), which can induce a reduction in the mechanical characteristics of the material which can be incompatible with its use. Moreover, the wear of the electrode does not always make it possible to obtain sufficient dimensional precision with regard to the accepted tolerances.

The invention proposes a simple, effective and economical solution which makes it possible to solve all or part of the problems of the machining operations of the prior art.

The invention proposes for this purpose a machine for machining cell(s) in a metal part, this machine comprising:

- a frame which is configured to carry the metal part to be machined,

- a machining tool which is carried by a support and which has a generally elongated shape along a machining axis, and

- moving devices configured to move the part relative to the tool, or vice versa,

characterized in that it further comprises:

- a tank filled with a dielectric liquid and in which said part is intended to be immersed,

and in that the machining tool comprises an upstream longitudinal portion which comprises an electroerosion machining electrode and which is intended to be immersed with the part in the dielectric liquid, and a downstream longitudinal portion which comprises a spindle broaching machining.

The proposed technical solution aims to take advantage of the advantages of the two machining technologies, die-sinking EDM and broaching.

This technical solution consists in the production of a machine allowing the machining of a cell using a roughing operation by spark erosion by sinking with an electrode specially designed to minimize the energy required, followed by a finishing operation by broaching, the assembly being carried out in a single movement of the machining tool vis-à-vis the workpiece or of the workpiece vis-à-vis the tool 'machining. For this, the machining tool comprises, one after the other, first an upstream portion (with reference to the direction of movement or machining along said axis) which comprises the electrode machining by electroerosion, and then a downstream portion which comprises the spindle or the set of spindles for machining by broaching.

The machine according to the invention may comprise one or more of the following characteristics, taken separately from each other, or in combination with each other:

• the machining axis is vertical; it is therefore understood that the size of the machine on the ground is limited;
• the machining tool is fixed and the moving members are configured to move the part relative to the tool along a first vertical axis, along a second horizontal axis, or even also around said first axis;
• said downstream portion comprises at most three consecutive spindle elements, and preferably at most two consecutive spindle elements; the number of spindle elements of the machining tool is therefore limited and much lower than the spindles of the prior art, which makes it possible to significantly reduce the length or the height of the machining tool;
• said upstream portion comprises an electrode holder for supporting the electrode, this upstream portion being fixed to the support by means of the electrode holder;
• the machining tool comprises an intermediate spacer arranged between said upstream and downstream portions;
• the spacer has a dimension along said machining axis, which is greater than a dimension of the cell to be machined in the part along this axis;

- the tank contains a volume of dielectric liquid such that the upper level of this liquid in the tank passes through the spacer; the upstream portion of the tool located below this level is immersed in the liquid, and the downstream portion of the tool is located above this level;

• said upstream portion comprises an internal duct for the circulation of dielectric liquid, this duct having an elongated shape along said machining axis and opening out at the two opposite longitudinal ends of the upstream portion;
• the intermediate wedge comprises an internal channel for the circulation of dielectric liquid, a first end of which is connected in leaktight manner to one of the open ends of said conduit, and of which a second opposite end is configured to be connected by a pump, or even also a filter, to said tank;
• the machine further comprises at least one nozzle for projecting cutting liquid onto said downstream portion of the machining tool, said at least one nozzle being connected by a pump, or even also a filter, to said tank; it is therefore understood that the cutting fluid which is configured to lubricate the machining tool and limit its rise in temperature is the same as the dielectric fluid; in other words, the dielectric liquid that is used for EDM machining is also used as cutting liquid during broaching machining; the liquid projected by the nozzle(s) onto the machining tool is then intended to flow into the tank for recycling.

The present invention also relates to a method for machining cell(s) in a metal part, by means of a machine as described above, this metal part being a rotor disk for an aircraft turbine engine, and the machining tool being used to make cells at the outer periphery of the rotor disc which are intended to receive blade roots.

Advantageously, the profile of the electrode is chosen in order to keep an extra thickness of material at the level of the zone to be machined of each cell, intended to be removed by broaching. This extra thickness ensures that the entire area affected by EDM will be removed during broaching machining.

Brief description of figures

Other characteristics and advantages will emerge from the following description of a non-limiting embodiment of the invention with reference to the appended drawings in which:

there is a schematic cross-sectional view of several cells and shapes which can be obtained by a machining operation by broaching;

there is a schematic perspective view of a spindle element for a broaching machining operation;

there is a schematic cross-sectional view of a fir tree cell and shows successive stages of material removal by broaching to obtain this cell;

there is a schematic view in axial section of a broaching tool and shows two successive teeth of this tool;

there is a view similar to that of the and shows the removal of material and the formation of a chip during the machining of a part by the teeth of the tool;

there is a schematic view of a prior art die-sinking EDM machine;

there is a schematic view of a machine for machining cells according to one embodiment of the invention, seen from above;

there is a schematic view of the machining machine of the , front view ;

there is a view similar to that of the and shows a step of a method according to the invention for machining cell(s) in a metal part;

there is a schematic view of the machining machine of the , side view, and shows the stage of the ;

there is a view similar to that of the and shows another step of the process, with an enlarged detail view;

there is a view similar to that of the and shows the other step of the process, with an enlarged detail view;

there is a partial schematic perspective view of a machining tool of the machine according to the invention;

there is a schematic cross-sectional view of an upstream portion of the machining tool;

there is a partial schematic view of the machining tool; And

there is a partial schematic view with partial tear-off of the machining tool.

Claims (11)

Machine (20) for machining cell(s) (34) in a metal part (24), this machine comprising:
- a frame (22) which is configured to carry the metal part (24) to be machined,
- a machining tool (26) which is carried by a support (28) and which has a generally elongated shape along a machining axis (LL), and
- moving members (30) configured to move the part (24) relative to the tool (26), or vice versa,
characterized in that it further comprises:
- a tank (42) filled with a dielectric liquid (44) and in which said part (24) is intended to be immersed,
and in that the machining tool (26) comprises an upstream longitudinal portion (26a) which comprises an electroerosion machining electrode (46) and which is intended to be immersed with the part (24) in the dielectric liquid (44), and a downstream longitudinal portion (26b) which includes a spindle (48) for machining by broaching. Machine (20) according to claim 1, in which the machining axis (LL) is vertical. Machine (20) according to claim 1 or 2, in which the machining tool (26) is fixed and the moving members (30) are configured to move the workpiece (24) relative to the tool along a first axis vertical, along a second horizontal axis, or even around said first axis. Machine (20) according to one of the preceding claims, in which said downstream portion (26b) comprises at most three consecutive spindle elements, and preferably at most two consecutive spindle elements. Machine (20) according to one of the preceding claims, in which the said upstream portion (26a) comprises an electrode holder (50) supporting the electrode (46), this upstream portion (26a) being fixed to the support (28 ) through the electrode holder (50). Machine (20) according to one of the preceding claims, in which the machining tool (26) comprises an intermediate wedge (54) disposed between the said upstream and downstream portions (26a, 26b). Machine (20) (20) according to the preceding claim, in which the intermediate wedge (54) has a dimension (U1) along the said machining axis (LL), which is greater than a dimension (U2) of the cavity to be machined in the workpiece (24) along this axis. Machine (20) according to one of the preceding claims, in which the said upstream portion (26a) comprises an internal duct (51) for the circulation of dielectric liquid (44), this duct (51) having an elongated shape along the said axis of machining (LL) and opening at the two opposite longitudinal ends of the upstream portion (26a). Machine (20) according to the preceding claim, dependent on claim 6 or 7, in which the spacer block (54) comprises an internal channel (56) for the circulation of dielectric liquid (44), one end of which is connected in a leaktight manner at one of the open ends of said pipe (51), and of which a second opposite end is configured to be connected by a pump (56), or even also a filter (58), to said tank (42). Machine (20) according to one of the preceding claims, in which it further comprises at least one nozzle (60) for projecting cutting fluid onto said downstream portion (26b) of the machining tool (26), said at least one nozzle (60) being connected by a pump (56), or even also a filter (58), to said tank. Method of machining cell(s) (34) in a metal part (24), by means of a machine (20) according to one of the preceding claims, this metal part (24) being a rotor disc for an aircraft turbomachine, and the machining tool (26) being used to produce cells (34) at the outer periphery of the rotor disk which are intended to receive blade roots.