EP1525949A1 - Automated method of polishing mechanical titanium or titanium alloy workpieces - Google Patents

Abstract

The method involves moving a wheel (110) with respect to a surface of a semi-finished mechanical part with a determined speed. The wheel comprises an abrasive belt driven in rotation and applied on the part with a determined pressure. The belt comprises super-abrasive grains of industrial diamond or cubic boron nitride. The wheel is supported on a head (100) mounted on a vertical mast (20). An independent claim is also included for a method for manufacturing blades of turbo engines.

Description

The present invention relates to the field of polishing parts mechanical titanium or titanium alloy. It relates in particular turbomachinery blades and in particular large blades such as than fan blades in a turbojet engine and aims in particular at a method of manufacturing such blades implementing said polishing.

For the polishing of mechanical parts, we generally look for low-cost abrasive materials that withstand stress and cause low pollution. Pollution in this area consists of grains of abrasive material that lodge in the mass of the room. She must be avoided imperatively in the blades made of titanium or alloy turbojet titanium.

It is conventionally used for polishing vanes strips abrasive silicon carbide. The band is mounted on a wheel rotated tangentially to the surface of the part. The displacement of the wheel relative to the surface of the room is controlled by a program according to the desired geometry. Parameters such as tape speed on the surface, the speed of movement of the wheel relative to the workpiece as well as the pressure exerted on the surface are determined in such a way as to remove the required thickness of material and ensure a certain surface condition. There is a description of a machine polishing abrasive belt in US Patent 5,193,314

However, this material is not entirely satisfactory.
The bands wear out quickly. For turbojet fan blades, for example, two strips per piece are consumed when geometrically conforming from a semi-finished blank.
The abrasive material pollutes titanium. Precautions must be taken to avoid it.
Deposition of the abrasive on the strips that are commercially available is generally performed by electrostatic means. The regularity of the deposit is not optimal. It leads to a certain dispersion in terms of removal of material. Polishing is not homogeneous. It follows the need to make manual adjustments of the removal of material associated or not rebalancing thicknesses.

Abrasive belt polishing is used in particular in the context of of a method of geometrical conformity of semi-finished blades forging for example. A polishing thickness is removed by polishing determined matter. However the conventional abrasive material removes an insufficient quantity of material by passage of the wheel with the band, which requires additional operations of removal of material and thickness control. Thus a compliance process geometry of a semi-finished forged blade include machining chemical before polishing. After polishing the room a first time with a certain grain, we must proceed again to a chemical machining and manual retouching on straight electric grinders or with reels or other portable machine.

The present invention aims at overcoming the disadvantages encountered with abrasive belts of the prior art.

According to the invention the method of automated polishing of parts mechanical titanium or titanium alloy semi-finished, by means of a machine with an abrasive belt mounted on a tangential contact wheel driven in rotation with a speed and applied with a pressure determined, the wheel moving relative to the surface of the piece with a determined speed, is characterized in that the abrasive belt is consisting of super-abrasive grains of industrial diamond or nitride cubic boron.

After testing, it was surprisingly found that the use of such strips solved problems with abrasive belts conventional.

The abrasive layer of the tape is more accurate. For example in the case of Diamond band formation is done through an electrochemical deposit more homogeneous. The super abrasive grains are supported by a layer of nickel which is itself secured to a polyester base. The layer of nickel absorbs heat and avoids work hardening.

Due to the greater accuracy of the abrasive layer of the strip, the quantities of material are removed with a very thick dispersion low. This low dispersion has a significant advantage in the the geometrical alignment of blades from semi-finished parts with determined thickness. The removal of material is carried out with a deviation from the setpoint sufficiently low for that one stays inside the tolerance band of shape of the blade. We thus avoids the need for manual adjustments by grinding.

• Force d'application de la roue sur la surface de la pièce : 137N à 196N
• Vitesse de défilement de la bande : 4,6 m/s à 18,6 m/s
• Plage d'avance relative de la roue par rapport à la pièce 3,4 m/min à 6,7 m/min.

In particular, for polishing the blades, when it is desired to reduce a specific thickness resulting from the forging of part or the machining, the machine parameters are fixed in the following intervals:

• Application force of the wheel on the workpiece surface: 137N to 196N
• Web speed: 4.6 m / s to 18.6 m / s
• Relative speed of the wheel relative to the workpiece 3.4 m / min at 6.7 m / min.

The extra thickness is between 2/10 and 4/10 mm.

Advantageously, the contact wheel is grooved with grooves arranged obliquely with respect to the axis of rotation of the wheel. The angle is in particular 25 to 35 °.

More particularly the contact surface of the wheel with the band Abrasive has a hardness of 70 Shore.

• la figure 1 représente schématiquement une machine de polissage pour la mise en oeuvre du procédé de l'invention,
• la figure 2 est une vue latérale de la machine de la figure 1,
• la figure 3 représente en coupe une bande utilisée dans le cadre de l'invention.

The invention is described below in more detail with reference to a non-limiting embodiment and with reference to the accompanying drawings in which:

• FIG. 1 schematically represents a polishing machine for carrying out the method of the invention;
• FIG. 2 is a side view of the machine of FIG. 1,
• Figure 3 shows in section a band used in the context of the invention.

The machine has five or six axes of freedom. An example of realization 1 is shown in Figure 1. It is for example a machine commercially available and supplied by Metabo. We see a table 10 with two jaws 11 and 13 between which the elongated piece such as a compressor blade, is kept horizontal. All of the piece with its support can move in this direction X or turn on itself around this axis in the direction U by means of appropriate electric motors Mx and Mu. Above the table, a head 100 is mounted on a vertical pylon 20 and can move along its Z axis. The head 100 can also move in rotation W around this axis. Z axis. Suitable motor means Mz and Mw are provided for the training of the head in these two directions. Finally the head 100 can be move horizontally in the Y direction perpendicular to the direction X and rotate in the direction V around this axis. Driving means My and Mv provide these trips. The head 100 supports a wheel 110 of movable contact about an axis which is fixed relative to itself. A motor mounted on the head 100 ensures the drive of the wheel 110 by the an abrasive belt which is mounted on the periphery of the wheel.

All the motor means is connected to a control box which comprises a control unit with programming means and memories incorporating in particular the geometrical data of the room to polish.

To polish the piece, the strip is applied locally tangentially to its surface by exerting a determined pressure and it sets it in motion. She turns with the wheel around her axis.

The desired removal value and surface condition depend both on the grain of the band but also machine parameters applied as well as characteristics of the contact wheel.

• la force (N) exercée par la roue de contact sur la pièce,
• la vitesse déplacement relative de la bande le long de l'axe de la pièce, ici l'axe X,
• la vitesse de défilement de la bande (m/s) sur la pièce dans la direction de rotation de la roue.

The parameters of a machine thus configured are:

• the force (N) exerted by the contact wheel on the part,
• the speed relative displacement of the strip along the axis of the part, here the X axis,
• the running speed of the web (m / s) on the workpiece in the direction of rotation of the wheel.

These parameters are determined for a well-defined wheel, as well geometrically only according to the quality of the material that constitutes it. By example we use a wheel of a specified width, 25 mm with a determined outer diameter, 120 mm. The wheel has on the surface grooves inclined at 30 °, 3 mm wide and 17 mm apart. The material at the periphery of the wheel is a rubber, hardness determined, for example 70 shore.

Such a machine is used in connection with Geometric conformity and finishing by polishing a semi-finished part.

These operations include a number of steps which are described below. The semi-finished part coming from the forging station has a shape and dimensional characteristics close to the finished part. However, his odds are not yet final because of a determined thickness. In the context of precision forging, this allowance is fixed from 2/10 to 4/10 mm. The polishing operation Automated is intended to remove this extra thickness.

Before polishing, we prepare the semi-finished part

We start with the so-called tri-thickness control by which we check the dimensional data of the part and, if necessary, proceed to the masking the parts of the surface whose thickness is insufficient by report to the data. This rebalancing of the thicknesses can be achieved by application of an adhesive tape.

The next stage of preparation consists of a chemical machining. By medium, the titanium alloys are chemically dissolved in a bath composed of nitric acid, hydrofluoric acid and other agents such as wetting agents or water. The immersion time in the bath determines the amount of material removed. The advantage of machining chemical is to remove a uniform material thickness, regardless of its shape.

If necessary, the two operations are repeated until a determined allowance that the polishing operation is responsible for eliminating.

The polishing operation by passing the workpiece through a machine having an abrasive belt is in itself known. We proceed to a polishing said first appearance.

Conventionally, a strip is used, the abrasive of which is carbide silicon and whose grain is for example 120. We remove a quantity of material of 0.25 +/- 0.1 mm.

Due to the nature of the abrasive belt, the dispersion in quantity of removed material is important.

A second tri-thickness type control mentioned above associated with a chemical machining is necessary.

This control is followed by a step of manual adjustment on a drum; this operation is delicate and can only be performed by qualified personnel. When the blades are large, these manual operations are the possible cause of occupational diseases such as MSDs (musculoskeletal problems).

Finishing polishing is done with a band with a grain larger end. However, because of the dispersion, the removal is, for example, 0.1 mm +/- 0.05 mm. Final validation of geometry with retouching manual is possibly necessary.

According to the invention, a strip is used with super-abrasive grains such as industrial diamond grains or boron nitride cubic.

FIG. 3 shows a schematic section of a strip 200 in representing its structure; the support 210 is a synthetic material based for example polyester. On this support are hung 220 grains of nickel. These are themselves used to support super-abrasive particles such as industrial diamond or cubic boron nitride. The Electrochemical deposition which ensures the formation of a homogeneous abrasive layer.

Such abrasive belts are commercially available and provided for example by the companies 3M, Saint Gobain Abrasives or KGS

Thanks to the homogeneity of its structure, such a band makes it possible removal of material with low dispersion thickness. The precision may be 0.01mm for a strip with a grain of 220 (= 74 μm).

• La plage de force de pression exercée par la roue de contact sur la pièce est de 137N à 196N.
• La plage d'avance de la table est de 3,4 m/min à 6,7 m/min.
• La plage de la vitesse de défilement de la bande abrasive diamant est de 4,6 m/s à 18,6 m/s. La roue de contact utilisée a les caractéristiques suivantes
• Roue de largeur de 25 mm avec un diamètre extérieur adapté à la géométrie des pièces.
• Rainures définies de façon à avoir suffisamment d'agressivité en terme d'enlèvement de matière.
• Matériau constituant la roue adapté à l'opération et de type caoutchouc.

The parameters of the machine have been defined to remove in one passage a thickness of 3/10 mm at most:

• The pressure force range exerted by the contact wheel on the workpiece is 137N to 196N.
• The table advance range is 3.4 m / min to 6.7 m / min.
• The speed range of the diamond abrasive belt is 4.6 m / s to 18.6 m / s. The contact wheel used has the following characteristics
• Wheel of 25 mm width with an outside diameter adapted to the geometry of the pieces.
• Grooves defined to have enough aggressiveness in terms of removal of material.
• Material constituting the wheel adapted to the operation and rubber type.

Once the semi-finished part has been prepared so as to present a overthickness in relation to the desired dimension, precisely defined as either by chemical machining, geometric adjustment by retouching manual (for example using carbide milling cutters on straight grinders electrical devices) or a combination of both operations, one reaches the room presenting the desired dimensions directly by polishing by means of such bands. There is no need for manual adjustment between two polishings one said first appearance, the other said finishing. We stay remarkably within the form tolerance imposed by the specifications.

Polishing first appearance with a 60 grit diamond band (= 250 μm) removes a quantity of material of 0.3 mm +/- 0.05 mm and ensures a state of area of 1.8 μm.

Polishing in the final appearance using a 220 grit diamond band (= 74μm) removes a quantity of material of 0.1mm +/- 0.01mm and ensures a surface state of 0.8 μm.

The final validation operation which consists of a dimensional control and appearance can be carried out without the use of the drum or machine portable polish.

It is also within the scope of the invention to carry out polishing first aspect or blank by any known means such as machining chemical, manual polishing or any mechanical polishing, as far as where polishing finishing or said in final appearance is done by the polishing technique with diamond band.

More generally, the first aspect polishing is performed on a overthickness defined to allow removal of material between 0.1 mm and 0.8 mm, preferably between 0.2 mm and 0.4 mm and more particularly, as reported previously, 0.3 mm +/- 0; 05 mm.

Finishing polishing using the diamond band with a fine grain, according to the invention, is made in such a way as to ensure a removal of material between 0.01 and 0.2 mm +/- 0.01 mm and preferably 0.1 mm +/- 0.01 mm.

Claims (7)

Process for automated polishing of semi-finished titanium or titanium alloy mechanical parts by means of a machine with an abrasive belt mounted on a tangential contact wheel driven in rotation with a speed and applied with a determined pressure, the wheel moving relative to the surface of the workpiece with a determined speed, characterized in that the abrasive belt consists of super-abrasive grains of industrial diamond or cubic boron nitride. Method according to the preceding claim for the polishing of turbomachine blades having a determined thickness, whose machine parameters are set in the following intervals: Application force of the wheel on the workpiece surface: 137N to 196N. Band speed: 4.6 m / s to 18.6 m / s. Relative speed of the wheel relative to the workpiece 3.4 m / min at 6.7 m / min. Process according to the preceding claim, the excess thickness of which is between 1/10 mm and 8/10 mm, preferably between 0.2 and 0.4 mm for a first appearance polishing. Process according to one of Claims 1 and 2, the excess thickness of which is between 0.01 mm and 0.2 mm, preferably 0.1 mm for a polishing in final appearance. Method according to one of the preceding claims, the wheel of which contact is grooved. Process for manufacturing titanium or alloy turbojet blades of titanium comprising making a semi-finished dawn with a overthickness and the elimination of said overthickness by a step comprising at least one polishing step in the final appearance according to one of 1, 2, 4 and 5, the excess thickness before elimination is determined in such a way as to allow the removal of material during a polishing operation first aspect between 0.1 mm and 0.8 mm thickness, preferably between 0.2 mm and 0.4 mm and more particularly 0.3 mm = / - 0.05 mm. Process according to the preceding claim, the elimination of which overthickness is achieved by first-aspect polishing by means of a diamond band.

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