Abstract:
The invention includes a double helix thread cutting tap with both right-handed and left-handed helical chip removal flutes. The double helix thread cutting tap places a positive rake angle on all sets of thread cutting teeth for optimum performance and minimal cutting pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Pursuant to 35 U.S.C. §119(e), this application claims priority to U.S. provisional patent application No. 60/846,674, filed Sep. 22, 2006, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to a device for tapping threads into various substrates with a wide range of hardness and machinability with a decrease in cutting pressure and an increase in surface finish and tap life. 
     BACKGROUND OF THE INVENTION 
     Existing thread cutting taps are generally designed and manufactured to perform within a relatively narrow range of hardness and machinability. This requires the user to maintain an inventory of several taps of the same size to accommodate different substrate materials in order to maximize the performance of the taps and the quality of the female threaded holes. These taps, which include helical flute taps and straight flute taps, often produce poor surface finishes and high cutting pressure. Right or left-handed helical flute taps have one sharp rake angle and one dull rake angle on the thread teeth cutting edges. In both cases, the side opposite the positive rake angle (sharp cutting edge) must be a negative rake angle (dull cutting edge). Negative rake angles increase cutting pressure and drag on the tap leading to tap breakage. Straight flute taps have only 0° rake angles on both upper and lower cutting edges of the thread teeth which also increase cutting pressure that leads to tap breakage. 
     For the foregoing reasons, there is a need for a device for tapping threads that overcomes the significant shortcomings of the known prior art as delineated hereinabove. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, an embodiment of the invention provides a double helix thread cutting tap with both right-handed and left-handed helical chip removal flutes. In one embodiment, the double helix thread cutting tap includes at least two opposing left-handed flutes and two opposing right-handed flutes. The double helix thread cutting tap places a positive rake angle on all sets of thread cutting teeth for optimum performance and minimal cutting pressure. This translates into longer tap life and better surface finishes as well as the ability to cut all substrates with one tap. 
     The thread cutting tap described herein is capable of cutting both hard and soft metals as well as those that have varying degrees of machinability. This is extremely valuable to the user because of the many different metals, plastics, bones, wood and other substrates that may require female threads to be cut in order to accept a threaded fastener of some type. 
     In one aspect, the double helix thread cutting tap overcomes the negative aspects of the existing taps, such as high cutting pressure, poor surface finishes, and the need to have a special tap for each type of material to cut threads into. In another aspect, the double helix thread cutting tap effectively reduces cutting pressure by providing sharp cutting edges on alternating upper and lower cutting surfaces of the opposing threaded margins. In yet another aspect, the double helix thread cutting tap makes it possible to cut both hard and soft materials by having low hook angled cutting teeth faces separated by high hook angled cutting teeth faces. In a further aspect, and in addition to benefit of double helix thread cutting tap&#39;s improved cutting ability is that it also provides a better surface finish on both upper and lower thread surfaces of the finished female threaded hole. The improved surface finish provides a better contact surface for the fastener which translates into higher clamping forces and resistance to loosening and backing out. 
     Accordingly, it should be apparent that numerous modifications and adaptations may be resorted to without departing from the scope and fair meaning of the claims as set forth hereinbelow following the detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an isometric view of one example of an embodiment of a thread cutting tap with two sets of right-handed, opposing helical chip removal flutes. 
         FIG. 2  shows a side view of the tap of  FIG. 1  with right-handed helical flutes. 
         FIG. 3  shows a side view rotated 90 degrees from  FIG. 2  with a left-handed helical flute ground into the leading end of the tap intersecting the right-handed helical flute. 
         FIG. 4  shows an end view of the tap of  FIG. 3  and opposing left- and right-handed helical chip removal flutes with details of the low hook angles and high hook angles of the thread cutting teeth faces. 
         FIG. 5  shows a close-up of a left-handed helical flute with emphasis on the positive rake angle of the upper cutting surfaces of the cutting teeth. 
         FIG. 6  shows a close-up of a right-handed helical flute with emphasis on the positive rake angle of the lower cutting surfaces of the cutting teeth. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The description and figures describe an embodiment of the tap having right-handed cutting surfaces that only cut when rotated in a clockwise direction. In another embodiment, the tap may have left-handed cutting surfaces that only cut when rotated in a counterclockwise direction. 
     Considering the drawings, wherein like reference numerals denote like parts throughout the various drawing figures, reference numeral  10  is directed to a thread cutting tap. 
     As shown in  FIGS. 1 ,  2 , and  3 , one embodiment of the thread cutting tap  10  is defined by upper driving end  11 , intermediate shank  14 , threaded portion  12  and bottom end  19 . Threaded portion  12 , having cutting teeth  42 , includes a plurality of right-handed, helical chip removal flutes  15  and  16  dispersed diametrically opposite each other around the circumference of threaded end  12 . Cutting edges  13  are form relieved to provide sharpened cutting edges. 
       FIG. 2  shows right-handed helical flutes  15  and  16  with a starting point at the bottom end  19  and traversing through the threads dissipating into intermediate shank  14  forming cutting edges and chip removal flutes. Flutes  15  are uniform in shape. A front elevation view of partial left-handed helical flute  17  in flute  16  is shown at end  19  of right-handed flutes  16  only. Radial form relief  18  is ground to create land  20  ( FIG. 3 ) located adjacent to end  19  between flutes  15  and  16  to provide cutting edge relief so the tap can begin cutting as it engages the hole to be tapped. 
     Partial left-handed helical flute  17  (see  FIG. 3 ) is shown in an overhead view at end  19  of right-handed helical flutes  16 . Right-handed helical flutes  15  do not contain partial left-handed helical flute  17 . 
     Right-handed helical flutes  15  (see  FIG. 4 ) are interspersed between right-handed helical flutes  16 , such that flutes  15  alternate with flutes  16  around the circumference of threaded portion  12 . Only right-handed flutes  16  have additional partial left-handed helical flutes  17  shaped to create cutting edges  25 . Right-handed helical flutes  15  create cutting edges  26 . Cutting edges  26  of flutes  15  are formed to create a high hook angle (about +7 to about +15°). Partial left-handed helical flute  17  is ground into right-handed helical flute  16  at end  19  to create low hook angles (about 0 to about +5°). 
     The high hook angle of cutting edge  26  is formed to cut soft materials such as aluminum and soft steel. The low hook angle of cutting edge  25  is formed to cut tougher materials such as cast iron, ductile iron and stainless steel. 
     Tap  10  includes an equal number of right-handed helical flutes  15  and right-handed helical flutes  16  containing partial left-handed flutes  17 . Tap  10  may be made with any number of flutes  15  and flutes  16 , as long as there are equal numbers of flutes  15  and flutes  16  and they alternate around threaded portion  12 . 
       FIG. 5  is for reference only to show a simple view of a left-handed helical chip removal flute  37  cut into threading tap  38 , creating upper cutting edges  30  and lower cutting edges  31 . Because of the way that helical chip removal flute  37  bisects through the thread teeth at an angle to the longitudinal axis of the tap, upper thread tooth flanks  27  and cutting faces  28  converge, producing upper thread teeth cutting edges  30  which form rake angles less than 90 degrees or sharp cutting edges. This causes the lower thread tooth flanks  29  to converge with cutting faces  28 , producing lower thread teeth cutting edges  31  with rake angles greater than 90 degrees or dull cutting edges. 
       FIG. 6  is for reference only to show a simple view of a right-handed helical chip removal flute  39  cut into threading tap  40 , creating upper cutting edges  35  and lower cutting edges  36 . Because of the way that right-handed helical chip removal flute  39  bisects through the thread teeth at an angle to the longitudinal axis of the tap, upper thread tooth flanks  32  and cutting faces  34  converge, producing upper thread teeth cutting edges  35  which form rake angles greater than 90 degrees or dull cutting edges. This causes the lower thread tooth flanks  33  to converge with cutting faces  34 , producing lower thread teeth cutting edges  36  with rake angles less than 90 degrees or sharp cutting edges. 
     As described above, tap  10  has a high hook angle on at least two opposing thread teeth faces of the right-handed helical flutes and a low hook angle on at least two opposing thread teeth faces of the partial left-handed helical flutes. With radial relief on all leading edges nearest end  19 , tap  10  will cut easier and last longer than existing taps and can be used in most soft substrates, such as bone, plastic, aluminum, brass, and bronze, to harder and tougher substrates, such as stainless steel, cast iron, ductile iron, titanium, and hardened steels. 
     Thread cutting tap  10  can be used with any thread profile, including, but not limited to, American standard threads, metric threads, Buttress threads, hook threads, dovetail threads, and self locking threads. 
     Thread cutting tap  10  is used by holding it by shank  14 , placing threaded portion  12  into a drilled hole of a preferred diameter and rotating it until the cutting edges dig in and cut the thread form into the side walls of the drilled hole. The rotation continues until tap  10  has succeeded in cutting the preferred thread profile to a desired depth. The rotation is then reversed and tap  10  is backed out of the drilled hole revealing the female helical threads within the drilled hole that can receive a male threaded fastener of a preferred embodiment. 
     The invention has been described above with the reference to the preferred embodiments. Those skilled in the art may envision other embodiments and variations of the invention that fall within the scope of the claims.