Abstract:
A method for cutting metal includes providing a rotating cutting tool and making a first cut in the material using a first tooth of the cutting tool, such that an amount of heat is conducted into the material. A second cut is made in the material using a second tooth of the cutting tool, before the heat dissipates from the material. The time between the first cut and the second cut is such that the heat softens the material and allows the second tooth to more easily cut the material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]    This application claims priority to U.S. provisional patent application No. 60/370,777 filed Apr. 8, 2002, the entire content of which is incorporated herein by this reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to an apparatus and method of cutting materials utilizing a rotating cutting tool. More specifically, the invention includes a cutting process that uses the heat generated by the cutting process to more efficiently cut materials.  
         BACKGROUND OF THE INVENTION  
         [0003]    In the process of metal cutting, when a tool cuts a metal, heat is generated by shear stresses, plastic deformation, and friction in the cutting region. Generally this heat is distributed into three regions. One portion flows into the tool, another portion flows into the chip, and the third portion is conducted into the workpiece. The surface of the workpiece is thermally softened by this third portion of heat. The heat that flows into the workpiece is conducted from the surface into the bulk, and the rate of this heat transfer depends on the thermal properties of the workpiece.  
           [0004]    A rotating cutting tool, such as a milling cutter, includes one or more teeth that cut material in a progressive manner. Between each cutting path of successive teeth, heat is conducted into the workpiece and is lost to the environment. For example, the heat may be conducted away into the workpiece-holding device or may be convected into the surrounding environment. Accordingly, the next tooth is unable to take advantage of the thermal softening caused by the previous tooth. There is a need in the art for an improved cutting system that cuts the thermally softened material, which requires lower specific cutting forces and results in lower power consumption, improved tool life, and improved material removal rates.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The present invention, according to one embodiment, is a method for cutting metal including providing a rotating cutting tool, making a first cut in the material using a first tooth of the cutting tool, such that an amount of heat is conducted into the material, and making a second cut in the material using a second tooth of the cutting tool, before the heat dissipates from the material, such that the heat softens the material and allows the second tooth to more easily cut the material. In one embodiment, the time between cutting passes is determined using the following equation:  
             T=T   (t=0)   +[T   s   −T   (t=0) ]{1 −erf[X/{square root} 4 αt]}   
           [0006]    Where, T is a transient temperature, T (t=0)  is an initial temperature, T s  is a temperature after the first cutting pass by the cutting tool, erf is an error function, X is a distance into the material from a top surface, α is a thermal diffusivity of the material, and t is the time between the first cut and the second cut. The result of cutting a material using the HFTP regime is a reduction in specific cutting forces, high utilization of heat, lower peak tool temperatures, higher tool life, and improved material removal rates.  
           [0007]    While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description. As will be apparent, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a flowchart showing a method of cutting or milling materials according to the present invention.  
         [0009]    FIGS.  2 A- 2 D show various stages of the workpiece cutting process.  
         [0010]    [0010]FIG. 3 shows a workpiece undergoing a multiple tooth pass cutting process, including a corresponding thermal profile of the cutting teeth and the workpiece, according to one embodiment of the present invention.  
         [0011]    [0011]FIG. 4 shows a workpiece undergoing a multiple tooth pass cutting process, including a corresponding thermal profile of the cutting teeth and the workpiece, according to another embodiment of the present invention.  
         [0012]    [0012]FIG. 5 shows a schematic view of a cutting tool according to one embodiment of the present invention.  
         [0013]    [0013]FIG. 6 shows an isometric view of a cutter according another embodiment of the present invention.  
         [0014]    [0014]FIG. 7 shows a sectional view of a cutter in a plane perpendicular to the central axis according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 1 is a flow chart showing a method  100  of cutting materials according to the present invention. As shown in FIG. 1, the first tooth of a multiple tooth cutting tool cuts the workpiece (block  102 ). This cutting process generates heat caused by forces between the cutting tool and the workpiece (block  104 ). Generally, this heat is distributed into three portions. One portion of the heat goes into the cutting tool (block  106 ), another portion goes into the chip or waste created by the cut (block  108 ), and the remaining portion goes into the workpiece (block  110 ). The heat conducted into the workpiece softens the surface of the workpiece (block  112 ). Depending on the thermal properties of the workpiece material, this heat from the surface gets transported into the bulk of the workpiece at a particular rate of conduction. The next tooth then cuts the workpiece before too much of the heat is transferred into the bulk of the workpiece (block  114 ). This process results in cutting material in a high-frequency tooth pass (“HFTP”) regime.  
         [0016]    The HFTP regime takes advantage of the thermal properties of materials, especially stronger materials such as titanium and titanium alloys, steel, alloy steels, and other non-ferrous metals. According to one embodiment of the present invention, a suitable time period between successive tooth passes is calculated using the following one-dimensional heat transfer equation:  
           T=T   (t=0)   +[T   s   −T   (t=0) ]{1 −erf[X/{square root} 4 αt]}   
         [0017]    Where, T is a transient temperature, T (t=0)  is an initial temperature, T s  is a temperature after the first cutting pass by the cutting tool, erf is an error function, X is a distance into the material from a top surface, α is a thermal diffusivity of the material, and t is the time between the first cut and the second cut. The result of cutting a material using the HFTP regime is a reduction in specific cutting forces, high utilization of heat, lower peak tool temperatures, higher tool life, and improved material removal rates.  
         [0018]    This heat transfer equation is used to calculate a suitable time between successive cutting actions. In one embodiment, the time between cutting passes is from about 0.8 to about 1.2 multiplied by t in the above equation. In another embodiment, the time between cutting passes is from about 0.9 to about 1.1 multiplied by t in the above equation. In yet another embodiment, the time between cutting passes is about t, as determined by the above equation. This time is then used to determine a frequency at which the material of a workpiece is cut. The frequency of the cutting tool or cutter is defined as the number of times a material is cut in a second. Thus, frequency is the number of tooth passes per second. The cutter frequency depends on the combination of the revolutions per minute (“RPM”) of the cutting tool and the number of teeth per around its circumference.  
         [0019]    In one embodiment, frequency of the cutting tool for the HFTP regime is at least about 95 tooth-passes-per-second. This frequency can be used for cutting different materials, including titanium and titanium alloys, steel and steel alloys, and other non-ferrous metals and materials.  
         [0020]    FIGS.  2 A- 2 D show the effect of applying the HFTP regime to a workpiece. As shown in FIG. 2A, a first tooth  202  of the cutting tool enters the workpiece  204 . In this illustration, the tool is moving from right to left of the view as it progresses into the cut. In FIG. 2B, the first tooth  202  finishes cutting and exits the workpiece  204  at the left. In the cutting process, a chip  203  is generated. Also, due to the cutting action, heat is generated and gets distributed into the tool  202 , the chip  203  and the workpiece  204 . The transfer of heat into the workpiece  204  is shown by line  207  in FIG. 2B. FIG. 2C shows the start of the cutting process by a second tooth  206 . As the cutting process is based on to the HFTP regime, accurate time delay exists between successive tooth passes. In FIG. 2C, the resulting heat  207  generated from the cutting action of first tooth  202  is shown near the surface of the workpiece  204 . Because of this heat  207 , the workpiece  204  material in the surface region remains softened. While this heat  207  remains on the surface of the workpiece  204 , the second tooth  206  enters the workpiece  204  and progresses into the cut. As shown in FIG. 2D, the second tooth  206  finishes cutting the workpiece  204  before the heat  207  dissipates. Chip  208  is generated as a result of the cutting action.  
         [0021]    [0021]FIG. 3 shows another embodiment of cutting a workpiece according to the HFTP regime. As shown in FIG. 3, two cutting teeth  302  and  306  are simultaneously engaged in cutting a workpiece material  310 . Heat is generated by the cutting action of the tooth  302 , and is distributed into the tooth  302 , the chip  304 , and the workpiece  310 . The heat that goes into workpiece  310  is represented by the lines  312 . The second tooth  306  then follows the first tooth  302  within a suitable time period calculated using the above equation, to take advantage of the softening of the workpiece  310  caused by the heat  312 .  
         [0022]    [0022]FIG. 4 shows yet another embodiment of cutting a workpiece according to the HFTP regime. As shown in FIG. 4, a cutting tool  420  has four cutting teeth  402 ,  406 ,  410 ,  414 . The cutting tool  420  has a plurality of teeth but only four are shown for representation purpose. The spacing and time interval between these successive teeth is designed according to the HFTP regime, as detailed above. Heat generated by the cutting action of the tooth  402  is distributed into the tooth  402 , the chip  404 , the workpiece  418 . This heat, which is shown by the line  405  on the workpiece, softens the material in front of the next tooth  406 . As a result, the cutting forces experienced in cutting action by the tooth  406  will be smaller compared to that experienced by the first tooth  402 . The heat generated by cutting action of tooth  406  is distributed into the tooth  406 , the chip  408 , and the workpiece  418 . This heat, which is shown by the line  409 , on the workpiece softens the material ahead of the next tooth  410 . As a result, the cutting forces experienced in cutting action by the tooth  410  will be smaller compared to a workpiece that has not been softened. The heat generated by cutting action of tooth  410  is distributed into the tooth  410 , the chip  412 , and the workpiece  418 . This heat, which is shown by the line  413 , on the workpiece softens the material ahead of the next tooth  414 . As a result the cutting forces experienced in cutting action by this tooth  414  will be smaller yet.  
         [0023]    [0023]FIG. 5 shows a schematic view of a cutting tool  500  according to one embodiment of the present invention. The cutting tool  500  may be an end mill, face mill, or any other similar cutting tool. FIG. 5, for example, shows an end mill with a straight flute. The cutting tool  500  includes a cylindrical tool body  502  and a shank  504 . This cylindrical body  502  may be a hollow or a solid body with an axis  506  passing through the center along the length of the body  502 . The tool body  502  extends from the shank  504  to an end face  508 . The cylindrical surface  510  is the surface between the end face  508  and the shank  504 . The cylindrical surface  510  carries plurality of flutes or grooves  512 . In one embodiment, the cylindrical surface  510  includes at least six grooves  512 , which originate at the circumference of the end face  508  and run throughout the cylindrical surface  510  of the tool body  502 . The flutes  512  may be straight or helical. For example, FIG. 5 shows twelve straight flutes  512 . The flutes  512  may have different shapes depending on designs and application including but not limited to a parabolic flute shape.  
         [0024]    A cutting edge  514  is formed by all outermost points on a flute  512 , which are on the cylindrical surface. As known in the art, a face mill will also have cutting edges along points on flute running in radial direction on end face. The angle of helix which is defined by an angle between cutting edge  514  and central axis, may vary from 0 to 60 degrees. For example the cutting tool in FIG. 5 has straight flutes  512 , so the angle of helix is zero. The flutes  512  may or may not be equidistant from each successive flute  512 . A through hole  518  along the length of the cutter may be provided for air-blow or for coolant circulation to keep peak tool temperatures at lower levels. Additional holes may or may not be provided along flutes  512  so as to direct coolant or air in a way to assist chip evacuation, cooling the tool  500 .  
         [0025]    The cutting tool  500  material may be any of the tool steels in general, including, for example, high speed steels, solid carbide, tool steel with carbide coatings, or an indexable insert cutter. The cutting tool  500  may also be impregnated with different materials including, for example silicon carbide, aluminum oxide, diamond, cubic boron nitride, garnet, zirconia or similar abrasive materials. In one embodiment, the cutting tool  500  may have an edge preparation depending on the use. The edge preparations that can be used include a T-land, a sharp-edge radius, or a ground and honed edge. The tool  500  material may have a coating on it. The tool  500  may also have an air blow option for ease in chip removal and a coolant option for keeping the tool temperatures low.  
         [0026]    The shank  504  is designed so that it is capable of insertion and securing into a spindle. Thus, the shank  504  could be of any shape and design suitable for a particular milling machine. The shank  504  designs may include a taper, a V-flange, or straight. As is known in the art, face mill does not have a shank. The shank  504  material may be similar to the tool  500  or may be different. For example, the shank  504  and the tool  500  may be made up of different materials and welded together to make a uniform single-body tool.  
         [0027]    [0027]FIG. 6 shows an alternative embodiment of a cutting tool  501  having twelve flutes  512 . As shown in FIG. 6, the flutes  512  have an angle of helix of twenty degrees. This cutter also has holes  518  to direct coolant onto the tool  501 .  
         [0028]    [0028]FIG. 7 shows a sectional view of the cutting tool  500 . As shown in FIG. 7, the diameter of tool  500  is shown by the dimension  516 . In one embodiment, the tool  500  diameter may vary from about 6 to about 300 mm, depending on the type of application. As shown in FIG. 7 an angle formed between plane of a flute and a radius of the tool  500  passing through the cutting edge in that plane is called radial rake angle  520 . The tool  500  may have a range of radial rake angles from positive to negative.  
         [0029]    Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.