WO2008087502A1

WO2008087502A1 - An assembly for finishing a three-dimensional surface of a workpiece and machine comprising such an assembly

Info

Publication number: WO2008087502A1
Application number: PCT/IB2007/053767
Authority: WO
Inventors: Costanzo Perico
Original assignee: Costanzo Perico
Priority date: 2007-01-18
Filing date: 2007-09-18
Publication date: 2008-07-24
Also published as: ITBG20070006A1

Abstract

The present invention relates to an assembly (1) for finishing a three-dimensional surface (8) of a workpiece (5). The assembly (1) according to the invention comprises a first device (10) with a material-removing head (11) for removing material from said three-dimensional surface (8). The first device (10) is structured so as to provide one or more degrees of freedom of movement to the material-removing head (11). The assembly (1) also comprises a second device (10), for supporting the workpiece (5), that provides one or more degrees of freedom to said workpiece (5). According to the invention, the sum of the degrees of freedom of movement provided by the first device (10) and the second device (20) is at least five. Control means (9) are also provided to control the first device (10) and second device (20) so that material is removed from the three-dimensional surface (8) according to a direction substantially normal to said surface.

Description

AN ASSEMBLY FOR FINISHING A THREE-DIMENSIONAL SURFACE OF A WORKPIECE AND MACHINE COMPRISING SUCH AN ASSEMBLY

DESCRIPTION

The present invention relates to a machine for finishing a three-dimensional surface of a workpiece, such as the blade of a gas turbine or compressor.

As known, the finishing of a three-dimensional surface represents the final swarf removing step that a workpiece undergoes before it is used. In particular, this procedure is used to bring the dimensions of the workpiece to within the required tolerances in terms of its dimensions and geometry (shape and roughness). In the precision mechanics sector, the surface finishing, polishing and/or belt grinding operations are obviously very important and for this reason they are still done by hand in many cases. This situation represents a technological drawback since any manual operation has a severe fallout on the production time and consequent costs. It is also worth emphasising that, though the human variable in the production cycle sometimes enables high quality peaks to be achieved, it fails to guarantee the repeatability and constancy of the product's final quality.

In the construction of dies or blades for turbo machines, such as those used in steam turbines and/or compressors for turbogas turbines, the finishing operation is still done manually in the majority of cases, by operators using abrasive tools (e.g. files or abrasive paper) to remove tenths or hundredths of a millimetre from the workpiece in order to bring the dimensions and/or shapes of the surfaces of the workpiece within the required tolerances. It is well-known that this abrasive action on the surface generates airborne dust that may be inhaled by the operators. In other words, a "manual finishing" operation intrinsically gives rise to working conditions that are never entirely risk-free for the operator's health. Though the use of conventional face masks helps to alleviate the problem, it fails to improve the situation significantly.

Moreover in the context of the manufacture of dies or turbine blades, semiautomatic machines have been developed that comprise abrasive means guided by a "copying mechanism". An example of such a technical solution, which has now become obsolete, is described in US 2606406. The machine described in this American patent comprises a sample blade that is placed in rotation around its longitudinal axis. A feeler is placed in contact with the blade during a revolving movement of it and is simultaneously displaced along its longitudinal axis. The movement of the feeler is transmitted by a transmission mechanism to an abrasive belt located in the vicinity of a blade being finished, which is placed in rotation in time with the sample blade. The transmission mechanism enables the abrasive belt to move along the blade being finished, substantially copying the positions occupied by the feeler during its displacement along the sample blade.

Like many other similar solutions, this technical solution poses several problems deriving particularly from the operating principle lying behind the machine. In fact, the finishing is done using a sample blade as a reference, that is liable to wear due to the continuous contact with the feeler element. In addition to demanding the regular replacement of the sample blade, this disadvantage also limits the precision of the finish on the blade, which is directly influenced by the wear caused by the feeler.

The machine in question also tends to be rather complicated from the structural standpoint. In particular, the mechanism that transmits the motion from the feeler to the abrasive belt is mechanically rather complex. Among other disadvantages, the large number of components involved increases the risk of breakdowns and machine failures, substantially interfering with the machine's reliability.

In the light of these considerations, there is clearly a need to realise new technical solutions enabling the above-mentioned drawbacks to be overcome.

The main technical aim of the present invention is thus to provide an assembly for the three- dimensional surface finishing of a workpiece that enables the above-mentioned drawbacks to be overcome.

Within the context of this technical aim, one object of the present invention is to provide a finishing assembly that enables a reduction in the machining times and consequently in the cost of the finishing operation and of the production cycle as a whole.

Another object of the present invention is to provide a finishing assembly with an operating principle that enables a good-quality surface finish to be achieved in an entirely repeatable manner.

Another object of the present invention is to provide a three-dimensional surface finishing assembly that does not give rise to working conditions potentially hazardous to the health of the operators employed in the production cycle.

Another, not necessary last, object of the present invention is to provide an assembly for finishing the three-dimensional surface of a workpiece that is reliable and easy to manufacture at a competitive cost.

This technical aim and these objects, which will emerge more clearly from the description that follows, are achieved by an assembly for finishing a three-dimensional surface of a workpiece characterised in that it comprises: - a first automatic device comprising a material-removing head for removing material from said three-dimensional surface, said first device providing one or more degrees of freedom of movement to said material-removing head; a second device for supporting the workpiece that provides one or more degrees of freedom of movement to the workpiece, the sum of the degrees of freedom of movement provided by said first and second devices being at least five;

- control means that control the first and second devices so that said removal of material is performed according to a direction substantially normal to the three-dimensional surface.

One of the main advantages of the finishing assembly according to the invention is that it enables a reduction in the time and cost involved to finish the workpiece. The automation of the process enabled by the presence of the control means reduces to a minimum any action required of the operators, with evident consequent advantages in terms of reliability. Further characteristics and advantages will become more apparent from the description of preferred, but not exclusive, embodiments of the invention, non-limiting examples of which are given in the attached drawings, wherein: figure 1 is a perspective view of a first embodiment of a finishing assembly according to the present invention;

- figure 2 is a perspective view of a second embodiment of the finishing assembly according to the present invention;

- figure 2 A is a perspective view of further embodiment of the finishing assembly according to the present invention;

- figure 3 is a view of a first possible embodiment of a material-removing head of a finishing assembly according to the present invention;

- figure 4 is a perspective view of another embodiment of a finishing assembly according to the present invention; figure 5 is a view of the material-removing head of the finishing assembly shown in figure 4; figure 6 is a perspective view of another embodiment of the assembly for finishing a three-dimensional surface according to the present invention; figure 7 is a perspective view of a machine comprising an assembly for finishing a three- dimensional surface according to the present invention.

With reference to the above-mentioned figures, the assembly 1 according to the invention comprises a first device 10 provided with a material-removing head 11 for removing material from a three-dimensional surface 8 of a workpiece 5. The expression "workpiece" is used here to indicate any type of semiprocessed part, made of any type of material, for which it is necessary to perform a surface finishing procedure in order to complete the production cycle. The first device 10 is designed so as to provide one or more degrees of freedom of movement to said material-removing head 11. As explained in more detail later on, the first device 10 is designed to enable the material-removing head 11 to perform translational and/or rotational movements along pre-established axes in order to come into contact with the three- dimensional surface 8 according to a direction normal thereto.

The assembly 1 according to the invention also comprises a second device 20 for supporting the workpiece 5 requiring surface finishing. The expression "surface finishing" is generically used here to indicate all those mechanically similar procedures, e.g. polishing, lapping or grinding, that are traditionally performed on a workpiece in order to bring the shape and/or size of the surfaces of the workpiece to within pre-established tolerances. The second supporting device 20 is designed so as to provide one or more degrees of freedom of movement to said workpiece 5. In other words, in addition to supporting the workpiece 5, the supporting device 20 also enables its controlled movement that is advantageously exploited to remove material from its surface according to the methods described below. According to the present invention, the sum of the degrees of freedom of movement provided by the first device 10 and by the second device 20 is at least five.

As explained above, the assembly 1 also comprises control means 9 that control the first device 10 and the second device 20 so that material is removed in a direction substantially normal to said three-dimensional surface 8. The control means 9 control the first device 10 and the second device 20, simultaneously moving the material-removing head 11 and the workpiece 5 to enable the head to perform its abrasive action in a direction constantly orthogonal to the three-dimensional surface 8.

The movement of the material-removing head 11 and workpiece 5 is controlled entirely by the control means 9, which substantially oblige the material-removing head 11 to follow a tool path established in advance as a function of the shape and size of the workpiece 5. In particular, the tool path is defined on the basis of a theoretical model of the three-dimensional surface 8 that needs to be finished.

It is clear from the above explanation that the control means 9 automate the procedure for finishing the three-dimensional surface 8 by controlling at least five axes. That means that the sum of the degrees of freedom of movement of the head 11 and workpiece 5 could also, for instance, be six or seven, as explained later on.

The automation of the finishing process clearly eliminates the human variable that currently influence productivity and the reliability of this stage of the production process. The orthogonal condition maintained throughout the removal of material also enables a fine finish to be achieved in an entirely repeatable manner, with obvious advantages from the qualitative standpoint.

Figures 1 and 2 are views of first and second possible embodiments, respectively, of a finishing assembly 1 according to the invention. More precisely in these solutions, the finishing assembly is used to machine the body of a blade intended for use, for instance, in the construction of a stage of a gas turbine. This is obviously just one possible, and consequently not exclusive, application of the assembly according to the invention, which could equally be used, for instance, to finish the surfaces of dies or other workpieces with single- and/or double-curve surfaces.

In the solution shown in figure 1, the first device 10 provides the material-removing head 11 with four degrees of freedom of movement, while the second device provides the workpiece 5 with only one degree of freedom. More precisely, the four degrees of freedom provided to the material-removing head 11 correspond to three translational movements along three corresponding axes of translation X₅Y₃Z (orthogonal to one another) and one rotational movement around a first axis of rotation (reference A).

In the same solution, the degree of freedom provided to the workpiece 5 corresponds to a rotational movement around a second axis of rotation (reference B). In practical terms, in this solution, the control means 9 control the movement of the material-removing head 11 along the axes of translation and around the first axis of rotation A, which preferably coincides with one of the axes of translation X₅Y₃Z. At the same time, the control means 9 also govern the rotation of the workpiece 5 around the second axis of rotation B₃ preferably chosen so that it is parallel to a plane defined by two axes of translation (e.g. the axes X₅Y in figure 1). The second axis of rotation B is preferably chosen so that it is parallel to two planes defined by the set of three axes of translation X, Y₅Z and orthogonal to the third plane defined by said set of three axes. More precisely, in the solution shown in figure I₃ the second axis of rotation B is orthogonal to the first axis of rotation A and parallel to the planes defined by the pairs of axes XY and XZ₅ and consequently orthogonal to the third plane YZ.

In an alternative embodiment to the one previously described, shown in figure 2, the first device 10 is configured so as to provide the material-removing head 11 with three degrees of freedom of movement, while the second device 20 provides the workpiece 5 with two degrees of freedom of movement. More precisely, the three degrees of freedom for the material- removing head 11 correspond to two translational movements respectively along a first axis of translation X and a second axis of translation Y, which are orthogonal to one another, and a rotational movement around a first axis of rotation A. The two degrees of freedom of movement for the workpiece 5 correspond to a rotation around a second axis of rotation B and a translational movement along a third axis of translation Z, orthogonal to the first and second axes of translation X and Y, along which said material-removing head 11 can move. Again in the technical solution shown in figure 2, the third axis of translation Z is orthogonal to the plane defined by the first X and second axes of translation Y, while the second axis of rotation B is orthogonal to the first axis of rotation A, around which the material-removing head 11 turns.

The two solutions described above clearly represent two possible embodiments, but shall not be considered exhaustive of the inventive concept. The control means 9 perform a simultaneous control over five degrees of freedom of movement obtained by combining the degrees of freedom provided to the material-removing head 10 and to the workpiece 5 respectively by the first device 10 and by the second device 20. Said sum can evidently be the result of different combinations, two of which are described above. In a further embodiment, for instance, the second device 20 could be designed to provide four degrees of freedom of movement to the workpiece 5, while the first device 10 could instead afford only one degree of freedom of movement to the material-removing head. In other words, all other possible combinations functionally equivalent to those described herein shall be considered as coming within the scope of the present invention.

In the solution shown in figure 2 A, the first device 10 provides the head 11 with four degrees of freedom of movement, while the second device 20 provides the workpiece 5 with two or more degrees of freedom of movement. In other words, the sum of the degrees of freedom of movement provided by the two devices 10,20 is six or more. In a first option, for instance, the first device 10 may be functionally similar to the one shown in figure 1, while the second device 20 provides the workpiece 5 with a further degree of freedom of movement in addition to that of the second axis of rotation B. In particular, said additional degree of freedom may correspond to a rotation around a third axis of rotation D substantially orthogonal to the second axis of rotation B.

In another possible embodiment, the second device 20 could advantageously provide the workpiece 5 with a further degree or freedom of movement corresponding to a rotation around a fourth axis of rotation A' substantially orthogonal to the second B and third axes of rotation D, i.e. substantially parallel to the first axis of rotation A around which the head 11 revolves. This is on the understanding that said further degree of freedom of movement could also correspond to a translational movement along an axis of translation (e.g. axis Z, shown in figure 2).

The increasing of the number of the degrees of freedom of movement of the workpiece advantageously enables an improvement in the precision of the finishing process, which benefits the final quality of the workpiece 5.

Figure 3 is a perspective view of a possible embodiment of a material-removing head 11 according to the present invention. In particular, the material-removing head 11 comprises a frame 13 and a housing 29 (see figure 1) removably connected to the frame 13. The material- removing head 11 comprises at least one material-removing tool, supported by the frame 13, which comprises at least one operative portion for removing material from the three- dimensional surface 8 of the workpiece 5. From the practical viewpoint, said operative portion can be considered as the part of the material-removing head 11 liable for the abrasion of the three-dimensional surface. The control means 9 thus move the material-removing head 11 and the workpiece 5 so that the contact between said operative portion and the three-dimensional surface 8 is performed according to a direction normal to said surface.

As shown in figure 3, according to a possible embodiment of the invention, the material- removing tool consists of at least a first abrasive belt 15 driven by a first rotary motor 17 also provided on said material-removing head 11.

According to a preferred embodiment of the invention, the assembly 1 comprises a third device, controlled by said control means 9, that provides one or more degrees of freedom of movement for the material-removing tool with respect to the housing 29. By this solution, for instance, the abrasive belt 15 can be rotated around an axis of rotation R orthogonal to the first axis A, around which the head 11 is turned by the first device 10. Said third device consequently moves independently of the first device 10, enabling a further-improved control over the finishing process.

The frame 13 comprises a first supporting portion 13A that supports the first rotary motor 17, and a second portion 13B that extends from said first supporting portion 13 A, defining a substantially L-shaped configuration for the frame 13. As shown again in figure 3, the first abrasive belt 15 is operatively installed between at least one drive pulley 18 A, mounted on the shaft of the first rotary motor 17, and at least a first sliding roller 19 provided at one end 24 of the second portion 13B of the frame 13. In particular, said first roller 19, co-operating with the abrasive belt wound around it, configures the above-mentioned operative portion of the material-removing head 11 that, as explained previously, is responsible for the removal of material from the three-dimensional surface 8. As shown in figure 3, the second portion 13B of the frame 13 is in the shape of a substantially elongated extension from the first portion 13 A of the frame 13. The first roller 19 is hinged to the end of said extension, thereby configuring the operative portion of the material-removing head 11.

Figure 3 shows, moreover, that the frame 13 also advantageously supports a first plurality of transmission rollers 26 that basically define a path for the movement and tensioning of the first abrasive belt 15. Due to the effect of its relative motion, the first belt 15 has an abrasive action on the three-dimensional surface 8 in correspondence of the operative portion defined by the above-mentioned end 24 of the second portion 13B. More in detail, the control means 9 of the assembly 1 control the movement of the material-removing head 11 so that the end 24 of the second portion 13B of the frame 13 approaches the three-dimensional surface 8 of the workpiece 5 according to an orthogonal direction.

According to a preferred embodiment of the invention, the material-removing head 11 preferably comprises extractor means 35 provided to remove the products of the abrasion process generated by the action of the first belt 15 on the three-dimensional surface 8. In the embodiment illustrated in figure I₅ for instance, the extractor means comprise an extractor tube 36 operatively connected to the housing 29 that, as mentioned earlier, is removably attached to the frame 13. Figure 1 also shows a second tubular element 37 advantageously provided to house the wiring of the electrical connections needed to operate the first motor 17. Figure 4 is a perspective view of another embodiment of the assembly 1 according to the present invention that differs from those of figures 1 and 2 in that the material-removing head 11 comprises a first and a second operative portions to obtain different degrees of finishing on the three-dimensional surface 8.

More in detail, in this solution, the material-removing head 11 comprises a second abrasive belt 15B and a second rotary motor 17B that moves said second belt. The second motor 17B is advantageously supported by the first supporting portion 13A of the frame 13, which also comprises a third portion 13C extending from the first 13 A in the opposite direction to the one in which the second portion 13B of said frame extends.

Figure 5 shows the material-removing head 11 shown in figure 4, but without the housing 29. As shown, the second abrasive belt 15B is operatively installed in a manner entirely similar to the first abrasive belt 15, using a second drive pulley 18B mounted on the shaft of the second rotary motor 17B and at least a second sliding roller 19B provided at a corresponding end 24B defined by the third portion 13C of the frame 13. As in the case of the first abrasive belt 17, the frame 13 preferably supports a second plurality of transmission rollers 26B that define a path for the movement and tensioning of the second abrasive belt 15B. As mentioned previously, in the operative configuration in question, the two abrasive belts 15 and 15B can advantageously be chosen so that they achieve a different degree of abrasion. The first abrasive belt 15 can be used, for instance, to perform a first rough finishing action on the three-dimensional surface 8 of the workpiece 5, while the second belt 15B can be used to fine finish and polish said surface. Continuing the same hypothesis, the switch from the rough grinding configuration to the fine polishing configuration can be controlled by the control means 9 used to control the degrees of freedom of movement of the material-removing head 11 and workpiece 5. For instance, if we consider the operation of the assembly 1 shown in figure 4 as being consistent, at least in terms of the combination of degrees of freedom of movement, with the one shown in figure 1, the control means 9 can change the belt by inducing a pre-set rotation of the material-removing head 11 around the above-mentioned first axis of rotation A.

Figure 6 shows a further possible embodiment of the finishing assembly 1 according to the present invention that differs in that the material-removing head 11 comprises scanning means for recording the profile of the three-dimensional surface 8 of the workpiece 5. More in detail, the scanning means are used to scan the three-dimensional surface in order to reconstruct a "real model of the three-dimensional surface". According to a preferred embodiment, laser scanning means are used, comprising a laser sensor 7 that records the characteristics of the three-dimensional surface 8. More precisely, the control means 9 govern the first device 10 and the second device 20 to enable the laser sensor 7 to take the characteristics of the three- dimensional surface 8 according to a direction that is maintained normal to said surface. In other words, the control means 9 govern the movement of the material-removing head 11 and workpiece 5 in a manner entirely similar to when they are used to finish the three-dimensional surface 8, i.e. by making the laser sensor 7 follow a tool path defined by a theoretical model of the three-dimensional surface 8.

According to a preferred embodiment, the control means 9 comprise data processing means that in turn comprise first data analysis means, second comparative means and third data displaying means. In particular, the first data analysis means are used to analyse the real data recorded by the laser sensor 7. By the second comparative means, these real data are compared with a theoretical model of the three-dimensional surface 8 to establish the latter's consistency with or deviation from the model. The third display means are then used to display said consistency and/or deviation of the real data taken by the laser sensor with respect to the theoretical model of the three-dimensional surface. For this purpose, the third display means may, for instance, generate a three-dimensional model of the surface 8 of the workpiece 5, identifying said consistency and/or deviation point by point. Details relating to the structure and operation of the laser sensor are described in the co-pending patent application for an industrial invention No. ITBG2007A000005 entitled "An assembly for analysing a three-dimensional surface of a workpiece and machine comprising said assembly" filed by the same applicant, the description of which is incorporated here for reference. The use of means for scanning the three-dimensional surface is clearly advantageous in that it enables the real characteristics of said surface to be recorded. In particular, said scanning operation can advantageously be done before undertaking any finishing so as to set up the latter operation in the light of the conditions actually recorded. At the same time, scanning the three-dimensional surface after the finishing process can advantageously enable a quality control on the workpiece obtained. It is also important to emphasise that using the laser sensor 7 guided along five axes makes the scanning of the three-dimensional surface extremely speedy. This means, for instance, that the traditional quality control conducted on random samples can be replaced by a quality control implemented on every single workpiece. With reference once again to figures 1 or 2, the second supporting device 20 advantageously comprises first and second supporting elements 21 and 22, that support two substantially opposite ends of the workpiece 5. More in particular, in the solutions shown, the two supporting elements 21 and 22 substantially consist of tailstocks aligned with one another so as to define the second axis of rotation B. More precisely, at least one of the two supporting elements is complete with drive means, governed by the control means 9, that turn the workpiece 5 around said second axis of rotation B. If the workpiece 5 is particularly long or of considerable size, both supporting elements are complete with drive means for turning the workpiece 5. Thus, relatively low-power drive means may be chosen to obtain said rotation, with obvious advantages from the point of view of the final production costs. Solutions functionally equivalent to the one just described shall nonetheless be considered as coming within the scope of the present inventive concept.

The present invention also relates to a machine 2 for finishing workpieces comprising a finishing assembly 1 according to the above description. Figure 7 schematically shows a machine 2 according to the invention complete with an outer housing 40 defining a volume 41 that enables the installation of the finishing assembly 1. More in detail, the outer housing 40 comprises a sliding wall 47 that defines a passage that the operator can use to place the workpiece 5 on the second supporting device 20, or for the set-up of the first device 10. The control means 9 can advantageously be installed on a portion of the outer housing 40 so as to be readily accessible to the operators. As shown, the structure of the machine 2 can advantageously comprise a supporting surface 43 on which the second device 20 is installed according, for instance, to the above specified technical solution. A part of the structure of the machine 2 is then used to install the first supporting device 10 as well as all of the drive means needed for its operation. The technical solutions adopted for the finishing assembly enable the stated technical aim and objects of the invention to be fully achieved. In particular, the finishing assembly according to the invention completely automates the finishing procedure, enabling a high-quality and entirely repeatable finish to be achieved. The assembly according to the invention thus enables a considerable reduction in the current times and costs of the finishing procedure. The finishing assembly according to the invention is susceptible to numerous modifications and variants, all coming within the scope of the inventive concept; moreover, all the parts may consist of other, technically equivalent alternatives.

In practice, any materials may be used and their contingent shapes and sizes adjusted according to need and the state of the art.

Claims

1. An assembly (1) for finishing a three-dimensional surface (8) of a workpiece (5), characterised in that it comprises:

- a first device (10) comprising a material-removing head (11) for removing material from said three-dimensional surface (8), said first device (10) providing one or more degrees of freedom of movement to said material-removing head (11);

- a second device (10) for supporting said workpiece (5) and providing one or more degrees of freedom of movement to said workpiece (5), the sum of the degrees of freedom of movement provided by said first device (10) and said second device (20) being at least five; control means (9) that control said first device (10) and said second device (20) so that said removal of material is performed according to a direction substantially normal to said three-dimensional surface (8).

2. An assembly (1) according to claim 1, characterised in that said material-removing head (11) of said first device (10) has four degrees of freedom of movement, said second device (20) provding one degree of freedom of movement to said workpiece (5).

3. An assembly (1) according to claim 2, characterised in that said four degrees of freedom of movement of said material-removing head (11) correspond to three translational movements along three axes of translation (X, Y₅Z) orthogonal to one another and to a rotational movement around a first axis of rotation (A), said one degree of freedom of movement of said workpiece (5) corresponding to a rotation of said workpiece (5) around a second axis of rotation (B).

4. An assembly (1) according to claim 3, characterised in that said first axis of rotation (A) of said material-removing head (11) corresponds to one of said axes of translation (X₅Y₃Z), said second axis of rotation (B) being parallel to a plane defined by two of said axes of translation (X₅Y₅Z).

5. An assembly (1) according to claim 4, characterised in that said first axis of rotation (A) corresponds to one of said axes of translation (X₅Y₅Z), said second axis of rotation (B) being parallel to two planes defined by said axes of translation (X₅Y₅Z) and being orthogonal to the third plane defined by said axes.

6. An assembly (1) according to claim 5, characterised in that said axes of rotation (A₅B) are perpendicular to one another.

7. An assembly (1) according to claim I₅ characterised in that said material-removing head (11) of said first device (10) has three degrees of freedom of movement, said second device (20) providing two degrees of freedom of movement to said workpiece (5).

8. An assembly (1) according to claim 7, characterised in that said three degrees of freedom of movement of said material-removing head (11) correspond to two translational movements along a first (X) and a second (Y) axes of translation orthogonal to one another, and a rotational movement around a first axis of rotation (A), said two degrees of freedom of movement of said workpiece (5) corresponding to a rotation of said workpiece (5) around a second axis of rotation (B) and a translational movement along a third axis of translation (Z) orthogonal to said first (X) and said second (Y) axes of translation.

9. An assembly (1) according to claim 8, characterised in that said third axis of translation (Z) is orthogonal to the plane defined by said first (X) and said second (Y) axes of translation, said first (A) and said second (B) axis of rotation being perpendicular to one another.

10. An assembly (1) according to claim 3, characterised in that said second device (20) provides at least a second degree of freedom of movement to said workpiece (5) corresponding to a rotation of the workpiece around a third axis of rotation (D) substantially orthogonal to said second axis of rotation (B).

11. An assembly (1) according to claim 10, characterised in that said second device (20) provides a third degree of freedom of movement to said workpiece (5) corresponding to a rotation of the workpiece around a fourth axis of rotation (A') substantially orthogonal to said second (B) and said third (D) axes.

12. An assembly (1) according to claim 10, characterised in that said second device (20) provides a third degree of freedom of movement to said workpiece (5)corresponding to a translational movement of the workpiece along an axis of translation (X₅Y₅Z).

13. An assembly (1) according to one or more of the claims from 1 to 12, characterised in that said material-removing head (11) comprises a frame (13) and a housing (29) removably connected to said frame (13), said material-removing head (11) comprising at least one material-removing tool supported by said frame (13), said material-removing tool comprising at least one operative portion for removing material from said three- dimensional surface (8) of said workpiece (5).

14. An assembly (1) according to claim 13, characterised in that it comprises a third device which provides at least one degree of freedom of movement to said at least one material- removing tool with respect to said housing (14).

15. An assembly (1) according to claim 13 or 14, characterised in that said material-removing head (11) comprises a material-removing tool in the form of a first abrasive belt (15), said material-removing head (11) comprising a first rotary motor (17) for operating said first abrasive belt (15).

16. An assembly (1) according to claim 15, characterised in that said frame (13) comprises a first supporting portion (13A) that supports said rotary motor (17) and a second portion (13B) that extends from said first portion (13A) in a substantially L-shaped configuration, said first abrasive belt (15) being operatively installed between at least one drive pulley (18A) installed on the shaft of said first rotary motor (17) and at least one first roller (19) provided at one end of said second portion (13B) to define said operative portion of said abrasive belt (15).

17. An assembly (1) according to claim 16, characterised in that said second portion (13B) of said frame (13) has an elongated portion extending from said first portion (13A), said first roller (19) being provided at the end of said second elongated portion to configure said operative portion of said abrasive belt (15).

18. An assembly (1) according to claim 16 or 17, characterised in that said material-removing head (11) comprises a plurality of transmission rollers (26) that define a path for the movement and tensioning of said abrasive belt (15).

19. An assembly (1) according to one or more of the claims from 15 to 18, characterised in that said material-removing head (11) comprises a second abrasive belt (15B) and a second rotary motor (17B) for the movement of said second abrasive belt (15B).

20. An assembly (1) according to claim 19, characterised in that said first supporting portion (13A) of said frame (13) supports said second rotary motor (17B), said frame (13) comprising a third portion (13C) that extends from said first supporting portion (13A) along the same line but in the opposite direction to that of said second portion (13B), said second abrasive belt (15B) being operatively installed between at least one second drive pulley (18B) mounted on the shaft of said second rotary motor (17B) and at least one second roller (19B) provided at one end of said second portion (13B).

21. An assembly (1) according to claim 20, characterised in that said material-removing head (11) comprises a second plurality of transmission rollers (26B) that define a path for the movement and tensioning of said second abrasive belt (15).

22. An assembly (1) according to one or more of the claims from 19 to 21, characterised in that said second abrasive belt (15B) has a different abrasive grain from said first abrasive belt (15).

23. An assembly (1) according to one or more of the claims from 1 to 22, characterised in that said first device (10) comprises means for scanning said three-dimensional surface (8) of said workpiece (5).

24. An assembly (1) according to claim 23, characterised in that said scanning means comprise a laser sensor (7), said control means (9) controlling said first device (10) and said second device (20) so that said laser sensor (7) records the characteristics of the three- dimensional surface (8) according to a direction normal to said surface.

25. An assembly (1) according to claim 24, characterised in that said control means comprise processing means including:

- first means for analysing the data recorde by said laser sensor;

- second means for comparing said data recorded by said laser sensor with a model of said three-dimensional surface; third means for displaying the consistency and/or deviation of said data recorde by said laser sensor with said model of said three-dimensional surface.

26. An assembly (1) according to claim 25, characterised in that said display means generate a three-dimensional model of said surface (8), highlighting said consistency and/or said deviation.

27. A machine (2) for finishing a workpiece characterised in that it comprises an assembly for finishing a three-dimensional surface (8) according to one or more of the claims from 1 to 26.