Abstract:
A process for preparing the cutting edge of a cutting tool having a rake face and a clearance face. The process comprises the elimination of material from the cutting edge by providing a series of rapidly recurring electrical spark discharges in a gap located between the tool edge and a counterface. The spark discharges vaporize and melt the tool edge to form a desired radius.

Description:
FIELD OF THE INVENTION 
     The invention relates to a process for honing the edge of a cutting tool using a series of controlled and rapid spark discharges. 
     BACKGROUND 
     Industrial tools for cutting and shaping materials are fabricated from hard materials in order to maintain their edges and to withstand the concentrated stresses that are present at the cutting edge. These tools are frequently fabricated from materials including high speed steel (HSS), cemented carbide, ceramic, polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or similar ultra-hard materials. 
     Investigations have found that imparting a small radius on the cutting edge of an industrial tool on the order of several micrometers has various advantages. A tool with an edge appropriately prepared is typically less susceptible to catastrophic chipping during machining, which leads to a manyfold improvement in tool life. In addition, edge preparation improves the overall surface quality of the machined parts. The process of imparting a very small radius on the cutting edge of a cutting tool is known as edge honing. Due to the aforementioned advantages, edge honing has become a critical element in the manufacture and performance of industrial cutting tools. 
     A variety of different devices and methods have been developed to hone the edges of cutting tools. U.S. Pat. No. 5,178,645 to Nakamura, et al. discloses a method for honing the edge of a PCD cutting tool by applying a YAG laser to the cutting edge of the tool. The tool is inclined with respect to the laser beam and moved to hone the cutting edge. Alternatively, the laser beam is adjusted while the tool is in a fixed position. The laser beam processing parameters are pre-selected such that a radius forms along the exposed portion of the cutting edge. 
     The method of U.S. Pat. No. 5,709,587 to Shaffer consists of directing a pressurized fluid stream that is comprised of an abrasive grit entrained in a fluid. The fluid stream is directed against the sharp cutting edge of an elongated rotary tool for a pre-selected time to transform the sharp cutting edge into a relatively honed edge. 
     The apparatus of U.S. Pat. No. 6,287,177 to Shaffer comprises a base with a rotating brush with abrasive bristles mounted to a variable speed motor. A mount with a fixture for holding the cutting tool is attached to the base. The mount has a translational mechanism for controlling the position of the edge of the cutting tool relative to the rotating brush. The edge of the cutting tool is honed by controlling the movement and position of the cutting tool through the volume of the abrasive bristles, which results in the formation of a tapered edge. 
     The method of U.S. Pat. No. 7,063,594 to Engin, et al. consists of immersing the cutting edges of a cutting tool in an abrasive liquid bath. The bath contains an abrasive granular media, which is circulated through the bath such that the cutting edges are disposed within the flow path of the abrasive media. Alternatively, the cutting edges of a cutting tool are immersed in an abrasive liquid bath and the cutting tool is rotated. The abrasive media is comprised of very small abrasive granules such that a radius along the full length of the cutting edges forms after prolonged exposure. 
     It is very difficult with the foregoing approaches to achieve a consistent and repeatable radius along the full length of the cutting edge. Variation in the edge radius affects cutting performance in terms of efficient chip formation, which has adverse implications in terms of tool life and the quality of the machined surface. 
     In addition to problems with generating consistent hone radii, these methods are also expensive, and are limited when applied to ultra-hard tool materials like such as PCD or PCBN, on account of their extreme hardness. The demand for such tools is on the rise, and hence the need for better honing methods. 
     Accordingly, an improved process for honing the edges of cutting tools with consistent and repeatable results is highly desired. 
     SUMMARY OF THE INVENTION 
     The present invention is a process of preparing the cutting edge of a cutting tool having a rake face and a clearance face, with a cutting edge at least partially disposed therebetween. The process comprises the melting and vaporization of material from the cutting edge by providing a series of rapidly recurring electrical spark discharges in a gap located between the tool edge and a counterface. The spark discharges vaporize and melt the tool edge to form the desired radius. 
     In some embodiments, the gap can range from approximately 2 μm to approximately 100 μm. 
     The spark discharges form a radius on the cutting edge from 2 μm to a pre-determined threshold which includes a radius of infinity that corresponds to a plane surface, known as a chamfer. 
     In some embodiments, the process comprises flushing of the discharge gap by a gaseous or liquid dielectric fluid. The fluid may contain abrasive or metallic particle additives. 
     In some embodiments, the removal of discharge material is enhanced by the use of an electrolyte or a dielectric. 
     In some embodiments, the cutting tool is positioned in various orientations relative the tool axis for symmetric or asymmetric edge preparation. 
     In some embodiments, including when preparing the edges of a complex tool, the tool is moved relative to a fixed counterface in two or more dimensions. In other embodiments, the counterface is moved relative to the fixed tool in two or more dimensions. In still other embodiments, both the tool and counterface are moved relative each other in two or more dimensions. 
     The counterface is a metallic or electrically conducting material such as aluminium or graphite. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  depicts a side view of a cutting tool disposed a predetermined distance from a counterface which is submerged in a tank containing a dielectric fluid. 
         FIG. 2  is a flow diagram illustrating a process for honing the edge of a cutting tool according to the present invention. 
         FIG. 3   a  shows the cutting tool and counterface of  FIG. 1  and a spark that occurred following the application of a pulse voltage applied between the cutting tool and the counterface. 
         FIG. 3   b  shows the cutting tool and counterface of  FIG. 3   a  with material removed due to the heating associated with the spark. 
         FIGS. 3   c  to  3   g  shows the cutting tool and counterface of  FIG. 3   a  with additional material removed due to the heating associated with additional sparks. 
         FIG. 4   a  shows a side view of a cutting tool obliquely orientated relative to a counterface with an asymmetric radius formed on the cutting edge. 
         FIG. 4   b  shows a side view of a cutting tool with a flat chamfer edge formed on the cutting edge. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is now described with reference to the drawings. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It may be evident, however that the present invention may be practiced without these specific details. 
     Referring now to  FIG. 1 , a cutting tool  10  is disposed a predetermined distance from a counterface  20 . Cutting tool  10  has a rake face  11 , clearance face  12  and edge portion  15 , and is fabricated from an electrically conductive material. Counterface  20  is comprised of a pre-selected electrically conductive material such as copper, aluminium, graphite or steel and has a relatively flat, planar surface  22 . Counterface  20  is submerged in a tank  25  containing a dielectric fluid  30 . Dielectric fluid  30  is re-circulated through a filter  35  attached to tank  25 . 
     Referring now to  FIG. 2 , a flow diagram  200  of a process for honing the edges of a cutting tool using controlled and rapid spark discharges is shown in accordance with the present invention. At  201 , cutting tool  10  and counterface  20  are provided. Counterface  20  is submerged in tank  25  containing dielectric fluid  30 , and edge portion  15  is spatially disposed at a predetermined distance and angle from surface  22  of counterface  20  at  202 . Typically, the distance between edge portion  15  and surface  22  can range from approximately 2 μm to approximately 100 μm. 
     At  203 , a pulse voltage having pre-determined electrical parameters is applied between cutting tool  10  and counterface  20 , which function as the electrodes. After the electrical resistance of dielectric fluid  30  has been overwhelmed, a spark  40  occurs at the closest point between edge portion  15  and counterface  20 , as shown in  FIG. 3   a . The temperature of the plasma channel associated with spark  40  results in the removal of material from both edge portion  15  and counterface  20  as shown in  FIG. 3   b . The resulting particles formed from the removed material are flushed away by dielectric fluid  30  and are filtered from dielectric fluid  30  by filter  35 . A person of skill in the art will appreciate that a gaseous dielectric may substituted for dielectric fluid  30 . 
     Additional pulse voltages are applied between cutting tool  10  and counterface  20  until a pre-determined threshold of machining time has been reached at  204 . As material is continually removed, the gap between the tool and the counterface increases and, if a consistent gap is required in order to achieve the desired results, the cutting tool  10  may be fed towards the counterface  20  to compensate for the increase in gap or vice versa at  205 . Alternatively, both cutting tool  10  and counterface  20  are moved simultaneously. 
     Since first electrical discharge or spark  40  will take the path of least electrical resistance between edge portion  15  and counterface  20 , the heat associated with each successive spark,  40   b ,  40   c ,  40   d ,  40   e ,  40   f ,  40   g  will melt and/or vaporize and remove a small amount of material from each of cutting tool  10  and counterface  20  as shown in  FIGS. 3   b  to  3   g . After a pre-determined number of pulse voltages have been applied between cutting tool  10  and counterface  20 , a uniform radius will form along the entire length of edge portion  15 . Optionally, material removal may be aided electrolytically by the use of an electrolyte (not shown). 
     In some embodiments, cutting tool  10  is oriented perpendicularly to counterface  20  to form a symmetric uniform radius. Optionally, cutting tool  10  is obliquely orientated relative to counterface  20  to form an asymmetric radius such that more material may be removed from either the rake face  11  or clearance face, as shown in  FIG. 4   a.    
     The process may also be applied to complex tools by moving the tool relative to the counterface in two or more dimensions. 
     The formation of a uniform radius along the entire length of edge portion  15  is influenced by the ratio of the amount of material removed from edge portion  15  and counterface  20 . The ratio of material removed, known as the wear ratio, is affected by several parameters, including but not limited to the choice of polarity, the electrical parameters, the spatial position of edge portion  15  relative to counterface  20 , and the choice of materials for the cutting tool  10  and counterface  20 . 
     If the wear ratio is set too low, edge portion  15  will machine directly into counterface  20  with minimal material removal from edge portion  15 . If the wear ratio is too high, a flat chamfer edge will form on edge portion  15 , and minimal material will be removed from counterface  20 , as shown in  FIG. 4   b . In some embodiments, a chamfer edge is desirable. Where the desired radius of the cutting edge is extremely high a chamfer edge will form. 
     Several experiments were conducted using a die-sink electro-discharge machine to determine the influence of the various parameters on the wear ratio. Other apparatus which employ the process of the present invention are contemplated. The results of these experiments are summarized below. 
     Experiment I 
     Experiment I was conducted using a copper counterface, a HSS AISI T15 cutting insert, a voltage of 180 V, current of 2.4 A, and an on-time and off-time of 3.7 μs for 60 seconds. Copper was chosen as the counterface material since it is a common electrode material, and copper electrodes experience minimal wear when the electrode polarity is set to negative. 
     It was observed in Experiment I that when the polarity of the copper counterface was set to negative, this resulted in the formation of a flat or chamfer edge on the cutting insert, as opposed to a radius. 
     When the polarity of the copper counterface was set to positive, a significant amount of material was removed from the copper counterface and considerably less material was removed from the cutting insert. A proper radius did not form on the cutting insert as a result. Based upon the observed results, the wear ratio for a copper counterface and a HSS cutting insert was not within the desired range for forming an ideal radius on the cutting insert. 
     Experiment II 
     Experiment II was conducted using a steel counterface, a HSS AISI T15 cutting insert, a voltage of 180 V, current of 2.4 A, and an on-time and off-time of 3.7 μs for 60 seconds. Steel was chosen as the counterface material since the material removal rate is higher when set to positive polarity. 
     It was observed in Experiment II that when the polarity of the steel counterface was set to negative, this resulted in the formation of a flat edge on the cutting insert, although the flattening was less severe compared to Experiment I. Based upon the observed results, the wear ratio for a steel counterface and a HSS cutting insert was not within the desired range for forming an ideal radius on the cutting insert. 
     Experiment III 
     Experiment III was conducted using an aluminium counterface, a HSS AISI T15 cutting insert, a voltage of 180 V, current of 2.4 A, and an on-time and off-time of 3.7 μs for 60 seconds. 
     It was observed in Experiment III that an ideal radius formed on the cutting insert which increased linearly with increased machining time. Although the wear ratio was found to be in the ideal range, the surface finish on the edge of the cutting insert was rough. 
     Experiment IV 
     Experiment IV was conducted using an aluminium counterface, carbide and HSS cutting inserts were machined using identical EDM machine parameters of 100 V, current of 1.8 A, and an on-time and off-time of 0.6 μs. 
     It was observed that the amount of material removed per spark is highly influenced by the spark energy. As such, a larger radius was generated in the same period of machining time by increasing the energy. In addition, the cutting edge radius developed faster on the carbide cutting insert compared to the HSS cutting insert. It was also observed that when the discharge current was increased, the rate of the cutting edge radius formation did not increase linearly with the increasing current. 
     Unlike the result of Experiment III, the surface finish on the edge of the cutting insert was considerably smoother, and the finish on the carbide cutting insert was smoother than the finish on the HSS insert. 
     Experiment V 
     Experiment V was conducted to determine the reproducibility of the present invention. Six cutting inserts fabricated from HSS and carbide were machined using the parameters from Experiment IV, for a machining time of 80 seconds. From the results of Experiment V the radius along the cutting edge of the HSS cutting inserts varied by a maximum of 2.2 μm and the highest standard deviation was 1.5 μm. The variability along the cutting edge of the carbide cutting inserts was larger compared to the HSS results, however the mean radius was 50.2 μm as opposed to 50.0 μm for the HSS inserts. 
     Based upon the results of Experiment V, The maximum deviation along the edge of a prepared HSS insert was 3.8% of the desired radius compared to 35% for commonly used methods such as abrasive brush edge honing methods. 
     The reproducibility of the process according to the present invention is superior to the prior art processes for honing the edge of cutting tools. The present invention is readily adaptable for honing cutting tools fabricated from ultra-hard materials, since most PCD and PCBN cutting inserts are fabricated with a metallic binder, typically cobalt. A worker skilled in the art will appreciate that the process parameters include selecting the material for counterface  20 , the spatial parameters of edge portion  15  relative to counterface  20 , the electrical parameters and the threshold of machining time. 
     Although the foregoing experiments were conducted using a die-sink electro-discharge machine, a person of skill in the art will appreciate that the present invention is readily adapted for other similar machines, including electrochemical discharge machines. 
     Given that edge honing typically improves the life expectancy of cutting tools several fold, the economic advantages associated with the present invention are significant, and will greatly benefit the tool manufacturing industry. 
     Although the description above contains many specific details, these should not be construed as limiting the scope of the embodiments but as merely providing illustrations of some of the presently preferred embodiments. Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given.