Abstract:
Rough machining of a workpiece is performed by a numerically controlled machine tool using an adaptive toolpath technique. Material removal rate and machine efficiency are increased by forming a pre-roughing slot in the workpiece along medial axes, and machining the remainder of the workpiece using a toolpath that begins inside the pre-roughing slot and spirals outwardly in smooth curves.

Description:
BACKGROUND INFORMATION 
     1. Field 
     The present disclosure generally relates to automated machining techniques, and deals more particularly with a method of optimizing toolpaths used by automatically controlled cutting tools. 
     2. Background 
     CAD/CAM (computer aided design/computer aided manufacturing) systems may be integrated with CNC (computer numerically controlled) systems to provide rapid, efficient machining of workpieces. CAM systems use various toolpath strategies to guide a cutting tool during rough machining operations in which the workpiece is machined to near net-shape. In one toolpath strategy, commonly referred to as constant-offset or parallel-offset, the degree of radial engagement of the cutting tool with the workpiece varies as the cutting tool moves along the toolpath. While the material removal rate (MRR) may be optimized along straight cuts, the feed rate must be reduced when the tool enters sharp curves or corners where the radial engagement increases. 
     In order to increase machining efficiency and reduce tool wear/breakage, adaptive type toolpath strategies have been developed in which full radial engagement of the cutting tool with the workpiece is maintained substantially throughout the rough machining operation. Based on full radial engagement of the cutting tool, the CAM system may calculate an axial cutting depth and feed rate that result in higher utilization of the available machine tool power. Although adaptive toolpath strategies may significantly increase cutting efficiency, the machine utilization rate is still not fully optimized. This is because, while a toolpath program may call for constant cutting tool feed rate, the machine may not be able to actually achieve a constant feed rate due to its inertial mass. For example, when the cutting tool reaches an outer cut area boundary, the machine is required to decelerate, stop, reverse direction and re-accelerate. The loss of efficiency due to these frequent changes in toolpath velocity and direction may be particularly severe along narrow or tight regions of a workpiece. In these regions, numerous, short back-and-forth passes of the cutting tool are necessary, each of which requires the cutting tool to decelerate, change direction and reaccelerate. 
     Accordingly, there is a need for a method of rough machining a workpiece using an adaptive toolpath strategy that increases machining efficiency, particularly along narrow or tight regions of an area being machined. There is also a need for a machining method that maintains the feed rate of a cutting tool at a substantially constant velocity while the cutting tool remains in substantially full engagement with the workpiece. 
     SUMMARY 
     According to the disclosed embodiments, an automated machining method is provided using an adaptive toolpath strategy that utilizes a greater amount of a machine tool&#39;s available power in order to increase material removal rate and reduce machining time. The need to decelerate, change direction and reaccelerate a cutting tool is significantly reduced. Long, smooth toolpaths are generated which allow a substantially constant feed rate to be maintained during substantially the entire rough machining operation. The method creates a pre-roughed condition that can be used as a starting point for rough machining that employs substantially 100 percent tool engagement and a feed rate matched to the maximum operating constraint the machine. 
     According to another embodiment, a method is provided of rough-machining a workpiece using a numerically controlled machine tool. The method comprises selecting a boundary defining the shape of an area of the workpiece to be machined, determining a medial axis of the shape, using the medial axis to generate a first set of toolpath data for guiding the movement of a cutting tool, machining a pre-roughing slot in the workpiece using the first set of toolpath data, generating a second set of toolpath data based on the geometry of the slot, and rough machining the workpiece using the second set of toolpath data. Determining the medial axis of the shape is performed using a computer implemented medial axis transform algorithm, and determining the medial axis includes generating multiple curves representing the shape, and then thinning the multiple curves by deselecting at least certain of the multiple curves. Generating the second set of toolpath data is performed using an adaptive toolpath algorithm. The medial axis transform algorithm includes a scaled medial axis transform. Deselecting the at least certain multiple curves is automatically performed using a programmed computer. Alternatively, deselecting the at least certain multiple curves includes visually presenting the multiple curves to a human, and the deselection is performed by the human. Rough machining the workpiece is commenced by locating the cutting tool within the slot and moving the cutting tool spirally outward from the slot in generally smooth curves. Generating the second set of toolpath data includes selecting a toolpath pattern that spirals outwardly from the slot. The rough machining includes moving the cutting tool spirally outward from the slot. 
     According to still another embodiment, a method is provided of rough-machining a workpiece having at least one narrow region. The method includes inputting a cut boundary to a computer that defines the shape of an area of the workpiece to be machined, using the computer to perform a medial axis transformation of the shape, including generating a set of medial axis curves describing the shape, selecting certain of the medial axis curves for use in generating a pre-roughing toolpath, using the computer to generate the pre-roughing toolpath based on the selected medial axis curves, automatically machining a pre-roughing slot in the workpiece using the pre-roughing toolpath to guide the cutting tool, using the computer to generate a roughing toolpath based on the geometry of the pre-roughing slot, automatically machining a remainder of the area of the workpiece using the roughing toolpath to guide the cutting tool. Using the computer to perform a medial axis transformation of the shape includes performing a scalar medial axis transformation. Automatically machining the remainder of the area of the workpiece includes using the roughing toolpath to guide the cutting tool in generally smooth curves spiraling outward from the pre-roughing slot. Selecting certain of the medial axes is performed automatically by the computer. Automatically machining the remainder of the area of the workpiece includes maintaining a substantially constant radial depth of cut throughout the roughing toolpath. 
     The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims. The illustrative embodiments, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an illustration of a functional block diagram of a system for machining a workpiece using a medial axis transform to optimize an adaptive toolpath strategy. 
         FIG. 2  is an illustration of a perspective view of a cutting tool in full radial engagement with a workpiece. 
         FIG. 3  is an illustration of a plan view of a workpiece showing the cut boundary of an area to be rough machined. 
         FIG. 4  is an illustration of a plan view of the workpiece showing medial axis skeletal curves computed for the shape defined by the cut boundary shown in FIG. 
         3 . 
         FIG. 5  is an illustration similar to  FIG. 4  after thinning of the medial axis skeletal curves. 
         FIG. 6  is an illustration of a plan view of the workpiece showing the location of the pre-roughing slot and the pre-roughing toolpath used to cut the slot. 
         FIGS. 7-9  are illustrations of perspective views showing the pre-roughing slot being cut based on the thinned medial axis skeletal curves shown in  FIG. 5  and the toolpath shown in  FIG. 6 . 
         FIG. 10  is an illustration of a plan view of the workpiece showing an optimized adaptive toolpath based on the geometry of the pre-roughing slot shown in  FIG. 6 . 
         FIG. 11  is an illustration of a flow diagram of a method of machining a workpiece using medial axis curves to optimize an adaptive toolpath strategy. 
         FIG. 12  is an illustration of a flow diagram showing additional details of the method shown in  FIG. 11 . 
         FIG. 13  is an illustration of a flow diagram of aircraft production and service methodology. 
         FIG. 14  is illustration of a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , a workpiece  24 , which may comprise a solid material such as aluminum or titanium, may be machined using a cutting tool  22  driven by a machining center  20 . The machining center  20  may comprise an automated multi-axis machine that displaces the cutting tool  22  and the workpiece  24  relative to each other. The machining center  20  may be automatically controlled using a programmed controller, such as the CNC (computer numerically controlled) controller  26 . The machining center  20  may comprise, for example and without limitation, a 3-axis or 5-axis mill having a rotating spindle (not shown) for rotating the cutting tool  22 . The cutting tool  22  may comprise, without limitation, an end-mill  22  shown in  FIG. 2 . The CNC controller  26  may be coupled with a CAD/CAM (computer aided design/computer aided manufacturing) system  28  having CAM software (not shown) and access to one or more software programs  30 . In the illustrated embodiment, the software programs  30  include at least a suitable MAT (medial axis transform) algorithm, and an adaptive toolpath generator  36 . A general purpose programmed computer  23  provided with user interfaces (e.g. input devices, displays, etc.) is coupled with the CADS/CAM system  28  and the CNC controller, and has access to the software programs  30 . 
     Referring to  FIG. 2 , the cutting tool  22  rotates  40  about an axis  41  corresponding to the machine tool spindle (not shown), which in the illustrated example is the Z-axis in an orthogonal XYZ coordinate system  38  of the machining center  20  in  FIG. 1 . The machining center  20  moves the cutting tool  22  along a toolpath  48  that is generated by the CAD/CAM system  28  and controlled by the CNC controller  26 . The cutting tool  22  has a radial depth of cut  44  in the X-Y plane, controlled by the machining center  20 . In the illustrated example, the radial depth of cut  46  is a full slot cut  42  in which the cutting tool  22  has maximum radial engagement with the workpiece  24 . The machining center  20  moves the cutting tool  22  over the workpiece  24  along the toolpath  48  in the X-Y plane. The cutting tool  22  has an axial depth of cut  46  determined by the CAD/CAM system  28  and controlled by the CNC controller  26 . 
       FIG. 3  illustrates a typical workpiece  24  having a workpiece area  25  that requires rough machining. The workpiece area  25  is defined by a cut area boundary  50 , and may include one or more narrow regions  56 . As used in this description, “narrow region” includes but is not limited to tight spaces, small areas, corners, channels or other narrowings within the cut area boundary  50 . In accordance with the disclosed embodiments, the workpiece area  25  may be rough machined using an adaptive toolpath strategy that is specifically optimized to the shape of the area  25  to be machined. As will be discussed below in more detail, this optimization is achieved by computing the medial axis skeletal curves of the area  25  to be rough machined, cutting a pre-roughed slot in the workpiece  24 , and then generating an optimized adaptive toolpath that is based on the geometry of the pre-roughed slot and is used to complete the rough machining process. By optimizing the adaptive toolpath in this manner, inefficiencies associated with acceleration and deceleration of the cutting tool  22  in the narrow regions  56  may be substantially reduced, 
     Attention is now directed to  FIG. 4  which illustrates a set of medial axis skeletal curves  68  that have been computed for the shape of the workpiece area  25  defined by the cut area boundary  50  shown in  FIG. 3 . The medial axis skeletal curves  68  includes a set of inner curves  60   a ,  68   b ,  68   c , and a series of outer, diagonal skeletal curves  68   d . The medial axis skeletal curves  68  are computed by the CAD/CAM system  28  using the computer  23  ( FIG. 1 ) and the MAT  34 . The MAT  34  may comprise any suitable, commonly available MAT algorithm, such as without limitation, a scaled medial axis transform that automatically thins a set of generated medial axis curves. Generally, a medial axis transformation is a mathematical technique used to extract the shape of a polygon—a process that is sometimes referred to as finding its skeleton, and the medial axis is sometimes referred to as the topological skeleton of a shape. The medial axis of a shape is a set of all points having more than one closest point on the shape&#39;s boundary. In two dimensions, the medial axis of a planar curve S is the locus of the centers of circles that are tangent to the curve S and in two or more points, where all such circles are contained in S. Stated in another way, the medial axis curve is the locus of the center of all those circles that fit within the cut area boundary  50  and are not within another circle. 
       FIG. 4  shows the initial results of the medial axis transformation of the area  25 , defined by the cut area boundary  50 , and is representative of an image that may be displayed to a human user on a computer screen (not shown) forming part of the user interfaces  32  ( FIG. 1 ). Because it may not be necessary or efficient to use all of the medial axis curves yielded by the medial axis transformation, the medial axis curves  68  may be thinned, i.e. reduced in number, through a selection process, in which the curves that are most useful or practical are selected, and the remaining curves are ignored or “deselected”. This selection process may be performed automatically by the CAD/CAM system  28 , or semi-automatically by a user providing appropriate selection instructions to the CAD/CAM system  28 . In the present example either the CAD/CAM system or the user may, for example, deselect the outer, diagonal curves  68   d , resulting in a display similar to that shown in  FIG. 5  in which the skeletal curves  68  have been thinned to a generally centrally located skeletal curve  68   a , and two outer skeletal curves  68   b ,  68   c.    
     Referring now to  FIG. 6 , using the location of the thinned medial axis skeletal curves  68  shown in  FIG. 5 , CAD/CAM system  28  ( FIG. 1 ) generates a pre-roughing slot toolpath  75 . The cutting tool  22  follows the pre-roughing toolpath  75  to cut a pre-roughing slot  72  in the workpiece  24  that substantially follows the thinned medial axis skeletal curves  68 . Referring to  FIGS. 7-9 , cutting of the pre-roughing slot  72  begins with the cutting tool  22  cutting a circular recess  70  in the workpiece  24 , centered at one end of the thinned medial axis curves  68  shown in  FIG. 5 . Then, as shown in  FIG. 8 , cutting tool  22  proceeds to cut the full length of the slot  72 , ending in a second circular recess  70  ( FIG. 9 ) centered at the other end of the thinned medial axis curves  68 . Depending on the desired final axial depth of the slot  72 , it may be necessary for the cutting tool  22  to make several passes though the workpiece  24 , in which a layer of material is removed with each pass. The slot may be cut to the final axial depth to which the workpiece  24  is rough machined, or, alternatively, successive layers of the slot  72  may be machined as the remainder of the workpiece  24  is being rough machined as described below. 
     Attention is now directed to  FIG. 10  which illustrates an optimized adaptive toolpath  76  that is generated by the CAD/CAM system  28  based on the geometry of the pre-roughing slot  72 . The cutting tool  22  begins the roughing process at a starting position  78  that is inside the pre-roughing slot  72 . The cutting tool  22  then moves in broad, substantially smooth curves around the slot  72 , forming an outward spiral pattern. The toolpath  76  is substantially continuous within the narrow regions of the workpiece  24 , and does not require either reversal of the direction of travel of the cutting tool  22 , or substantial deceleration and acceleration of the cutting tool  22 . 
     Attention is now directed to  FIG. 11  which broadly illustrates the steps of a method of machining a workpiece  24  using an adaptive toolpath technique optimized through the use of a medial axis transform, as previously described. Beginning at step  80 , an area  25  of the workpiece  24  to be machined is selected, which is the cut area boundary  50 . At step  82 , at least one skeletal curve  68  for the selected area  25  is computed. At step  84 , a pre-roughing slot  72  is machined in the workpiece  24 , substantially along the computed skeletal curve  68 . At step  84 , a toolpath  76  is generated that is based on the geometry of the pre-roughing slot  72 . At step  84 , the cutting tool  22  is moved along the optimized toolpath  76  to rough machine the workpiece  24  to the desired axial depth. At step  86 , an optimized toolpath based on the geometry of the pre-roughing slot is generated. At step  88 , the cutting tool is moved along the optimized toolpath to rough machine the workpiece. 
       FIG. 12  illustrates additional details of the method shown in  FIG. 11 . Beginning at  90 , an NC (numerically controlled) operation is generated which may include selecting a CAD file that digitally describes the workpiece  24  to be machined, and generating a CNC program to perform the machining operation. At step  92  a cut area boundary  50  is selected defining the area  25  to be rough machined. The cut area boundary  50  may be selected, for example, such that it excludes areas of the workpiece  24  that have already been rough machined. The selection process in step  92  may be performed by a user, or automatically under computer control. 
     At step  96 , the medial axis skeletal curves are computed for the cut area boundary  50  selected in step  94 , and at step  98 , the computed medial axis skeletal curves may be displayed to a human user on a display screen. At step  100 , medial axis skeletal curves computed at step  96  and displayed at step  98  are selected for use in a pre-roughing operation in which a first set of pre-roughing toolpath data is generated that is used to machine the pre-roughing slot  72 . The selection process in step  100  may be performed by a user, or automatically by a computer  23 . At step  100 , the computed skeletal curves are thinned to a desired set. At step  102 , a second set of roughing toolpath data is automatically generated by the computer  23  which define a toolpath for creating the pre-roughing slot  72 . Finally, at step  104 , an optimized adaptive toolpath  76  is generated by the computer  23  based on the geometry of the pre-roughing slot  72  and the cut area boundary  50  selected at step  94 . Following generation of the optimized adaptive toolpath  76  at step  104 , the workpiece  24  may be rough machined. 
     Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine, automotive applications and other applications requiring machined workpieces. Thus, referring now to  FIGS. 13 and 14 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  106  as shown in  FIG. 13  and an aircraft  108  as shown in  FIG. 14 . Aircraft applications of the disclosed embodiments may include, for example, without limitation, machining any of a variety of parts and components of solid material, and particularly metals such as aluminum and titanium. During pre-production, exemplary method  106  may include specification and design  110  of the aircraft  108  and material procurement  112 . During production, component and subassembly manufacturing  114  and system integration  116  of the aircraft  108  takes place. Thereafter, the aircraft  108  may go through certification and delivery  118  in order to be placed in service  120 . While in service by a customer, the aircraft  108  is scheduled for routine maintenance and service  122 , which may also include modification, reconfiguration, refurbishment, and so on. 
     Each of the processes of method  106  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 14 , the aircraft  108  produced by exemplary method  106  may include an airframe  124  with a plurality of systems  126  and an interior  128 . Examples of high-level systems  126  include one or more of a propulsion system  130 , an electrical system  132 , a hydraulic system  134 , and an environmental system  136 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries. 
     Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method  106 . For example, components or subassemblies corresponding to production process  102  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  108  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages  114  and  116 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  108 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  108  is in service, for example and without limitation, to maintenance and service  122 . 
     The description of the different illustrative embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different illustrative embodiments may provide different advantages as compared to other illustrative embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.