Abstract:
A method of laser drillings a series of holes at spaced-apart hole locations in a workpiece comprising continually delivering laser pulses to the workpiece, and positioning the workpiece relative to the laser head from one hole location to a next hole location within a time between two consecutive pulses by moving both the laser head and the workpiece. The positioning includes synchronizing on an ongoing basis the workpiece with the pulse frequency.

Description:
TECHNICAL FIELD 
   The invention relates generally to drilling machines and, more particularly, to an improved high speed laser drilling machine and method. 
   BACKGROUND OF THE ART 
   Providing cooling holes in combustion chamber liners and shield heat liners of gas turbine engines usually requires drilling hundreds of holes with variable inclination angles. Known processes of drilling such holes include the use of a pulsed laser in a drilling machine having five degrees of freedom where the worktable receiving the workpiece and the laser head are driven by ball screws. Generally, the table is translated along and rotated about perpendicular horizontal axes (x,y) and the laser head is translated along a vertical axis (z). 
   The limited capabilities of the feed drives and the dynamic constraints due to the high moving mass and inertia of such a drilling machine limits the linear and angular accelerations of the relative movement between the laser head and the worktable. Thus, the positioning between two consecutive holes on the workpiece usually takes more time than the time between two pulsations of the pulsed laser. Accordingly, the laser shutter must be closed and opened each time the machine moves from one hole to the next As a result, a major part of the cycle time between consecutive holes is spent in positioning the worktable and/or laser head and opening/closing the laser shutter, which largely increases the duration of the drilling process. 
   Accordingly, there is a need to provide an improved high speed laser drilling machine and method. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide an improved high speed laser drilling machine and method. 
   In one aspect, the present invention provides a method of laser drilling a series of holes at spaced-apart hole locations in a workpiece, comprising the steps of continually delivering laser pulses at a given laser pulsed frequency from a laser head to the workpiece while keeping a shutter of the laser head open, and positioning the workpiece relative to the laser head from one hole location to a next hole location within a time between two consecutive pulses by moving both the laser head and the workpiece. 
   In another aspect, the present invention provides a method for laser drilling a plurality of holes into a workpiece, the method comprising: producing laser pulses at a given frequency with a laser head; successively moving at least one of the workpiece and the laser head to a series of relative positions with at least one of a variable speed and an irregular pattern, a time necessary to move between consecutive ones of the relative positions being at most equal to a time between two consecutive ones of the laser pulses; drilling a hole in the workpiece at each one of the series of relative positions with at least one of the laser pulses. 
   In another aspect, the present invention provides a method for laser drilling a plurality of holes into a workpiece, the method comprising: producing a pulsed laser beam with a laser head, the pulsed laser beam having a given pulse frequency, successively aligning each of a plurality of desired hole locations with the pulsed laser beam through relative movement between the workpiece and the laser head, one of the workpiece and the laser head being rotated about two different axes of rotation; and drilling a hole at each of the desired hole locations by coordinating the relative movement with the laser pulse frequency. 
   In another aspect, the present invention provides a high speed laser drilling machine for drilling a hole in each of a plurality of desired holes locations of a workpiece, the high speed drilling machine comprising: a laser head producing laser pulses in a constant manner; a worktable adapted to retain the workpiece; means for moving at least one of the worktable and the laser head to produce a relative movement between the worktable and the laser head along five degrees of freedom, the means for moving successively aligning the laser head with each of the hole locations at a desired hole angle during a time between successive laser pulses; and means for controlling the means for moving to coordinate the relative movement with the laser pulses so that the laser pulses drill a hole in the workpiece at each of the hole locations at the desired hole angle. 
   In another aspect, the present invention provides a high speed drilling machine comprising for drilling a plurality of holes in a workpiece, the high speed drilling machine comprising: a worktable adapted to receive the workpiece; a fixed base and a body extending therefrom; at least three legs connecting the worktable to the fixed base, each of the legs having a high speed linear actuator such as to have a variable length, the legs moving the worktable along at least three degrees of freedom; a laser head connected to the body and producing laser pulses in a constant manner to drill a plurality of holes in the workpiece. 
   In another aspect, the present invention provides a high speed drilling machine for drilling a plurality of holes in a workpiece, the high speed drilling machine comprising: a fixed base and a body extending therefrom; a worktable connected to the fixed base and adapted to receive the workpiece; a first laser portion connected to the body through at least a first high speed linear actuator to be translatable along at least a first direction; a second laser portion connected to the first laser portion through a first high angular acceleration rotational drive to be rotatable about a first axis; a laser head connected to the second laser portion through a second high angular acceleration rotational drive to be rotatable about a second axis, the laser head producing laser pulses in a constant manner to drill a plurality of holes in the workpiece. 
   Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
       FIG. 1  is a perspective view of a drilling machine according to an embodiment of the present invention; 
       FIG. 2  is a perspective view of a drilling machine according to another embodiment of the present invention; and 
       FIG. 3  is a perspective view of a drilling machine according to a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a high speed drilling machine  10  including a fixed base  12 , a body  14  supported thereon, a low inertia circular worktable  16  movably connected to the base  12  and receiving a workpiece  18 , and a laser assembly  20  movably connected to the body  14  and located above the worktable  16 . Throughout this specification, the term “laser assembly” and other related terms are intended also to encompass other drilling or machining processes using a pulsing medium. Similarly, the term “drilling” and other related terms are intended to encompass other machining processes removing matter from a workpiece, such as trepanning, cutting, etc. 
   A tripod base  24  is received on top of the base  12  and rigidly connected thereto. Three legs  22  extend from the tripod base  24  and support the worktable  16 . The legs  22  extend radially from the worktable  16  in a regulary angularly spaced apart manner. Each leg  22  is connected to the tripod base  24  through a revolute joint  26  having an axis of rotation perpendicular to the leg  22 . Each leg  22  also includes a high speed linear motor drive  28  comprising a strut base  30  which is slidable under the action of a high speed linear motor (not shown). A strut  32 , preferably made of carbon fibre, is rigidly connected to each strut base  30  at one end and connected at another end to the worktable  16  through a spherical joint  34 . Actuation of each linear motor drive  28  varies the length of the corresponding leg  22  and causes movement of the worktable  16 . This configuration allows the worktable  16  to be moveable along three degrees of freedom, namely a translation Tz along the vertical direction, a rotation Ry about a horizontal axis Ay and a rotation Rx about a horizontal axis Ax perpendicular to the axis Ay. 
   The body  14  includes four identical vertical pillars disposed in a rectangular pattern about the worktable  16 . Two y-axis high speed linear motor drives  40  extend horizontally in a parallel manner, each one being connected to two of the pillars. Each of the y-axis linear motor drives  40  receives an interfacing platform  42  which is slidable under the action of a high speed linear motor (not shown) to produce a translation Ty. A controller (not shown) provides motion synchronization between the two y-axis linear motor drives  40  such that the two interfacing platforms  42  move as a single unit An x-axis high speed linear motor drive  44  is rigidly connected to the two interfacing platforms  42  such as to extend perpendicularly to the y-axis linear motor drives  40 . The x-axis linear motor drive  44  receives a laser head  36  of the laser assembly  20  which is slidable under the action of a high speed linear motor (not shown) to produce a translation Tx. A laser focusing lens  38  extends downwards from the laser head  36  over the worktable  16 . This configuration allows the laser assembly  20  to be movable along two degrees of freedom, namely the perpendicular translations Tx and Ty, which are both perpendicular to the translation Tz. 
   Thus, the drilling machine  10  provides five degree-of-freedom relative positioning between the laser head  36  and the workpiece  18  on the worktable  16 , i.e. three perpendicular translations, Tx, Ty and Tz, and two rotations about perpendicular axes, Rx and Ry. The configuration of the drilling machine  10 , particularly the parallel kinematics structure formed by the legs  22 , tripod base  24  and worktable  16 , provides great motion flexibility and low load capacity on the axes of the linear motor drives  28 ,  40 ,  44 . The two y-axis linear motor drives  40  provide a better accuracy and stabilize the support of the high moving mass of the laser assembly  20 . Preferably, the tripod geometrical features are reconfigurable to provide the motion flexibility required by the products. The revolute joints  26  and/or the spherical joints  34  can be displaced during setup time to obtain the desired geometry. A high sampling rate real-time open arehitecture controller controls the linear motor drives  28 , 40 , 44  to obtain the desired five degree-of-freedom motion through an inverse kinematic model and synchronizes the relative movement between the workpiece  18  on the worktable  16  and the laser head  36  with the laser pulse frequency. 
   In use, the laser head  36  produces laser pulse having a preferable frequency of at least 4 Hz, more preferably between 10 Hz and 20 Hz. A number of desired hole locations and hole angles are programmed into the controller. The controller directs the linear motor drives  28 , 40 , 44  to move the worktable  16  and laser head  36  to successively align the desired hole locations with the pulsed laser beam at the desired angle, while coordinating the movement with the laser frequency such as to move the worktable  16  and laser head  36  during the time between consecutive pulses. In other words, after a hole is drilled, the next desired hole location is aligned with the laser head  36  at the desired angle before the laser head  36  produces a next laser pulse. 
     FIG. 2  illustrates another embodiment of a high speed drilling machine  110  including a fixed base  112 , a body  114  supported thereon, a worktable  116  movably connected to the base  112  and receiving a workpiece  118 , and a laser assembly  120  movably connected to the body  114  and located above the worktable  116 . 
   The base  114  includes an horizontal y-axis high speed linear motor drive  140  receiving an interfacing platform  142  which is slidable under the action of a high speed linear motor (not shown) to produce a translation Ty. The interfacing platform  142  is rigidly connected to an horizontal x-axis high speed linear motor drive  144  extending perpendicularly to the y-axis linear motor drive  140 . The x-axis linear motor drive  144  receives the worktable  116  which is slidable under the action of a high speed linear motor (not shown) to produce a translation Tx. This configuration allows the worktable  116  to be moveable along two degrees of freedom, namely perpendicular horizontal translations Tx and Ty. 
   The body  14  is shaped as an inverted L extending from the base  116 . The top portion of the L receives a vertical z-axis high speed linear motor drive  128  extending perpendicularly to the x-axis and y-axis linear motor drives  140 , 144 . A laser cavity  148  of the laser assembly  120  is received in the linear motor drive  128  to be slidable under the action of a high speed linear motor (not shown) to produce a translation Tz. A first rotary portion  146  is connected at the bottom of the laser cavity  148  through a first high angular acceleration direct drive  150  having an axis of rotation Az parallel to the translation Tz. A laser head  136  is connected to a side of the rotary portion  146  through a second high angular acceleration direct drive  151  having an axis of rotation Ah perpendicular to the axis of rotation Az. A laser focusing lens  138  extends downwards from the laser head  136  over the worktable  116 . The rotary portion  146  and laser head  136  have a minimal moment of inertial and each include mirrors such that the laser beam coming from the laser cavity  148  can reach the laser focusing lens  138 . This configuration allows the laser head  136  to be movable along three degrees of freedom, namely the translation Tz, a rotation Rz about the axis Az and a rotation Rh about the axis Ah. 
   Thus, the drilling machine  110  provides five degree-of-freedom relative positioning between the laser head  136  and the workpiece  118  on the worktable  116 , i.e. three perpendicular translations, Tx, Ty and Tz, and two rotations about perpendicular axes, Rz and Rh. No inverse kinematic model is required for this layout since the required motion of the motor drives  128 , 140 , 144 , 150 , 151  is directly transformed from the coordinates of the desired hole angles by appropriate control means. 
     FIG. 3  illustrates a further embodiment of a high speed drilling machine  210  including a fixed base  212 , a body  214  connected thereto, a worktable  216  integral with the base  212  and receiving a workpiece  218 , and a laser assembly  220  movably connected to the body  214  and located above the worktable  216 . In this embodiment, the worktable  216  is fixed and the laser assembly  220  provides the five degree-of-freedom movement. 
   The body  214  includes two vertical pillars extending with the worktable  216  therebetween. Two y-axis high speed linear motor drives  240  extend horizontally in a parallel manner, one on top of each pillar. Each of the y-axis linear motor drives  240  receives an interfacing platform  242  which is slidable under the action of a high speed linear motor (not shown) to produce a translation Ty. A controller (not shown) provides motion synchronization between the two y-axis linear motor drives  240  such that the two interfacing platforms  242  move as a single unit. An x-axis high speed linear motor drive  244  is connected to the two interfacing platforms  242  such as to extend perpendicularly to the y-axis linear motor drives  240 . The x-axis linear motor drive  244  receives another interfacing platform  243  which is slidable under the action of a high speed linear motor (not shown) to produce a translation Tx. A z-axis high speed linear motor drive  228  is rigidly connected to the interfacing platform  243  such as to extend perpendicularly to the x-axis and y-axis linear motor drives  240 , 244 . A laser support  252  is received in the z-axis linear motor drive  228  and is slidable under the action of a high speed linear motor (not shown) to produce a translation Tz. A first rotary portion  246  is connected at the bottom of the laser support  252  through a first angular acceleration direct drive  250  having an axis of rotation Az parallel to the translation Tz. A second rotary portion  247  is connected to the first rotary portion  246  through a second high angular acceleration direct drive  251  having an axis of rotation Ar extending at an angle from the axis of rotation Az, a preferred angle being 45 degrees. A laser head  236  is rigidly connected to the second rotary portion  247 , with a laser focusing lens  238  extending from the laser head  236 . The laser focusing lens  238  forms a complementary angle with the axis of rotation Ar such that it is possible to bring the focusing lens  238  parallel to the axis of rotation Az, a preferred complementary angle being 45 degrees. 
   This configuration allows the laser head  236  to be movable along five degrees of freedom, for five degree-of-freedom relative positioning between the laser head  236  and the workpiece  218  on the worktable  216 , i.e. three perpendicular translations, Tx, Ty and Tz, a rotation Rz about the axis Az and a rotation Rr about the axis Ar. Moreover, this configuration reduces the amount of translation necessary to compensate for any offset due to the rotations. The two y-axis linear motor drives  240  provide a better accuracy and stabilize the support of the high moving mass of the laser assembly  220 . 
   In all embodiments, high resolution linear feedback encoders, preferably with a resolution of 0.00004 inch, are provided for the purpose of accurate positioning. The motor drives  28 , 40 , 44 /  128 , 140 , 144 , 150 , 151 /  228 , 240 , 244 , 250 , 251  are preferably cog free brushless DC servomotors, more preferably IntraDyn drives manufactured by Bosh Rexroth, but can be any other motor drives capable of sufficient speeds and accelerations. The linear motor drives  28 , 40 , 44 /  128 , 140 , 144 /  228 , 240 , 244  can advantageously provide for 10 g acceleration with a maximum speed of 5 m/s. The angular motor drives  150 , 151 /  250 , 251  can advantageously provide for an angular acceleration of 500 rad/s2 with a peak angular speed of 1200 rpm. Such high speed and high acceleration capabilities enable the relative positioning between the laser head  36 , 136 , 236  and the worktable  16 , 116 , 216  to happen on a rate that matches the frequency of the pulsed laser, e.g. preferably at east 4 Hz, more preferably 10 to 20 Hz. 
   In use, the laser head  36 , 136 , 236  produces a laser pulse at a given frequency, in a continual manner. A relative movement between the laser head  36 , 136 , 236  and the workpiece  18 , 118 , 218  is produced by moving the laser head  36 , 136 , 236  and/or the worktable  16 , 116 , 216 . Each of the desired hole locations of the workpiece  18 , 118 , 218  is successively aligned with the laser head  36 , 136 , 236  such that a hole can be drilled. The relative movement between the workpiece  18 , 118 , 218  and the laser head  36 , 136 , 236  is coordinated so that the time elapsed between the alignment of consecutive desired hole locations with the laser head  36 , 136 , 236  is less than or equal to the time between the firing of two consecutive pulses, e.g. the positioning is done in at most 0.1 s for a laser frequency of 10 Hz. 
   The laser shutter is not closed during the relative movement between the workpiece  18 , 118 , 218  and the laser head  36 , 136 , 236 , since the positioning is done in between consecutive pulses. Thus, a laser pulse frequency of 10 Hz represents a drilling rate of 600 holes per minute for holes requiring only one laser pulse to be completed, which can represent up to a 20 fold increase in drilling rate when compared with prior drilling machines driven by ball screws. The laser drilling machine  10 , 110 , 210  can improve the productivity of laser drilling operations of combustion chambers, combustion chamber liners, and other workpieces necessitating a high number of holes, by minimizing the time wasted in non-drilling tasks such as positioning of the workpiece  18 , 118  and/or the laser head  36 , 136 , 236  while eliminating the need to open and close the laser shutter. 
   The relative movement with five degrees of freedom between the laser head  36 , 136 , 236  and the workpiece  18 , 118 , 218 , including the two rotations of the laser head  136 , 236  or workpiece  36 , allows the relative movement between the workpiece  18 , 118 , 218  and the laser head  36 , 136 , 236  to be done along an irregular pattern, i.e. with numerous changes of direction, such that the drilling machine  10 , 110 , 210  can effectively drill holes that are irregularly distributed (i.e. with variable distances between adjacent holes), as well as drill holes in workpieces which are not axisymmetric. The variable speed of the relative movement between the laser head  36 , 136 , 236  and the workpiece  18 , 118 , 218  allows the positioning to be done between two consecutive pulses even when the holes are distributed along an irregular pattern. 
   Also, the two rotations of the laser head  136 , 236  or workpiece  36  allow the drilling machine  10 , 110 , 210  to drill successive holes having different hole angles, as well as to drill holes having a composite angle. 
   The coordinated movement of both the laser head  36 , 136  and the workpiece  18 , 118  allows for faster and easier relative positioning from one hole location to the next. 
   The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, the drilling machine  10 , 110 , 210  can be used with other types of workpieces requiring a high number of holes, of any shape, and of any material appropriate for laser drilling. The invention is also intended to encompass other drilling processes using a pulsing drilling medium. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.