Abstract:
A method and tooling for inspecting a contour of an edge of a cutout formed in a disk, each cutout fixedly receiving a turbine blade. A first device receives the disk containing the cutouts. A second device has a sensor associated with the movement of an instrument, the first device securing the disk at a predetermined orientation to the sensor. The disk is secured to the first device, the instrument being directed along the cutout defining a path substantially parallel to a surface of the contour, the instrument remaining in physical contact with the surface of the contour. The sensor transmits a signal associated with instrument movement to an algorithm to convert the signal to two-dimensional positions along the tangent path along the surface of the contour. The acceptability of the edge contour of the cutout is determined by comparing the two-dimensional positions to predetermined ranges of values.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to a method and apparatus for inspecting turbine engines. More specifically, the present invention relates to method and apparatus for inspecting the surface contour of an edge of a cutout formed in a disk for fixedly receiving a turbine blade. 
   In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high-pressure turbine are mounted on one shaft, and the fan and low-pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. 
   The turbine is composed of a disk having a plurality of complex, closely toleranced slots machined along the periphery of the disk to receive a portion of the turbine blades referred to as a dovetail. The contour of slots is typically formed by broaching. Broaching is a production technique whereby a cutter, typically referred to as a broach, is used to finish cutouts or apertures of circular, square or irregular sections. In broaching, the action of the broach itself serves as a clamping medium so that the operation may be completed in a minimum of time. The cutting tool or broach is typically provided with many teeth graded in size such that each takes a small chip when the tool is directed through the previously prepared leader aperture. 
   In the case of disks for securing the dovetails of turbine blades, broaching produces an amount of disk material tear out that extends past the surface of the disk opposite the surface in which the broach is first directed, or exit side of the disk. This tear out is in the form of jagged discontinuities or burrs typically formed along substantially the entire periphery of the broached cutout or aperture. Material along the broached area contains reduced strength properties. Since this broached region is highly stressed in operation, care must be taken to remove these discontinuities to avoid the possibility of stress-induced cracking, which can significantly reduce component life. The process of removing the tear out discontinuities is referred to as breaking the edge or producing an edgebreak or breakedge. Material was typically removed from the slot peripheries, such as by machining techniques, to form small rounded edges along the exit side of the disk. Unfortunately, it has been recently discovered that the rounded edges of a considerable number slot peripheries were of insufficient size to remove discontinuous regions formed by broaching, and that follow-up machining or other techniques of material removal is now required to enlarge the radius of the rounded edge or form a chamfer to preserve component life. Thus, a significant number of disks of turbine engines in operation are likely in need of this retrofit operation. To determine whether the disk requires retrofit machining, an inspection is conducted. These disk re-inspections are in addition, of course, to the on-going need to inspect newly fabricated turbine disks. 
   Further complicating matters, commonly used techniques for inspecting edgebreaks are time-consuming and prone to error. For example, a wax-and-trace technique involves manually placing heated wax to the surface of the disk, waiting for the wax to cool, properly removing the wax, i.e., without deforming or distorting the wax impression, and then visually aligning the impression for mechanical trace to ensure that the trace direction is normal to contour centerlines. Not only did this technique expend up to ten minutes for each peripheral slot, but gage repeatability and reproducibility approached 50 percent. In other words, the impression associated with the wax-and-trace technique, even if done correctly, could account for about one half of the total permissible range of dimensional variation of the edge contour of the slot. Due to the time-consuming nature of the wax-and-trace technique, inspections for edgebreaks may be further limited, thus increasing the probability of missing a defect. 
   What is needed is a method or apparatus for inspecting turbine disks that can be performed more quickly and having improved gage repeatability and reproducibility. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method of inspecting a contour of an edge of a cutout of a plurality of cutouts formed in a disk, each cutout of the plurality of cutouts for fixedly receiving a turbine blade. The method includes the steps of: providing a first device for receiving the disk; providing a second device having a sensor and an instrument in association with the sensor, the first device securing the disk at a predetermined orientation to the sensor; securing the disk to the first device; positioning the instrument adjacent a cutout of a plurality of cutouts formed in the disk; directing a tip of the instrument over a point of tangency along an edge of a cutout of the plurality of cutouts, the tip following a path along the surface of the contour that is substantially coincident with a plane defined by an axis of tangency with the point of tangency and a line substantially normal to the edge of the contour, the instrument being in physical contact with the surface of the contour; transmitting a signal from the sensor based upon movement of the instrument to an algorithm; converting the signal received by the algorithm to a plurality of positions along the path; and comparing the plurality of positions to predetermined ranges of values for determining acceptability of the contour of an edge of the cutout of the plurality of cutouts. 
   One advantage of the present invention is it requires significantly less time to inspect disk slot end contours for receiving turbine blades. 
   Another advantage of the present invention is it has enhanced gage repeatability and reproducibility. 
   A further advantage of the present invention is that it is can be easily performed. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a perspective view of a turbine disk. 
       FIG. 1A  is an enlarged partial plan view of a turbine disk. 
       FIG. 1B  is a side view taken along section  1 - 1  of  FIG. 1A  of the present invention. 
       FIG. 2  is a perspective view of the inspection apparatus of the present invention. 
       FIG. 3  illustrates a flow chart detailing the inspection method of the present invention. 
       FIGS. 4-14  are perspective views showing the sequence of steps to perform the inspection of a turbine disk using the inspection apparatus of the present invention. 
   

   Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  illustrates one embodiment of an inspection apparatus  10  for inspecting a dovetail edgebreak contour formed in a turbine disk  200 . Referring to  FIGS. 1 and 1A , turbine disk  200  includes a disk portion  206  extending to a sleeve portion  208 . A bore  204  is formed through both disk portion  206  and sleeve portion  208  to engage a shaft of a jet engine to turn the turbine disk  200 . The periphery of disk portion  206  is commonly referred to as a root form  202 . Root form  202  includes a plurality of substantially uniformly spaced, radially outwardly extending posts  210 . Each post  210  includes a plurality of dovetail protrusions  212  and dovetail indentions  214  that are formed on opposite sides along the length of the post  210 . A cutout or slot  222  is defined by facing surfaces of adjacent post  210  connected by a base  216 . Cutout or slot  222  receives a dovetail portion  218  of a turbine blade  220 . 
   Cutout or slot  222  preferably includes an axis of symmetry  224 . Coincident with both base  216  and axis of symmetry  224  is a point of tangency  226 . Base  216  preferably includes a curved contour that provides symmetric points of tangency  230 ,  232  with opposed sides of slot  222  having dovetail indentions and protrusions  212 ,  214 , such as along an axis  228 . As further discussed below, the inspection apparatus  10  includes a process to inspect slot  222 , such as at points of tangency  224 ,  230  and  232 , although it is to be understood that inspection apparatus  10  can be used to inspect any number of desired points of tangency along the slot  222 . 
   Referring back to  FIG. 2 , inspection apparatus  10  includes a first device  12  and a second device  14  that interact to permit an accurate, convenient and quickly performable means of obtaining edgebreak dimensions for comparison with predetermined levels. First device  12  can include a table  16 , which supports a plate  18  having a substantially flat surface  20 . Flat surface  20  of plate  18  supports a workpiece carriage  22  having a base portion  23  that extends to a neck portion  29 . Opposite the end of neck portion  29  that is connected to base portion  23  is an end having an aperture  27  for receiving a locating pin  26 , such as a ball lock pin. Interposed between the ends of neck portion  29  is a pivot  24  that includes a fastener that is directed through both the neck portion  29  and plate  18 , pivot  24  having an axis of rotation  25 . In other words, workpiece carriage  22  slidably rotates about pivot  24  having an axis of rotation  25  along surface  20  of plate  18 . Pegs  30  limit the angular range of rotation of workpiece carriage  22  about pivot  24  to ensure workpiece carriage  22  fully remains within the footprint of plate  18 , which in a preferred embodiment enables workpiece carriage to rotate approximately 180 degrees. To secure workpiece carriage  22  in a desired position, aperture  27  of neck portion  29  is aligned with one of a plurality of table apertures  28  formed in plate  18 , and locating pin  26  is directed through both aperture  29  and table aperture  28 . The plurality of table apertures  28  define different locating positions of the workpiece carriage  22 . It is to be understood that any number of table apertures  28  may be formed in plate  18  and that any number of angular spacings between adjacent table apertures  28  or arrangements may be selected as desired. For example, a preferred embodiment can contain table apertures  28  in a first position, or home position, a second position that is achieved by rotating the workpiece carriage  22  approximately 90 degrees counterclockwise about pivot  24  from the first position, and a third position which is achieved by rotating the workpiece carriage  22  approximately an additional 90 degrees counterclockwise about pivot  24  from the second position. Thus, the third position is approximately 180 degrees from the first position. 
   A point of reference for use with the Figures is position  84  which is adjacent one side of table  16  that is opposite a table  44  of the second device  14 . From point of reference  84 , in the first position, the base portion  23  extends to the right of pivot  24 , defining a three o&#39;clock position. Similarly, for purposes herein, the second position defines a twelve o&#39;clock position, and likewise, the third position defines a nine o&#39;clock position. 
   To permit workpiece carriage  22  to secure turbine disk  200 , workpiece carriage  22  includes a substantially vertically extending spindle  36  mounted on a cross-slide table  42  that is rotatable about an axis of rotation  38  independently of axis of rotation  25 . It is preferable that spindle  36  defines a substantially cylindrical profile with a sufficient conical element such that the diameter of the end of the spindle  36  opposite surface  20  is less that the diameter of the base of the spindle  36  so that the bore  204  of the turbine disk  200  is substantially self-centered in a substantially vertical orientation when the turbine disk  200  is lowered over the spindle  36 . Preferably, spindle  36  is covered by a sleeve  32  that extends to a base  34  that surrounds the periphery of sleeve  32  adjacent the surface  20  of plate  18 . Sleeve  32  and base  34  are preferably composed of a layer of a relatively soft material as compared to the turbine disk  200 , such as any number of hard machinable plastics, non-metallic materials or other non-marring materials, so as not to damage the bore  204  of the turbine disk  200  when the turbine disk  200  is installed over the spindle  36 . However, sleeve  32  and base  34  are composed of material that is also rigid enough not to flex excessively when compressed between the turbine disk  200  and the workpiece carriage  22 , thus providing a repeatable, consistent orientation of the turbine disk  200  when installed. 
   To install turbine disk  200  on spindle  36 , the turbine disk  200  is positioned above the workpiece carriage  22  and the center of the bore  204  is aligned with the axis of rotation  38 . After alignment between the spindle  36  and turbine disk  200  is achieved, the bore  204  of the turbine disk  200  is directed into contact with the sleeve  32  and base  34 . 
   In addition to permitting rotational movement about axis  38 , cross-slide table  42  additionally incorporates the ability to make minute adjustments in either or both of two mutually perpendicular axis directions along the surface  20  of plate  18 , such as X and Y axes. Due to the considerable weight of turbine disk  200 , an air bearing  40  is preferably incorporated beneath workpiece carriage  22 . Air bearing  40  includes an interface with a pneumatic source (not shown), which provides a layer of compressed air to be trapped beneath the surface beneath the air bearing  40  and the surface  20  of plate  18  to provide a substantially frictionless interface between workpiece carriage  22  and surface  20  of plate  18 . 
   Second device preferably includes a table  44  that supports a base  48  having a substantially vertical column  50  extending from the base  48 . Column  50  is rotatable in directions  68  or  74  about its axis  52 . An arm  54  is slidably movable along column  50  in a vertical direction  72  and a horizontal direction  76 , arm  54  preferably being maintained in a substantially horizontal position while being moved with respect to column  50  in vertical direction  72 . Secured to one end of arm  54  is a tracer unit  46  that includes a tracer head  56  that has a wand  58 . When used to take readings from a substantially vertical surface, such as a slot surface  222  of turbine disk  200  when the bore  204  is positioned substantially coincident with axis  38  of spindle  36  ( FIG. 6 ), wand  58  is preferably oriented at an angle  78  that is substantially vertical while also providing a horizontal component when wand is actuated along its axis  66 . Wand  58  extends from the tracer head  56  to a stylus  60 , stylus  60  preferably extending substantially perpendicular to wand  58 , and terminating at a tip  62 . When tip  62  is placed in contact with a surface of an object to be inspected with the wand  58  being directed to travel along its axis  66 , preferably toward tracer head  56 , a pair of sensors (not shown) preferably housed within the tracer head  56  provide signals to an algorithm, such as a computer program (not shown). One of the sensors is associated with movement of the wand  58  along its axis  66 , while the other sensor is associated with angular rotation  82  about axis  80  of the wand  58  in response to contact between the tip  62  and the surface of the slot  222 . The algorithm converts the signals from the sensors into data points such as X and Y axis coordinates. The data points can then be compared to a predetermined range of values that are considered acceptable. Optionally, the data points can be transferred to a display (not shown), printed, or saved to a memory device for future reference or machining. 
   Preferably, a control program or algorithm executed by a microprocessor, or control device, or control panel (not shown) can be used to control the operation of the tracer head  56 . This movement includes any individual or combinations of movement of the wand  58  of tracer head  56  along its axis  66 , horizontal movement of the tracer head  56  along an axis  76  of arm  54 , vertical movement of the combined tracer head  56  and arm  54  along axis  52  of column  50 , angular rotational movement  82  of the combined wand  58  and tip  62  about axis  80  and rotational movement of the combined tracer head  56  and arm  54  about axis  52  of column  50 . Alternately, all movements or any combination of these movements can be manually performed by an operator, if desired. 
     FIG. 3  illustrates a flow chart detailing the inspection process using the inspection apparatus  10  of the present invention to inspect a dovetail edgebreak contour formed in a turbine disk  200 . The control process of  FIG. 3  can be performed as a separate control program executed by a microprocessor of control device, or a control panel or control process can be implemented as a sub-program in the control program for the inspection apparatus  10 . 
     FIGS. 4-14  illustrate the sequence of steps of the process to perform the inspection of turbine disk  200  using the inspection apparatus  10  of the present invention. The process begins, after starting the process in step  100 , with initialization of the inspection apparatus  10  in step  105  as shown in  FIG. 2 , which includes moving the tracer head  56  to a home position and moving the workpiece carriage  22  to a home position, such as the 3 o&#39;clock position, if the tracer head  56  and workpiece carriage  22  are not already at their home positions. Although the process for moving the workpiece carriage  22  can be controlled by an operator, it is to be understood that the entire process, i.e., moving the workpiece carriage  22  and the tracer head  56 , as well as loading/unloading the turbine disk  200 , can be completely automated. Once the inspection apparatus  10  has been initialized, in step  110 , the turbine disk  200  is loaded, or installed over the spindle  36  which extends from the workpiece carriage  22  as previously discussed and as shown in  FIGS. 4-6 . 
   After the turbine disk  200  has been loaded in step  110 , the tracer head  56  is positioned in step  115  by an appropriate combination of rotation  74  of the tracer head  56  about axis  52 , travel along axis  76  of arm  54 , rotation of the wand  58  about axis  80  and vertical travel  72  along vertical axis  52  as shown in  FIG. 7 . Once the tracer head  56  has been positioned so that the tip  62  contacts the surface of a slot  222  to be inspected, the tip  62  begins tracing the contour of the slot  222  in step  120  as the tip  62  is directed toward the tracer head  56  along axis  66 . For example, if the turbine disk  200  is positioned in the three o&#39;clock position, tip  62  traces the contour of the slot  22  along point of tangency  230  ( FIG. 1A ). It is critically important that the tip  62  is directed along a substantially linear path that is substantially normal to the point of tangency  230  of the slot  222 , with an acceptable variance of about plus or minus three degrees between axes defining the path and the surface of the slot  222 . 
   For example, referring to  FIGS. 1A-1B , in the three o&#39;clock position, wand  58  and tip  62  contact slot  222  at contact point  238  and are preferably directed in travel direction  86  which is along axis  66 , and that is coplanar with axis  228  that is normal to normal line  236 , normal line  236  being substantially normal to an upper surface  250  and coincident with point of tangency  230 . Point of tangency  230  is along an edge of base portion  216  of slot  222 . When tip  62  is directed along direction  86 , tip  62  traces a path from contact point  238  to point of tangency  230 , or normal segment  240  which is coincident with line  236 , then to termination point  246  that is located along the upper surface  250  of post  210 . However, if wand  58  and tip  62  are directed along an alternate axis of travel  88  having an angle of deviation  90  from travel direction  86 , the path of travel of tip  62  is different. Although the contact point  238  is the same in both instances, directing the wand  58  and tip  62  along direction  88  causes the tip  62  to trace a path  244  which extends from contact point  238  to contact point  242  along the edgebreak of the turbine disk  200 , then to termination point  248  that is located along the upper surface  250  of post  210 . It is clear that normal segment  240  is coincident with the line normal to the point of tangency  230 , or line  236 , and defines the shortest distance along the edgebreak of the turbine disk  200 . Any deviation from the line normal to the point of tangency  240 , such as deviation segment  244 , increases the length of this line as seen by the tip  62  and sensed by the sensors associated with its movement, can provide erroneous information to the operator as to whether the edgebreak is within permissible tolerances as previously discussed. 
   It is to be understood that instead of point of tangency  230  being coincident with upper surface  250 , point of tangency  230  can alternately be coincident with lower surface  252 , which is opposite and substantially parallel to upper surface  250 . Therefore, the discussion of the immediately preceeding paragraph is inverted. Axis  228 , point of tangency  230  and termination points  246 ,  248  are coincident with lower surface  252 . That is, instead of the tip  62  being directed from contact point  238  to point of tangency  230  then to termination point  246  along upper surface  250 , the tip  62  is directed from contact point  238  to point of tangency  230  then to termination point  246  along lower surface  252 . Thus, other that a preference between the upper surface  250  and the lower surface  252 , the concept of operation is otherwise the same. 
   For the tracer head  56  to trace the surface contour of the slot  222  as the wand  58  is directed along axis  66 , the sensors associated with movement of the wand  58  and tip  62  along axis  66  and rotation of the wand  58  and tip  62  about axis  80 , respectively, transmit a signal in step  125  to the control panel, which then converts the signal to data points, such as X,Y coordinates, in step  130  as previously discussed. After the signal has been converted to data points, the data points are compared to predetermined value ranges in step  135  to determine whether an acceptable contour, i.e., such as chamfer, radius or other smooth surface transition, has been produced along the edgebreak of the slot  222 . Step  135  includes saving the data points, and other information, such as acceptability of the edgebreak, to a memory device. 
   Once the data points have been compared in step  135 , it may be desirable to inspect the same portion of a different, or additional slot  222 . If the control panel (or operator) determines in step  140  to conduct a similar trace on an additional slot  222 , the turbine disk  200  is rotated about axis  38  to locate the next desired slot  222  to inspect. Once the turbine disk  200  has been rotated to the next slot  222  for inspection, steps  115  through  135  are performed in a similar manner as previously discussed. After step  135  has been performed, the control panel (or operator) determines in step  140  whether to conduct the same trace on an additional slot  222 . If an additional trace in step  140  is not to be performed, control of the process is shifted to step  150 , wherein the control panel (or operator) determines in step  150  whether to perform an additional trace for the same or other slot  222  on the turbine disk  200 . If an additional trace is to be performed, the turbine disk  200  is moved along the surface  20  of plate  18  in step  155  to a new position, such as the twelve o&#39;clock position. Moving the turbine disk  200  involves rotating the turbine disk  200  about axis  25  that is coincident with pivot  24  by removing the locating pin  26  from the table aperture  28 , and initiating the air bearing  40  as previously discussed and shown in  FIGS. 8 and 9 . 
   Prior to or contemporaneously with rotating the turbine disk  200  about axis  25 , tip  62  is retracted along axis  66  until there is sufficient clearance between the tip  62  and the turbine disk  200  or rotated about axis  80  to avoid damage to the tip  62  or wand  58 . Once the turbine disk  200  has been sufficiently rotated about axis  25  and the aperture  27  of the neck portion  29  aligning with the corresponding table aperture  28 , the locating pin  26  is directed through the aperture  27  and the table aperture  28  to fix the position of both the workpiece carriage  22  and turbine disk  200 . After fixing the position of the turbine disk  200 , the turbine disk  200  is then rotated about axis  38  in a direction  70  ( FIG. 9 ) if required, to provide a favorable orientation for the tip  62  to engage a different portion of the previously inspected slot  222  or even a different slot  222 . Additionally, it may be necessary to provide minor adjustments, such as X,Y axis adjustments as provided by the cross-slide table  42 , to permit access to a different region of the slot  222 . However, it is believed that if tables  16  and  44  are properly spaced, the effective length of arm  54  is adjusted and if the neck portion  29  of the workpiece carriage  22  includes additional apertures so that the effective rotational radius defined by rotating the workpiece carriage  22  about pivot  24  can also be modified without requiring a change to the table apertures  28 , many, if not all configurations of turbine disks can be accommodated by the inspection apparatus  10 . 
   Once the turbine disk  200  is moved along the surface  20  of plate  18  in step  155  to the twelve o&#39;clock position, as shown in  FIG. 9 , the tracer head  56  is moved into position in step  115  so that the tip  62  contacts the slot  222 . However, the tracer head  56  is already substantially in the desired position since it had been moved during the previous step  145 . Therefore, to position the tip  62  in contact with a different region of the slot  222  of the turbine disk  200 , all that should be required is to actuate the wand  58  along axis  66  or rotate the wand  58  about axis  80 , although minor adjustments to move the tracer head  56  along axis  72  and/or minor adjustments to the cross-slide table  42  may be required to move the turbine disk  200  with respect to the workpiece carriage  22 . After the tracer head  56  has been positioned, steps  120  through  135  are achieved in a substantially identical manner as previously described. Upon completion of step  135 , the control panel (or operator) determines in step  140  whether to conduct the same trace on a different slot  222 . If the same trace is to be performed on a different slot  222 , the turbine disk  200  is rotated about axis  38  in step  145  in a manner as previously discussed. Control is then returned to steps  115 - 135  as previously discussed. 
   After steps  115  through  135  have been performed, the control panel (or operator) determines in step  140  whether to inspect an additional slot  222 . If an inspection of an additional slot in step  140  is not to be performed, control of the process is shifted to step  150 , wherein the control panel (or operator) determines in step  150  whether to perform an additional trace on the same or other slot  222  on the turbine disk  200 . If the control panel determines that an additional trace on the same slot  222  is to be inspected, in step  155 , the workpiece carriage  22  is moved along the surface  20  of the plate  18  about pivot  24  to the nine o&#39;clock as previously discussed and as shown in  FIG. 6 . Once the turbine disk  200  has been moved, steps  115  through  135  are performed in a manner substantially similar as previously discussed. After step  135  has been performed, the control panel (or operator) determines in step  140  whether to perform the same trace on an additional slot  222 . If the same trace on an additional slot  222  in step  140  is to be performed, control of the process is shifted to step  145 , then to steps  115  through  135  as previously discussed. After step  135  has been performed, the control panel (or operator) determines in step  140  whether to perform the same trace on an additional slot  222 . If the same trace on an additional slot  222  in step  140  is not to be performed, control of the process is shifted to step  150 , wherein the control panel (or operator) determines in step  150  whether to conduct an perform an additional trace for the same or other slot  222  on the turbine disk  200 . If the control panel determines that an additional trace for the same slot  222  is not to be inspected, the apparatus settings are initialized at step  160 , which preferably moves the tracer head  56  and the workpiece carriage  22  to their respective home positions as previously discussed. After the apparatus settings are initialized, the turbine disk  200  is removed from the inspection apparatus  10  at step  165  before the process terminates at step  170 . 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.