WO2011020775A1

WO2011020775A1 - Tool for machining a cmc by milling and ultrasonic abrasion

Info

Publication number: WO2011020775A1
Application number: PCT/EP2010/061793
Authority: WO
Inventors: Christophe Gérard Régis GROSBOIS; Damien Hebuterne; Alexis Perez-Duarte
Original assignee: Snecma
Priority date: 2009-08-21
Filing date: 2010-08-12
Publication date: 2011-02-24
Also published as: FR2949204B1; FR2949204A1; US20120184184A1

Abstract

The invention relates to a tool for machining hard materials such as metal-matrix or organic-matrix composite materials, comprising a cylindrical sonotrode (22) connected, along the axis (A) of said cylinder, to an assembly vibrating at a predetermined ultrasonic frequency and rotated about said axis, and at least one nozzle (3), on the surface to be machined and at the terminal end (24) of the sonotrode, for ejecting a liquid in which abrasive particles are suspended, said particles being vibrated at said ultrasonic frequency by the sonotrode so as to form a drilling head along the axis (A) of the cylinder, characterized in that the circumference (23) of the sonotrode is covered with particles of a so-called superhard material in order to form a milling head capable of moving in a plane that is substantially perpendicular to said vibratory axis (A).

Description

MACHINING MACHINE FOR CMC BY MILLING AND

ULTRASOUND ABRASION

The field of the present invention is that of the machining of materials and more particularly that of high hardness materials such as ceramic matrix (CMC) or organic matrix (CMO) composites.

Ceramic matrix composite materials are renowned for their difficulty in machining and their abrasive action on tools that attempt to machine them. They are usually cut or rectified by water jet, but then with low precision, and few materials, apart from the diamond, are able to machine them properly while having sufficient lifetimes.

Diamond is most commonly used in machining tools in a polycrystalline form, or PCD (Poly Crystalline Diamond). PCDs are obtained by sintering diamond particles with a chemical mechanical binder, such as cobalt, under high pressure and at high temperature. The cobalt binder allows cohesion of the diamond grains and the combination of the two gives interesting cutting properties to the final tool. However, the cobalt matrix is insufficiently resistant and the diamond grains are gradually torn off during machining, which makes the PCD insufficiently effective in machining ceramics.

Next to this machining by mechanical contact between the tool and the workpiece, there is known a method of machining composites by ultrasound. It is described in particular in patent EP 0362449 of the National Office for Aerospace Studies and Research (ONERA). It relates to a tool, or sonotrode, connected to an assembly vibrating at an ultrasonic frequency which transmits these vibrations to an abrasive, such as boron carbide. The abrasive is suspended in a liquid that is sent to the workpiece between the end of the sonotrode and the workpiece. Particles have the effect of micro-hammering the piece and eroding it. The tool gradually sinks into the room, reproducing its own shape. This method assumes that the gap between the sonotrode and the material is properly controlled. Also known is a French patent application FR 2534512 which describes a sonotrode intended to polish a workpiece after machining. This acts in a manner similar to that described in the ONERA patent, namely that it is subjected to longitudinal vibratory movement in the presence of a jet comprising abrasive particles. It is also driven in a rotational movement on itself, performed at a relatively slow speed (500 to 5000rpm), which is intended to prevent the occurrence of wear of the sonotrode which would not be homogeneous circularly. This rotation movement does not participate in milling.

Whatever the method envisaged for machining parts in CMC or CMO, it does not allow a machining capacity combining good accuracy and a satisfactory cutting speed.

The object of the present invention is to overcome these drawbacks by proposing a method for machining ceramic matrix or organic matrix composite materials which operates at a relatively high speed and which does not result in excessively rapid wear of the tool. used.

For this purpose, the subject of the invention is a machine for machining hard materials such as composite materials with a metal matrix or an organic matrix, comprising a cylindrical sonotrode connected to a vibrating assembly along the axis of said cylinder at a frequency ultrasound determined and rotated about said axis, and at least one nozzle on the surface to be machined, at the end end of the sonotrode, a liquid in which abrasive particles are in suspension, said particles being vibrated at said ultrasonic frequency by the sonotrode so as to form a piercing head along the axis of the cylinder, characterized in that the periphery of the sonotrode is covered with super hard material particles to form a milling head adapted to move in a plane substantially perpendicular to said vibratory axis.

The sonotrode thus acts by a combination of milling actions, both by the terminal end and by its periphery, which makes it possible to increase the speed at which the machining takes place, and / or to attack very hard materials while maintaining a reasonable depth of In a particular embodiment, the so-called super hard material is polycrystalline diamond.

In another embodiment the super hard material is cubic boron nitride.

Preferably, said abrasive particles are made of boron carbide.

Alternatively said abrasive particles are made of polycrystalline diamond.

The invention also relates to a method for machining hard materials such as metal matrix or organic matrix composite materials, using a machine as described above, the sonotrode vibrating in a direction substantially perpendicular to the surface to be machined and moving in a plane substantially perpendicular to its direction of vibration.

Advantageously, the sonotrode is driven in vibrations at a frequency substantially equal to 20 KHz.

Preferably, the speed of rotation of said sonotrode is between 10,000 and 40,000 rpm.

Advantageously the machine operates in successive passes, the depth of each pass being less than or equal to 0.5 mm.

Preferably, the axial forward speed is greater than 500 mm / min.

Even more preferably, the axial forward speed is between 500 and 1000 mm / min.

The invention will be better understood, and other objects, details, features and advantages thereof will appear more clearly in the following detailed explanatory description of an embodiment of the invention given as a purely illustrative and non-limiting example, with reference to the accompanying schematic drawings.

On these drawings:

- Figure 1 is a schematic view of the operation of an ultrasonic milling machine according to the prior art;

- Figure 2 is a schematic view of the operation of a milling machine by rotation and vibration of the tool, according to the prior art; - Figure 3 is a schematic view of the operation of an ultrasonic milling machine which is associated with a rotation of the tool, according to one embodiment of the invention;

- Figures 3, 4 and 5 are successive schematic views of the progress of a machining operation with a machine according to the invention;

- Figure 7 is a detailed view of the machining of a workpiece by the tool of a machine according to the invention.

Referring to Figure 1, we see an ultrasonic machining machine, similar to that described in patent application EP 0362449, for machining a workpiece 1 positioned in front of a sonotrode 2. The machine transforms an alternating electric current at a frequency of approximately 20 KHz, which corresponds in the air to the ultrasound range, into mechanical vibrations of the same frequency which are applied to the sonotrode 2. The sonotrode 2 is driven by a movement vibratory back and forth in a direction A parallel to its axis of symmetry. The sonotrode 2 serves as a machining tool through very hard abrasive particles, such as boron carbide, which are projected against the material to be machined. Nozzles 3 are positioned for this purpose, next to the sonotrode 2, and send on the surface to be machined, at the end end 4 of the sonotrode, a jet of water in which the abrasive particles are in suspension. Because the water transmits the ultrasonic frequencies well, these particles are excited by the vibrations of the sonotrode 2 and animated by a vibratory movement on the same frequency of 20 KHz. They then enter the surface to be machined, causing a deformation which is followed by a removal of material in the form of micro-chips.

Referring now to Figure 2, there is shown a machine, such as milling cutter or milling cutter, machining by rotation of a tool 12, which is also associated with a vibration of the tool. The cutter 12 is covered at its end end forming drill 14 and on its circumference milling head 13, an abrasive material, such as polycrystalline diamond. The tool is conventionally rotated by the machine and brought into contact with the material 1 to be machined. In addition to its rotational movement, the tool 12 is animated with a vibratory movement in the direction A parallel to its axis of rotation. symmetry that causes a periodic attack of the material, like a percussion drill.

In Figure 3 we see a machining machine according to the invention, suitable for machining very hard materials such as composite materials CMC or CMO. It comprises, as before, a tool 22, which is here both office milling head and sonotrode. It is for this reason, on the one hand animated vibratory movement along its longitudinal axis at a frequency of 20 KHz to act as sonotrode through its terminal end 24 and, secondly, driven in rotation to act as a milling head via its cylindrical periphery 23. This periphery 23 is covered over a certain height of so-called super hard materials, such as polycrystalline diamond or cubic boron nitride, which have a hardness substantially equal to that of the diamond. Its end 24 may be smooth, unlike the tool of Figure 2, or be covered with diamond to improve its abrasion resistance. On either side of the rotary sonotrode 22 are, as in the case of FIG. 1, placed nozzles 3 which project abrasive particles suspended in a flow of water oriented towards the composite material 1. These particles, d a diameter of a few tens of microns, may be boron carbide, silicon carbide or polycrystalline diamond.

With reference to FIGS. 4 to 6, the method used for machining CMCs or CMOs using a machine as described above proceeds as follows:

Tool 22, coated with polycrystalline diamond, typically has a diameter of between 5 and 15 mm and is rotated at a speed of between 10,000 and 40,000 rpm. It is also vibrated, along the axis A, at a frequency of 20 kHz by an acoustic block consisting of piezoelectric ceramics, whose mechanical amplitude can be adjusted and vary between 5 and 100 microns. This vibration amplitude is compatible with the imperative of controlling the gap between the sonotrode and the material to be machined. A jet of water containing particles of boron carbide or diamond is injected in front of the rotary sonotrode 22 by means of the nozzles 3.

The tool 22 is first positioned facing the surface to be machined that it attacks (FIG. 4) in the same way as a sonotrode of the prior art, with axial progression. Once a predetermined depth, known as the depth of pass, is reached, the probe continues to vibrate and to be fed by the nozzles, but begins to move laterally (FIG. 5) to attack the material by its periphery 23 covered with polycrystalline diamond. From this moment, the progression of the machining (FIG. 6) is carried out simultaneously by an abrasion of the material situated opposite the end end 24 of the rotary sonotrode 22 due to the abrasive particles moved by the ultrasounds, and by a mechanical attack by the lateral surface 23 of the sonotrode. When the material 1 has been machined over its entire length, the operator starts a new pass by returning to the starting point and resuming the operations at the level of FIG.

Referring to FIG. 7, the manner in which the rotary horn 22 enters the material to be machined 1 is seen. Its periphery 23 attacks the material on a height h1, while the end 24 digs the surface of the material to a second height. h2. Finally, the height of the pass obtained is equal to the sum of the two heights hl + h2.

This generates a lateral progression in the material, in the manner of a milling cutter and in practice a milling depth of up to 0.5 mm per pass is obtained, which can be supported with feed rates. advancement between 500 and 1000 mm / min, that is to say speeds that can be considered high for machining a composite material CMC or CMO.

The combination of an ultrasonic machine, with a water jet mixed with abrasive particles, with a rotating spindle driving a diamond-type tool thus makes it possible to obtain precise machining and to produce complex shapes with speeds of up to advancement, which did not allow the techniques of earlier machines used separately.

Although the invention has been described in connection with a particular embodiment, it is obvious that it includes all the technical equivalents of the means described and their combinations if they fall within the scope of the invention.

Claims

1. Machine for machining hard materials such as metal matrix or organic matrix composite materials, comprising a cylindrical sonotrode (22) connected to a vibrating assembly along the axis (A) of said cylinder at a determined ultrasonic frequency and driven in rotation about said axis, and at least one nozzle (3) for sending on the surface to be machined, at the end end (24) of the sonotrode, a liquid in which abrasive particles are suspended said particles being vibrated at said ultrasonic frequency by the sonotrode so as to form a piercing head along the axis (A) of the cylinder,

characterized in that the periphery (23) of the sonotrode is covered with super hard material particles to form a milling head adapted to move in a plane substantially perpendicular to said vibratory axis (A).

2. Machine according to claim 1 wherein the super hard material is polycrystalline diamond.

3. Machine according to claim 1 wherein the super hard material is cubic boron nitride.

4. Machine according to one of claims 1 to 3 wherein said abrasive particles are made of boron carbide.

5. Machine according to one of claims 1 to 3 wherein said abrasive particles are made of polycrystalline diamond.

6. A method of machining hard materials such as metal matrix or organic matrix composite materials, using a machine according to one of claims 1 to 5, the sonotrode (22) vibrating in one direction ( A) substantially perpendicular to the surface to be machined and moving in a plane substantially perpendicular to its direction of vibration.

7. The method of claim 6 wherein the sonotrode (22) is driven in vibration at a frequency substantially equal to 20 KHz.

8. Method according to one of claims 6 or 7 wherein the speed of rotation of said sonotrode is between 10 000 and 40

000 rpm.

9. Method according to one of claims 6 to 8 wherein the machine operates in successive passes, the depth of each pass being less than or equal to 0.5 mm.

10. Method according to one of claims 6 to 9 wherein the axial forward speed is greater than 500 mm / min.

11. The method of claim 10 wherein the axial forward speed is between 500 and 1000 mm / min.