Abstract:
A cutting tool incorporates a body terminating in cutting edges distal from a chuck mount and having an axial bore for reduced mass to raise the natural frequency of the tool. In certain of the embodiments, the body is preformed from a steel or carbide blank into a cylindrical pipe forming the hollow bore prior to grinding of the cutting edges. Filling of the bore with a light weight polymer to further absorb vibration can also be employed.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is divisional of copending U.S. application Ser. No. 12/502,462 filed on Jul. 14, 2009 which is in turn a continuation-in-part of application Ser. No. 11/757,547 filed on Jun. 4, 2007 both entitled INCREASED PROCESS DAMPING VIA MASS REDUCTION FOR HIGH PERFORMANCE MILLING by Keith A. Young, Eric I. Stern, Thomas L. Talley, and Randolph B. Hancock and having a common assignee with the present application the disclosure of which is incorporated herein by reference. 
    
    
     FIELD 
     This invention relates generally to the metals machining and more particularly to a bore relieved milling tool having reduced mass for process damping and high performance milling. 
     BACKGROUND 
     Finish machining of deep pocket aircraft structural components is limited by deflection and chatter. Modern designers are consistently pursuing weight reduction opportunities in metallic structure. Machined parts with deep pockets and small corner radii require long slender end mills to cut the corners. Long slender cutting tools are more susceptible to chatter and vibration than shorter more rigid tools. Long cutting tools exhibit lower natural frequencies, which reduces the process damping effects which can stabilize chatter. This requires small cuts and slower cutting speeds to avoid chatter, which can increase manufacturing costs. Current methods to increase machining rates include using higher cutting speeds and tools with more cutting edges. Both of these techniques can result in more chatter for longer cutting tools. 
     Current methods exist to reduce cutting tool vibration and chatter. These include using an eccentric relief on the cutting tool to enhance the rubbing of the cutter on the machined part. This rubbing will also stabilize the cutting tool. The use of an eccentric relief is a benefit for shorter cutting tools, but the effect is not useful for longer tools, when the resonant frequency of the cutting tool creates a wavelength that is longer than the eccentric relief. 
     It is therefore desirable to provide modified cutting tools which retain or increase process damping effects to stabilize chatter. 
     SUMMARY 
     The embodiments disclosed herein provide a method for fabrication of a cutting tool incorporating a body terminating in cutting edges distal from a chuck mount and having an axial bore. In certain of the embodiments, the body is preformed from a steel or carbide blank into a cylindrical pipe forming the hollow bore. 
     For exemplary embodiments of the method, a threshold established for a cutting tool mass for resonant frequency and stability followed by reducing the cutting tool mass below the threshold through introducing a central bore in the tool for a reduced wall shank. Establishing a threshold for a cutting tool mass is accomplished by determining pocket depth and tool length and calculating frequency response of the tool based on a spindle configuration for a given stiffness to determine mass. A threshold mass is then identified for desired frequency response and the mass is adjusted to obtain stability lobes positioned for maximized depth of cut. 
     In alternative embodiments, the axial bore is filled with a vibration absorbing material. A light weight polymer is used in exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is an isometric side view of an embodiment of the reduced mass tool; 
         FIG. 2  is a bottom axial view of the embodiment of  FIG. 1 ; 
         FIG. 3  is a side view of the embodiment of  FIG. 1 ; 
         FIG. 4  is an isometric view of a filled embodiment of the tool; 
         FIG. 5  is an illustration of the cutting profile effects of low frequency vibration in a tool without process damping; 
         FIG. 6  is an illustration of the cutting profile with process damping provided by a tool incorporating the present invention; 
         FIG. 7  is a graph depicting stability lobes for depth of cut with respect to cutting speed for a tool without process damping; 
         FIG. 8  is a graph depicting resulting stability lobes for depth of cut with respect to cutting speed for a tool employing process damping provided by the present invention; 
         FIGS. 9A through 9C  demonstrate tools with solid shank and various diameter central bores to demonstrate the method of the present embodiments; 
         FIG. 10  is a graph of resonant frequency response for the tools shown in  FIGS. 9A-9C ; 
         FIGS. 11A through 11D  graphs depicting resulting stability lobes for depth of cut with respect to cutting speed for the tools of shown in  FIGS. 9A-9C ; and, 
         FIG. 12  is a flowchart of the method for tool fabrication for optimizing tools of the embodiments disclosed. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the tool disclosed herein are applicable to rotating milling cutters and stationary boring cutters where the work piece rotates instead of the tool. As shown in  FIG. 1 , an embodiment of the reduced mass tool  10  is hollow; incorporating a center bore  12 .  FIG. 2  demonstrates that for this embodiment, the center bore employs a large diameter  14  with respect to the overall diameter of the tool  16  and is aligned with the axis of rotation of the tool. Additionally, the tool shank  18  is necked down or relieved to further reduce mass with the cutting edges  20  formed at a first end of the tool and a chuck attachment  22  formed at the opposite end. 
     The cutting tool mass is reduced by pre-forming the carbide or steel blank into a cylindrical pipe before grinding the cutting edges. For the embodiment shown, a reduction of over half the mass of a conventional tool is achieved. The mass reduced form increases the resonant frequency of a milling cutter, as the example embodiment, without significantly reducing the tool stiffness. This allows the tool to cut with approximately the same static deflection, but with significantly reduced dynamic deflection and chatter, as will be discussed in greater detail subsequently. In alternative embodiments, boring of the center hole in the completed tool or prior to heat treating or sintering and grinding of cutting edges is accomplished. 
     In alternative embodiments, the large hole in the center of the cutting tool is filled with a vibration absorbing material such as a light weight polymer  24  as shown in  FIG. 4  to further absorb vibration. An exemplary polymer is silicon RTV 664B produced by General Electric. Alternative filler materials such as metallic or nonmetallic shot or pellets, a viscous liquid, oil or water, a resin, or another metal with higher material damping are anticipated in exemplary embodiments. 
     Testing of embodiments shown herein has shown a significant reduction in cutter vibration. The cutting tool with less mass vibrates at a higher frequency. The natural frequency, Wn, of the resulting mechanical system is given by Wn=sqrt(k/m), where k is the stiffness and m is the mass. As mass is reduced, the natural frequency is increased by the square root of the mass. Dynamic stiffness of the milling cutter is measured using impact testing with an accelerometer attached to the tool. By striking the tool with a mallet, the dynamic stiffness of the cutter is reported by a displacement Frequency Response Function (FRF) monitored on an oscilloscope output from the accelerometer. Tuning of resonant frequency by modifying the central hole diameter in the cutting tool can be accomplished for specific machining requirements such as tool rotational speed as desired. However, for most embodiments, achieving the highest frequency while maintaining necessary tool stiffness is desirable. 
     Creating higher frequency response on the tool allows smearing by an eccentric relief or clearance ramp  34  of the tool which is not possible at lower frequency. As shown in  FIG. 5 , low frequency vibration of a tool without incorporation of the present invention creates cutting scallops  30  in working machine part  32  which exceed the effective capability of clearance ramp  34  on cutting edge  20  with tool rotational direction generally indicated by arrow  36 .  FIG. 6  demonstrates the higher frequency contact of the cutting edge in a tool comparable to the disclosed embodiments providing a smoother surface. For the embodiment shown, the clearance ramp is modified to incorporate a eccentric relief grind to enhance smearing on the rake face. 
     Similarly, a stability zone prior to onset of chatter of the tool is achieved for cuts of, greater depth as shown in  FIGS. 7 and 8 . For a tool without the present invention, the “no chatter” region  40  is limited to a an onset value  42  for depth of cut based on cutting speed as shown in  FIG. 7 . Certain stability lobes  44  are present at higher cutting speeds. Employing the present invention provides a significant stability zone  46  to a much higher onset value for chatter as shown in  FIG. 8 . Additionally, the stability lobes  44 ′ are increased in area providing increased functionality for machining soft metals. The tool frequency changes via mass removal can be employed to align a stability lobe with the top speed of a spindle for improved machining rates. 
     Exemplary data has been obtained for comparative tools shown in  FIG. 9A-9C  using a standard tool  50  with solid shank  18  having a 1 inch diameter  51 , a first reduced wall tool  52  having a shank  18  with a 0.33 inch center bore diameter  54  and a second reduced wall tool  56  having a shank  18  with a 0.46 inch center bore diameter  58 . Length  59  of each of the tools is 4.050 inches from the chuck attachment  22  to the extent of the cutting edges which are not shown in detail in  FIGS. 9A-9C . As previously discussed with respect to  FIG. 5 , increasing the resonant frequency of the tool provides for enhanced performance through smearing by the clearance ramp. The resonant frequency is increased from that of standard tool  50  shown in trace  60  of  FIG. 10  by implementing a center bore to remove sufficient mass in the tool. For the standard tool, resonance of the spindle is shown in inflection point  61  at approximately 1200 Hz. The resonance of the tool  50  is shown at inflection point  62  at approximately 1380 Hz. The difference between spindle shaft frequency and cutting tool, frequency needs to be a multiple of the spindle rotation frequency. The natural frequency desired can be calculated as described above or through the use of finite element modeling. How much mass needs to be removed from the cylinder of the tool shank to make the difference between the spindle resonant frequency and the tool resonant frequency by the spindle rotation frequency can be approximated for a cylindrical tube with open ends of inner radius r 1 , outer radius r 2 , length h and mass m by the equations
 
 I   z =½ m ( r   1   2   +r   2   2 )
 
 I   x   =I   y = 1/12( m )[3( r   2   2   +r   1   2 )+ h   2 ]
 
Or when defining the normalized thickness t n −t/r and letting r=r 2 , then
 
 I   z   =mr   2 (1− t   n +½ t   n   2 )
 
With a density of ρ and the same geometry
 
 I   z =½ πρh ( r   2   4   −r   1   4 )
 
     A threshold for mass removal from the tool shank by the center bore to achieve both a desired resonant frequency increase and added stability as discussed below can be determined. Trace  63  shows the resonant frequency of the spindle at inflection point  64  comparable to the solid tool and of first reduced wall tool  52  at inflection point  65  also comparable to the solid tool. Trace  66  for second reduced wall tool  56  shows the resonant frequency of the spindle is consistent with the other two tools however an increase of approximately 100 Hz at inflection point  67 . Second reduced wall tool  56  employing a center bore of 0.46 inches exceeds the threshold in the exemplary spindle and an increase of 100 Hz over the other two tools can be achieved. 
     As previously described, modification of the stability lobes for prevention of chatter is also accomplished by mass removal by the center bores. As shown in  FIGS. 11A-11D  individually and then combined for the solid shank tool, trace  70 , the first reduced wall tool, trace  72 , and the second reduced wall tool, trace  74 , the increase in stability of the second reduced wall tool  56  provides a significantly widened stability lobe  76  providing performance improvement for depth of cut over the other tool configurations. Stability lobes are calculated for a limiting depth of cut, Blim, using the published Thusty Method as Blim=1/(2*Ks*Re(G)*mu), where G is the tool compliance function, Ks is specific cutting pressure of the material, mu is a coefficient related to number of flutes and radial immersion. Blim is the maximum stable depth of cut. In the example of a 10 flute cutter, the depth of cut was increased from 0.020″ for the standard tool  50  having a sold wall to 0.050″ by using the embodiment of the second reduced wall tool  56  as shown in  FIG. 11D . For a two flute cutter, the depth of cut would be increased from 0.1″ to 0.25″ 
       FIG. 12  demonstrates the method for design and fabrication of tools achieving the desired performance improvement through mass reduction by implementing a center bore. A pocket depth and associated tool length is determined, step  1202 . Based on the spindle configuration the frequency response of the tool is calculated for a given mass stiffness to determine a desired mass, step  1204  and a threshold is determined for establishing the desired resonant frequency increase, step  1206 . Mass of the tool is adjusted by varying the diameter of a center bore to obtain stability lobes allowing the desired depth of cut, step  1208 . A carbide or steel blank is then formed into a pipe having the determined wall thickness to achieve the required center bore diameter for the determined mass, step  1210  or alternatively, a solid tool is machined to create the center bore, step  1212 . In varying methods, the tool is then machined for a reduced diameter shank as a supplemental means for mass reduction for desired center bore diameter and wall thickness at desired stiffness, step  1214 . Heat treating or sintering and grinding of cutting edges is then accomplished to complete the tool, step  1216 . 
     The embodiments disclosed have been tested and provide the ability for use for pockets up to 4 inches in depth. At this depth, the new hollow reduced mass cutting tool is more than twice as productive as a prior art solid counterpart. Pockets of up to 8 inches in depth are anticipated to be within the capability of the tool. The embodiments disclosed herein allow more productive use of long, slender end mills, which are traditionally problematic. 
     Having now described exemplary embodiments for the invention in detail, as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.