Abstract:
Described herein is a system for generating a plurality of coolant beams that converge at a focal point for advanced heat transfer. The system utilizes a variable strength activation of coolant and superposition of coolant beams generated by multiple actuators for increased cooling strength increase, thereby avoiding activation saturation in conventional systems. Each coolant beams is activated to carry an ultrasonic or megasonic vibration component. In addition, the system includes a coolant activation assembly having a plurality of actuators for generating the coolant beams. The coolant activation assembly further includes supporting components for positioning the actuators so that all of the coolant beams generated by these actuators converge at the focal point. Experimental results show that the system provides significantly improved workpiece quality in a machining process. Compared with the most advanced existing system, this system offers a further improvement of up to 12.30% on surface roughness of the finished workpiece.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 60/193,564, filed Dec. 8, 2008, which is incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention is related in general to the field of machining and in particular to a system for providing cooling in a machining or chemical process that requires cooling. 
       BACKGROUND OF THE INVENTION 
       [0003]    As the most important manufacturing process in modern industry, machining is defined as the process of removing material from a workpiece in the form of chips. To perform the machining operation, relative motion is introduced between the tool and the workpiece. This relative motion is achieved in most machining operations by means of a primary motion, called cutting speed and a secondary motion, called feed. The shape of the tool and its penetration into the workpiece surface, combined with these motions, produce the desired shape of the resulting workpiece surface. 
         [0004]    Common machining operations, such as drilling, turning, milling, and grinding, are capable of generating certain part geometries and surface textures. For example, the turning operation uses a cutting tool to remove material from a rotating workpiece to generate a cylindrical shape. As another example, grinding, which is the most precision machining process, generates smooth surfaces and fine tolerances. 
         [0005]    In particular, grinding involves removing materials by creating a contact between a grinding wheel and a workpiece. Each grain of the grinding wheel removes a chip from the surface of the workpiece material and generates a surface finish. Material removal is done by individual grains whose cutting edge is bounded by force and path. The initial cutting interface is characterized by elastic deformation, followed by plastic flow of workpiece material. As discussed in J. Kopac and P. Krajnik, “High-performance grinding—A review,” Journal of Materials Processing Technology, Vol. 175, No. 1-3, pp. 278-284, 2006, which is hereby incorporated by reference, penetration between two hard materials influences the kinematics and contact condition. 
         [0006]    A major limiting factor in any machining process is thermal damage caused by heat. In a machining process, energy is converted to heat, which is concentrated within the cutting zone. The high temperature produced can cause various types of thermal damage to the workpiece, such as burning, phase transformation, softening of the surface layer with possible rehardening, unfavorable residual tensile stresses, cracks, and reduced fatigue strength. To some extent, heat can also increase tool wear and reduce tool life. 
         [0007]    Heat damage can be reduced by applying cooling fluid, also known as coolant, to remove the heat created by the interaction between the workpiece and the cutting tool and to lubricate the surfaces between them to reduce the amount of friction in the cutting zone. Because the coolant removes heat by way of conduction, the colder the fluid, the more effective the heat transfer. The fluid is also used to flush away chips. In addition, when the cutting fluid is applied to the cutting zone, it will initially undergo nucleate boiling. This process enhances the rate of heat transfer between the workpiece and the fluid. 
         [0008]    There are four categories of cutting fluids based on composition, as suggested in K. Blenkowski, “Coolants and lubricants: part 1—the truth,” Manufacturing Engineering, pp. 90-96, 1993 and J. A. Webster and C. Cui, R. B. Mindek, “Grinding fluid application system design,” CIRP Annuals, Vol. 44, No. 1, pp. 333-338, 1995, both of which are hereby incorporated by reference for everything they describe. The publication by P. Q. Ge, L. Wang, Z. Y. Luan, and Z. C. Liu, “Study on service performance evaluation of grinding coolants,” Key Engineering Materials, Vol. 258-259, pp. 221-224, 2004, further shows that no fluid is perfect for all aspects of machining processes. This Ge et al. publication is also incorporated by reference for everything it describes. The significances of cooling, grinding forces, and thermal behavior have also been studied. In particular, it has been shown that water-based emulsions have better cooling effect, but generally lead to higher grinding forces. 
         [0009]    Surface profile and roughness of a machined workpiece are two of the most important product quality characteristics and in most cases a technical requirement for mechanical products. Achieving the desired surface quality is of great importance for the functional behavior of a workpiece. Surface quality of a workpiece is generally indicated by surface roughness, surface physical and chemical performance, surface fluctuation, surface hardness, and residual stress. 
         [0010]    Beyond machining processes, many mechanical or chemical systems also generate a significant amount of heat during their operations, due to frictions between components, combustions, or chemical reactions in the working zone. Cooling by way of cooling media or coolant such as gas or fluid is often needed to minimize thermal damage and maintain normal system performance in these systems. 
         [0011]    Conventional cooling methods for reducing thermal damages include cryogenic cooling, spray cooling, air cooling, active cooling, megasonic cooling, actively cooled and activated cooling. The limitations of these conventional cooling methods are discussed below. 
         [0012]    Cryogenic cooling utilizes a jet of liquefied gas such as liquid nitrogen. In this method, cooling is realized through a very high temperature gradient generated by contrast between the high temperature in the working zone and the very low temperature of the liquid nitrogen. The method has been shown to be effective in grinding ductile materials. However, for brittle materials, the very high temperature gradient may present a problem due to the possibilities in generating excessive thermal stresses on the surfaces of brittle materials. In addition, the method requires frequent replenishment of liquid nitrogen, which is uneconomical for long term use and requires great care for safety. 
         [0013]    Spray cooling is a frequently used method of heat removal in many machining processes. However, it is not practical in precision machining processes. 
         [0014]    In the air cooling method, the temperature is typically reduced to −10° C. ˜60° C. The temperature gradient is still quite large. However, in terms of specific heat and thermal conductivity, the physical properties of chilled air are more unfavorable than those of water based coolant. The delivery speed is generally up to 100 m/s, which is approximately 40-200 times of the one for water-based coolant, thereby causing a high level of noise. 
         [0015]    In the active cooling method, an active cooling system is utilized to reduce the machining temperatures in the cutting zone through force convection. The active cooling system includes a coolant tank connected to an evaporator of the heat pump for heat exchange to remove the machining heat so as to reduce temperature in the working zone. 
         [0016]    In ultrasonic or megasonic cooling, a floating nozzle having an integrated ultrasonic or megasonic transducer is utilized to provide coolant to cool the cutting zone. The surface quality improvement in the ultrasonic and megasonic cooling is attributed to the fluid cavitation effect. For example, previous studies have shown that megasonic cooling allows an increase in the grinding ratio by about 2 times and an improvement in the surface roughness by 20 to 30%. The temperature gradient mechanism was not utilized and as such cooling effectiveness improvement was limited. 
         [0017]    In an actively cooled and activated coolant method, the cooling mist generated through a high frequency activation is able to take away heat from the cutting zone by way of evaporation effect. It has been shown that the actively cooled and activated cooling can achieve a 22.9% of average surface quality improvement in depth of cut tests and a 23.77% of average surface quality improvement in table speed tests. In these tests, an average improvement up to 36.68% in roughness value (Ra) has been obtained. 
         [0018]    However, these conventional cooling methods are often insufficient to provide cooling necessary for producing high-quality workpieces. In particular, in conventional ultrasonic and megasonic cooling, there is a technical limitation in the piezoelectric activation component which imparts an upper limit in the activation strength, thereby limiting the cooling effect. In addition, it is also desired for a cooling system to have the capability to adjust the strength of cooling provided to the working or cutting zone so that the cooling effect is optimized for a given process. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    Described herein is a system for providing cooling in a machining process or a system that generates heat. Unlike conventional systems that provide a limited cooling effect, various embodiments of this system can provide an adjustable cooling strength by generating multiple activated cooling medium beams. In these embodiments, the coolant are cooled by an active cooling system and activated by an actuator assembly. These coolant beams, which can be controlled individually, are arranged to form a focal point to produce an enhanced cooling effect. 
         [0020]    One objective of the system is to provide an effective way to significantly cool a working zone and improve workpiece quality at low cost without using expensive system components. 
         [0021]    Still another objective of the system is to provide adjustable cooling that can be optimized for the conditions and requirements of a machining process or a system. 
         [0022]    According to some embodiments, a system is provided for cooling a machining process. The system includes a coolant supply for providing a coolant flow, and an actuator assembly for receiving the coolant flow and forming a plurality of coolant beams for cooling a cutting zone, where the plurality of coolant beams faun a focal point. 
         [0023]    According to other embodiments, a method is provided for cooling a cutting zone in a machining process. The method includes receiving a coolant flow from a coolant supply, and generating a plurality of coolant beams from the received coolant flow, wherein the plurality of coolant beams form a focal point within the cutting zone. 
         [0024]    According to still other embodiments, an actuator assembly is provided for cooling a cutting zone in a machining process. The actuator assembly includes at least one coolant inlet for receiving coolant from a coolant supply, and a plurality of coolant outlets, where each of the plurality of coolant outlets generates a coolant beam and the plurality of coolant beams form a focal point. 
         [0025]    One application of the system described herein is for providing enhanced cooling in a machining process such as a grinding operation. In this respect, multiple cooled activation medium beams are arranged to point to a focal point in the grinding zone to increase the cooling strength. The activation causes evaporation that takes away the heat from the working zone. This, together with the active cooling, provides a better machining condition so that better surface quality can be achieved on the workpiece. 
         [0026]    The system can also be used in many other mechanical or chemical processes as long as a cooling medium is used in the processes. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0027]      FIG. 1  depicts a diagram illustrating a system for providing cooling in a machining process, including an actuator assembly and a coolant supply; 
           [0028]      FIG. 2A  depicts one embodiment of an actuator assembly in  FIG. 1  for generating a plurality of coolant beams converging at a focal point; 
           [0029]      FIG. 2B  depicts another embodiment of the actuator assembly in  FIG. 1  for generating a plurality of coolant beams converging at a focal point; 
           [0030]      FIG. 2C  depicts one embodiment of the actuator assembly in  FIG. 1  where the coolant beams are formed within a horizontal plane; 
           [0031]      FIG. 2D  depicts another embodiment of the actuator assembly in  FIG. 1  where the coolant beams are formed within a vertical plane; 
           [0032]      FIG. 2E  depicts still another embodiment of the actuator assembly in  FIG. 1  where the coolant beams are formed within a plane that is at an angle from the horizontal position; 
           [0033]      FIG. 2F  depicts still another embodiment of the actuator assembly in  FIG. 1  where the plurality of actuators are positioned on a spherical surface; 
           [0034]      FIG. 2G  depicts one embodiment of the actuator assembly having a plurality of actuators and supporting components; 
           [0035]      FIG. 2H  depicts an actuator assembly in  FIG. 2G  in its installation position with respect to a grinding machine; 
           [0036]      FIG. 2I  depicts a side view of an actuator and its supporting components according to  FIG. 2G ; 
           [0037]      FIG. 3A  depicts a cooling system utilizing multi-level cooling and activation coolant; 
           [0038]      FIG. 3B  depicts a diagram of an active cooling system; 
           [0039]      FIG. 4  depicts a testing system for verifying the enhanced vibration at the focal point of the actuator assembly; 
           [0040]      FIG. 5  depicts another testing system for verifying the enhanced cooling effect in a grinding process; 
           [0041]      FIG. 6  depicts testing results showing a relationship between vibration amplitude and ejection distance; 
           [0042]      FIG. 7  depicts the relationship between the vibration amplitude and the number of coolant beams; 
           [0043]      FIG. 8  depicts the relationship between the loading current and the number of coolant beams; 
           [0044]      FIG. 9  depicts the relationship between the surface roughness and the number of coolant beams; 
           [0045]      FIG. 10  depicts the relationship between the standard deviation of loading current and the number of workpieces obtained from the experiment shown in  FIG. 8 ; 
           [0046]      FIG. 11  depicts the relationship between the standard deviation of surface roughness and the number of workpieces obtained from experiment shown in  FIG. 8 ; 
           [0047]      FIG. 12  depicts the relationship between the range of surface roughness and the number of workpieces obtained from the experiment shown in  FIG. 8 ; 
           [0048]      FIG. 13  depicts the main effects for surface roughness; 
           [0049]      FIG. 14  depicts the interaction for surface roughness; 
           [0050]      FIG. 15  depicts the effects of coolant temperature on the loading current; 
           [0051]      FIG. 16  depicts the effects of number of coolant beams on the loading current; 
           [0052]      FIG. 17  depicts the effects of coolant temperature on surface roughness; 
           [0053]      FIG. 18  depicts the effects of number of coolant beams on surface roughness; 
           [0054]      FIG. 19  depicts Table 1, showing the experimental conditions of the experiments in  FIGS. 4 and 5 ; 
           [0055]      FIG. 20  depicts Table 2, showing the measurement conditions of the experiments in  FIGS. 4 and 5 ; 
           [0056]      FIG. 2I  depicts Table 3, showing a summary of the data collected during the experiments in  FIGS. 4 and 5 ; 
           [0057]      FIG. 22  depicts Table 4, showing the surface roughness of measured in the experiment in  FIG. 5 ; 
           [0058]      FIG. 23  depicts Table 5, showing the effects of the coolant temperature t c  on the loading current I; 
           [0059]      FIG. 24  depicts Table 6, showing the effects of the number of coolant beams n a  on the loading current I; 
           [0060]      FIG. 25  depicts Table 7, showing the effect of t c  on the surface roughness R a ; 
           [0061]      FIG. 26  depicts Table 8, showing the effect of n a  on R a ; 
           [0062]      FIG. 27  depicts Table 9, showing the rate of variation for R a  against the coolant temperature, R a ′(t c ); and 
           [0063]      FIG. 28  depicts Table 10, showing the rate of variation for R a  against the number of coolant beams, R a ′(n a ). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0064]    Now turning to the drawings, depicted in  FIG. 1  is an embodiment of a system  100  for providing enhanced cooling in a machining process. As shown in the figure, system  100  includes a coolant supply  101  and a coolant activation assembly  108  that draws cold coolant from the coolant supply  101 . The coolant activation assembly  108  generates a plurality of coolant beams  109 , which is applied to a working zone  110  so that the heat generated within the working zone  110  is taken away by the coolant. The heated coolant, which absorbs and carries away the heat, is returned to the coolant supply  101 , which cools and circulates the coolant back to the coolant activation assembly  108 . As discussed above, the coolant can be gas or fluid as used in any existing cooling system. 
         [0065]    The coolant supply  101  further includes a cooler  106  for cooling the heated coolant, a pump  102  for driving the coolant through the cooler  106  to form a coolant flow and deliver the cold coolant flow to the coolant activation assembly  108 , and a flow meter  104  for monitoring the coolant flow supplied to the coolant activation assembly  108 . In particular, the cooler  106  usually takes the form of a heat exchanger that cool the coolant coming from the working zone  110 . 
         [0066]    According to a further embodiment, the working zone  110  includes a cutting area in a machining process such as grinding or drilling. In this embodiment, heat is generated within the working zone  110  due to frictions between a workpiece and a cutting tool such as a grinding wheel or a drill bit. Alternatively, the working zone  110  includes other mechanical or chemical processes that require cooling. 
         [0067]    The coolant activation assembly  108  includes one or more actuators for generating activated coolant beams  109 . Each of the actuators includes a piezoelectric component for imparting a vibration component to the coolant beams  109  for providing enhanced cooling to the working zone  110 . 
         [0068]      FIG. 2A  depicts one embodiment of the coolant activation assembly  108 , where six actuators  206  are arranged along an arch  203  such that the coolant beams  208  generated by these actuators converge at a focal point  214  within the working zone  110 . Each of the actuator  206  is oriented such that the coolant outlet  207  of the actuator is pointing toward the focal point  214 . As shown in  FIG. 2A , each actuator  206  also has a coolant inlet  204  for receiving coolant from the incoming cold coolant flow  202  coming from the coolant supply  101 . The heated coolant that flows through the working zone is collected to form a heated coolant flow  212 , which is returned to the coolant supply  101 . 
         [0069]    As discussed above, each actuator  206  in  FIG. 2A  has an integrated activation element such as a piezoelectric element for generating an ultrasonic or megasonic vibration component in the coolant beam  208 . 
         [0070]    The actuator  206  for generating activated coolant beam is conventional and well known in the art. For example, the actuator  206  can take the form of a coolant nozzle described by K. Suzuki et al. “Grinding performance improvement by a special coolant superimposed with the megasonic vibration,” Key Engineering Materials, Vol. 238-239, pp. 183-188, 2003, and K. Suzuke, et al. “Effects of megasonic floating nozzle on grinding performance for hard materials,” Key Engineering Materials, Vol. 257-258, pp. 311-314, 2004, all of which are hereby incorporated by reference in their entireties and for everything they describe. As descried in these literatures, as the coolant enters the actuator  206  and contacts the piezoelectric component in the actuator  206 , the piezoelectric component imparts an ultrasonic vibration to the cutting fluid passing through the actuator, thereby resulting additional energy added to the coolant beam output from the coolant outlet  207 . 
         [0071]    Alternatively, the technique described in Y. Gao et al. “Spatial distribution of cooling mist for precision grinding,” Key Engineering Materials, Vol. 389-390, pp. 344-349, 2009, which is hereby incorporated by reference in its entirety and for everything it describes, can also be used to generated the activated coolant beams  208  with an ultrasonic or megasonic vibration component. In this literature, due to high frequency vibration, mist is generated with in the work zone after an activated coolant beam touches the wheel or workpiece surface. As a result, heat transfer is enhanced through the activation. 
         [0072]      FIGS. 2G ,  2 H and  2 I illustrate another embodiment of the coolant activation assembly  108  for installation on a grinding machine for cooling workpiece in a grinding process. As shown in  FIG. 2G , the actuator assembly  108  includes six actuators  206 . Each actuator  206  is incorporated into a housing  236 , which is in turn attached to one end of a vertical arm  234 . The other end of the vertical arm  234  is attached to a screw bolt that allows arm  234  to be affixed to a horizontal arm  233  through a slot  235  and a screw nut  232 . 
         [0073]    As shown in  FIG. 2I , the vertical arm  234  can be screwed up or down along the vertical direction (i.e., y direction) to adjust the vertical position of the actuator  206 . In addition, the horizontal position (i.e., z direction) of the actuator  206  can be adjusted by moving the horizontal arm  234  in slot  235  of the horizontal arm  233 . The actuator housing  236  is attached to the vertical arm  234  through a pivot point  237  that allows the yaw angle of the actuator to be adjusted. 
         [0074]      FIG. 2G  further shows that the horizontal arm  233  is attached to a center plate  238  through a pivot point  239  that allows the horizontal arm  233  to be rotated, thereby adjusting the oriental of the actuator  206 . The center plate  238  has a shape suitable for installation on a machine such as a grinding machine or a milling machine. 
         [0075]      FIGS. 2G and 2I  further show that each actuator  206  has a power connection  242  for supplying electrical power to the actuator, a coolant inlet  204  for receiving coolant from the coolant supply  101 , and a coolant outlet  207  for forming a coolant beam. During the assembling and installation, the vertical position, the horizontal position, and the yaw angle of each actuator  206  are adjusted so that the coolant beams output from the coolant outlet  207  converge at the focal point. 
         [0076]      FIG. 2H  illustrates the coolant activation assembly depicted in  FIG. 2G  in its installation position with respect to a grinding machine  245 . 
         [0077]    According to some other embodiments, the actuator assembly is not limited to the structure depicted in  FIGS. 2A ,  2 G,  2 H, and  2 I. The actuator assembly  108  can include any number of actuators and take any shape; as long as the coolant beams  208  generated by these actuators converge at a focal point in the working zone. 
         [0078]    For example,  FIG. 2B  shows an alternative embodiment of the actuator assembly  118  having six actuators  206  arranged on a line  215 . Each actuator  206  is oriented so that the coolant beams  208  converge at the focal point  214 . 
         [0079]      FIG. 2C  depicts still another embodiment of the actuator assembly  118  having five actuators  206  arranged in a horizontal plane  218  that passes through the focal point  214 . The figure shows a view of the assembly from the back of the center actuator toward the focal point  214 , which is blocked by the center actuator in this view. 
         [0080]      FIG. 2D  depicts still another embodiment of the actuator assembly  118  having five actuator  206  arranged in a vertical plane  219 . Similar to  FIG. 2C , all of the five actuators  206  are oriented so that the coolant beams form a focal point  214  within the plane  219 , which is also block by the center actuator in this view. 
         [0081]      FIG. 2E  depicts still another embodiment of the actuator assembly  118  having five actuator  206  arranged in an oblique plane  220  that form an angle θ with the horizontal plane  218 . Similarly, all of the coolant beams reside in the plane  220  that passes through the focal point  214 . 
         [0082]      FIG. 2F  depicts still another embodiment of the actuator assembly  118  having four actuators  206  arranged on a spherical surface  220 . Similar to those described above, the actuators  206  are oriented so that the coolant beams  208  form a focal point  214 . 
         [0083]    As discussed above, one skill in the art will readily recognized that other arrangements of the actuators are possible so long as the coolant beams converge at a focal point and the vibration components carried by the coolant beams have substantially similar strength at the focal point. One skill in the art will further recognize that the distnace between each of the coolant outlets and the focal may or may not be similar and the initial strength of the vibration component can be adjusted so that all of the vibration components have substantially similar strength. 
         [0084]    Now turning to  FIG. 3A , depicted therein is another embodiment of a system for providing enhanced cooling to a machining process. Similar to system  100 , system  300  utilizes active cooling and activated coolant techniques. In particular, system  300  includes a coolant activation assembly  308  similar to that depicted in  FIGS. 2A-I . In addition, system  300  utilizes a multi-level cooling device  306  for providing enhanced cooling. 
         [0085]    The multi-level cooling device  306  includes a plurality of coolers  306 A-C connected in series for providing various stage of cooling. As the heated coolant returned from the working zone  110  passes through the plurality of coolers, the heat is extracted from the coolant. The system  300  further includes a valve  330  for controlling the coolant flow rate provided to the coolant activation assembly  308 . 
         [0086]    As discussed above, one application of the systems depicted in  FIGS. 1-3  is for providing cooling in a precision machining process such as grinding. As shown in  FIGS. 1-3 , to achieve better workpiece quality, a number of cooling medium activation units are used for generating multiple coolant beams. 
         [0087]    The heated cooling medium (i.e., coolant) collecting from the working zone is pumped into a cooling device  106  or  306 . The cooling device utilizes forced convection to cool the heated medium. The temperature can be measured by using a temperature sensor for monitoring and controlling purposes. The medium temperature can be controlled by using a controller or by switching the pump  102  or  302  on or off. 
         [0088]    As shown in  FIG. 2A-I , multiple actuators can be control independently to provide adjustable cooling. Due to the pressure generated by the pump and by the activation element (i.e. actuator), the cooled and activated medium is ejected by each actuator at a velocity to form a cooling medium beam. For multiple activation units, a number of cooled and activated medium beams are formed. 
         [0089]    In order to provide enhanced cooling, the positions of the activation units can be adjusted to allow all of the cooling medium beams to form a focal point so as to jointly affect the area of interest within the working zone  110 . The adjustment can be realized through a number of suitable adjustable fixtures that adjust the positions and orientations of the activation units as shown in  FIG. 2G-I . 
         [0090]    The area of interest is typically the area where significant heat is generated during a machining or chemical process. In general machining processes, this area is the one where materials are removed. In a grinding process, this area is called contact point or grinding zone. The focal point formed by the multiple cooling medium beams resides within the area of interest when the coolant activation assembly is corrected installed on the machine. 
         [0091]    As shown in  FIGS. 2A-I , the actuators are arranged to activate the cooling medium and direct the cold and activated cooling medium to the focal point. The actuators are attached or integrated in a supporting structure similar to that shown in  FIGS. 2G-I , which allows each actuator to be independently adjusted. 
         [0092]    After leaving the coolant outlet of an actuator, a cold and activated medium beam travels in the open space at a velocity which is determined by the momentum of the cooling medium beam. When the initial velocity is in a horizontal direction, the height of the beams drops due to the gravitational force, as the beams travel further away from the coolant outlets. Consequently, each activated coolant beam has a maximum traveling distance in the open space, beyond which the coolant beam loses its momentum and the cooling strength is substantially decreased. The maximum traveling distance of a cold and activated medium beam depends on the pressure generated by the pump and the strength of the ultrasonic or megasonic activation provided by each actuator. In order to achieve optimal cooling strength, the actuators should be located close to the focal point so as to provide a beam travel distance shorter than the maximum travel distance. 
         [0093]    According to a further embodiment, when a plurality of actuators are used to generate the coolant beams, each actuator can be controlled independently. For example, one or more actuators can be turned off so that fewer beams are generated when less cooling is needed. On the other hand, when the heat continues to accumulate within the working zone and more cooling is needed to maintain the optimal cooling, additional actuators can be engaged to generate more coolant beams, thereby increasing the cooling strength. In this case, the number of medium beams may be different from the number of actuators. As a result, the cooling strength can be adjusted by increasing or decreasing the number of coolant beams to optimize for each process. 
         [0094]    According to another embodiment as depicted in  FIG. 3B , each of the coolers  106  and  306 A-C can be replaced with an active cooling system  340  similar to those described in Y. Gao et al. “An active coolant cooling system for applications in surface grinding,” Applied Thermal Engineering, Vol. 23, No. 5, pp. 523-537, 2003; Y. Gao et al. “Effects of actively cooled coolant for grinding brittle materials,” Key Engineering Materials, Vol. 291-292, pp. 233-238, 2005; and Y. Gao et al. “Effects of actively cooled coolant for grinding ductile materials,” Key Engineering Materials, Vol. 339, pp. 427-433, 2007, all of which are hereby incorporated by reference in their entireties and for everything they describe. 
         [0095]    Specifically, the active cooling system  340  receives the heated coolant returned from the working zone and passes it through an evaporator  346 , which uses an internal refrigerant circulation to extract the heat from the coolant. The internal refrigerant circulation of the active cooling system  340  additionally includes a compressor  344  and a condenser  342 . Compared with the coolers  106  and  306 A-C, the active cooling system  340  can provide cold coolant with a steady low temperature even when the temperature of the heated coolant fluctuates. 
         [0096]    In the cooling system described above, either gas or fluid can be used as the cooling medium or coolant, provided that suitable actuators are used to activate the coolant. Activation can be realized by imparting ultrasonic or megasonic waves (i.e., vibrations) onto the cooling medium such as air, water, or oil-based coolant. 
         [0097]    It should be readily understood by one skilled in the art that the cooling strength of the above-described system is determined in part by the number of coolant beams forming the focal point, the beam traveling distance from the coolant outlet to the focal point, and the coolant temperature. In order to demonstrate the effectiveness of systems  100  and  300 , a number of experiments are carried out. In addition, due to the differences in machine characteristics and in materials, suitable values of the parameters must be determined through a number of experiments to choose the parameter values that give the best results. Diagrams of these testing systems are depicted in  FIGS. 4-5  and their results are shown in  FIGS. 6-18 . 
         [0098]    Specifically,  FIG. 4  depicts the diagram of a system  400  for testing the cooling effect provided by the active cooling and activated coolant. The testing system  400  includes a PCB piezo sensor  408 , a current amplifier  410 , and an oscilloscope  412 . The PCB piezo sensor  408 , which is installed at the focal point of an actuator assembly  402 , detects the aggregated vibration at the focal point resulting from the converging activated coolant beams  406  and converts the mechanical vibration into electronic signals. The resulting electronic signals, when amplified by the current amplifier  410 , can be visualized and measured by the oscilloscope  412 . 
         [0099]    Similar to that in  FIG. 2A , the actuator assembly  402  includes six independently controlled actuators for generating the coolant beams  406 . Because the cooling strength provided by the systems  100  and  300  is determined in part by the vibration amplitude (A) of the vibration component carried in the coolant beams, which in turn is determined in part by the number of coolant beams (n a ) and the beam travel distance (i.e., ejection distance d e ), one of the objective of the experiment is to demonstrates the effects of the number of beams (n a ) and the ejection distance d e  on the vibration amplitude A. Therefore, during the experiments, the number of coolant beams n a  and the ejection distance d e  are adjusted, while the vibration amplitude A is measured. 
         [0100]    Another testing system  500  shown in  FIG. 5  utilizes a grinding operation performed on a grinding machine to demonstrate the effectiveness of the active cooling and activated coolant systems  100  and  300 . As depicted in  FIG. 5 , the testing system  500  includes a current probe  512 , an amplifier  514 , and an oscilloscope  516 . The current probe  512  is used to tap into the wire connecting the wheel motor  506  and the inverter  508  to collect the wheel current signal I, which is amplified by the amplifier  514  and then visualized and measured by the oscilloscope  516 . 
         [0101]    Because the load current I and the workpiece surface roughness are directly determined by the machining force, which in turn is determined by the cooling strength, the load current I, the workpiece surface roughness Ra, and surface morphology are recorded during the experiment, and their relationship with the number of coolant beams na and the coolant temperature tc are demonstrated in  FIGS. 8-18 . 
         [0102]    The experimental conditions and the data measured from the experiments depicted in  FIGS. 4 and 5  are shown in  FIGS. 19 and 20 . 
         [0103]    As the experimental results in  FIGS. 23-28  and  FIGS. 6-18  demonstrate, the cooling systems and methods described above is more effective and advantageous than existing systems, in that it provides an enhanced cooling effect that can be adjusted and optimized by simply increasing or decreasing the number of coolant beams used to form the focal point. At the same time, because a large vibration amplitude can be generated by aggregating a plurality of coolant beams with relatively weak vibrations, the requirements on the system components are lowered and costs are reduced. In some extreme cases, a vibration amplitude that exceeds the limits of individual actuators can be generated, without causing stress or damage to the system components. 
         [0104]      FIGS. 6-7  illustrate the experimental results showing the vibration amplitude A as a function of the ejections distance de ( FIG. 6 ) and as a function of the number of beams na ( FIG. 7 ). As seen in  FIG. 6 , as the ejection distance de is increased from 3 to 9 mm, the vibration amplitude A remains stable. On the other hand,  FIG. 7  shows that the vibration amplitude A increases nearly linearly with the number of coolant beams na. 
         [0105]      FIG. 8  shows that both active cooling and activated coolant increases the effectiveness of the machining process as the current, so as the force, is increased. It also shows that using more coolant beams further increases the effectiveness.  FIG. 9  shows similar effectiveness, where the surface roughness of the workpiece is decreased when more coolant beams are applied to the grinding process and/or when both the active cooling and activated coolant are utilized. 
         [0106]      FIGS. 10-12  show that the average variation of the results is approximately 3-5% for standard deviation and approximately 10% for individual measurement result. This shows that multiple measurement points and averaging are necessary as demonstrated in  FIG. 20 . These data demonstrates that the results are stable and thus reliable. 
         [0107]    The results showing the coolant temperature tc, wheel speed ns, depth of cut dc, and number of coolant beams na are further listed in  FIGS. 21-22  and depicted in  FIGS. 13-14 . 
         [0108]    As further shown in  FIGS. 15 and 23 , by using active cooling and multiple coolant beams, the machining effectiveness in terms of loading current, which is related to machining force, increases up to 20.66%. In terms of surface roughness, the improvement is up to 22.04%, as shown in  FIGS. 17 and 25 . 
         [0109]      FIGS. 16 and 24  show that, compared with the single coolant beam approach, in terms of current that is related to machining force, a further improvement of approximately 10.35% is achieved by using the multi-beams coolant activation assembly having six actuators.  FIGS. 18 and 26  show that, in terms of surface roughness, a further improvement of approximately 12.3% is achieved by the multi-beams coolant activation assembly having six actuators. 
         [0110]      FIGS. 25-28  further show various results when the active cooling temperature tc≦8° C. and the number of activated beams na≧3. 
         [0111]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0112]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.