Abstract:
The present invention discloses a five-axis flank milling system for machining a curved surface and a tool-path planning method. The method generates a tool path comprising a series of cutter locations by optimization with minimizing machining errors. The tool path planning method includes a reciprocating tool path planning method and a multi-pass tool path planning method. The reciprocating tool path planning method eliminates the “forward only” limitation. The tool is allowed to move backward in certain regions, producing smaller machining errors compared with forward only cutter movement. Furthermore, the multi-pass tool path planning method computes various tool paths applied to finish milling multiple times. Each path can be chosen to be generated by minimizing undercut error, overcut error, or the total machining error. The machining errors are reduced in a progressive manner, resulting in better machining quality than single pass tool path.

Description:
PRIORITY CLAIM  
       [0001]    This application claims the benefit of the filing date of Taiwan Patent Application No. 100143480, filed. Nov. 28, 2011, entitled “A FIVE-AXIS FLANK MILLING SYSTEM FOR MACHINING CURVED SURFACE AND A TOOLPATH PLANNING METHOD THEREOF,” and the contents of which is hereby incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a five-axis flank milling system for machining curved surface and a tool-path planning method thereof, and more specifically, the tool-path planning method of the present invention can minimize machining error by applying reciprocating tool motion and multi-pass tool path. 
       BACKGROUND OF THE INVENTION 
       [0003]    Five-axis machining is commonly used to produce complex geometries in automobile, aerospace, energy, and mold industries. With additional degrees of freedom in its tool motion, five-axis machining offers better shaping capability and productivity compared to three-axis machining. Tool path planning is a difficult task in most five-axis machining operations. Two major concerns are tool collision avoidance and machining error control. 
         [0004]    Five-axis machining operations can be categorized into two types: end milling and flank milling. In flank milling, material removal mainly occurs on the tool flank through line contact with the cutting teeth. From a geometric perspective, to completely avoid machining error is not possible in five-axis flank milling when a cylindrical cutter is used to produce curved surfaces. The machined surface is considered acceptable in practice as long as the amount of machining error is limited within a given tolerance. 
         [0005]    Five-axis flank milling is often applied to produce ruled surfaces. A simple method of tool path generation in this case is to let the cutter follow the ruling lines of the machined surface. This is the tool motion used most frequently in current industry, despite of its serious machining error produced on twisted surfaces. 
         [0006]    Most prior art developed geometric algorithms that adjust individual cutter locations for reducing machining error. The adjustment of one cutter location is independent from the others. Such a greedy approach does not consider the machining errors generated between consecutive cutter locations, thus leading to sub-optimal solutions with a larger machining error, as disclosed in Taiwan patent application number 96147909. Therefore, the same patent developed a tool path planning method for five-axis flank milling of ruled surfaces based on global optimization methods. The developed method can precisely control the machining error produced on the machined surface through the optimization process with machining error minimization as the objective. 
         [0007]    The tool path planning method mentioned above suffers from unsatisfactory quality of optimal solutions due to two assumptions. The first assumption is that the cutter must make contact with the boundary curves. Also, tool motion is designed for moving forward only. Both assumptions greatly restrict the solution space in search for optima, resulting in worse tool paths. 
       SUMMARY OF THE INVENTION 
       [0008]    Therefore, in order to overcome the deficiency mentioned above, a scope of the present invention is to provide a five-axis flank milling system for machining ruled surfaces. This system comprises an interface module, an arithmetic module, a machining module, and a control module. 
         [0009]    The interface module reads the geometric definition of the workpiece to be machined on a workpiece. The machining module comprises a cutting tool for removing material from a given stock material. The control module is coupled with the arithmetic module and the machining module for controlling the machining module to produce the workpiece with the cutting tool according to the tool path generated. And the arithmetic module is coupled with the interface module for generating a tool path according to the surface geometry to be machined and the user commands. 
         [0010]    However, the tool path of the present invention includes, but is not limited to, the description above in actual applications, the tool path comprises a first tool motion and a second tool motion. The first tool motion and the second tool motion are constructed with a first index and a second index respectively according to the surface geometry to be machined and the user commands. The first tool motion and the second tool motion have a first error value and a second error value respectively. In addition, the first tool motion and the second tool motion are used for removing the material of a first bulk and a second bulk from the stock material respectively. The first index and the second index are defined by the user commands. 
         [0011]    Furthermore, another scope of the invention is to provide a tool path planning method for five-axis flank machining of curved surfaces. Material is removed from the stock by a cutting tool according to the tool path generated, following: step S 11 : preparing a curved surface; step S 12 : reading user commands; and step S 13 : generating the tool path based on the curved surface and the user commands. Wherein, the tool path comprises a first cutter location, a second cutter location, and a third cutter location, and the three cutter locations correspond to a first tool motion and a second moment, respectively, the first tool motion is ahead of the second tool motion. 
         [0012]    Another scope of the invention is to provide a tool path planning method for five-axis flank machining of curved surfaces. The method comprises step S 21  to step S 24 . The step S 21  and S 22  are similar with the step S 11  and S 12  mentioned above, thus the steps need not be elaborated any further. At step S 23 , constructing a first tool motion with a first index according to the curved surface and the user commands, wherein the first tool motion has a first error value; and step S 24 : constructing a second tool motion with a second index according to the curved surface and the user commands, wherein the second tool motion has a second error value. Moreover, the first index and the second index are corresponded to the user commands, the sequence of the first tool motion and the second tool motion is run independently of the summation of the first error value and the second error value. 
         [0013]    In addition, the first tool motion and the second tool motion are used for removing material of a first bulk and a second bulk from the stock respectively, and the sequence of the first tool motion and the second tool motion is run independently of the summation of the first bulk and the second bulk. 
         [0014]    In conclusion, the present invention discloses a five-axis flank machining system for curved surfaces and includes a tool-path planning method of reciprocating tool motion M 1  and a multi-pass tool path planning method M 2 . By eliminating the “forward only” limitation of traditional tool-path planning methods, the present invention is able to move the cutting tool backward first; then resume forward, so as to produce a machined curved surface of a smaller error. Furthermore, the multi-pass tool path planning method M 2  is able to minimize machining error by applying various tool paths on the stock progressively for multiple times, wherein each of the tool paths is generated in accordance with the same surface to be machined. 
         [0015]    Many other advantages and features of the present invention will be manifested by further descriptions and the accompanying sheet of drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic diagram illustrating an initial tool path and the representative matrix thereof. 
           [0017]      FIG. 2  is a flowchart illustrating a tool-path planning method of reciprocating tool motion of the invention. 
           [0018]      FIG. 3A  is a schematic diagram illustrating an initial tool path of the reciprocating tool path planning method according to an embodiment of the invention. 
           [0019]      FIG. 3B  is another schematic diagram illustrating an initial tool path of the reciprocating tool path planning method according to an embodiment of the invention. 
           [0020]      FIG. 4A  is a schematic diagram illustrating the first tool motion according to an embodiment of the reciprocating tool path planning method of the invention. 
           [0021]      FIG. 4B  is a schematic diagram illustrating the second tool motion according to an embodiment of the reciprocating tool path planning method of the invention. 
           [0022]      FIG. 4C  is a schematic diagram illustrating the tool path according to an embodiment of the reciprocating tool path planning method of the invention. 
           [0023]      FIG. 5  is a flowchart illustrating a multi-pass tool path planning method according to an embodiment of the invention. 
           [0024]      FIG. 6  is a function block diagram illustrating a five-axis flank milling system for machining curved surface according to an embodiment of the invention. 
       
    
    
       [0025]    To facilitate understanding, identical reference numerals have been used, where possible to designate identical elements that are common to the figures. 
       DETAILED DESCRIPTION 
       [0026]    The invention discloses a five-axis flank milling system for machining curved surface and a tool path planning method thereof. The word “tool path” in the description is defined as the motion of cutting tool which consists of a series of cutter locations; the word “work-piece” is defined as the material to be machined; and the word “curved surface” means a desired surface machined from the work-piece. Besides, the five-axis flank milling system for machining curved surface and a tool path planning method thereof are represented as “machining system” and “planning method” respectively. 
         [0027]    The planning method of the invention is utilized to generate a tool path for a cutting tool to remove material from a work-piece along the tool path according to the user input commands. Additionally, the present invention provides two methods to minimize machining errors, and the two methods are the tool-path planning method of reciprocating tool motion M 1  and the multi-pass tool path planning method M 2  respectively. 
         [0028]    Please refer to  FIG. 1 .  FIG. 1  is a schematic diagram illustrating the tool contact point of an initial tool path on the surface to be machined and the representative curve parameters thereof. As shown in  FIG. 1 , the initial tool path of convention  9  is formed by selecting points on the two boundary curves  91  and  92  respectively, determining the cutter center points of both tool ends by offsetting those points along the surface normal directions with a distance of tool radius, and then generating the tool axis by connecting the offset points. However, the tool contact points are restricted to the boundary curve  91  and  92 . The tool motion is forwarding only. Thus the optimized tool path of convention  9  cannot result in minimal machining errors due to a smaller restricted solution space. 
         [0029]    Therefore, the present invention provides a reciprocating tool path planning method M 1  to solve the problem mentioned above. More specifically, please refer to  FIG. 1 ,  FIG. 2 ,  FIG. 3A , and  FIG. 3B .  FIG. 2  is a flowchart illustrating a reciprocating tool path planning method of the invention.  FIG. 3A  and  FIG. 3B  are the schematic diagrams illustrating an initial tool path of the reciprocating tool path planning method according to an embodiment of the invention respectively. As show in the figures, the reciprocating tool path planning method M 1  comprises step S 11 , S 12 , and S 13 . 
         [0030]    Step S 11  is to prepare a curved surface to be machined. More specifically, at step S 11 , a three-dimensional surface is obtained from a data source or by other methods. Step S 12  is to read user commands, wherein the commands comprises an overcut error minimization command, an undercut error minimization command, or a total error minimization command, the number of cutter locations, the density of linear interpolation, and other parameters for computing the tool path. 
         [0031]    And step S 13  is to generate an initial tool path  9  according to the curved surface and the user command. In order to illustrate the difference between the present invention and the prior art, please refer to  FIG. 1  again. The initial tool path  9  is determined by points on the two boundary curves  91  and  92 . On the initial tool-path  9  of prior art, the points u 0   A  to u n-1   A  and u 0   B  to u n-1   B  on the two boundary curves  91  and  92  of the curved surface  90  should be corresponded and arranged in order from least to greatest, so that the cutting tool can program a forward-only tool-path. 
         [0032]    Compared to the prior art, the present invention breaks the restriction of the points. More specifically, the points u 0   A  to u n-1   A  and u 0   B  to u n-1   B  on the initial tool path  9  must be arranged in a ascending order in the corresponding curve parameters. The situations of u i   A &gt;u i+1   A  or u i   B &gt;u i+1   B  is allowed in computing the initial tool path of present invention, more specifically, the i+2 cutter location can be positioned between the and the i and the i+1 cutter locations, so as to make the tool motion partly backward. Therefore, the tool path planning method can move the tool backward and then resume moving forward in some regions were machining error can be reduced compared to forwarding only tool motion. 
         [0033]    In order to illustrate the relative relation of each cutter location in a reciprocating tool path plan, please refers to  FIG. 3A  and.  FIG. 3B . As shown in the figures, the initial tool path  9  comprises a first cutter location P 1 , a second cutter location P 2 , a third cutter location P 3 , and a fourth cutter location P 4 . The four cutter locations are corresponded to a first tool motion, a second motion, and a third motion, respectively. 
         [0034]    Wherein, the first tool motion is ahead of the second tool motion, the second tool motion is ahead of the third tool motion. Three cutter locations P 1 , P 2 , P 3  and the above boundary curve  91  (or called first curve) are assigned with a first coordinate C 1 , a second coordinate C 2 , and a third coordinate C 3  respectively, meanwhile, the curve length D 2  between the first coordinate C 1  and the second coordinate C 2  is greater than the curve length D 1  between the first coordinate C 1  and the third coordinate C 3 . 
         [0035]    After encoding the cutter locations described above, evolutionary optimization methods (genetic algorithm, particle swarm optimization, ant colony optimization, and/or simulated annealing) can be applied to compute a reciprocating tool path. The total error on the machined surface serves as an objective in the optimization process, which searches for an optimal tool path with an initial tool path  9 . 
         [0036]    In addition, the present invention further provides a multi-pass tool planning method M 2  for improving the effectiveness of machining system. The multi-pass tool planning method M 2  is utilized to generate a tool path  8  for a cutting tool to remove material from a work-piece along the tool-path  8 . 
         [0037]    Wherein, the tool path  8  comprises at least a first path  81  and a second path  82 . Please refer to  FIG. 4A to 4C ,  FIG. 4A  is a schematic diagram illustrating the first path according to an embodiment of the invention;  FIG. 4B  is a schematic diagram illustrating the second path according to an embodiment of the invention; and  FIG. 4C  is a schematic diagram illustrating the tool path according to an embodiment of the invention. 
         [0038]    More specifically, the multi-pass tool planning method M 2  computes several passes of tool path that constitutes a complete tool path with different indexes, so as to minimize the errors of curved surface  90  by machining in a progressive manner. To be noticed, each pass of tool path is constructed with a corresponding index. And the several passes of tool path comprises a first path  81  and a second path  82 , these two paths represent a tool path in a corresponding machining process. Either overcut error, undercut error, or the total error of the machined surface can be chosen as the objective in each machining process with the tool path planning method of the present invention. 
         [0039]      FIG. 5  is a flowchart illustrating the multi-pass tool planning method according to an embodiment of the invention. As shown in  FIG. 5 , the multi-pass tool planning M 2  comprises steps S 21  to S 24 , wherein the steps S 2  land S 22  are in essence the same as the steps S 11  and S 12  of the reciprocating tool path planning method M 1 , thus the steps need not be elaborated any further. 
         [0040]    Step S 23  is to construct a first pass of tool path  81  with a first index according to the surface  90  and the user commands, wherein the path  81  produces a first error value; and S 24  is to construct a second pass of tool path  82  with a second index according to the surface  90  and the user commands, wherein the path  82  produces a second error value. 
         [0041]    For example, overcut error minimization and undercut error minimization are chosen to be the objectives in the first index and the second index respectively. The first pass of tool path  81  comprises cutter locations generated by using overcut error minimization command; and the second pass of tool path  82  comprises cutter locations by using undercut error minimization command. In the tool path optimization process, the search priority is to eliminate overcut error and undercut error, respectively. 
         [0042]    The amount and distribution of stock material left on the workpiece are different after each machining process. Thus, the workpiece geometry from which the tool path is computed is different from the first pass of tool path  81  and the second pass of tool path  82 , although the reference surface is the same curved surfaces  90 . 
         [0043]    The machining process of prior art usually adopts rough milling first and then finish milling. This machining strategy is to maximize the machining productivity in the rough milling and to achieve quality surface finish in the finish milling with different tools and machining parameters. Tool path planning of the rough milling is normally based on the offset geometry of the surface to be machined while the finish milling is based on the surface to be machined. Uniform material is expected to remain on the workpiece after the rough milling and to be removed by finish milling. A major difference between the prior art and the present invention is that the multiple passes of tool path generated by the planning method of the present invention are all applied in finish milling. The successive tool paths are calculated to reduce machining error in a progressive manner. 
         [0044]    The present invention also discloses a five-axis flank milling system for machining curved surfaces with the reciprocating tool path planning method M 1  and the multi-pass tool path planning method M 2  described previously. The system guides a cutting tool to remove material from a work-piece along the tool path generated by the two methods. The resultant tool path produces a smaller error on the machined surface compared to the tool paths generated by prior art.  FIG. 6  is a function block diagram illustrating a five-axis flank milling system for machining curved surface according to an embodiment of the invention. Wherein, the system  1  comprised an interface module  10 , an arithmetic module  20 , a machining module  30 , and a control module  40 . 
         [0045]    The interface module  10  inputs the geometric definition of the surface to be machined and user commands; wherein the curved surface and the commands have been described previously. The arithmetic module  20  is coupled with the interface module  10  for computing tool path based on reciprocating tool path planning method M 1  and the multi-pass tool path planning method M 2 . And the control module  40  is coupled with both the arithmetic module  20  and the machining module  30  for machining the work-piece according to the tool path computed. In actual applications, the system  1  described above can be a five-axis machine tool connected with a computer. 
         [0046]    The reciprocating tool path planning method M 1  eliminates the “forward only” limitation of traditional tool path planning methods. The cutting tool can move forward first; then partially backward and resume moving forward in some regions on the surface to be machined as long as such reciprocating tool motion further reduce machining errors. The multi-pass tool path planning method M 2  computes several passes of tool path that constitutes a complete tool path with different indexes, so as to minimize machining errors in a progressive manner. 
         [0047]    The above disclosure should be construed as limited only by the metes and bounds of the appended claims.