MACHINING METHOD AND APPARATUS

Abstract: A method of machining a workpiece at a plurality of working positions, employing a pulsed laser under external control, comprises effecting continuous relative movement of the workpiece and the laser the rate of movement being synchronised with the laser firing so that on successive laser pulses the laser beam is aligned with different work positions on the workpiece. Apparatus for machining a workpiece according to this method comprises a mechanism for effecting continuous relative movement of the workpiece and a pulsed laser operable under external control, a position transducer to determine the relative positioning of the workpiece and the laser, and a closed feedback control loop to ensure that each time the laser is pulsed it is aligned with a predetermined work position on the workpiece, the rate of movement being similar to the rated pulse repetition rate where possible.

Description: The invention relates to machining methods and apparatus, and particularly to cutting or drilling (covered herein by the generic term "machining") with a pulsed laser which has to effect a separate machining operation at each of a plurality of spaced positions on a workpiece. With such a machnining operation, it is usual to complete machining at one position on the workpiece, employing the requisite number of pulses of the pulsed laser for this purpose, and then to effect a relative indexing movement of the workpiece and the laser whilst the latter is switched off. The indexing time is generally large as compared with the actual machining time, so that the laser is actually idle for most of the time and the aim of the invention is to improve productivity in this respect. DE-A1-2719275.2 discloses a method of machining a predetermined pattern of discrete areas on an object using a radiation source. The object is moved continuously and the radiation source is pulse modulated to machine each discrete area by impingement of a pulse modulated beam of coherent radiation from the source. Each region of impingement, corresponding to each of said discrete areas is maintained at a substantially constant position on the object by movement of the beam during the time in which the respective discrete area is being machined. EP-A2-0048487 discloses a process for the production of a code disc for optical angle-step signallers or angle coders, including the step of producing light permeable code tracks with a code pattern by partially removing a light impermeable layer applied to the code disc, the pattern applied to the code disc being precisely centred with respect to a shaft which bears the code disc. The code disc is firstly connected to the shaft which is accommodated in bearings and then a laser beam transmitter is used to burn the code pattern into the light impermeable layer. On each occasion, the laser beam transmitter is positioned relative to the code disc via an angle-step control unit and the shaft which bears the code disc is connected to an angle-measuring device and is brought to a predetermined angular position prior to each actuation of the laser beam transmitter. This invention provides a method of machining a workpiece comprising:- providing a pulsed laser to be directed along a path at said workpiece, and mounting said workpiece in a movable holder for effecting adjustment of the attitude wherein the method includes the steps of simultaneously firing said laser along a fixed path whilst continuously adjusting the attitude of said workpiece by movement of said holder such that on successive laser pulses the laser beam is aligned with different work positions on said workpiece, the firing of said laser along said fixed path being synchronised with the movement of said holder. Preferably said workpiece is mounted on a rotary holder, the laser firing being synchronised with the rate of rotation. A circular arrangement of holes may then be machined in the workpiece. It is further preferred that said laser is focussed to determine the diameter of holes machined in said workpiece. In any of the above methods a transducer may be provided on said holder, the position of said holder determining the output of said transducer, said output being fed to laser control circuitry to affect the pulsing of said laser. Also in any of the above methods the rate of movement of said workpiece may be closely related to the rated pulse repetition rate of said laser, said laser being fired each time a new work position is presented. Preferably said workpiece makes a plurality of passes of said laser. The invention also provides apparatus for machining a workpiece comprising:- a pulsed laser having a power supply and focussing means and capable of firing under external control along a path at a workpiece, a continuously movable workpiece holder to hold a workpiece to be acted on in the path of the laser, position transducer means, and control means wherein the laser is mounted at a fixed location to direct a laser beam along a fixed path and in that said laser is fired under control of said control means such that said laser is fired each time a new work position on the workpiece is presented, said position transducer means and said control means is connected to said holder and said laser to synchronise movement of said holder with firing of said laser via a closed feedback control loop. Preferably said holder is a rotary holder. It is further preferred that the movement of said holder is repeated so as to allow repeated passes of the workpiece past the laser. As will be appreciated the method of the invention is of particularly advantageous applications to the pulsed-laser drilling of a ring of holes in a workpiece, which can be indexed with a purely rotational relative movement of workpiece and laser about the centre of the pitch circle of the holes. A typical example is the drilling of a ring of holes in a spinnaret or in a combustion ring for an aircraft gas turbine engine, and the pulsing of the laser is linked to the rotational position of the workpiece such that successive laser pulses impinge at successive hole positions on the workpiece. Thus the number of complete rotations of the workpiece during a complete working cycle is equal to the number of laser pulses required to "break through" when drilling a hole, as a separate static operation, and no time is wasted by the indexing movement which entirely occurs in the off time of the laser between pulses. A closed feedback control loop may be utilised to ensure that the laser is pulsed each time it is aligned with a predetermined work position on the workpiece, the feedback control loop being such that the laser is pulsed when aligned with successive work positions on the workpiece during the continuous relative movement of the latter. In a preferred embodiment a rotary workpiece holder or mounting is driven at an approximately constant speed by an electric motor, and a position feedback loop includes an angular position transducer rotatabley coupled to the workpiece holder or mounting to determine each instant at which a laser, focussed on the workpiece, is fired at the workpiece. The rotary speed is chosen such that the rate at which work positions pass the laser approximately equals the rated pulse repetition rate of the laser to be utilised, thus enabling the maximum average power of the laser to be applied constantly to the workpiece with minimal laser dead time. It will, of course, be appreciated that any arrangements of the work positions can be accommodated with appropriate continuous relative movement of the workpiece. However, the invention is mainly appropriate for percussion laser drilling of holes in a component with the holes disposed in a circular or linear arrangement. The invention will now be further described with reference to the accompanying schematic drawings which illustrate, by way of example, an embodiment of the invention applicable to the laser drilling of a circle of peripheral holes in an annular workpiece. A workpiece W of 45.72cm (18 inch) diameter, 0.30 cm (120/1000 inch) thick stainless steel, is mounted on a workpiece support comprising a rotary table 1 which is rotated by a 2.1 Nm torque D.C. servomotor 2 via a gearbox 3, a shaft 4 and a worm and wheel arrangement 5. Angular position transducers are provided on the motor shaft and on the worm and wheel arrangement 5. The transducers 6, 7 are typically incremental encoders (Heidenhaim) with associated electronics giving a square wave quadratic output with five fold interpolation so as to give an output of 500 pulses per degree of rotation at the table 1. YAG solid state pulsed laser (M34 Coherent General Inc, U.S.A) L, operative under external control is focussed on to the surface of the workpiece W and operates to drill a succession of holes indicated as a , b , c , d , e ..... x in the through the periphery of the annular workpiece W as the latter rotates. The laser L is modified in a known manner such that an external signal can be used to cause the laser to fire (normal input signal +15v, switch to 0v causing firing). The laser L, its associated power supply 9 and the other sytem components described above are all controlled by a central control arrangement 8 which is shown in further detail in Figure 2. The main functional components of the arrangement 8 are as follows: 1.0 Control Elements Figure 2 shows the following major elements: 1.1. Transducer/encoder decode logic 1.1, taking signals from the transducers 6, 7 and producing three main outputs: a) a direction signal, indicating the direction of travel of the transducer and attached axis (table 1) b) a clock signal, giving a square wave output indicating incremental movement at the transducer c) a datum signal, indicating the "Home Marker" position on the transducer. 1.2. An Up Down Counter 1.2 This is fed by the three signals described in 1 above and holds an absolute count indicating the current position at the axis relative to the "Home Marker" position on the transducer. 1.3. A Digital Comparator 1.3 This is used to compare the current value held in the Up/Down Counter 1.2. with a value passed from the CPU 1.4. 1.4. A CPU 1.4 This is used to both control the overall function of the system for reset and power-up and also to control the transfer of data from the Hole Position Data Table 1.6 to the Comparator 1.3. 1.5. A Fire-Pulse Generator 1.5 This circuit produces a short pulse whenever the output of the comparator 1.3 indicates equality of the values from the counter 1.2 and the CPU/Table 1.4, 1.6. This pulse is used by the laser to initiate the firing of the flashlamps. 1.6 A Hole Position Data Table 1.6 This is a pre-programmed look up table accessed by the CPU which has a unique position entry for each hole to be drilled. 2.0 Principle of Operation 2.1. Initialisation To initialise the system the table 1 is positioned such that about a 5° rotation is required to reach the home marker (zero) position indicator and the laser shutter is forced closed. 2.2. Operation The table is commanded to move in a positive direction by the controller 8 at an approximately constant speed on passing through the home marker position, the following will occur as a result of the datum signal being detected. i) the laser shutter will be opened ii) the Up/Down counter 1.2 will be reset to zero iii) the CPU will access the first entry in the data table 1.6 and pass it to the Comparator 1.3 As the table 1 continues to move, the count value from the transducer 6 in the Up/Down counter 1.2 will eventually become equal to that stored in the comparator 1.3. At this time an equal signal will be passed to the Fire-Pulse Generator 1.5 causing the laser to fire a single pulse. The equal signal from the comparator also goes to the CPU 1.4 which will then access the next entry in the data table 1.6 and pass it to the comparator 1.3. The above sequence is reapeated until in the case of a rotary table 1 the "Home Marker" position is again reached at which time the Up/Down 1.2 is again zeroed and the CPU 1.4 resets its data pointer to top of the Data Table 1.6. The system is thus ready to begin a second revolution of the axis without interruption to the axis motion. As will be appreciated from the above description, the position of the table 1 is indicated by the output from the transducers/encoders 6,7, particularly transducer 6 on the worm drive. This information is used by the controller 8 to cause the laser to fire when the workpiece W is aligned with the laser at any of positions A, B, C, D, E.......X for holes to be drilled. The peripheral speed of the workpiece W is usually arranged such that the rate of presentation of hole positions to the laser L is approximately equal to the rated pulse repetition rate of the laser thus reducing the laser dead time to a minimum. However, it is to be noted that the firing rate of the laser L is determined by the position of the table 1, not by time and so the time between firing pulses can vary. Typically, 3 holes are drilled per degree of rotation and the table rotates at approximately 6 o s⁻¹ giving about 500 holes per revolution. As the position of the table 1 and workpiece W can be accurately determined and used to fire the laser L, it is not necessary that the laser should penetrate the workpiece in one firing as the system allows multiple passes of the workpiece until the required hole depth is drilled e.g. 5 passes to drill a 0.050 cm (20/1000 inch) diameter hole 4mm deep. It will be appreciated that although in the above description the holes are equally spaced, the invention is not restricted in this respect since any selected pattern of irregularly spaces holes may be machined. However, where there are large spacings between groups of holes, the laser shutter will need to be closed and this can be achieved under control using a coarse position indicator, e.g. transducer 7. Furthermore, by using information from more than one axis to synchronise the laser movement, for example by the use of an X-Y table, any desired hole pattern can be produced in accordance with a central machine programme.

Claims: A method of machining a workpiece (W) comprising:- providing a pulsed laser (L) to be directed along a path at said workpiece (W), and mounting said workpiece (W) in a movable holder (1) for effecting adjustement of the attitude of said workpiece (W) of said laser (L), characterised in that the method includes the steps of simultaneously firing said laser (L) along a fixed path whilst continuously adjusting the attitude of said workpiece (W) by movement of said holder (1) such that on successive laser pulse the laser beam is aligned with different work position on said workpiece (W), the firing of said laser (L) along said fixed path being synchronised with the movement of said holder (1). A method as claimed in Claim 1, characterised in that said workpiece (W) is mounted on a rotary holder (1), the laser firing being synchronised with the rate of rotation. A method as claimed in Claim 2, wherein a circular arrangement of holes is machined in said workpiece (W). A method as claimed in any of Claims 1 to 3, characterised in that said laser (L) is focussed to determine the diameter of holes machined in said workpiece (W). A method as claimed in in any of Claims 1 to 4, characterised in that a transducer (7) is provided on said holder (1), the position of said holder (1) determining the output of said transducer (7), said output being fed to laser control circuitry (8) to affect the pulsing of said laser. A method as claimed in any of Claims 1 to 4, characterised in that the rate of movement of said workpiece (W) is closely related to the rated pulse repetition rate of said laser (L), said laser being fired each time a new york position is presented. A method as claimed in any of Claims 1 to 6, characterised in that said workpiece (W) makes a plurality of passes of said laser. Apparatus for machining a workpiece comprising:- a pulsed laser (L) having a power supply (9) and focussing means and capable of firing under external pressure along a path at a workpiece (W), a continuously movable workpiece holder (1) to hold a workpiece to be acted on in the path of the laser, position transducer means (6, T), and control means (1.1 to 1.6); characterised in that the laser (L) is mounted at a fixed location to direct a laser beam along a fixed path and in that said laser is fired under control of said control means (1.1 to 1.6) such that said laser is fired each time a new work position on the workpiece is presented, said position transducer means and said control means is connected to said holder and said laser to synchronise movement of said holder with firing of said laser via a closed feedback control loop. Apparatus as claimed in Claim 8, characterised in that said holder (1) is a rotary holder. Apparatus as claimed in Claim 9, characterised in that movement of said holder (1) is repeated so as to allow repeated passes of the workpiece past the laser (L).

Inventors: SUTTIE, DAVID GEORGE
Applicants: ELLIOTT INDUSTRIES LIMITED
Application year: 1989
Main classification: B23K26
IPC codes: B23K26