Patent Publication Number: US-2013243541-A1

Title: Combined thermal fit and rotational locking  machine tool assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a nonprovisional patent application of U.S. Provisional Patent Application No. 61/610,724, filed Mar. 14, 2012, and titled COMBINED THERMAL INTERFERENCE FIT AND ROTATIONAL LOCKING MACHINE TOOL ASSEMBLY, which is incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a machine tool holders and cutting tools, and more particularly to a machine tool assembly for a machine tool spindle. 
     Cutting tools and holding systems for milling machines, machining centers, and other machine tools typically have a clearance fit that allows easy assembly and disassembly of the cutting tool and holder, and a mechanism that provides secure clamping of the cutting tool relative to the tool holder. 
     For example, a first machine tool holding system, for example, as shown in  FIG. 6 , shows a tool holder having a through hole formed axially through the tool holder and threaded holes and associated set screws oriented radially through the tool holder. The cutting tool includes flats formed on one side of its shank. The shank is inserted into the through hole of the tool holder and the set screws are tightened, engaging them against the flats of the tool shank. Accordingly, the engagement of the set screws and shank flats secure the holder and tool axially and rotationally, and the conventional machine tool assembly is ready for operation. 
     However, the engagement of the set screws with the shank flats presses the cutting tool against the side wall of the axial through hole, opposite the set screws. Thus, securing the tool shank relative to the tool holder forces the cutting tool to be eccentric to the tool holder and thus to the tool rotation. 
     This eccentricity is often referred to as Total Indictor Reading or TIR. TIR is the enemy of tool service life. Even a very small reduction of TIR can increase tool life substantially. Excessive heating and/or vibration caused by TIR can drastically reduce the usable lifespan of a cutting tool. With excess TIR and the shank and through hole of the tool holder and cutting tool not in firm contact around their full circumference and length, excessive vibration and subsequent excessive heating can develop. For example, as the cutting tool contacts the material being machined, eccentricity and the clearance gap in fit between the cutting tool and tool holder result in vibration that transforms what should be a continuous shaving of material from the work piece into non-continuous, rapid bites that overheat and prematurely wear the cutting tool. If the cutting surface is a carbide insert, or the whole cutting tool is carbide, the vibration and premature wear is even greater due to the brittleness of carbide compared to conventional tool steel. 
     Excess TIR and premature cutting tool wear can increase the scrap rate of potentially expensive parts, especially when an out of tolerance finish cut results. Excess TIR also causes uneven wear between blades or inserts of the cutting tool, necessitating replacement before all have worn out. 
     To maximize the service life of the cutting tool and reduce the scrap rate of parts being machined, it is desirable to reduce the eccentricity and improve the fit of the cutting tool and tool holder interface and thus reduce TIR, vibration, and heating. 
     A second machine tool holding system, for example, as shown in U.S. Pat. No. 4,955,764, includes a tool holder, a cutting tool, a spring collet, and a collet nut. The tool holder generally includes a through hole axially formed through the tool holder, both for receiving the spring collet and the cutting tool on the distal chuck end (tool end), and for supplying coolant from the machine tool spindle to the work piece, either through or around the cutting. 
     The distal end of the through hole in the tool holder includes a tapered recess or chuck that is shaped to receive and compress the spring collet. The spring collet includes a through bore that receives the shank of the cutting tool. Tightening of the collet nut onto the distal end of the tool holder axially drives the spring collet deeper into the tapered recess, compressing the spring collet radially, and thus clamping shank of the cutting tool within the interior bore of the spring collet, axially centering and fixing the cutting tool within the tool holder, thus eliminating any eccentricity and TIR such as that discussed above for the first conventional tool holder system. 
     Although the collet nut and associated tapers of the collet and collet chuck portion of the tool holder do compress and clamp cutting tool centered within the spring collet, the strength with which the spring collet clamps the cutting tool is sometimes insufficient, especially with the higher machining forces, especially torque, transmitted with the materials, feed rates, and RPM that can be withstood by modern cutting tools or inserts, for example, carbide inserts on indexable cutting tools. The cross sectional shapes of the cutting tool and the through hole of the tool holder are round, so a relative rotation (twist) between the tool holder and the cutting tool may still occur during use. For example, the relatively small diameter of the shank of the cutting tool that is held by the interior bore of the collet may, under sufficient operating torque, result in the cutting tool shank rotating within the collet. Such rotation can also cause axial pullout of the cutting tool from the tool holder because of the twisting action. 
     Rotational slippage and axial pullout can cause damage to the cutting tool and/or work piece. To avoid rotational slippage and axial pullout, feed rates and RPM must be limited, which is often impractical. 
     A third machine tool holding system, for example, U.S. Pat. No. 5,311,654 and U.S. Pat. No. 6,339,868, include a tool holder, having a tool mounting or holding portion, and a cutting tool. The tool holder generally includes a through hole axially formed through the tool holder, both for receiving the cutting tool on the distal end (tool end), and for supplying coolant from the machine tool spindle to the work piece, either through or around the cutting. 
     The distal end of the tool holder comprises the tool mounting portion and is generally a cylindrical elongate member, optionally having a tapered exterior surface, through which the through hole forms a cylindrical receiving bore for the matching cylindrical shank of the tool holder. The relative diameter and the tolerances of the inner surface of the receiving bore (ID) and the outside surface of the tool shank (OD) are such that the ID of the receiving bore is smaller than the OD of the tool shank. This relationship provides a thermal interference fit. Thus, to couple the cutting tool to the tool holder, the tool mounting portion of the tool holder is heated, thereby expanding the diameter of the ID of the receiving bore so that the tool shank easily slides into the receiving bore, and as the tool holder cools, the tool shank is firmly clamped within the smaller diameter receiving bore. 
     This tool holding system radially centers and axially and radially fixes the cutting tool within the tool holder, thus substantially reducing or eliminating eccentricity and TIR such as that discussed above for the first conventional tool holder system; however, in order for the strength with which the receiving bore clamps the cutting tool to be sufficient to the cutting tool shank rotating within the tool holder, the interference fit must be strong enough (ID&gt;&gt;OD) that uncoupling the cutting tool from the tool holder is very difficult, if not impossible to do. 
     This is especially true for indexable cutting tools, which typically have a tool shank made of tool steel that is the same or very close to that used for the tool holder. The tool shank and tool holder, being constructed of the same or a very similar material, have very similar thermal properties. 
     More specifically, the desired force with which the tool shank is held in the receiving bore of the tool holder prevents extraction without first expanding the ID of the receiving bore by heating; however, if the tool holder is heated, the tight contact and similar thermal properties of the tool holder and the cutting tool shank also very quickly heats the tool shank, expanding its OD at a rate that prevents extraction of the shank from the bore, even when the ID of the receiving bore expands. Yet the use of such indexable cutting tools is increasing, and the desired interference fit when using modern, high feed rate, high RPM indexable cutting tools must being even stronger than earlier cutting tools, making separation of the cutting tools and tool holder even more difficult and often impossible. Providing an indexable cutting tool with a carbide or other metal shank having a substantially different thermal expansion properties than the tool holder is generally cost prohibitive. 
     A fourth machine tool holding system, for example, U.S. Pat. No. 7,527,459, discloses a cutting tool having a stepped shank that is matingly received with a lower end of a tool holder. The cutting tool stepped shank is separated into three sections, a conical, tapered wedging section, a drive section having a polygonal cross-section, and a cylindrical section having an interior thread. The tool holder has an interior bore that includes three stepped sections that matingly receive the stepped tool shank. The cutting tool shank is drawing into the tool holder bore using a threaded pull stud/draw bar, mating the various sections. The mating of the drive section with its respective mating section prevents rotational slippage. The conical mating sections radial center the cutting tool to the tool holder; however, a distinct problem with conical, tapered mating sections that are not long in length is that eccentricity, or axial misalignment or tilt as this reference calls it, can occur. The eccentricity of the conical sections not perfectly aligning upon mating causes TIR from the axis of the cutting tool being misaligned with the axis of the tool holder. The cylindrical section of the tool shank mating with its respective cylindrical bore is provided to minimize the tilt; however, without an extended constant diameter, elongate cylindrical mating surface between tool holder and tool shank, some axial misalignment may remain. Additionally, pull stud/draw bar type tool holders are not always desirable or even usable in all application, for example, when coolant fluid is delivered through the center of the tool holder and cutting tool. Furthermore, the complex stepped sections and surfaces required on both the cutting tool and the tool holder make the tool holding system very expensive to manufacture. 
     Therefore, an improved and economical machine tool holding assembly that prevents tool rotation in the holder under high torque conditions while providing elimination of eccentricity and allowing easy, reliable coupling and uncoupling of the tool holder and cutting tool is desired. 
     SUMMARY 
     The present invention may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof. 
     The main objective of the invention is to provide a machine tool holder assembly that prevents a relative rotation and substantially reduces or eliminates eccentricity between a cutting tool and a tool holder, while also providing for coupling and decoupling of the cutting tool and tool holder. 
     An illustrative embodiment of a machine tool assembly has a tool holder, a keyed recess, and a cutting tool. The tool holder has a hole axially formed through at least a portion of the length of the tool holder, and a receiving bore axially formed with the hole at a tool mounting portion of the tool holder. The keyed recess is located within the tool holder, for example, at the base of the receiving bore. The keyed recess can be formed integrally with the tool holder, or can be formed in an adaptor screwed or otherwise rotationally secured in the bore. The keyed recess has multiple corners formed around an inner surface of the recess. 
     The cutting tool can have a non-standard diameter shank and multiple edges formed on the machine tool end of the shank. Coupling of the cutting tool and holder includes heating the holder until the diameter of the bore expands sufficiently to receive the tool shank. Decoupling of the cutting tool and holder includes holding the tool holder, heating the holder to expand the diameter of the bore, and pulling the shank from the receiving bore in the tool holder. 
     When inserted within the bore of the holder and into the keyed recess, the multiple edges of the cutting tool abut with the multiple corners of the recess. Because the cross sectional shape of the recess is multilateral and has multiple corners which the multiple edges abut, the combination of the corners and the edges prevent relative rotation between the tool holder and the cutting tool. The fit between the centers the cutting tool within the tool holder and prevents axial movement of the cutting tool relative to the tool holder. 
     The process of decoupling the tool holder and cutting tool can optionally including heating the tool holder to expand the bore diameter and reduce or eliminate the force required to pull the shank from the bore; however, the rapid transfer of heat to the cutting shank counteracts some of the benefit of expanding the bore diameter; thus, selecting a thermal fit, for example, a transitional fit, that eliminates eccentricity and axial pullout while cutting, yet allows the cutting tool shank and tool holder to be consistently decoupled without damage to the tool or holder, is desirable. 
     Additional features of the disclosure will become apparent to those skilled in the art upon consideration of the following detailed description and drawings of the illustrative embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view in partial section of a first embodiment of a machine tool holding assembly in accordance with the present invention; 
         FIG. 1B  is an enlarged cross sectional end view of a keyed end of a cutting tool and keyed recess of a tool holder of the first embodiment of  FIG. 1A ; 
         FIG. 2  is a cross sectional side view of the machine tool assembly in  FIG. 1   
         FIG. 3  is a perspective view in partial section of a coupling step of the first embodiment of a machine tool holding assembly in accordance with the present invention; 
         FIG. 4  is an enlarged cross sectional end view of a keyed end of a cutting tool and a keyed recess of a tool holder of a second embodiment of the machine tool holder in accordance with the present invention; 
         FIG. 5A  is a cross sectional side view of the machine tool assembly in  FIG. 1A ; 
         FIG. 5B  is an end view of the machine tool assembly in  FIG. 1A ; and 
         FIG. 6  is a side perspective view of a first prior art machine tool holding assembly. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1 to 2 , a first illustrative embodiment of a machine tool holder and assembly in accordance with the present invention comprises a tool holder  10  and a cutting tool  50 . 
     The cylindrical tool holder  10  has a main body  12 . The main body  12  has a connecting section  14 , a tool mounting section  16 , a flange section  18 , and a through hole  20 . The outer surface  22  of the connecting section  14  is tapered or semi-conical for engaging the machine spindle (not shown). The tool mounting section  16  is defined at a distal or the cutting tool end. The interior surface of the hole  20  in the tool mounting section  16  defines a keyed recess  30  and a receiving bore  40  for receiving a shank portion  52  of the cutting tool  50 . The connecting section  14  of the main body  12  is defined at an opposite machine tool end from the tool mounting section  16 . The flange section  18  is located between the connecting section  14  and the tool mounting section  16 . 
     The hole  20  is optionally a through hole that defines not only the receiving bore  40 , but also a coolant flow path from the machine spindle (not shown) to and/or past the cutting tool  50 . For example, the cutting tool  50  can included a coolant hole  51 , or the receiving bore  40  can have one or more slots  44  ( FIG. 2 ) cut axial in the inner surface  42  of the bore through which coolant fluid can pass around the tool shank  52 . In the illustrated embodiment, the through hole  20  is axially formed through the connecting section  14 , the flange section  18 , and the tool mounting section  16 . 
     The receiving bore  40  in the illustrative embodiment includes a first segment  46  and a second segment  48 . The first segment  46  has an inner diameter (ID)  47  dimensioned and toleranced sufficiently less than the dimension and tolerance of the outer diameter (OD)  53  of the shank  52  so that a fit ensuring axial alignment of the cutting tool  50  and tool holder  10  is provided. For example, a transitional fit for which thermal expansion of the receiving bore  40  provides for easy insertion and extraction of the tool shank  52 . The second segment  48  is axially formed between the first segment  46  and the keyed recess  30  and has a diameter larger than ID  47  of the first segment  46 , and is provided as a relief for the typical grinding process used to finish the first segment  46  of the bore  40  to the desired diameter and tolerance. 
     Referring to  FIGS. 2 and 3 , advantageously, the fit between the outer cylindrical surface  53  of the shank  52  defined by the cutting tool  50  and the inner cylindrical surface  42  of the receiving bore  40  defined by the tool holder  10  is a transitional fit that is closer to an interference fit than a clearance fit, for example, a fit for which thermal expansion of the receiving bore  40  provides for easy insertion and extraction of the tool shank  52 . More specifically, the OD  54  of the tool shank  52  is dimensioned and toleranced to provide a thermal transitional fit providing little to no interference, and little to no clearance, for example, less interference than a typical industry standard dimensioned and tolerance ID  46  ( FIG. 2 ) of the receiving bore  40 , such as the tool receiving bore of Shrink Fit Tool Holders sold by Techniks, Inc., of Indianapolis, Ind. The desired fit does not provide clearance that allows insertion or extraction with mechanical forces or thermal expansion, including in the worst case of the specified tolerances, thus reducing TIR for the tool holder  10  upon coupling with the cutting tool  50 , but also not so tight so that the cutting tool shank  52  can&#39;t be consistently extracted from the receiving bore  40 . For example, it is desirable to provide a thermal transition fit that allows extraction by a hand (with gloves) upon heating the tool holder  10 , for example, to about 300-800 degrees F., and more typically about 300-400 degrees F., depending on the diameter of the receiving bore  40  and tool shank  52 . But without heating of the tool holder  10 , the shank  52  is firmly held within the receiving bore  40  of the tool holder  10 . For example, for a tool holder  50  formed from 4340, 4140, or H13 tool steel and a tool holder  10  formed from H13 or 8620 tool steel, an OD  54  of the tool shank  52  and an ID  31  of the associated bore  40  in the tool holder  10  sized and tolerance to provide the desired thermal transition fit according to the present invention can be as follows, in inches: 
     
       
         
           
               
               
             
               
                   
                   
               
               
                   
                 Tolerance 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Shank OD 
                   
               
               
                   
                 1.0000 
                 −0.0012/−0.0014 
               
               
                   
                 0.7500 
                 −0.0005/−0.0007 
               
               
                   
                 0.5000 
                 −0.0004/−0.0006 
               
               
                   
                 Bore ID 
               
               
                   
                 1.0000 
                 −0.0012/−0.0015 
               
               
                   
                 0.7500 
                 −0.0008/−0.0010 
               
               
                   
                 0.5000 
                 −0.0006/−0.0008 
               
               
                   
                   
               
            
           
         
       
     
     Thus diameter of the OD  54  of the shank  52  is about equal too, or is slightly larger than the diameter ID  31  of the bore  40 . In an alternative embodiment, the OD  54  of the shank  52  of the cutting tool  50  is a standard dimension and tolerance for cutting tools used in non-thermal interference holding systems, and the dimension and tolerance of ID  42  of the receiving bore  40  is determined to provide the desired thermal fit in accordance with the above embodiment. 
     Referring to  FIG. 2 , the recess  30  can be integral with the tool holder body  12 , formed axially in hole  20  beginning at the base  49  of the second segment  48  of the receiving bore  40 , and thus in communication with the bore  40 . The inner surface  31  of the recess defines multiple corners  32  and/or multiple interleaving flats  33  at intervals around the interior circumference of the recess, and thus has a non-circular cross-section. The multiple corners  32  and flats  33  are axially aligned with the receiving bore  40 . Referring to  FIG. 2 , further features of recess  30  can similarly be defined integrally by the tool holder body  12 , including the multiple corners  32 , associated flats  33 , and the bottom  34 . The recess  30  can be in open communication with the hole  20  formed in the connecting section  16  of the tool holder  10 , or can be closed off. 
     With reference to  FIGS. 2 , the cutting tool  50  has a cutting end  58 , a shank  52 , and a keyed end  56 , defined by a portion of the shank  52  opposite the cutting end. Cutting tools with related features to keyed end  56  sometimes refer to such features as the driving end or tang. When assembled with the tool holder  10 , the cutting end  58  is located outside the tool mounting section  16  and the keyed end  56  and at least a portion of the shank  52 , is located within the tool mounting section  16 , specifically clamped in place by the receiving bore  40 . 
     The keyed end  56  has an outer surface defining multiple edges  55  at intervals around its periphery. The recess  30  is sized and the multiple corners  32  of the recess are formed to receive the keyed end  56  such that the edges  55  abut the corners  32 , thus preventing relative rotation of the cutting tool  50  about the machine tool body  12 . Additionally or alternatively, surfaces  57  between the edges  55  of keyed end  56  and flats  33  between corners  32  of recess  30  are cooperatively adjacently positioned as shown in  FIG. 1B  to prevent relative rotation. Specifically, depending to the relative cross-sections and fit, the engagement of the keyed end  56  into the recess  30  may impede all relative rotation, for example, the cross-sections and dimensions of the keyed end  56  and recess  30  providing a slip fit, or the cross-sections and fit may allow only partial rotation before abutting of the edges  55  and corners  32  and/or associated flats  33  and  57  prevents further rotation. The flats  33  and  57  between edges  55  and corners  32  may be, but are not required to be planar surfaces, so long as the cooperation of features of the recess  30  and keyed end  56  prevent all relative or at least continuing rotation. 
     The keyed end  56  can have a cross-sectional shape the same as that of the recess  30 , so the edges  55  respectively abut the corners  32 . For example, the cross sectional shape of the keyed end  56  can be rectangular and provide four edges  32 . Alternatively, the cross-sectional shape of the recess  30  may be different from that of the keyed end  56 . For example, the cross-sectional shape of the recess  30  can be hexagonal and the cross sectional shape of the keyed end  56  can be triangular. The present invention does not limit the cross sectional shapes of the keyed end  56  and the recess  30  as a number of geometrically differing, but engage cross sections are known in the art that prevent continuing rotation of the cutting tool relative to the recess  30  and thus the machine tool body  12 . 
     As shown in  FIG. 3 , the cutting tool  50  can have a coolant aperture  51  formed through the cutting tool  50  and communicating with the recess  30 . Accordingly, coolant supplied by the machine spindle (not shown) can flow through the hole  20  and the cutting tool aperture  41  to cool the cutting tool  50  and a work piece. Additionally, the base  34  of the recess  34  or the base  49  of the second segment  48  can provide a stop surface to prevent axial translation of the cutting tool toward the connecting section  14  (machine spindle end), thus fixing the cutting tool axially relative to the machine tool holder body  12 . 
     For either embodiment, in order to couple the tool holder  10  and cutting tool  50 , the body  11  of the tool holder must be heated until the ID  46  of the receiving bore  40  surface  42  expands to a diameter at least equal to the OD  54  of the unheated cutting tool shank  52 . The tool holder  10  and cutting tool  50  are then assembled as shown in  FIG. 3  before the diameter  46  of the surface  42  of receiving bore  40  contracts from cooling. 
     Upon cooling, the fit of the receiving bore  40  and tool shank  52  ensure perfect alignment, removing eccentricity and TIR typical of prior machine tool holder assemblies, reducing uneven wear of cutting end  58  (including inserts  59 , if applicable) and thus substantially improving the service life of the cutting tool  50  and reducing scrap parts rates because of the reduced vibration and heating achieved by eliminating the eccentricity from the coupling of the tool holder  10  and cutting tool  50 . Additionally, the interface of the keyed end  56  and recess  30  prevents torsion applied to the cutting tool  50  in operation from rotating the cutting tool  50  relative to the tool holder  10 . And advantageously, the reduction in the tightness of the fit between the bore  40  and tool shank  52 , compared to that of the prior art thermal interference fit systems, achieved by using the engagement of the keyed end  56  into the recess  30  to prevent rotation, allows the cutting tool shank  52  to be removed or more easily removed from the receiving bore of the tool holder  10 . To decouple the tool holder  10  from the cutting tool  50 , the two must be pulled apart, for example, by heating the tool mounting section  16  of the tool holder  10  and pulling the cutting tool  50  axially so that the shank  52  is pulled out from the bore  40 . Alternatively, at least for the first illustrative embodiment, the tool holder body  11  can be held and the tool shank  30  pressed distally from the receiving bore by applying mechanical pressure on the tool shank  52  through the hole  20 . 
     The tool holder  10 , including the shaft  38  can be made from typical tool steel, for example 8620. The cutting tool  50  also can be made from typical tool steel, for example H13. Both the tool holder  10  and the cutting tool  50  also can be heat treated according to processes typical for machine tools. 
     Referring to  FIG. 4 , an alternative second illustrative embodiment of the machine tool holder  100  includes a locking adaptor  21 . The locking adaptor  21  is mounted securely in the through hole  20  of the tool holder  100 . A shoulder  24  is defined between a first end  26  and a second end  28  of the locking adaptor  21 . The shoulder  24  of the adaptor  21  abuts the abutting surface  19  defined by the through hole  20 . 
     The locking adaptor  21  may be similar to a set screw used to limit axial depth of a cutting tool shank  52  in the tool holder body  11 ; however, in this case the recess as described for the first illustrative embodiment is defined in the second end  28  of the adaptor  21  and receives the keyed end  56  of the cutting tool shank  52 . 
     An external thread  23  is formed around the first end  26  of the adaptor  21  and is screwed into a matching threaded portion  25  of the through hole  20 . The external thread  23  must be oriented so that locking adaptor  21  will tighten and not loosen during cutting. Because machine tool spindles typically rotate clockwise, the external thread  23  and associated matching threaded portion  25  are typically right-hand threads (which is opposite of that typically used for set screws). 
     Locking adaptor  21  can also include a coolant hole  27  axially formed through the locking adaptor  21  and communicating with the recess  30 , thus allowing for liquid coolant flow from the machine spindle (not shown) to the cutting tool  50 . 
     While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described and that all changes and modifications that come within the spirit and scope of the invention as defined in the claims and summary are desired to be protected.