Patent Publication Number: US-2007122249-A1

Title: Wood screws capable of cutting wood

Description:
This application claims the priority benefit of Taiwan Patent Application Serial Number 094142072 filed Nov. 30, 2005, the full disclosure of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1Field of the Invention  
      The present invention relates to a screw, and more particularly, to a wood screw.  
      2. Description of the Related Art  
      Referring to  FIG. 1 , it illustrates a conventional wood screw  100 . The wood screw  100  has a head  110  for receiving a fastening tool, a shank  120  extending from the head  110  and including a parallel shank portion  124  and a tapered portion  122  formed at the tip of the parallel shank portion  124 , and a helical thread  130  formed on the tapered portion  122  and the parallel shank portion  124 .  
      The wood screw  100  can be screwed directly into an article, such as wood without drilling a hole prior to driving. However, the wood screw  100  has the drawbacks that the driving torque required for fastening becomes greater and greater while driving the screw  100  into the wood because the friction caused by that the parallel shank portion  124  and thread  130  are in contact with the wood is getting great. Therefore, the wood is apt to be cracked while the screw  100  is driven into the wood.  
      In order to solve these problems, the Taiwan Patent Publication Number M259860, entitled “A WOOD SCREW CAPABLE OF CUTTING WOOD”, discloses a wood screw  200  capable of cutting wood (see  FIG. 2 ). The wood screw  200  has a head  210  for receiving a fastening tool, a shank  220  extending from the head  210  and including a parallel shank portion  224  and a tapered portion  222  formed at the tip of the parallel shank portion  224 , an engagement thread  230  formed on the parallel shank portion  224 , and a cutting thread  240  formed on the tapered portion  222  and connecting with the engagement thread  230 . The engagement thread  230  has a helix angle greater than that of the cutting thread  240 . While screwing the wood screw  200  into a wood, the cutting thread  240  continues to cut the wood fiber. The driving torque required for the wood screw  200  to be drilled into a wood will therefore be reduced in comparison with the screw  100  having no cutting thread. The cracking of the wood can also be avoided while the wood screw  200  is to be drilled. However, the cutting thread  240  having a helical thread to cut the wood fiber is difficult to be formed on the wood screw  200 . This is because that the helix angle of the engagement thread  230  is required to be different from that of the cutting thread  240  and therefore the thread rolling dies for making the wood screw  200  are difficult to align with each other while the engagement thread  230  and cutting thread  240  are simultaneously formed on the wood screw  200  through the thread rolling dies.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a wood screw that its parallel thread formed on the tapered portions can first drill a wide hole and then cut the wood fiber; therefore make ease of the drilling and avoid cracking the wood.  
      In one embodiment, the wood screw according to the present invention includes a head and a shank. The top of the head has a recess for receiving a fastening tool. The shank extends from the head. The shank has a parallel shank portion aligning with the screw axis, a tapered portion formed at the tip of the parallel shank portion, and a pointed end formed at the tip of the tapered portion. The parallel shank portion has an engagement thread formed thereon for engaging with the wood. The wood screw has at least one parallel thread formed on the tapered portion which aligns with the screw axis.  
      In another embodiment, the wood screw according to the present invention further includes at least one guiding thread. The guiding thread is formed on the tapered portion and between the parallel thread and engagement thread. The guiding thread is capable of cutting and expanding the wood fiber during screwing and making ease of engaging the engagement thread with the wood.  
      In a further embodiment, the wood screw according to the present invention includes at least one cutting thread. The cutting thread is formed on the tapered portion and has a parallel thread extending from the pointed end and a guiding thread extending from the tip of the parallel thread and reaching the tip of the parallel shank portion and not connecting to the engagement thread.  
      The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a conventional wood screw.  
       FIG. 2  illustrates another conventional wood screw with a cutting thread.  
       FIG. 3  illustrates a wood screw according to the first embodiment of the present invention.  
       FIG. 4  illustrates a wood screw according to the second embodiment of the present invention.  
       FIG. 5  illustrates a wood screw according to the third embodiment of the present invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 3 , it illustrates a wood screw  300  according to the first embodiment of the present invention. The wood screw  300  has a head  310  and a shank  320 . The top  314  of the head  310  has a recess  312  for receiving a fastening tool. The shank  320  extends from the head  310  and defines an axis  390 . The shank  320  has a parallel shank portion  324 , a tapered portion  322  formed at the tip of the parallel shank portion  324 , and a pointed end  326  formed at the tip of the tapered portion  322 . The parallel shank portion  324  has an engagement thread  330  formed thereon. The engagement thread  330  can engage with the wood and provide a resistance to pull-out. The wood screw  300  has at least one parallel thread  340  formed on the tapered portion  322  and, preferably, parallel to the axis  390 . The parallel thread  340  has substantially the shape of a spindle, that is, the parallel thread  340  has the largest thickness in the middle portion and tapers toward each end. The parallel thread  340  may be symmetrical or asymmetrical along its length. Preferably, the wood screw  300  according to the present embodiment has at least two parallel threads  340  symmetrically to each other formed on the tapered portion  322 .  
      The parallel thread  340  of the wood screw  300  according to the present invention can cut the wood fiber while the wood screw  300  is screwed into a wood. The driving torque required for the wood screw  300  to be drilled into a wood will therefore be reduced in comparison with the conventional screw  100 . The cracking of the wood can also be avoided while the wood screw  300  is to be drilled.  
      Referring to  FIG. 4 , it illustrates a wood screw  400  according to the second embodiment of the present invention. The wood screw  400  has a configuration similar to that of the wood screw  300 , where identical reference numerals have been used when designating substantially identical elements that are common to the figures. The wood screw  400  further includes at least one guiding thread  450  formed on the tapered portion  322  and between but not connecting with the parallel thread  340  and the engagement thread  330 , the guiding thread  450  extends for at most one turn. The guiding thread  450  has a helix angle α greater than the helix angle β of the parallel thread  340  and smaller than the helix angle θ of the engagement thread  330 . The helix angle is defined as the angle of the thread helix with respect to the axis  390 . The largest crest diameter D 2  of the guiding thread  450  is greater than the root diameter d of the engagement thread  330  and smaller than the crest diameter D 1  of the engagement thread  330 . Preferably, the wood screw  400  according to the present embodiment has at least two parallel threads  340  symmetrically to each other formed on the tapered portion  322  and at least two guiding threads  450  symmetrically to each other formed on the tapered portion  322 .  
      The guiding thread  450  of the wood screw  400  according to the present invention can cut and expand the wood fiber while the wood screw  400  is screwed into a wood and then guide the engagement thread  330  to engaging with the wood. The reason is that the guiding thread  450  has a smaller helix angle and therefore can cut and expand wood fiber easier. The engagement of the guiding thread  450  with the wood forces the wood screw  400  first to be driven into the wood and then a less driving torque is required to engage the engagement dread  330  with the wood. Such design can also keep the wood from splitting. Since wood fibers are continuously cut and expanded by the guiding thread  450 , and then the engagement thread  330  starts to engage with the wood.  
      Referring to  FIG. 5 , it illustrates a wood screw  500  according to the third embodiment of the present invention. The wood screw  500  has a configuration similar to that of the wood screw  400 , where identical reference numerals have been used when designating substantially identical elements that are common to the figures. The wood screw  500  has at least one cutting thread  550  formed on the tapered portion  322  that the cutting thread  550  is similar to the combination of the parallel thread  340  and the guiding thread  450  of the wood screw  400 . The cutting thread  550  includes a parallel thread  552  extending from the pointed end  326  and a guiding thread  554  extending from the tip of the parallel thread  552  and reaching the tip of the parallel shank portion  324 , but not connecting with the engagement thread  330 . The guiding thread  554  has a helix angle α′ greater than the helix angle β′ of the parallel thread  552  and smaller than the helix angle θ′ of the engagement thread  330 . The largest crest diameter D 2 ′ of the guiding thread  554  is greater than the root diameter d of the engagement thread  330  and smaller than the crest diameter D 1  of the engagement thread  330 . The parallel thread  552  is preferably parallel to the axis  390 . The guiding thread  554  preferably extends for at most one turn. The cutting thread  550  may be symmetrical or asymmetrical along its thread helix. Preferably, the wood screw  500  according to the present embodiment has at least two cutting threads  550  symmetrically to each other formed on the tapered portion  322 .  
      The parallel thread  552  and the guiding thread  554  of the wood screw  500  have the effects identical to the effects shown by the parallel thread  340  and the guiding thread  450  of the wood screw  400 . Thus, any further illustrations of the cutting thread  550  are omitted herein. The differences between the wood screw of the present invention and the wood screw disclosed in the Taiwan Patent Publication Number M259860 are the facts that the cutting thread of the wood screw according to the present invention has variable thread leads and does not connect with the engagement thread. The cutting thread having a parallel thread according to the present invention can more efficiently cut the wood during drilling than the cutting thread having a helical thread in the prior wood screw. Additionally, when the wood screws according to the present invention are made by two pairs of thread rolling dies, there is no need to carefully align the two pairs of thread rolling dies with each other since the guiding thread is not required to connect with the engagement thread.  
      Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.