WO1997017173A1

WO1997017173A1 - Machining cell and method for machining of a piece

Info

Publication number: WO1997017173A1
Application number: PCT/FI1996/000584
Authority: WO
Inventors: Keijo Sundelin; Ismo Sundelin
Original assignee: Oy Robotic Technology Systems Finland Ltd.
Priority date: 1995-11-03
Filing date: 1996-10-31
Publication date: 1997-05-15
Also published as: AU7300296A; FI955274A; FI955274A0

Abstract

The invention concerns a machining cell for machining of a piece (P). The machining cell comprises a machining robot (100), material being removed from the surface of the piece by means of a tool (60) attached to said robot (100) in order to shape the surface of the piece as desired. The machining cell comprises a workbench (10), to which the piece (P) to be machined is attached for the time of machining. The machining cell comprises a system of measurement (90) which measures the piece to be machined, by means of which system the precise dimensions of the surface to be machined are measured. The equipment comprises a central computer (70), to which the measurement data received concerning the piece (P) to be machined are transferred from the system of measurement (90). The commands to the control unit (50) of the machining robot are produced based on the measurement data obtained concerning the piece and produced by the system of measurement (90).

Description

Machining cell and method for machining of a piece

The invention concerns a machining cell and a method for machining of a piece.

From the prior art, machining cells are known, in which a robot has been fitted to machine a piece which has been attached to a workbench. Various numerically controlled systems are known, in which the ultimate dimensions of the piece are achieved as programmed in advance. However, from the prior art, no system is known in which a precise machining result is obtained based on separate measure¬ ment which is based on precise observation of the face. In the present application, a novel system is described, which is meant in particular for grinding of propellers and in which, based on measurement by means of cameras, the grinding of the face of the propeller is carried out after the casting stage.

The system in accordance with the present invention comprises an equipment for attaching the piece, a what is called workbench robot. The piece is attached onto the axis X of the workbench robot. The piece is positioned so that the symmetry axis of the piece coincides with the geometrical central axis of the shaft of rotation of the equipment for attaching the piece. The equipment comprises a separate lifting system, by whose means the piece can be raised to the desired working heights and measurement height.

According to the invention, the piece is attached in a precise position by means of a separate fastening equipment, preferably a locking cylinder. When the piece to be machined is a propeller equipment, the locking force is applied through the locking cylinder to conical pieces placed at the ends of the hub of the piece inside the hub, which conical pieces centre the piece and press the piece between the cones, whereby the piece is centred and fixed. The equipment in accordance with the invention comprises a separate system that observes and measures the piece, in which system an essential part consists of detecting devices, preferably detectors that receive the detecting ray from the point to be measured, preferably cameras, for example line matrix cameras. The detector device can also be other than a camera. The principle is determination of the location of a point on the piece based on triangulation, in which connection, by means of two detecting devices, measurements signals are produced from the face to be measured, from the measurement point produced on said face, at the same time to each of the observing detector devices. By means of the arrangement, the measurement rays arriving from the measurement point to the detectors and the directions of said rays are observed. When the directions of the vectors arriving in the detectors are known, it is possible to determine the precise position of the measurement point on the face to be measured and, further, the precise location of the measured point on the face to be measured at the intersection point of the measurement vectors Vi and V₂. When the positions of the cameras are known, it is possible to determine the precise location of the measurement point in relation to any system of coordinates whatsoever. In this way it is possible to determine the precise dimensions of the piece, the precise profile and the shape of its face.

In a preferred embodiment, the point of light is scanned by means of a laser scanner on the face to be measured so that measurement lines are formed. The precise position of each line and, consequently, the precise positions of the points that form the line, can be computed. Further, adjacent light lines are formed, whereby me whole face can be mapped, and the shape profile of the face concerned can be created. After a propeller blade has been mapped from above and from below, a transfer takes place to the next blade, etc. The measurement rays are formed from the edge of the piece to be measured towards the central axis X, i.e., in the case of a propeller, from the tip of the propeller blade towards its root.

In the system in accordance with the invention, by means of the measurement data concerning the face that has been observed, the robot that machines the piece is controlled, and the robot further carries out the machining of the face that was measured. Based on the measurement values, the face that was measured is divided into different machining sectors, and the grinding takes place in the different sectors by means of optimal grinding disks and with optimal other machining parameters.

According to the invention, the piece to be machined, favourably a propeller, is attached, for the purpose of grinding after casting, in accordance with the invention, to a workbench by means of separate cones. In the construction in accordance with the invention the propeller can be raised to different working heights, in which case the propeller blade can be machined both from above and from below without detaching die propeller. In such a case, the measurement means are preferably placed both above and below the propeller blade. In the construction in accordance with the invention, as the propeller, attached to the workbench, can further be rotated to the desired angular position, the propeller can be machined, favourably ground, with one fastening all over.

Thus, the system in accordance with the invention comprises at least one robot and, further, a central computer for controlling the equipment. In the construction in accordance with the invention, the workpiece attached to the workbench, favourably a propeller, is machined with the same fastening so that there are two different machining stations: an upper machining station, in which the lower face of the propeller blade is ground, and a lower machining station, in which the upper face of the propeller blade is ground. Between said machining stations, there is a measure¬ ment station, in which the dimensions of the propeller are ground both from the upper face and from the lower face of the propeller blade, whereby the cross- sectional profile of the propeller blade, i.e. the dimensions of the propeller blade, can be determined across the entire length and width of the blade.

In a machining cell in accordance with the invention, when the piece to be machined is a propeller and when the machimng method is grinding, the grinding of the propeller blade is carried out preferably along radial machining paths from the tip of the propeller blade to the root of the propeller blade. Between the machining cycles, the piece is measured, and on the basis of the measurement values obtained the robot is controlled further until the ultimate desired dimensions of the piece to be machined are reached.

When the piece, for example a propeller blade L, is machined, a grinding disk rotated by the motor of the robot is operated.

The invention is characterized in what is stated in the patent claims.

The invention will be described in the following with reference to some preferred embodiments of the invention shown in the figures in the accompanying drawings, the invention being, yet, not supposed to be confined to said embodiments alone.

Figure IA is an illustration of the overall system in accordance with the invention.

Figure IB shows the concept as shown in Fig. IA in more detail.

Figure IC is a block diagram presentation of the transfer of data in the robot cell.

Figure 2A is a sectional view of the equipment for fastening of the workpiece to be machined.

Figure 2B is a block diagram illustration of the system of operation of the equipment as shown in Fig. 2A.

Figure 2C shows a robot that can be used in the invention for machining a piece.

Figure 3A illustrates the principle of measurement used in the system in accordance with the invention for grinding of a propeller.

Figure 3B illustrates the transfer of the measurement point parallel to the propeller blade, in which case the measurement points form an illuminated stripe on the face. Figure 3C illustrates the principle of operation of a laser scanner which operates as the source of light.

The system shown in Figs. IA, IB and IC consists of the following main compo- nents: workbench robot, i.e. workbench 10, grinding robot 100, robot control unit 50, grinding tools 60, central computer 70, programmable logic system 80, and measurement system 90. On the other hand, the measurement system 90 consists of a measurement computer 91 , cameras 91a-, , 19a₂... , and of rays of light 92a , ,92a₂...

The tools 60 are preferably grinding tools, for example grinding disks, by whose means the piece can be machined to the desired ultimate measures that have been fed into the system. The robot 100 has been fitted, by means of its motor, to rotate the machining tool 60 in order to remove material from the face of the piece. The machining tools 60 can also be other tools that remove material, such as, for example, sand-blasting means or equivalent.

From the source of light 9aι ,9a₂, signals, i.e. rays of light S, are applied to the face P of the workpiece to be measured, on which face they form a line of light.

Central computer

The principal controller and programming tool in the cell consisting of the work¬ bench robot 10 and the grinding robot 100 is a central computer 70, which com¬ prises a microcomputer based on an Intel 80486 processor and in which the operat- ing system is MS-DOS 6.2. The primary storage of the microcomputer comprises 16 megabytes for running the programs, and the fixed disk has 540 megabytes of storage capacity for storage of robot programs and data files. From the microcom¬ puter 70, there are data communication connections along RS-232 buses 71 to the robot 100, to its control unit 50, to the CNC controller 40 of the workbench 10, to the measurement system 90 and to the balancing detector as well as to the programmable logic 80. Further, the central computer 70 communicates through the buses 72,73 with the designing computer 95 and with the measurement computer 91. Fig. 2 A is a sectional view of the workbench construction. The figure illustrates the fastening bench equipment 10 for the workpiece. The fastening bench equipment is also called a workbench robot. As is shown in the figure, the fastening bench equipment 10 rests on the floor D, and the equipment extends from the floor D level D' to the storey K placed underneath. The equipment has been passed partly into the storey K placed below through an opening A in the floor. A deck k has been fixed to the floor D. Further, an annular plate k' and a sleeve shaft 11 that guides the equipment in the vertical direction (direction X) have been fixed to me deck k. The annular plate k' is attached to the deck k by means of a number of hexagonal socket head screws r. By rotating the hexagonal socket head bolts, it is possible to position the central axis of the workbench precisely vertically. The annular plate k' is locked in the vertical position by tightening the locking bolts r of the annular plate k', whereby the annular plate is pressed against the threadings on the hexagonal socket head bolts tightly into its position. Between the annular plate k' and the outer housing G₃ of the flange Gi of the shaft 16 there is a resilient bellows W, which protects the guide faces.

A sleeve shaft 11 or tailstock spindle guides the equipment by means of glide bearings in the vertical direction. Below the deck, the annular plate k' is connected with lifting cylinders 12aι ,12a₂, preferably hydraulic cylinders. The lifting cylinders 12aι ,12a₂ are connected by the piston rods 12a' , ,12a'₂ with an annular flange 13, in whose middle the shaft 14 of rotation proper of the equipment is mounted. Between the shoulder J of the shaft 14 and the portion of the annular flange 13 that is placed against the shaft 14, a bearing 15a_j is mounted, on whose support the shaft 14 revolves. Outside the shaft 14, there is a sleeve shaft 16, which is non-revolving and which has been fitted to glide in the vertical direction while guided by the tailstock spindle 11. At the end of the shaft 14, underneath the annular flange 13, there is a cogged pulley 17, which is rotated, by means of a cogged belt 18, by an electric motor, preferably an AC motor 19, connected with the annular flange 13. The drive pulley 19' on the output shaft of the electric motor 19 is engaged with the cogged belt 18, which rotates the cogged pulley 17 and, further, the shaft 14 coupled with the cogged pulley 17 and thereby the workbench 23 and the propeller P. At the top end of the shaft 14, there is a bearing 15a₂ between the shaft 14 and the flange Gi connected with the end of the middle sleeve shaft 16. Above the flange Gi , there is an annular plate G₂, on which there are balancing cylinders, for example an annular cylinder 20a _j, which are fitted to act upon the displaceable flange 21 , on which there are balancing detectors 22. On the balancing detectors 22 there is the workbench 23. Thus, when the locking is opened by means of the cylinder 27, the propeller P can be raised by means of the annular cylinder 20a i onto the support of the detectors 22 for balancing.

The fastening base 23 includes a sleeve shaft 25 and further a cone 24, which surrounds the sleeve shaft 25.

Through the hollow interior space Ui in the shaft 14 and through the hollow interior space u₂ in the sleeve shaft 25, a locking screw d has been threaded into the locking shaft 26 connected with the locking cylinder 27 placed at the end of the shaft 14.

By threading the nut Mi placed at the end of the locking screw d, the deck plate 31 can be pressed tightly against the loose bushing f placed on the top portion of the sleeve shaft 25, whereby the locking force of the locking cylinder is transferred by the intermediate of the cones 24,32, on which the propeller rests, both to the propeller P and to the fastening base, thus centering the propeller and pressing the fastening base 23 tightly against the lower constructions of the workbench. Further, there is the second cone 32 freely around the sleeve shaft 25. When the locking of the propeller P hub N in relation to the workbench is accomplished, first the nut Mi provided on the locking shaft 26 is threaded so that a distance of about 2 cm remains between the deck plate 31 and the nut. The ultimate locking is accomplished by means of the locking shaft 26 by using the locking cylinder 27. When the locking by means of the cylinder 27 is opened, i.e. when the piston rod 26 of the locking cylinder 27 is raised and when, at the same time, the shaft d is raised, the propeller P is brought into a balancing position by raising the measurement detectors into contact with the fastening base 23 of the workbench 10. Then, the propeller P rests on the balancing detectors, and the balance of the propeller P can be measured. Between the parts 16 and 11, there is a glide fitting. By means of the actuators 12a_j and 12a₂ the shafts 14,16 and the means connected with them are raised to different working levels and to a different measurement level. The raising movement is guided by the tailstock spindle 11. The actuators 12a_j and 12a₂ are preferably hydraulic cylinders.

The locking cylinder 27 is fixed to the lower end of the rotatable shaft 14 from its cylinder frame.

When the propeller P has been attached and locked in the desired machining station, it is raised by means of the lifting cylinder arrangements 12a_j,12a₂ to the desired working height. Thus, by using the lifting cylinders 12aι ,12a₂, it is possible, besides bringing the propeller P to the desired working height, also to bring the propeller to the measurement point in the vicinity of the cameras and the source of light. By operating the lifting cylinders 12aι ,12a₂, the whole construction unit is raised.

According to the invention, the workpiece, favourably a propeller, is attached, for the purpose of grinding after casting, to the workbench by means of separate cones 24,32. In the construction in accordance with the invention, the propeller can be raised to different working heights, in which case the propeller blade can be machined both from above and from below without detaching the propeller. Then, the measurement means are preferably placed both above and below the propeller blade. In the construction in accordance with the invention, as the propeller P can, when attached to the workbench 10, additionally be rotated to the desired angular position, the propeller can be machined, preferably ground, with the same fastening all over.

Into the lower end of the locking cylinder, a shaft O₂ has been threaded, to which shaft the inner circumference O₃ of an angle detector has been fixed. Thus, the inner circumference O₃ of the angle detector revolves along with the locking cylinder 27. The outer circumference O_j of the angle detector is journalled on the inner circum¬ ference: Rotation of the outer circumference along with the inner circumference is prevented by means of an arm O₄,O₅ fixed to the annular flange 13.

The pieces P to be ground are attached to the workbench robot 10 by first lifting them between two cones placed on the shaft of a particular fastening base and by lifting the fastening base onto the workbench. The workbench robot has two degrees of freedom: the movement of rotation and the movement of translation in relation to the vertical axis of the bench. Besides these movements of regulation, the workbench robot is provided with a locking-detaching movement necessary for attaching the piece to be machined, with a balancing movement, in which the fastening base is raised apart from the surface of the bench on support of balancing detectors, and with a hydraulic locking of the rotation movement, which ensures that the shaft 14 remains in the desired position.

The hydraulic system has been designed so that the tightening force is sufficient to keep the propeller between the cones in its position during the grinding process.

Balancing detectors

All propellers P are balanced statically after their machining has been completed. For static balancing of the propeller P, six weighing detectors are mounted on the bench. The measurement ranges of the detectors are arranged in two groups. In one group of three detectors, an individual detector can measure a mass of 0...200 kilograms, and in the other group a mass of 0...1000 kilograms. The detectors are placed on a hydraulic annular cylinder so that the three detectors in each group are fitted as equally spaced on the circumference of the annular cylinder. During measurement, the propeller is raised by means of the hydraulic cylinder onto the support of the three detectors, and the loading of the detectors is read by means of measurement units. The analog-digital converter of the measurements units is a 16- bit converter, in which case the larger detectors provide the measurement result with a resolution of 100 grams and the smaller ones with a resolution of 10 grams. The selection of the group of detectors takes place by means of fastenings. The fastenings have been designed so that just one group of detectors is loaded during measure¬ ment. For propellers whose weight is less than 600 kilograms, smaller detectors are used for the measurement, and for propellers of a weight of 600...3000 kilograms detectors of a higher range of measurement are used.

In static balancing of a propeller, a balancing weight is determined for the tip of a propeller blade. The magnitude of the permitted weight depends on the diameter of the propeller and on the category of precision. The balancing program employs input data obtained from the design data and consisting of the diameter of the propeller, the speed of rotation, and the classification, as well as of the measurement results of the three detectors obtained from weighing. For computing of the balancing weight and direction for the tip of the propeller blade, the principle of calculation of the centre of mass and the law of momentum are used.

Hydraulic translation movement

The translation movement of the rotating bench is accomplished by means of two hydraulic cylinders 12a_j,12a₂. The stroke length of the cylinders 12a_j,12a₂ is about 1000 millimetres, and the cylinders are capable of lifting a propeller P weighing more than 5000 kilograms. The movement of lifting the table permits grinding by means of the robot from underneath the propeller blade without turning the propeller P over. The control of the lifting movement is accomplished by means of a feed¬ back-connected proportional valve.

The hydraulic operations of the bench are controlled by a programmable logic 80. The function of the programmable logic is to carry out the logic assignments programmed in its memory and, on their basis, to control the hydraulic operations. This takes place by collecting data concerning the cell, by processing said data, and by controlling the output circuits. The data coming from the cell are received primarily from inductive approach detectors installed on the turning bench and from a controller made for manual control of the bench as well as from a control pro- gram. The hand controller has switches of their own for each operation, by means of which switches the performing of the movements is controlled. The signal level of the input/ output interface is, as a rule, 24 V DC or ± 10 V DC.

Fig. 2B illustrates the control of the movement of rotation of the workbench as a block diagram presentation.

The rotation of the workbench in relation to the vertical axis is accomplished by means of a CNC-controlled inverter operation. In this way the torning of the propeller can be carried out fully in accordance with the angle in degrees determined by the operator and sufficiently precisely. The output of the controllable AC motor is 2.2 kilowatt, and for its power transmission a gearbox and a cogged belt are used.

The control of the rotation of the workbench has been accomplished by means of a decentralized control system, wherein the microprocessor of the central computer 70 operates as the main controller, and the control of the movements themselves is carried out by means of a separate, commercial control card. In the control software of the central computer 70, command formation functions have been provided which support the data transfer protocol of the control card D.

As is shown in Fig. 2B, the system consists of a central computer 70 operating as the main controller, of a SWI (switch) controlling the data communication, of a card that takes care of the control of movement, of a frequency converter drive that carries out the rotation, and of data communication buses between said units. From the microcomputer the locating commands and stams enquiries are passed along the RS-232 bus to the SWI. This interprets the commands and transmits them further to the slave bus RS-485. From the slave bus the control card D receives the commands belonging to the card and controls the frequency converter in compliance with said commands. After the control card D has carried out the locating command, it reports that the location has been reached, in connection with a status enquiry. A slave bus solution permits the use of several locating cards and acmator devices in the same bus. The frequency converter T receives its speed feedback from a pulse detector installed at the end of the electric motor. The frequency converter T is controlled by the control card D, which reads the position data from the pulse detector directly attached to the shaft of the workbench. As the detector is attached to the shaft of the bench directly, plays that deteriorate the locating result are avoided, which plays always occur in connection with power transmissions and gearboxes. The resolution of the locating detector is 180,000 pulses per revolution, which means that the precision of one pulse corresponds to an angle of 0.002 degrees, and on a circumfer¬ ence of 1.5 metres to a resolution of about 0.005 millimetre. In factual locating, it is possible to reach a tolerance range just about twice that large. However, this is an adequate precision of positioning for a propeller in robotized grinding. The accuracy of reproduction of the robot itself is ± 0.4 millimetre.

Fig. 2C shows a prior-art robot that carries out machining, preferably grinding, as a separate illustration.

The robot 100 comprises a robot control unit 50, from which the control commands come to the robot mechanism. As is shown in the figure, the mechanism can comprise a set of arms Li .L^, which can be rotated in the vertical plane and of which the lower set of arms Li is additionally fixed to the base L₃ so that the whole unit can be rotated around the vertical axis. Thus, the robot 100 comprises a first arm L_], which can be pivoted in the vertical plane, and a connected second arm 1^, which is connected widi the first arm Li and which can also be pivoted in the vertical plane. Further, the arm L^ consists of two parts, and its outer part can be rotated in relation to the latter part around the central axis of the set of arms. The arm L^ is further connected with a wrist L₄ of the robot, which can be pivoted in relation to the arm 1^ and, further, it can be rotated around its central axis, whereby the tool 100 connected with the arm L₄ possesses six degrees of freedom. The tool 60, preferably a grinding disk, can thus be controlled in compliance with the desired path of movement while programmed by means of the commands coming from the control unit 50. It is preferable that the tool should have a maximal number of degrees of freedom of movement in order that the working paths could become sufficiently smooth so that they can take into account the forms of the machining tool.

As the robot 100, it is possible to use, as a rule, any available commercial solution whatsoever which possesses a sufficient working space for the machining of the workpieces concerned.

In a test cell, an IRB-6000 has been in use. The IRB-6000 is an electromechanical articulated robot with six degrees of freedom. The basic composition of the robot consists of a control unit 50 and a mechanical articulation manipulator. The modular construction permits different versions for the robot 100 both for the mechanical construction and for the electrical control. These include, e.g., different fastening bases, arms, and outer servo shafts with their controls. Manual control takes place by means of a portable control device, which is connected to the control unit through a cable. The programming proper is carried out as off-line programming.

Fig. 3A illustrates a measurement system 90 which employs two cameras 91a_{1 (}91a₂.

The cameras Ci and C₂, which are line matrix cameras, are placed precisely in certain positions in relation to the object F to be measured. From a laser source of light L a ray of light, preferably a ray of laser light, is produced onto the face of the target, whereby an illuminated point P is formed on the face. The illuminated point is monitored in each camera Ci and C₂, and by means of the camera it is possible to compute the directional vector V_j between the measurement point and the image point formed by the camera on the focusing plane. The same can be done in respect of the other camera C₂, and, thus, the intersection point of the vectors V_j and V₂ determines the location of the measurement point Pi when the precise locations of the cameras Ci and C₂ are known. Similarly, when the quantities mentioned above are known, the location of the point P can be determined in relation to any system of coordinates K whatsoever. By scanning the ray S on the face to be measured, a measurement line and, thus, for example, the profile of the propeller at a certain point over the width of the propeller are obtained. By forming these measurement lines across the entire width of the propeller by displacing the source of light or the target to be measured, the surface profile can be measured across the entire width of the piece to be measured, for example, exactly across the width of a propeller blade. After one propeller blade has been measured and machined, the next blade is rotated to the measurement position and machining position.

In the measurement system 90, onto the face of the propeller P blade L, , in one of its embodiments, the point to be measured can be formed by means of laser (LS). In the system, in order to mark the target, it is also possible to use a normal white stripe of light.

Functions of the sight system / measurement system

In the applicant's robotized grinding cell, the sight system is used both for the control of the robot and for quality control. The sight system is utilized in locating the propeller and in measurement of the propeller shape and in grinding that corrects the shape of the propeller. By means of the sight system, a measurement record complying with the ISO 484 standard is prepared concerning the propeller shape.

The measurement computer 91 is a microcomputer based on a 80486 processor, whose operating system is UNIX. The operating system provides the measurement program with a genuine multi-processing environment and with a X-Window System user interface. The cameras 92a_t,92a₂... are connected to the measurement computer 91 through two image processing cards. In connection with the assembly of the system, the image processing card is also connected with an extra monitor, by whose means the correct orientation of the cameras in relation to the target and the bright- ness of the image produced by the cameras are checked. Also, the system can be connected with a source of light 93a _j, which can be directed and whose automatic control is carried out directly from the measurement software. In order to measure the shape of the propeller, in the applicant's system eight cameras are employed. In order that the sight system could measure both sides of the propeller blades, the cameras have been attached as two groups of four cameras onto the floor and onto the ceiling of the machining cell. Since the propeller blade has no visible points for measurement, for the marking of the point to be measured it is necessary to use sources of light by whose means a stripe of light is produced on the face of the blade. The stripes of light created on the top face and on the bottom face of the propeller blade act as measurement points for the sight system. Around the cameras and the sources of light, separate shield housings have been constructed to protect them from grinding dust. The housings are provided with pneumatically operated doors, which have been connected to the central computer by the intermedi¬ ate of the logic system. The central computer gives commands to open the doors for the time of measurement and to close them upon completion of measurement, which opening and closing take place either manually controlled by the operator or automatically in connection with the measurement process.

Fig. 3B illustrates the formation of a stripe of light ei on the upper face of the propeller P. If the cameras 92a_j ...92a₈ and the sources of light 93a_j and 93a₂ are placed at both sides of the propeller P blade Li (Fig. IA), the ray of light is produced at both sides of the propeller blade L_j in the same vertical plane.

As the sources of light 93a₁,93a₂ it is possible to employ either light projectors or laser scanners (Fig. 3C).

The light projectors 93a_j are mounted stationarily on the wall of the cell so that the white stripes of light pass through the central axis X of the propeller P hub N. In the measurement of the propeller P, the propeller P is rotated step by step through the area of stripes of light and, at the same time, the sight system measures the profile of the propeller blade (Li) from each point of rotation. In this way, by means of the system, propellers can also be measured whose blades are highly twisted around the hub. Thus, the profiles to be measured are formed as fan-shaped on the propeller face starting from the hub and ending at the outer edge of the blade. From the point of view of measurement of the propeller, the most important area is at the tip of the blade, so that this area of the propeller must be measured as precisely as possible, However, in measurement taking place by means of a rotatable bench, the measurement profiles become more distant from one another at the outer edge of the propeller. On the contrary, at the root of the blade, there will be more frequent profiles even though its shape does not have an equally high significance for the operability of the propeller. This problem arising from the direction of the light stripe can be solved by measuring profiles of different lengths on the propeller blade. Thus, measurement points are not computed from the light stripe up to the hub at all angles of measurement of the propeller, but some of the profiles are, in a way, cut off in the middle of the blade. In this way the number of profiles to be measured can be increased at the tip of e blade without having unnecessarily dense measurement points at the root of the blade.

The locations of measurement of the profiles are determined as angles of rotation of the propeller and as starting points and end points of the area to be measured. For the computing, the measurement program charges a propeller geometry data file in its memory. Initially the program determines, from the data file, the shape of the outer edge of the propeller and the maximal opening angle of the of the edges of the propeller blade in relation to the hub. The value of the angle is divided by the number of the profiles to be measured, whereby the magnitude of the angle of rotation of one step is obtained. After this, for each angle value of rotation, the corresponding point is computed at the outer edge of the blade, which point acts as the end point of the measurement area. The starting point of the measurement area is obtained from the root of the propeller blade. This point is, however, varied at every other measurement angle so that its location is halfway on the blade. By means of the measure system, it is possible to determine the dimensions of the piece to be machined.

Fig. 3C illustrates the laser scanner LS operating as the source of light. In principle, it is a second alternative for measurement of the propeller that the whole blade Li ,!^... of the propeller P is measured at the same position of the rotatable workbench 10. In the way shown in Fig. 3C, in the place of light projec¬ tors, there is a laser scanner which produces a circular point Pi and which can be controlled so that the point can be created in the desired location on the face of the blade. When the point P, is displaced at a high frequency (about 500 cycles per second) back and forth on the face of the blade, the point Pi of light can be made to appear as a stripe of light in relation to the cameras (imaging frequency 25 cycles per second). With laser scanners the measurement becomes slightly quicker, and by means of the arrangement a slightly better measurement precision is obtained. The above is partly also affected by the fact that the speed of the point of light can be varied at different points of the stripe of light so that the intensity of the light visible to the cameras is uniform over the entire area of the stripe of light.

Onto the face of the blade, stripes of light can be applied in accordance with the same principles as when light projectors are used.

Connecting of the sight system to the central computer

The measurement of the propeller has been automated by means of a measurement program run in the central computer 70. Based on the design data and on the preset measurement parameters, the measurement program computes the number and locations of the necessary stripes of light and controls both the sight system and the workbench during the measurement. Physically the central computer 70 is connected to the measurement computer 91 through a RS-232 serial bus. By means of the data transfer protocol and the instruction set that have been developed between these two computers, it is possible to transmit the necessary commands from the central computer to the control of the measurement system 90. Measurement

Before the ready-computed measurement areas can be utilized in measurement of the propeller, it is necessary to know the rough location of the propeller in relation to the stripe of light. It is only on the basis of the location data that the planned meas¬ urement areas can be placed in the correct locations on the propeller blade. The location of the propeller can be determined readily in connection with its mounting on the rotatable workbench. This takes place simply so that the stripe of light is switched on manually, and one side of the propeller blade is turned to the stripe of light. This position is particularly suitable from the point of view of measurement, because in the measurement program the first profile to be measured is determined at this location.

After the cell controller has shifted the propeller P blade L_j to the desired measure- ment point, the measurement program requests to perform measurement by means of a command to the system. In connection with the measurement command, the system is notified, by means of parameters, of the area of the stripe of light that is to be measured and of the position of the rotatable bench as degrees. Also, as parameter data, the number of points to be measured from the stripe is reported.

Having received the measurement command, the sight system takes the images of the stripe of light and confirms to the central computer 70 that the operation has been completed. After this, the central computer 70 gives a command to mm the propeller P to the following point of measurement. In this way, the entire blade of the propeller P is processed. When the last measurement point has been reached, the central computer 70 reports to the measurement system 90 that the measurement of the propeller P blade Li has been completed. The measurement system 90 confirms receipt of the command and starts computing the measurement points from the images that were taken. The computed points are stored as a data file in the hard disk of the measurement computer. Further processing of the measurement results

The measurement profiles created by the sight system are not directly comparable with the geometry data used in designing. The geometry of a propeller blade is, as a rule, depicted by means of about ten circle-centred sections. The sections are divided to different percentage points based on the length. Similar locating of geometry points is also employed in the standardized measurement record. In order that the measured propeller could be compared with the designed geometry, the measurement results must be converted to the standardized form.

For this conversion, a program that has been prepared in connection with the sight system is used, which program carries out the fitting of the measurement points onto the geometric model and computes the values of the geometric points based on the measurement record. In order to create the measurement record, the values of the points in accordance with the standard must be computed from the measurement profiles. In the computing, it is necessary to use interpolation in order to determine the points, because the measurement points created by the sight system do not coincide with the points in accordance with the standard. The computing is started by forming continuous curves out of the measurement points. After this, from the curves, intersection points are computed for each measurement radius. From the intersection points, the curves in accordance with each radius are formed, from which curves the equivalent points in accordance with the geometry data file are computed. In this way, the measurement result is obtained in a form that can be compared, and at the same time a measurement record can be formed. The fitting of the measurement points in the geometric model already gives information valuable for the control of the robot. From said fitting, the location and the position of the propeller blade are found out. Based on these data, the measured propeller blade can be shifted to the planned grinding position, and the grinding paths of the robot can be corrected so as to correspond to the position of the blade concerned.

In the following, a preferred embodiment will be described: Attaching of the propeller

The piece to be ground (propeller/impeller) is attached to the workbench robot by means of a fastening base and two cones and a fastening screw. The fastening screw is drawn hydraulically to the ultimate fastening tightness.

Initial operations

The stripe of light of the measurement system is switched on manually. One of the propeller blades is positioned visually in a certain position and at a certain level in relation to the stripe of light provided by the sight system. This can take place either so that commands are given manually through the monitoring software of the cell computer to the equipment of rotation of the workbench robot, or so that the hydraulic locking of the fastening screw is opened so that the propeller can be rotated on support of the cone on the workbench quite easily by hand.

Control software:

The control software of the central computer 70 of the cell is started. The work name of the propeller is fed to the software as starting data. The work forms to be carried out are chosen from the menu: Measurement of propeller, shape -correcting grinding, finishing grinding, automatic balancing. From among the above, the operator can choose any combination whatsoever. The operator can also mark the checking points at the desired points in the grinding process. For example, after measurement the system waits for a confirmation of the operator if the operator wishes to check the measurement results. Or an interruption can be set to take place, for example, after the grinding of each blade, for visual inspection by the operator.

Further, from the menu of the control software, it is possible to choose the desired working order in respect of finishing grinding and correcting grinding. This because it may be necessary to make an exception to the normal working order (first the correcting grinding and then the finishing grinding of the whole blade) if it is noticed, for example, after the grinding of the first blade, that the quality of the cast face is to such an extent inferior that finishing grinding requires removal of a quantity of material larger than normal from the face of the blade. A change in the working order can also be required in the case of propellers of higher precision, because of little tolerances.

In any case, the selections of the work form that is being performed and of interrup¬ tions can be altered in the middle of the grinding process; the operation goes on to the end of the component process that is being run at the time concerned, after which the new forms and terms of operation will be effective.

Starting the process

After the operator has given an operation command from the menu of the control- room software, the control software starts the measurement process for measurement of the first blade. This takes place as follows:

Preliminary operations

The control software passes the commands through the serial interface to the control logic to open the doors of the cameras and to switch off any extra lights.

Measurement program and measurement system

The control program calls the measurement program of the measurement computer 91 of the measurement system 90 and transmits the name of the propeller P to be measured to said program, on the basis of which name the measurement program can charge a measurement data file of the correct name in its memory, or, if there is no measurement data file, the measurement program calls a sub-routine, which prepares the measurement data file based on the design data and the preset measure¬ ment parameters. Based on d e measurement data file, the measurement programs gives, through the serial interface, the workbench 10 the command to rotate the propeller P stepwise from one measurement sector to the other. At each sector, the measurement program receives a confirmation from the workbench concerning the completion of the rotation step. Likewise, through the serial interface, the measurement program gives the measurement system the command to carry out the measurement in the sector concerned. The measurement command consists of the starting point and end point of the sector to be measured, of the angle to be measured, and of the number of the measurement points to be computed. The measurement system switches the stripe of measurement light produced from the source of light onto the face of the blade on and off. By means of the cameras, images are taken of the blade widi and without a stripe of light.

Measurement and optimizing of the position of the blade

As a result of the difference between said images, an image can be determined in which there is information at the stripe of light only. From the images, from the grey shade values of the stripe, the centre line is determined in accordance with the principle of calculation of the centre of mass, after which the line is divided into the desired number of measurement points. When measurement points are obtained sufficiently densely across the area of the entire blade, on their basis it is possible to compute radius curves created on the face of the blade in relation to the centre point of the propeller, and when said curves are fitted in the design data, by means of the method of the least squares, the optimally "correct" position of the blade is found out in consideration of the design data.

After the blade has been measured, the measurement software confirms to the control program that the received command has been carried out. The received measurement data and the optimal positions of the blades are transmitted either through the serial interface or transferred through the local network to the cell computer upon completion of each measurement operation. The measurement program indexes the names of the data files of the measurement results obtained separately for each blade in order to avoid confusion.

Working order of grinding

The grinding is started from the pressure side (top side) of the propeller blades. Each blade is first ground to the correct shape regarding the pitch. After this, checking measurements are carried out in order to establish the correct shapes. Next, the whole blade face is ground clean, and finally it is finished with a finer disk. After this the blades are ground from the suction side in compliance with the thickness. Before the finishing of the faces at the suction side, the static balance of the propeller is measured. Based on the results of said measurement, me propeller is ground from the suction side to balance, and the static balancing of the propeller is carried out from the suction side. Since, before balancing, the blades were ground clean in accordance with the principle of minimal grinding, normally the balancing provides no problems because of the working reserves allowed by the tolerances.

Use of measurement results for making the grinding programs

Based on the work name of the propeller, the control program of the central computer 70 transfers the name of the design data file and the current number of the blade to be machined to the machining-path programs. In the design data file of the design computer 95, based on the design data proper of the blade face, there is also information on the classification of the propeller and on the dimensions of the hub. By means of the current number, the machining-path software is capable of search¬ ing the correct measurement data file in the measurement computer 91.

For the purpose of making the grinding programs, at the beginning of, or during, the work cycle, the operator determines the values of the parameters affecting the making of the grinding programs. Such parameters are, for example, the factors affecting the removal of material: grinding angles of the different tools, grinding speed, grinding pressure, and distance between successive machining paths. The measurement results newly obtained are used for making the grinding programs proper. The measurement data of each blade are compared with the design data in the machining-path program. Based on the classification of the propeller, the tolerance limits are determined for the pitch of the blade, for its thickness and shape. In compliance with these limits, the permitted tolerance limits are determined for each design point. Based on the comparison, the areas are determined in which there is extra material as well as the quantity of extra material. On the other hand, based on the grinding disk 60 used, the grinding pressure, the grinding angle, the distance between successive grinding paths, and the grinding speed, the quantity of material removed on one working cycle is known with a sufficient precision. Thus, it is possible to determine the necessary machining areas and the number of machining cycles in view of obtaining the desired surface shape.

Further, when the grinding programs are being prepared, the factual shape of the blade within the tolerance ranges is taken into account, and the grinding is per¬ formed on the basis of this information, i.e. within the scope of the standards, the blade is ground based on its facmal shape rather than based on ideal design data. This is permitted and justified in an attempt to minimize the amount of material to be removed. True enough, with little dimensional differences between the design data and the facmal shape and with advanced grinding tools, this matter has no major importance. Advanced tools are herein understood as referring to tools that are capable of regulating the grinding pressure.

Based on all of the information mentioned above, the machining-path software of the central computer 70 computes the necessary machining paths for the robot 100 and the number of machining cycles for the grinding of the blade surface and stores the programs thus obtained in the data file.

Grinding

After this, the control program of the central computer 70 charges the programs that were prepared into the memory of the control unit 50 by the intermediate of the serial interface, starts the grinding, and monitors the messages arriving from the robot or from elsewhere in the cell. Upon completion of the grinding cycle, the robot 100 notifies the control program of the central computer 70 thereof.

Checking measurement and finishing grinding of the blade face

After the correcting grinding has been performed, the checking measurement is performed and, if necessary, the correcting grinding is repeated. After the required tolerances have been reached, the entire blade face is finish-ground. By means of said working order (first the correcting grinding and then the finishing grinding), attempts are made to reduce the number of necessary grinding cycles. Of course, when the correcting grinding is performed, the material thickness to be removed during finishing grinding is taken into account. When all the measurement points on the blade face are within the tolerance ranges, the blade can be finish-ground definitively. The making of the program takes place with the same principles as regarding the correcting grinding. Likewise, the stage of carrying out involves nothing new.

Angles between the blades and rotating of the propeller

At this stage, the pressure side of the first blade has been ground, and the correct position of the propeller in respect of the rest of the cell is known in relation to this blade that was measured first. But since, in the assembly of the casting mould and in the casting process, minor changes of angle often occur in the positions of the moulds in relation to one another, the propeller blades can be placed, seen from above, at slightly different angles in relation to the axis of rotation, i.e., in a four- blade propeller, the centre lines of the blades are not placed with a spacing of 90 degrees in a plane in relation to the axis of rotation, but, rather, they are placed at angles of about 90 degrees. The propeller standards permit such a deviation (with propellers of S and I classes, the deviation is allowed to be ±1 degree, and with propellers of II and III classes ±2 degrees, i.e. a rather large deviation is permitted. In practice, errors as large as this do not occur. It follows from this that, if the reference of the position of the propeller is fixed permanently in relation to this first blade, owing to asymmetry, the other blades can be placed to such an extent aside from the reference point that the location of the blade is not optimal in view of the grinding, but material must be removed from the blade face unnecessarily. Thus, for each blade, its position correct from the point of view of grinding must be sought while, at the same time, taking into account the requirements imposed by the tolerances in this context.

The following blades to be measured will be rotated (control program) to the measurement position in compliance with the optimal position of this blade that was measured first. If the centre lines of the blades are not placed at angles of 90 degrees in a plane in relation to one another, at this stage this does not cause difficulties for the measurement, because the rays of light reflected onto the blade face do not have to be precisely at a certain point on the blade, but it is enough that the rays of light passed onto the blade face meet the face of the blade.

Each blade is measured and, thus, in accordance with said principles, for each blade, the reference position (angle position and height) optimal in view of the grinding is determined. The differences in height between the blades can be taken into account either by, by means of the workbench robot, shifting the propeller to the desired machimng height or by preparing the grinding programs in accordance with the correct height of the blade. The latter alternative is simpler, because, as a grinding program of its own is, in any case, prepared for each blade based on its measures, at the same time its difference in height can also be taken into account. Of course, at this stage, the differences in height between the blades are compared with the deviations permitted by the standards.

Machining of the suction side

After the pressure side of the propeller has been ground in respect of all the blades and after the faces have been finished, the control program gives the workbench robot a command, by the intermediate of the logic system, to raise the propeller to the level for grinding of the suction side.

The grinding is carried out in accordance with the same principles as the grinding of the pressure side. The suction side of the blade is ground first, in respect of the thickness, to the tolerances (upper limit), and after this the whole blade face is finish-ground. Before the blades are finished, the propeller is balanced. The blades to be ground are rotated into their grinding positions so that each blade is aligned wid its "correct" position.

Balancing

Even though the propeller blades have been ground almost identically at this stage, the balance of the propeller may, however, be inadequate at this stage. A reason for this can be, for example, a slightly eccentric location of the shaft hole in the hub in relation to the outer face of the hub.

At the balancing stage, the control program calls the balancing program, which gives the logic appropriate controls, whereby the hydraulic fastening of the propeller is opened and the three balancing detectors raise the propeller. The readings indicated by the detectors are read through the serial interfaces into the central computer, and on their basis the computed balance value of the propeller is compared with the value in compliance with the standard (in consideration of a certain margin of safety). Based on the result obtained, if the balance of the propeller is not correct, the propeller must be balanced. As a result of the computing, the location of any extra material is also found out in relation to the current position of the propeller.

The balancing mass obtained based on the balancing angle obtained in the computing and on the current position of the propeller is divided among the propeller blades. Then, the balancing mass can be divided either on one blade, or if the computing balancing mass is placed between two blades, on two blades. The balancing program registers the number of the blade in which the larger balancing mass is placed. Now, when the difference between rhe measured and the permitted deviation of balance is known and, moreover, when the density of the material to be machined and the quantity of material removed in one machining cycle are known, it is possible to compute the magnimde of the area to be used in the balancing grinding and the number of machining cycles. The magnitude of the area to be machined during balancing is divided in relation to the length of the radius to be set as parameter data (for example, 0.7 of the length of the radius), whereby the radius can be determined within which the balancing grinding is confined from the tip of the blade towards the hub.

Finally, the balancing program notifies the control program of the number of the blade that requires balancing and of the necessary number of machimng cycles. If no balancing is needed, the number of machining cycles is set as 0.

The control program starts the grinding of the blade normally. After a first balancing has been carried out in respect of grinding, the call of the balancing program is repeated until the reply mat is received gives the number of machimng cycles as 0.

Finishing of the faces

After this the blades of the suction side can also be finished with a fine disk.

Measurement record

The propeller is now ready in respect of the grinding of the blades. Finally, the whole propeller is still measured once, and, concerning the results, a measurement record in compliance widi the standards is drawn up automatically.

Claims

1. A machining cell for machimng of a piece (P), which machining cell comprises a machining robot (100), material being removed from the surface of the piece by means of a tool (60) attached to said robot (100) in order to shape the surface of the piece as desired, and which machining cell comprises a workbench (10), to which the piece (P) to be machined is attached for the time of machining, characterized in that the machining cell comprises a system of measurement (90) which measures the piece to be machined, by means of which system the precise dimensions of the surface to be machined are measured, and the equipment comprising a central computer (70), to which the measurement data received concerning the piece (P) to be machined are transferred from the system of measurement (90), and that the commands to the control unit (50) of the machining robot are produced based on the measurement data obtained concerning the piece and produced by the system of measurement (90).

2. A machining cell as claimed in claim 1, characterized in that the system of measurement (90) comprises cameras and sources of light above and underneath the piece to be machined, a ray of light being produced from the source of light onto the surface to be machined and the precise location of the ray of light being measured by means of the cameras, and the precise location of the line of light observed by means of the cameras is detected by means of the cameras, and, based on said camera observation, the data on the dimensions of the surface of the piece to be machined are produced, and that the system of measurement (90) comprises a measurement computer (91), to which the data of measurement obtained concerning the piece to be machined are transferred.

3. A machining cell as claimed in any of the preceding claims, characterized in that the source of light is a laser scanner, by whose means points of light are scanned on the surface to be measured, which points are seen in the camera as a line, the dimensions of the surface to be observed being determined on the basis of observation of said line.

4. A machining cell as claimed in any of the preceding claims, characterized in that, when the piece to be machined is a propeller, at each side of the blade to be measured there are at least two cameras and one source of light, and that the line of light is produced from the edge of the piece to be machined towards its middle portion.

5. A machining cell as claimed in any of the preceding claims, characterized in that there is a workbench in which there is a rotated shaft connected with means for fastening of the piece to be machined, so that the piece to be machined can be rotated by rotating the shaft by means of a motor.

6. A machining cell as claimed in any of the preceding claims, characterized in that there is an angle detector (29), by whose means the precise angular position of the workbench shaft (14) and, thus, the precise position of the workpiece (P) fitted to revolve along with the shaft are measured, and that, in the construction, die workpiece, preferably a propeller (P), is fitted symmetrically on the geometrical central axis (X) of the shaft.

7. A machining cell as claimed in any of the preceding claims, characterized in that me line of light produced from the source of light is guided from the edge of the piece to be machined through the geometrical axis (X) of the workbench and, if the piece (P) to be machined is a propeller, a ray of light (S) is directed from the source of light from the outer edge of the propeller blade through die centre of the hub (N) of the propeller (P), and after observation of each measurement line, me propeller (P) is rotated by rotating me shaft (14) of the workbench by means of the motor (19), the system including at least two sets of light-producing means, both above and below the propeller (P) blade (L_j) to be measured, the line of light being directed at the propeller blade (Li) in the same vertical plane both above the blade and below die blade.

8. A machining cell as claimed in any of the preceding claims, characterized in diat the machining tool (101) in the machining robot (100) is a grinding tool, preferably a grinding disk, and that the equipment comprises a number of different tools for the machining robot (100), the central computer (70) in the system compris¬ ing a software, the optimal tool (60) being chosen on the basis of the measurement data depending on the quantity of material to be removed, on the sort of material, on the surface finish required, and on the location of the surface to be machined.

9. A machining cell as claimed in any of the preceding claims, characterized in that the equipment comprises a designing computer (95), to which the data are fed concerning the desired ultimate geometry of the piece to be machined and from which the design data are transferred further to the central computer (70).

10. A machining cell as claimed in any of the preceding claims, characterized in that the workbench (10) includes actuators (12aι , 12a₂;19), preferably cylinders (12aι ,12a₂), which displace the workbench in relation to the floor level, and that the equipment includes a motor (19), preferably an electric motor, which is fitted to rotate the shaft (14) of the workbench (10), to which shaft the fastening means are fixed for attaching the piece (P), and that the shaft (14) is hollow, the rod (26) of the locking cylinder (27) being passed through the space (ui) in the interior of said shaft, said locking cylinder (27) being fitted to attach the piece (P) to be machined in a precise position against the cones (24,32), and the piece (P) to be machined, preferably to be ground, can be raised by means of the acmator (12a_{1 ?} 12a₂) to different working levels, in which case the workpiece can be machined by means of the tool (60) of the robot from both sides of the piece with the same attachment of the workpiece (P) to the workbench (10).

11. A machimng cell as claimed in any of the preceding claims, characterized in that the frame unit displaced by the cylinders (12aι ,12a₂) comprises separate balancing detectors (22) in connection with separate cylinder devices (20aι), preferably with an annular cylinder, in which case, after the locking cylinder (20a 1) has been opened, the piece to be machined, while fixed to the fastening base (23) of the workbench (10), can be raised onto the support of the balancing detectors (22), in which connection the balance and the overall mass of the piece to be machined can be measured.

12. A method for machimng a piece (P), in which method the piece (P) is attached to the workbench (10) and in which method a separate robot (100) that machines the piece is employed in order to machine the surface of the piece by means of the tool (60) of the robot to the desired dimensional values, characterized in the method, the dimensions of the surface of the piece are observed by means of separate devices, and that, for the time of machining of the piece, the piece is attached to the fastening base (23) of the workbench (10), and the piece (P) is raised by means of an acmator (12aι , 12a ), while it remains attached to the fastening base (23), so that the piece can be placed at different working levels in different machining positions.

13. A method as claimed in claim 12, characterized in that the piece is rotated on the workbench (10), and that, between me cycles of machining carried out by the robot (100), the dimensions of the piece are measured and, based on the measure¬ ment data, machimng of the surface of the piece is carried out.

14. A method as claimed in claim 12 or 13, characterized in that a ray of light is produced from the source of light onto the surface of the object to be measured, in which connection the ray of light forms a stripe of light, and that a source of light is fitted to produce a ray of light both onto the upper face and onto the lower face of the piece to be machined, and that there are actuators by whose means the piece can be raised to the desired working level for the time of machining, preferably grinding, in which connection, by rotating the workbench and by altering the height position of the workbench, it is possible to machine the same workpiece without opening its fastening to the workbench in between.

15. A method as claimed in the preceding claim, characterized in that the piece is attached centrally so that the symmetry axis of the piece coincides with the geometri¬ cal centre line (X) of the shaft of rotation in the workbench, in which connection a point of light or ray of light is produced from the source of light onto the face of the piece, and said point or ray of light is detected by means of a detector, and, based on the observation of the ray of light, the data concerning the profile of the surface of the piece are formed, the piece being rotated after each measurement by rotating the shaft (14) of the workbench (10) and, thereby, the piece (P) attached to the shaft, and that the source of light is a source of laser light, from which a point of light is produced onto the surface of the workpiece to be observed, and the point of light is displaced by scanning along a linear pam preferably from the edge of die workpiece towards its middle area, so that the measurement signal produced, preferably light, passes through the centre line (X).

16. A method as claimed in any of the preceding claims 12 to 15, characterized in that me piece (P) to be machined is a propeller, which is attached by means of cones (24,32) to the workbench while the cones (24,32) centre the piece so that it can be rotated around its geometrical central axis and around the geometrical central axis (X-axis) of the shaft (14), and that each measurement station is monitored by means of an angle detector (29) related to the shaft (14) of rotation.

17. A method as claimed in any of the preceding claims 12 to 16, characterized in that, when the piece to be machined is a propeller (P), in the method, each blade (21) of the propeller (P) is measured separately, and the top face and the bottom face of the blade (21) are machined separately to the desired tolerance values, in which connection, when the surface is machined, always between cycles of machining, measurement of the surface to be machined is carried out.

18. A method as claimed in any of me preceding claims 12 to 17, characterized in that, in the method, a measurement computer (91) of the measurement system (90) is employed, which computer has a data-transfer connection with the central com¬ puter (70), and that there is a separate designing computer (95), which also com¬ municates with the central computer (70), and that the central computer (70) communicates with the control unit (50) of the robot and with the logic (80) and with the control of the workbench (10).

19. A method as claimed in any of the preceding claims 12 to 18, characterized in that, when the robot (100) machines a surface with a tool (60), preferably a grinding disk or equivalent, a program is employed in which the surface to be machined has been divided into different machining areas in accordance with the requirement of machimng, in which case, in the case of different machining areas, the machining of the machining area concerned is carried out with the optimal values and by means of the machining tools regulated by die program.