Abstract:
A novel apparatus and method for laser-assisted micro-milling. The disclosed laser-assisted micro-milling system and method provides unique micro-milling capabilities for very difficult-to-machine materials, such as ceramics, high temperature alloys and composites. A low power laser beam is focused at a very small spot, thus producing a very high power density, the spot being located just ahead of a mechanical micro-milling cutter to preheat the material prior to machining. This localized heating thermally weakens the workpiece resulting in lower cutting forces, improved surface finish, and longer tool life. The system is capable of micro-milling difficult-to-machine materials that may be conductive or non-conductive with high material removal rates compared to existing systems and methods.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/927,996, filed May 7, 2007, which application is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to laser-assisted material processing, and more particularly to systems and methods for performing micro-scale laser-assisted machining. 
     BACKGROUND OF THE INVENTION 
     Micromachining can be difficult to apply to many engineering materials due to a variety of scaling induced factors including: low cutting speeds, high relative tool deflections and runout, and increased material strength at smaller size scales. Additionally, edge burrs which can easily be removed after macro-scale machining must be avoided in micromachining due to the lack of available finishing operations. A fundamental change in the cutting process occurs when the uncut chip thickness falls below a minimum value. Below this minimum chip thickness the work material is ploughed by the tool instead of being cleanly sheared away, resulting in increased cutting forces, surface roughness, and a decrease in machined edge quality. Some hard materials such as ceramics and high temperature alloys will further increase the wear on the cutting tool. 
     Specific cutting energy at the micro-scale is much higher than at the macro-scale owing to the well known size-effect in machining operations and the relative dullness of micro tools. This dullness is due to limits on how small the cutting edge radius can be made. Typically, conventional machining systems have an edge radius to diameter ratio of 1×10 −6  while micromachining systems often has a ratio greater than 0.005. These issues result in higher relative cutting forces which cannot be sustained by micro-sized cutting tools. For micromachining systems this typically leads to failure of the tool by complete fracture at the flute starting location. 
     Therefore, there is a need for a laser-assisted micromachining system which can precisely cut hard objects while maintaining a high edge quality and decreasing the wear on a cutting tool to achieve prolonged tool life. 
     SUMMARY OF THE INVENTION 
     A general object of the invention is to overcome problems associated with conventional micromachining. One aspect of the invention is a laser-assisted micro-milling system including a high-speed spindle which holds a micro-milling tool, in combination with a laser that is optically aligned sequentially with a beam expander, a focusing element, and a target spot substantially adjacent to a working end of the micro-milling tool. The workpiece material to be machined is locally preheated by a focused laser beam prior to machining. 
     According to another aspect of a laser-assisted micro-milling system according to the present invention, a tool holder on a machine frame holds a micro-milling tool, and a laser is optically aligned sequentially with a beam expander, a focusing element, and a target spot substantially adjacent to a working end of the micro-milling tool. The workpiece to be machined is locally preheated by a focused laser beam prior to machining. 
     Another aspect of the present invention is a laser-assisted micro-milling method with includes moving a workpiece material relative to a micro-cutting too, producing a laser beam, expanding the laser beam, and focusing the expanded laser beam on a target spot on the workpiece material so as to locally preheat the target spot prior to machining thereof with a micro-milling tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a front view of one embodiment of a laser-assisted micro-milling system according to the present invention, including the entire path of the laser beam. 
         FIG. 1B  illustrates a left side view of the laser-assisted micro-milling system of  FIG. 1A . 
         FIG. 2  illustrates components of the beam expander and the effect of those components on a laser beam. 
         FIG. 3  illustrates the slidable universal lens holder. 
         FIG. 4  illustrates a partial path of the laser beam in the laser-assisted micro-milling system and illustrates an embodiment with a slidable universal lens holder, a high-resolution digital camera, a nozzle, and an acoustic imaging sensor. 
         FIG. 5  illustrates a variety of cutting tools which can perform cutting actions on a workpiece material. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     One application of a novel apparatus for micromachining in accordance with the present invention is to facilitate machining of difficult-to-machine materials on a micro-scale.  FIG. 1  illustrates one embodiment of a laser-assisted micro-milling system  10  according to the present invention. The disclosed embodiment has a focusing element  30  and a beam expander  36  mounted along with a set of mirrors  25 ,  26 ,  27  and  28  on a fixture plate  42  which is designed to be mounted on the frame of a milling machine that has multi-axis movable computer controlled stage (not shown) which supports a workpiece material  24  to be cut by a micro-milling tool  22  held by a high-speed spindle  16 . The high-speed spindle  16  may operate at 90,000 revolutions per minute or higher and is capable of holding the micro-milling tool  22  such as a micro-endmill which may have a diameter of 10-200 microns (μm). 
     A laser beam  20  generated by a laser  12  is reflected by the first mirror  25  into the beam expander  36 , which increases the beam diameter to yield an expanded laser beam  20 ′. The laser beam  20  passes through the beam expander  36  and the expanded beam  20 ′ is reflected by the second mirror  26  into the focusing element  30 . The laser beam  20 ′ passes through the focusing element  30  to yield a focused laser beam  20 ″ ( FIG. 4 ), which is reflected by the third mirror  27  onto the fourth mirror  28 , which reflects the laser beam  20 ″ onto the workpiece material  24 . In the depicted embodiment the beam path from the fourth mirror  28  to the workpiece material  24  forms the final stage path  48  of the laser beam  20 ″. The final stage path  48  is the final straight-line beam path segment of the laser beam  20 ″, which interacts with the workpiece material  24 . 
     The angle of incidence the final stage path forms with the workpiece material is between zero and 45°, preferably less than 30°, and more preferably 10-15°. The lower the angle of incidence of a laser beam, the more circular the spot is on the workpiece and the greater effect it has on the workpiece material  24 . The angle of incidence is the angle formed between the final stage path of a laser and the perpendicular of the workpiece material where the laser beam interacts with the workpiece material. In instances where the workpiece material has an uneven surface the angle of incidence is understood to be the angle formed by the final stage path and the longitudinal axis of the micromachining tool. 
     Mirrors may be incorporated into the arrangement to make the beam path of the beams  20 ,  20 ′ and  20 ″ occupy a smaller work space by incorporating reflecting angles into the beam path. The first, second, third and fourth mirrors  25 ,  26 ,  27 ,  28  can be configured to reflect the laser beams  20 ,  20 ′ and  20 ″ and introduce an angle between zero and 180 degrees into the final stage path  48  of the laser beam  20 ″ so that the laser beam  20  will pass through the beam expander  36  and then the expanded beam  20 ′ will pass through the focusing element  30 . The first, second, third, and fourth mirrors  25 ,  26 ,  27 ,  28  can be coupled to the fixture plate  42  to optically align the laser  12 , beam expander  36 , and focusing element  30  in a more compact work space than if the laser  12 , beam expander  36 , and focusing element  30  were oriented in an optically equivalent straight line fashion. The various optical elements described above are considered optically aligned in that the beam  20  emitted from the laser  12  reflects off the mirror  25  and passes through the beam expander  36 , the expanded beam  20 ′ reflects off the mirror  26  and passes through the focusing element  30 , and then the focused beam  20 ″ reflects off the mirrors  27  and  28 . The relative positioning of the focusing element  30 , mirrors  27  and  28 , and cutting tool  22  determines the target spot on the workpiece  24 . The first mirror  25 , second mirror  26 , third mirror  27 , and fourth mirror  28  are available from a number of sources, for example Laser Research Optics, Part No. MM-0508-M-UC, 14 mm. The mirrors  25 ,  26 ,  27  and  28  need to be able to withstand the heat of the laser beams  20 ,  20 ′ and  20 ″. Molybdenum mirrors can be used to provide durability due to its intrinsically hard surface. 
     A variety of focusing elements are available from a number of sources, for example, a Selies LX focusing lens from Laser Research Optics, Part No. LX-0730-Z-ET1.8. A zinc selenide lens has a low absorption of energy, which is suitable for laser applications. A variety of focusing elements can be used in the disclosed embodiment. Focusing elements are understood to include a variety of focusing optics. Examples of focusing optics include various lenses, mirrors, prisms, diffractive optical elements, and zone plates. The focal length of the focusing element  30  affects the beam diameter as the laser beam  20 ′ passes through the focusing element  30 . The shorter the focal length of the focusing element  30 , the smaller the beam diameter is at the focal point after passing through the focusing element  30  compared to focusing elements having a longer focal length. The diameter of the laser beam  20 ″ at the focal point of the focusing element is known as the spot size. The spot size is proportional to the focal length of the focusing element  30 . The focal length of the focusing element  30  in the disclosed embodiment is less than 100 millimeters, and may be in the range of 10-20 mm for example. 
     The spot size is also affected by the diameter of the laser beam  20 ′ before it enters the focusing element  30 . A larger beam diameter before entering the focusing element  30  yields a smaller beam diameter at the focal point after passing through the focusing element  30  compared to a smaller diameter laser beam  20 ′ before entering the focusing element  30 . The spot size is inversely proportional to the diameter of the laser beam  20 ′ before passing through the focusing element  30 . Therefore the smallest spot size is achieved when the focal length of the focusing element  30  is smaller and the beam diameter before passing through the focusing element  30  is larger. 
     However the spot size cannot be made infinitely small because the wavelength of the laser beam represents the theoretical minimum diameter under perfect conditions. The wavelength of a typical CO 2  laser is 10.6 microns, and therefore 10.6 microns is the smallest theoretical spot size of typical CO 2  laser beam. The spot size is theoretical because it could only be achieved if lenses and mirrors could have zero aberrations and if a laser could have zero diffraction and divergence. Such an arrangement cannot be achieved in the physical world due to opposing physical optimization techniques. The effect of laser diffraction and divergence are minimized when the beam diameter is large and the focal length of a lens is short. However the effect of lens aberration is minimized when the beam diameter is small and the focal length of a lens is long. The opposing configurations to minimize optical imperfections prohibit the theoretical minimum spot size from being attained in the real world. 
     The beam expander  36  is used to enlarge the diameter of the laser beam  20 ′ before passing through the focusing element  30 .  FIG. 2  illustrates how the diameter of the laser beam  20  is expanded when passing through the beam expander  36 . The beam expander  36  can be comprised of an entrance optics plano-concave lens  50  and exit optics objective lens  52 . The beam expander power is equal to the ratio of the effective focal length of the exit optics objective lens  52  to the effective focal length of the entrance optics plano-concave lens  50 . The physical separation between the objective lens  52  and the entrance lens  50  is equal to the sum of their back focal lengths. The result of the laser beam  20  passing through the beam expander  36  is the laser beam  20 ′ having a larger beam diameter than the beam  20  that entered the beam expander  36 . The beam expander  36  is available from a number of sources, for example a Synrad COL 2.5 beam expander, which increases the diameter of an incoming beam by a factor of 2.5. Other beam expanders are also contemplated, including a flexible or adjustable beam expander, i.e., one capable of varying the output beam diameter. 
       FIG. 3  illustrates a slidable universal optics holder  34 . The slide housing  54  can be mounted on fixture plate  42 . Slide rails  56  are coupled to the slide housing  54 . The universal optics holder  34  is slidably coupled to the slide rails  56  whereby the position of the universal optics holder  34  can be adjusted along a path parallel to the beam path of the beam  20 ″. The focusing element  30  is coupled to the universal optics holder  34 , and adjusting the position of the universal optics holder  34  thereby adjusts the position of the focusing element  30 . The ability to adjust the position of the focusing element  30  allows the laser beam  20 ″ to be optimally focused on the workpiece material  24  as various focusing elements are used and when the position of the workpiece material  24  moves. The slidable universal optics holder  34  is available from a number of sources, for example Edmund Industrial Optics, Part No. NT38-531. 
       FIG. 4  illustrates an embodiment incorporating the slidable universal lens holder  34 . The diameter of the laser beam  20  is increased by the beam expander  36  as noted above. The expanded laser beam  20 ′ is reflected off the second mirror  26  and then passes through the focusing element  30 . The position of the focusing element  30  can be adjusted by sliding the universal optics holder  34  along the slide rails  56  which are parallel to the beam path of the beam  20 ″. After passing through the focusing element  30  the laser beam  20 ″ is reflected off the third mirror  27  onto the fourth mirror  28  and reflected off the fourth mirror  28  onto the workpiece material  24  at a point substantially adjacent to the micromachining tool  22 . 
     The optical arrangement shown in  FIG. 4  was designed with the intent of shortening the focal length and creating a very small spot size while substantially avoiding vibration problems. This setup allows the micromachining tool  22  to be held directly by the high speed spindle  16  and not by a complex arrangement of collets and bearings. In order to provide the requisite final stage path  48  of the beam  20 ″ and also to allow for adjustment of the focusing element  30 , the several mirrors  25 ,  26 ,  27 ,  28  are mounted to the fixture plate  42  in the disclosed embodiment. Among other alternative arrangements contemplated for use as part of the present invention, such as an arrangement which eliminates mirrors  25  and  26  by aligning the beam expander  36  and focusing element  30  on a line at an angle of about 30 degrees from vertical, e.g., such as the angle at which a high-resolution digital camera  58  is mounted in  FIG. 4 . As another alternative, focusing element  30  may be replaced by a parabolic mirror suitably oriented to receive the output beam  20 ′ from the beam expander  36  oriented at an angle of about 30 degrees from vertical and to reflect and focus that beam  20 ″ onto the target spot. 
     The focused laser beam  20 ″ may be between a nanometer and a millimeter in diameter for some applications, e.g., in the range of 20-800 μm, but the beam  20 ″ preferably has a diameter of 20-200 μm and, in one embodiment, has a diameter of about 80-150 μm. The focused beam  20 ″ provides the requisite heating immediately ahead of the micromachining tool  22  during laser-assisted micromachining. For example, with a 100-micron-diameter focused beam  20 ″ and a 100-micron-diameter micromachining tool  22 , the center of the focused laser beam  20 ″ is substantially adjacent and preferably 50-100 microns from the outer surface of the tool  22 . That is, the center-to-center spacing is preferably 100-150 microns. Substantially adjacent means the center-to-center distance ranges from equal to the average diameter of the cutting tool  22  and the spot size to ten times the average diameter of the cutting tool  22  and the diameter of the spot size. Separating the focused laser beam  20 ″ from the micromachining tool  22  helps shield the micromachining tool  22  from the heating effect of the laser beam  20 ″ and increases tool life. 
     The arrangement also allows for different diameter cutting tools to be used. Due to the small focal diameter of the laser beam, it is possible to elevate the workpiece material temperature to over 1000° C. with a low power laser. At the elevated temperature, the micromachining tool  22  removes material with a material removal rate higher than existing methods. In particular, this process provides an effective means of machining difficult-to-machine materials such as ceramics and high temperature alloys, which conventional mechanical micromachining tools cannot machine. A variety of lasers are available, for example a Synrad 10 W CO 2  laser with, for example, a 3.5 mm beam diameter. 
     The camera  58  can be mounted to the fixture plate  42 . A fifth mirror  60  is attached to the fixture plate  42  whereby the cutting action of the micromachining tool  22  is reflected to the high-resolution digital camera  58  by the fifth mirror  60 . The high-resolution digital camera  58  is understood to be optically aligned with the cutting action of the micromachining tool  22  in this orientation. This arrangement operates as an in-process control device to monitor the cutting action. A variety of high-resolution cameras can be used, for example Edmund Optics, Part number E0-3112. 
     An acoustic imaging sensor  62  can be incorporated into the laser-assisted micromachining system. The acoustic emissions generated during machining can be used for process monitoring and assessment. The cutting action propagates at frequencies in the 100-1000 kHz range, leading to almost zero background noise. The acoustic imaging sensor can be connected to a matching preamplifier and data acquisition card with software being used for all signal processing. Physical Acoustics software is an example of a suitable acoustic processing software. An acoustic imaging sensor  62  mechanically coupled to the workpiece material can measure the acoustic emissions generated by the micromachining tool  22  contacting the workpiece material  24 . The acoustic imaging sensor  62  can be used to quantitatively evaluate the effect of the laser-assisted micromachining system on the workpiece material  24 . The acoustic imaging sensor  62  can monitor the cutting tool contact with the workpiece material, tool wear, material removal temperature, and effects of depth of cut. There is a positive correlation between acoustic emissions and tool wear, and between acoustic emissions and axial depth of cut. The effectiveness of laser-assisted micromachining is shown by studies which report that acoustic emissions were reduced by up to 75% when the workpiece material was heated to above 350 degrees Celsius. Therefore there is a negative correlation between acoustic emissions and workpiece temperature. A variety of acoustic imaging sensors can be used, for example, Physical Acoustics, part number WDU. Additional description of acoustic emission sensors can be found in the following publications, which are incorporated by reference: Nakao, Y., et al., 2003, “Diamond turning using position and AE dual feedback control system,”  Precision Engineering , Vol. 27, 2003, pp. 117-124; Lee, D. E., et al., 2006, “Precision manufacturing process monitoring with acoustic emission,”  International Journal of Machine Tools and Manufacture , Vol. 46, 2006, pp. 176-188; and Tansel, I., et al., “Micro-endmilling-III. Wear estimation and tool breakage detection using acoustic emission signals,”  International Journal of Machine Tools and Manufacture , Vol. 38, 1998, pp. 1449-1466. 
     A nozzle (not shown) can be attached to the fixture plate  42  allowing for an adjustable flow of assist gas to be directed toward the cutting tool  22  while the laser-assisted micromachining system  10  is in use. Inert gas is used to quickly blow chips away from the zone where the cutting tool  22  interacts with the workpiece material  24 . This reduces the likelihood that chips removed from the workpiece material  24  interfere with the path of the laser beam  20 ″. Inert gas is used to prevent excessive oxidation or burning of the workpiece material and to reduce the residue which could accumulate on the laser-assisted micromachining system  10 . Liquid should not be used to remove chips from the cutting zone because accumulation of liquid on the workpiece material  24  can alter the effect the laser beam  20 ″ has on the workpiece material  24 . Another reason liquid is not used to remove the chips is because splash back can result in the liquid collecting on the components of the laser-assisted micromachining system  10  further altering the effect the laser beam  20 ″ has on the workpiece material  24 . 
     The micromachining tool  22  and the laser beam  20 ″ are in a fixed position during the machining operation while the workpiece material  24  moves relative to the micromachining tool  22  and laser beam  20 ″ to achieve the desired cutting action. The workpiece material  24  is detachably coupled to a three-axis precision computer controlled stage (not illustrated). The workpiece material can be coupled to the controlled stage using a standard vise, a vacuum type vise or other similar means to detachably couple the workpiece material  24  to the controlled stage. A controlled stage with more than three axes can also be implemented and a rotary work  10  holding device can be added to perform contouring operations. Also a controlled stage with less than three axis could be implemented. Since the cutting tool  22  and the laser beam  20 ″ are fixed the precision of the cutting will depend on the precision of the controlled stage. A controlled stage with a resolution of 0.5 micron is therefore preferred. The entire operation including the controlled stage movement and laser power is controlled by a CNC program. A variety of controlled stages can be used, for example, Aero tech, Part number A TS 125-100 (XYZ stages). 
     The laser-assisted micromachining system  10  can be configured to perform laser-assisted micromachining using different cutting tools  22  as illustrated in  FIG. 5 . This set-up can also include a rotary cutting tool  22  to perform contouring operations. Techniques to optimize the effect of the laser on the workpiece material can be found in the following publications, which are incorporated by reference: Tian, Y., et al., “Laser-assisted milling of silicon nitride ceramics,”  ASME Int. Conf. on Manuf. Science and Engineering , Oct. 8-11, 2006, Ypsilanti, Mich.; and Shelton, J. et al., “An experimental evaluation of laser-assisted micromilling of two difficult to machine alloys,”  Proceedings of MSEC 2008, 2008  ASME International Conference on Manufacturing Science and Engineering , Evanston, Ill. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. For example, while there are significant differences between milling and turning operations, certain principles of the present invention may be usefully applied in certain applications to laser-assisted micromachining on a lathe or other equipment using micro-cutting tools.