Patent Publication Number: US-2012034048-A1

Title: Self-tapping thread forming screw and corresponding thread roll die

Description:
FIELD OF THE INVENTION 
     This invention relates to a thread forming screw thread fastener for forming an internal thread in a work piece material such as synthetic plastics, resins and the like and corresponding thread roll die to manufacture the thread of the screw. 
     BACKGROUND OF THE INVENTION 
     Conventional screws often damage the material of the work piece, in particular when the material of the work piece is a synthetic plastic, resin and the like. Relatively high driving torques are typically required to fasten the screw in the work piece, which contribute to the likelihood of damaging the material of the work piece when the screw is fastened to the work piece. The screw may also strip-off, burst, become stuck in the material, and/or stress cracks may form in the work piece material. This damage to the work piece material and screw weakens the integrity of the joints formed between the work piece material and the screw. 
     Attempts have been made to improve screw thread design. However as mentioned, conventional screw threads do not adequately meet critical functional requirements, and the strength and integrity of the joints has been compromised. For example, some previous threads that have been disclosed include, U.S. Pat. No. 4,527,932, U.S. Pat. No. 5,544,993, U.S. Pat. No. 6,672,813, and U.S. Pat. No. 5,061,135. These previous screw thread designs have been employed in a few applications on work piece materials as attempts to provide sure insertion of the screw into the work piece. However, optimum results have not been achieved with conventional screw thread designs in areas such as the positive engagement, failure torque, torsional strength and the like. In particular, the assembly efficiency and flexibility of the screw into the work piece of conventional screw thread designs are not easily applied in product miniaturization applications where the design solution is hampered by weak integrity of joints of the screw thread formed with the work piece material. 
     Therefore, a thread forming screw thread and corresponding thread roll that alleviates the problems and drawbacks associated with the prior art is needed. 
     SUMMARY 
     An aspect of the invention is a self-tapping thread forming screw thread fastener for fastening into a hole formed by the fastener in a work piece, the fastener comprising a head having a drive to be engaged with a driver bit; and a shank with threads having a symmetrical thread profile with a minor diameter and a major diameter, the major diameter defining a thread crest of the screw thread, and the minor diameter defining the thread root of the screw thread, the profile of the screw thread defined by the surface of the screw thread between adjacent thread crests forming a thread flank, the thread flank having a first central portion of constant radius along the thread root, the radius common at the thread root, at each adjacent thread crests, and at two transition intersection points, the profile of the thread flank having a second and third portion defined by surfaces having different radii of curvature between each transition intersection point and each adjacent thread crest. 
     In embodiments the different radii of curvature of the second and the third portion varies relative to the constant radius of the central portion of the screw thread profile along the thread root. The different radii of curvature of the second and the third portion may be less than the constant radius of the central portion of the screw thread profile along the thread root. The surface of the screw thread profile of the second and third portions may be straight edges formed between a crest and an adjacent transition intersection point. The surface of the threads, thread crest, thread root and thread flanks are arranged to deform the ductile material without cracking or damaging the surrounding material of the work piece. The threads are arranged to deform the ductile material of the work piece against the surface of the thread flank. The second and third portions of the thread flank are arranged to deform the ductile material of the work piece against the surface of the central portion of constant radius of the thread flank. The radius of the central portion of the thread flank having constant radius defines a construction circle which determines the pitch of the thread. The pitch of the thread is determined by the tangents between adjacent construction circles sharing a thread crest. The thread pitch of the fastener urges ductile material displacement towards the thread root. 
     An aspect of the invention is a thread roll die for manufacturing a thread forming screw thread fastener for fastening into a hole formed by the fastener in a work piece, the fastener comprising: a head having a drive to be engaged with a driver bit; and a shank with threads having a symmetrical thread profile with a minor diameter and a major diameter, the major diameter defining a thread crest of the screw thread, and the minor diameter defining the thread root of the screw thread, the profile of the screw thread defined by the surface of the screw thread between adjacent thread crests forming a thread flank, the thread flank having a first central portion of constant radius along the thread root, the radius common at the thread root, at each adjacent thread crests, and at two transition intersection points, the profile of the thread flank having a second and third portion defined by surfaces having different radii of curvature between each transition intersection point and each adjacent thread crest. In an embodiment the thread roll die comprises a die face having the same profile as the thread forming screw. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that embodiments of the invention may be fully and more clearly understood by way of non-limitative example from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions, and in which: 
         FIG. 1A-D  shows the side perspective view of the thread forming screw ( FIG. 1A ), the cross-section of the screw taken along line A-A in  FIG. 1A  ( FIG. 1B ), enlarge view of the thread profile ( FIG. 1C ), and basic construction and geometry of the thread forming screw ( FIG. 1D ) in accordance with an embodiment of the invention; 
         FIG. 2A-B  shows the side view and the cross sectional view of the screw where the thread forming screw is rolled from a cylindrical header blank ( FIG. 2A ), and the cross section of the screw taken along line A-A in  FIG. 2A  ( FIG. 2B ) in accordance with an embodiment of the invention; 
         FIG. 3A-B  shows the side view of the roll thread die to formed the thread ( FIG. 3A ) and the face of the roll thread die ( FIG. 3B ) in accordance with an embodiment of the invention; 
         FIG. 4  shows the cross sectional view of an assembled thread forming screw fastener with engagement and material flow of the material of the work piece that the fastener is attached in accordance with an embodiment of the invention; and 
         FIG. 5A-B  shows a cross sectional view ( FIG. 5A ) and a graph ( FIG. 5B ) of a thread forming screw fastener showing the torque and insertion depth at three points during insertion into a work piece in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A thread forming screw thread fastener and a corresponding thread roll die for manufacture of the screw thread of the fastener are disclosed. The thread forming screw thread fastener is for fastening into a hole made by the fastener of a work piece composed of a low ductility material such as synthetic plastic, resin, glass filled nylon, polypropylene, polyphenelyne oxide, polystyrene, polyethylene, acrylonitrite butadiene styrene (ABS), polycarbonate and the like. More specifically, the thread forming screw fastener is a self-tapping thread which is able to tap an internal thread in an unthreaded work piece which may be made by, for example, die casting, molding, drilled hole and the like. The thread forming self-tapping screw has a symmetrical thread profile with a sharp transition crest and circular thread root which forms an internal thread in the work piece with low installation torque. A low installation torque is defined as the amount of torque applied to the thread forming screw configured in accordance with an embodiment of the invention during installation or engagement between the work piece and the screw, whereby the resulting in less stress cracks in the work piece, avoiding thread strip, while a higher clamp load is retained in the joint or the like. A lower installation torque may result in a lower friction that is transmitted on the work piece, whereby an effective cutting action is promoted by the specific engineered geometric design of the thread flank, while promoting efficient deformation of the work piece material during installation and having a high failure torque to enhance high clamping loads and improve joint integrity with mating parts of the plastic material of the work piece. Given an example of an embodiment of the invention, if a failure torque of the thread forming screw is 1.0 Nm, the targeted installation torque is expected to be below 0.25 Nm, which is may lead to an improvement of 10-15% over some attempts made with previous threads in conventional fasteners. Of course, the value of the failure torque of the thread forming screw and the targeted installation torque may depend on several factors including for example the type of material, length of engagement, boss diameter, size of screw, type of finishing of the thread forming screw and the like. 
       FIG. 5A-B  shows a cross sectional view ( FIG. 5A ) and a graph ( FIG. 5B ) of a thread forming screw fastener showing the torque and insertion depth at three points during insertion into a work piece in accordance with an embodiment of the invention.  FIG. 5A  shows the torque and insertion depth in a work piece at Point  1  which is a thread forming torque, when the screw starts to be assembled. Point  2  is where the screw is further inserted and the torque increases as seen in  FIG. 5B  until it touches the work piece base. At this point the screw has passed through the thread friction. The total of thread forming torque plus the torque to overcome the thread friction is called installation torque (Ti). At this point it is concluded that there is no clamp load (clamping force) that is reacting in the joint. When additional torque is applied to the screw further, the tension in the joint increases and the clamp force in the joint also increase until it reaches to a point whereby the material and or screw will fail, as at Point  3 . The clamp load may be increased gradually in relation to applied torque and insertion depth. 
     An aim in an assembly joint in accordance with an embodiment of this invention is to have a lower installation torque (Ti), and a high failure torque. A lower installation torque means a lower friction between the thread and work piece and which cause by efficient thread forming. Referring to  FIG. 5A-B , the torque between point  2  to point  3  provides a clamping force in the joint, if the gap between point  2  and  3  is narrow, it means either the installation torque is high and or the failure torque in the joint is low and thus provides a lower clamping force or load. This may occur because there is an insufficient load carrying capability and which may result in loosening when subjected to shock or vibration in real time applications, which contributes to failure in assembly joints. 
     In an embodiment, the thread forming screw consists from a size range from 0.8 mm through 3 mm in size for electronic devices application and capable to be extended to a larger range based on application needs. The material of the thread forming screw is generally produce with steel, stainless steel and other materials based on application needs. 
     Embodiments of the invention relate to a self-tapping screw having a symmetrical thread profile with a sharp transition between the flanks and circular thread root. The screw shank is surrounded by a helically circumferential cutting-edge which projects radially from the root of the thread. The thread flank is constructed with a sharp crest, for example with 30 degree angle, which is designed to deform and or cut the base material of the work piece. The work piece may not require to be tapped and the thread root is in circular shape which connected radially from one flank to the other to promote efficient flow of the material of the work piece and encourage deformation of the work piece material towards the root of the thread. The flanks themselves are provided with a constant pitch from the root of the thread so that there is a constant thread angle between the two flank lines. It will be appreciate that the thread is feasible to be constructed with single helix and double helix with the same pitch and geometry. 
     Embodiments provide a universal screw which, with a relatively low installation torque, assures a highly stressable connection between the screw and material of the work piece such as plastic, even with different consistencies of the plastic. A highly stressable connection is regarded when a common self-tapping screw having a thread flank with 60 degree angle increase radial force and hoop stress which may result in work piece cracks and burst. In an embodiment of the invention, this is accomplished by having a sharp thread crest. The sharp thread crest deforms the material to form a thread and efficiently promote the flows of material towards the root with a circular core. This reduces the radial stress between the screw and plastic, which prevents premature failure such as, boss burst, cracks and stripping. The improved fastener comprises having a sharp crest with a 30-degree isosceles triangle as flank, which is half of total flank height which intersect with the basic fundamental circle that form the thread root. 
     Due to smooth transition from sharp thread crest of the flank and continuing towards circular root of thread, the stress and friction during installation is reduced and enhances smooth material flow of the material of the work piece that the fastener is fixed. In accordance with an aspect of the invention there is provided a corresponding thread roll die for manufacturing the said thread. 
     With reference to  FIG. 1A-1D , a thread forming screw  10  in accordance with an embodiment of the invention as shown in  FIG. 1A-1D . In  FIG. 1A , a thread forming screw is shown having a symmetrical thread profile around the central axis  11  with a constant radius transition from intersection  46  and to the root of the thread  18 . In  FIG. 1A  the profile of the screw thread is shown, where the profile is the cross section taken along the longitudinal central axis  11  of the screw. The flanks  20  of the thread are developed from the intersections with two circles points  46  and meet the common point  50  that makes, for example in one embodiment, an isosceles triangle of 30 degrees  48  as shown in  FIG. 1D . The flanks  20 , threads root radius  18  and pitch  32  are constant throughout the threads. However the lead thread  30  and the thread end  28  can be manufactured with various shapes based on application needs. The fastener has a head  12  having a drive to be engaged with a driver bit. 
     Embodiments have a thread forming screw thread which has a capability of cutting and deforming a low ductility material such as synthetic plastic, resin, glass filled nylon, polypropylene, polyphenelyne oxide, polystyrene, polyethylene, acrylonitrite butadiene styrene (ABS), polycarbonate and the like. The sharp thread flank  20  with 30-degree angle  48  promotes cutting action, which will split through the plastic material and accommodate it to flow easily in the circular valley of the threads and flow out over to the root  18 . This minimizes radial stress in the pilot hole and or boss and eliminates stress cracks. The cross sectional configuration of the thread major diameter  26  and minor diameter  24  is cylindrical in shape. 
     In another aspect of the invention, a thread roll die  52  is shown for manufacturing the thread forming screw, the thread roll die  52  is comprises the mirror image profile of the thread forming screw with thread depth  56 , thread root radius  58 , die face  54 , thread pitch  60  and thread crest  62 . Embodiments of a screw thread, namely thread forming screw for plastic in accordance with the invention are disclosed. In addition, corresponding roll thread die as shown in  FIG. 3A-B  to manufacture the thread are disclosed. 
     The thread forming screw is to have a high torsional strength, efficient thread forming capabilities, which eliminates tapping process, lower thread forming torque, high failure and or stripping torque, improve assembly efficiency, reduce hoop stress and reusable with multiple assemblies. The physical characteristic of the present invention also attributes to a reliable joint, which provides comfortable margin between the assembly and failure torques. This wide torque range assures that all screws are fully tightened eliminating the possibility of stripped assemblies. The thread forming screw is engineered primarily for engineering plastics application, miniaturizations compatibility with smaller diameter and length as well feasibility to manufacture with lower cost. The material of the thread forming screw is generally produced with steel, stainless steel, and other materials which may be chosen based on application needs. 
     In the embodiment the thread forming screw  10  in  FIG. 1A-D  and  FIG. 2A-B , having a head  12 , recess  14 , flat in head shape and can be extended to various shapes such as circular, countersunk, oval and the like. The thread forming screw  10  can be formed from any suitable material, such as steel, stainless steel and the like, having appropriate requisite properties and is rolled form a cylindrical cold formed blank  40  with variable diameter and length based on requirements. The thread is extended from the bearing surface  44  of the head  12  and towards the thread end  28 , and is the total length of the screw. The thread length of the thread forming screw may be in the range of, for example, 2 mm and above based on sizes and application needs. The top region, from bearing surface  44  to the first complete thread  36  is the thread run-out  16 . The bottom region, thread end  28  is the lead thread, the thread end  28  is not limited to as flat surface as shown but also able to manufactured with variable shapes such as tapered, round, sharp, dog point and others as per intended application. The central is the full formed threads, with sharp flank  20 , circular thread root  18 , major diameter  26 , minor diameter  24 , thread pitch  32  and thread height  22 . The cross section configuration of the thread forming screw is cylindrical, this allow the mating part with an engagement by a full circumferences engagement enable to achieve strong contact in the mating members, which prevents vibration loosening, high pull out force, high failure torque, thread stripping and ability in multiple usage. 
     The thread forming screw thread according to the invention has a sharp flank  20  which formed isosceles triangle of 30 degree angle  48 , connected to intersection point  46  of basic construction circle and the concavity from intersection point  46  is continuous since it is formed by the basic root radius  18  having constant radii and whose centers are disposed in the same planes. The sharp thread flank  20  is able to cut the plastic material and the like  64  efficiently without any cracks, with low torque transmission during installation and maximize material engagements in the mating parts and this extended with a circular thread root radius  18  which allows smooth displacement flow of material  66  towards the root without any interruption. In this manner, the fastener has a shank with threads having a symmetrical thread profile with a minor diameter and a major diameter. The major diameter of the thread defines a thread crest of the screw thread, and the minor diameter of the thread define the thread root of the screw thread. The profile of the screw thread is defined by the surface of the screw thread between adjacent thread crests forming a thread flank. The thread flank has a first central portion of constant radius along the thread root. The constant radius is defined as the resulting circle shown in  FIG. 1D  formed by the surface of the profile of the central region portion of the thread flank. The radius of the same circle formed by the central region portion of the thread flank is also common at the two corresponding thread crests. The two corresponding thread crests formed adjacent to and on either side of the central constant radii surface of the central portion of the flank share the same radius. Two transition intersection points  46  are provided that defines a second and third portion, respectively, of the surface of the thread flank that provides a thread profile of the thread flank forming a sharp portion of the flank having a second and third portion defined by surfaces having different radii of curvature between each transition intersection point  46  and each adjacent thread crest  50 . The concavity from one transition intersections points to the other transition intersection point through root  18  is continuous since it is formed by arcs of the basic construction circles  46  having constant radii and whose centers  76  are disposed in same planes. The thread crest  20  is also shares the same radii and disposed on the same plane as the central flank portion of constant radius. The smooth radius crest  20 , flank  44  is able to deform the ductile material of the work piece, boss, plate or the like efficiently without any or minimizing cracks, minimal debris, low friction during installation and a maximum engagement of flank  44  in the mating parts. The wide spaced thread pitch  32  determined by the design of the screw of constant radius thread flank, in comparison with conventional screw threads that do not have configuration of thread flank of constant radius, allows maximum material displacement towards the thread root  18  without any interruption or damage to the internal structure of the ductile material of the work piece, boss, plate or the like. 
     Embodiments of the invention relate to a thread forming screw  10  as shown in  FIG. 1A-D . In  FIG. 1A , a thread forming screw is shown having a symmetrical thread profile around a central axis  11  with a constant radius transition in central flank portion between the transition intersection points  46  and including the root point  18  of the thread. The central flank portion of the thread are developed with a constant construction circle  46 . The pitch of the thread  32  may be determined by the tangent  78  between each of the adjacent construction circles proximate, on either side, or sharing a common or same thread crest  20 . The construction circles  46  are shown in  FIG. 1D . The root radius  18  is constant and continuous since the same arc of the basic circles  46  forms the root radius. The region between each transition intersection point and corresponding crest point, the radius is different than that of the construction circle. However, it will be appreciated that the lead thread  30  and the thread end  28  can be manufactured with tapered end, rounded, dog point, and/or sharp point based and the like on application needs. 
     An embodiment of the invention is an improved thread forming screw, which has a capability of deforming low ductile material such as magnesium, soft aluminum, thermoset plastics and other low ductility materials. The smooth radius flank  44  from the crest  20  steadily increases towards the roots, which accommodate the material to deform easily in the valleys of the threads and flow out over to the root  18 . Crack formation, boss burst and chipping are avoided and thus providing a positive and optimum flank engagement  56  in the joint. The cross sectional configuration of the thread major diameter  26  and minor or root diameter  24  is cylindrical in shape. 
       FIG. 3A-B  show the thread forming screw roll thread die  52  is disclose for manufacturing the thread, the thread roll die  52  is comprises of the same profile of the thread forming with thread die depth  56 , thread root radius  58 , die face  54 , thread pitch  60  and thread crest  62 . 
       FIG. 4A , show the cross sectional view of an assembled thread forming screw in plastic material and the like  64  which intended. Embodiments of the thread forming screw is specifically designed to eliminate tapping operation and forming strong thread by perfect material engagement of thread flank  20  and deformation of material towards the thread root  18  for a strong joint reliability. 
     It will be appreciated that embodiments of the invention may be configured as double helix threads for fast insertions into work pieces, however, for sake of clarity and to clearly explain embodiments of the invention have been described with respect to single threads. 
     Thus, a self-tapping thread forming screw thread fastener and a corresponding thread roll die are disclosed for fastening into a hole of a work piece composed of a material such as synthetic plastic, resin and the like. More specifically, the thread forming screw fastener is a self-tapping screw having a symmetrical thread profile with a sharp transition flank and circular thread root which forms an internal thread in the work piece with low torque while promoting efficient deformation of the work piece material during installation and having a high failure torque to enhance high clamping loads and improve joint integrity with mating parts of the low ductile material of the work piece. 
     While embodiments of the invention have been described and illustrated, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the scope of the present invention as defined by the appended claims.