Thread forming method and apparatus

Abstract: A method of producing threaded fasteners which results in seam free threads. The method employs a rolling die with a novel double form thread profile geometry wherein the angle of divergence of the groove defining walls of the die gradually varies between an obtuse pointing angle and an acute finish angle as the groove depth increases from a starting depth to the finish depth. The finish form of the die is maintained for a length which is commensurate with at least two and one-half revolutions of the fastener which is being formed.

Description: The present invention relates to thread forming dies and particularly to rolling dies having a unique thread profile geometry which will produce a seam-free thread. More specifically, this invention is directed to the production of seam-free threaded fasteners and particularly to the generation of rolled form threads characterized by an absence of seams, laps and craters. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character. The formation of a threaded fastener by subjecting a generally cylindrically-shaped portion of a preformed metal blank to a thread-rolling process is well known and will not be described in detail herein. It should suffice to note that a widely used technique, which enables quality threaded fasteners to be manufactured at high production rates, involves serial feeding of the preformed fastener blanks into the gap between a pair of "flat" thread-rolling dies. By imparting planar motion to a first one of the flat dies relative to the other die, a blank disposed between the dies will be caused to roll as it traverses the length of the other die. During the rolling of the blank, because of the forces applied thereto by the dies, the metal comprising the blank will flow so as to form a thread having a shape determined by the profile of the lands and grooves machined in the opposing faces of the dies. Fasteners having conventional rolled form threads, fasteners formed in accordance with the above-briefly described prior art technique which employs a pair of flat thread-rolling dies for example, are suitable for most applications. It is to be noted, however, that the generation of rolled form threads will typically produce a thread characterized by a crest seam or seams, i.e., a fissure at the crest of the thread. In accordance with SAE specification AS7456, issued February 19, 1991, seams at the roots of a helical thread or in the flanks of a thread below the "pitch line" or midpoint of the thread depth are generally not permissible, but a seam in the thread crest, which is sometimes referred to as a crest lap or crest crater, may be tolerated if not of excessive size. However, in many applications, especially where cleanliness is critical, a crest seam provides a potential haven for bacteria and other contaminants. Accordingly, a thread-rolling technique and apparatus which would produce a seam-free helical thread without a significant reduction in obtainable production rates has been long desired. It is to be noted that previous attempts to meet the above-mentioned long standing desire for a mass-produced seam-free rolled helical thread have encountered numerous obstacles. The most prevalent of these obstacles, where the use of flat thread-rolling die geometry is the technique of choice, has been slippage of the blank being worked during transfer of the thread form from the die to the workpiece as one of the two cooperating dies rotates the workpiece. Any such slippage will result in the production of a defective thread. An example of a prior art thread rolling die is disclosed in US-A-4862718. This discloses a die (10) for use in the formation of a helical thread in a metal blank, the die being provided with substantially parallel thread forming grooves in a face thereof which contacts the blank and having a thread profile defining groove geometry comprising: a dwell portion wherein the depth of the grooves is substantially constant, said dwell portion terminating at the discharge end of the grooves, the facing groove defining side walls diverging at an average acute angle which is commensurate with the finish form of the thread within said dwell portion, said dwell portion extending from the discharge end of the grooves toward the starting end of the grooves a distance which corresponds at least to 2.5 revolutions of the blank; and a second portion which extends from the starting end of the grooves to said dwell portion. The present invention is characterised in that the depth of the grooves in the second portion increases progressively and smoothly between said starting end and dwell portion, said side walls diverging at said starting end in accordance with a pointing form angle which exceeds 90°, said side walls undergoing a smooth transition from said pointing form angle to an acute finish form angle along that portion of the grooves where the depth thereof is increasing, said transition progressing outwardly from the base of the grooves toward the crest of the die whereby said side walls have a double form in the portion of the length thereof disposed between said groove starting ends and said dwell portion. The present invention overcomes the above-briefly discussed and other deficiencies and disadvantages of the prior art by providing a unique process for the generation of seam-free rolled-form helical threads. The present invention also encompasses a novel thread-forming die thread profile geometry which may be employed to implement this unique process. This novel die thread profile geometry causes a balanced radial flow of the blank material so as to avoid the formation of seams, laps, and craters in the course of production of machine screws and screws having "space type" threads. A machine screw is generally characterized by a helical thread having a crest width which is substantially equal to the root width, and the profile of the thread forming die is generated about the pitch line of the thread. A "space type" thread is characterized by a coarse pitch and a root which is wider than the thread crest. Fasteners having a "space type" thread include type B, type AB, type A, wood screws and lag screws along with the customized variations of such fasteners. A thread-rolling die in accordance with the present invention is characterized by a thread profile geometry which, at its starting end, has a pointed or sharpened form wherein the facing side walls or flanks which define each groove diverge at a wide angle. The die profile undergoes a smooth transition to the finish form, as the thread depth increases, from the starting end to a dwell portion of the die. During this transition, the thread profile geometry will have a "double" form, i.e., the transition will progress outwardly toward the crest as the pointed form evolves to the acute angle of the finish form. The pointed form will, as this evolution occurs, fade out at the start of the dwell region. A thread-rolling die for producing a "space type" thread in accordance with the present invention is characterized by a thread profile geometry which is generated about the diameter of the blank in which the thread is to be formed. This thread profile geometry, at its starting end, has a groove depth which equals approximately twice the penetration, i.e., the amount the blank cylinder penetrates into the die. This starting end groove depth will gradually increase to the finish form depth. The constant finish form depth extends, from the finish or discharge end of the die toward the starting end of the die, a length which is equivalent to at least 2.5 screw revolutions. This region wherein the groove depth is constant is known as the dwell region and, in the typical case, will be approximately one-third of the die length. The thread profile geometry of a thread rolling die for producing a "space type" thread in accordance with the invention is also characterized by an obtuse pointing angle, i.e., the angle defined by lines intersecting a pair of facing side walls or flanks which define a groove, which changes as a function of both thread diameter and pitch. As noted above, the die profile undergoes a smooth transition to the finish form, as the thread depth increases, from the starting end of the die to the beginning of the dwell region where the finish form depth is achieved. The transition will progress outwardly toward the crest as the pointed thread form evolves to the acute angle of the finish form and as the thread depth increases. Thus, the pointing form angle at the starting end of the die will gradually fade out between the starting end of the die and the start of the die dwell region. The present invention may be better understood and its numerous advantages will become apparent to those skilled in the art by reference to the accompanying drawings wherein like reference numerals refer to like elements in the several figures and in which: Figure 1 is a top view of one of a pair of cooperating flat thread-rolling dies in accordance with the invention, the other die appearing substantially the same when similarly viewed; Figure 2 is a side elevation view of the die of Figure 1 as employed in the manufacture of machine screws, the cooperating die appearing substantially the same when similarly viewed; Figure 3 is a partial end view, on an enlarged scale and taken in direction A-A of Figure 2, which schematically shows a die for use in the formation of a machine screw in accordance with the invention, the die being depicted at an intermediate stage in the formation thereof; Figure 4 is a partial end view, on the same scale as Figure 3 and taken along line A-A of Figure 2, which schematically shows the completed die; Figure 5 is a graphical representation of the thread profile cross-section of the die of Figure 4 at four points, including the starting and finish positions, along the length of the die, the four points being indicated on Figure 2; Figure 6 is a schematic cross-section of a machine screw rolled at varying thread fullness, Figure 6 illustrating the progressive increase in the profile of a thread produced in accordance with the invention; Figure 7 is a side elevation view of the die of Figure 1, Figure 7 being similar to Figure 2 but depicting a die for use in the manufacture of fasteners having a space-type thread; Figure 8 is an engineering drawing which will enable a designer to provide a thread rolling die in accordance with the invention for use in the production of fasteners having a "space type" thread; Figure 9 is a partial end view, taken along line A-A of Figure 7, which schematically shows the die of Figures 7 and 8 on an enlarged scale; and Figure 10 is a graphical representation of the thread profile cross-section of the die of Figures 7 - 9 at five points along the length of the die. With reference now to the drawings, a "flat" thread-rolling die in accordance with the invention is shown, in a top view, in Figure 1 and in a side view in Figures 2 and 7. Figures 1, 2 and 7 may be considered as showing either the stationary or "short" die or the reciprocating or "long" die of a pair of cooperating flat dies. This die, which is indicated generally at 10, has a starting end 12 and a finish or discharge end 14. The face 16 of the die is machined so as to have parallel lands and grooves 17 shaped in accordance with a thread-forming profile. As a cylindrically shaped metal fastener blank rolls from the starting end to the finish end of the dies as a result of the motion of the "long" die, while being subjected to compression, the material comprising the blank will flow to define a helical thread. The die 10, with the exception of the unique thread profile geometry to be described, is of conventional construction. With reference to Figure 3, which is an enlarged partial view taken along line A-A of Figure 2 depicting a die 10 for use in the manufacture of machine screws at an intermediate stage of the machining of the thread profile in the face 16 thereof, the solid lines indicate the thread form at the starting end 12 of the "short" die and the matching point on the "long" or reciprocating die. The broken line showing indicates the thread form at the finish end of the dies. The finish end thread depth is indicated at D and, by reference to Figure 2, it may be seen that the thread depth increases symmetrically, from the starting end 12 to the beginning of the "dwell" region of the die, about a pitch or groove line G.L. In the embodiment of Figures 2 - 6, the flanks of facing threads of the fastener to be formed, and thus the facing sides of the grooves of die 10 at the finish end, intersect at an angle of 60°. While the crest and roots of the die profile are depicted as flat in Figures 3 and 4, in actual practice these portions of the profile will be rounded as represented in Figure 5. In accordance with the present invention, the die thread profile is "pointed" i.e., the profile is provided with a sharpened thread form, at the starting end 12 of the "short" die, and the matching point on the "long" die. This "pointed" thread form is obtained by further machining, after the thread profile configuration of Figure 3 has been obtained, to produce the thread profile geometry of Figure 4. Figure 4 shows that, at the starting end of a die for producing a machine screw with a 60° finish form, the angle of divergence of the facing sides of the grooves is approximately 98°. The 98° "pointing" of the 60° thread form has, most unexpectedly, been found to eliminate the formation of crest seams. There will be a smooth transition from the 98° pointing to the 60° finish end thread form along that portion of the length of the die where the thread depth D is gradually increased, i.e., before the dwell region of the die. This region where thread depth gradually increases is indicated at 18 on Figures 1 and 2 and corresponds to that portion of the die, extending from the starting end, where the crest of the thread profile is machined at a predetermined taper. Figure 5 graphically depicts the actual die thread forming profile at the starting end and at three successively distant points along the length of the die. Figure 5 clearly shows the smooth transition between the pointed thread form at the starting end and the desired final thread form. Figure 5 also shows that, as the transition occurs, the groove profile has a double form. The pointed form fades out at the beginning of the dwell region. Curves 5A-5D may respectively be considered to be views taken along lines A-A, B-B, C-C, and D-D of Figure 2. Figure 6, progressing from curve No. 1 through curve No. 8, shows various stages in the formation of a seam-free machine screw thread in accordance with the present invention and employing the above-described tooling. As may be seen from Figure 6, as a cylindrical blank passes between a pair of the flat dies in accordance with the invention, and the thread is formed therein, the metal of the blank will flow in such a manner that the crest of each thread is formed smoothly by a balanced radial flow of material whereby crest seams and craters are avoided. The present invention is predicated upon the discovery that seam-free threads will result from a critical pointing of the thread form at the starting end of the thread-forming die. This critical pointing, i.e., the angle transcribed by the facing flanks of the die profile, is independent of pitch P. Referring to Figure 4, wherein the pointing angle  is twice the angle α, the pointing angle may be calculated as follows: Tan. α = .375P/.32476P = 49° 6′ 24″ where the thread depth D is .32476P, the width of the thread root and the crest of the thread are each 0.125P and  = 2α = 98° 12′ 48″ and, accordingly,  = 2 Tan. ⁻¹ (.375/.32476) Thus, in the practice of the present invention, to produce a helical thread having a 60° finish form, the thread form as defined by the profile of the die is "pointed" to an angle as close to 98° 12′ 48″ as permitted by manufacturing tolerances at the starting end of the "short" die (and at the matching point on the "long" die) and, from this pointing angle, there is a smooth transition to the desired 60° die finish end thread form. Referring now to Figures 1 and 7 - 11, a die intended for use in the manufacture of a fastener having a space type thread, a type B self-tapping screw having a 60° finish form for example, is shown. Figure 8 is an engineering drawing relating to die 10 of Figures 1 and 7. Figure 8 shows the parameters which must be taken into account in calculating the die profile geometry which will permit production of a seam-free space type thread. In Figure 8, the solid lines represent the thread form at the finish end of the "short" or stationary die and the matching point on the "long" or moving die. The broken line showing indicates the thread form at the starting end of the dies. Referring to Figure 8, the penetration, i.e., is the amount that the blank penetrates into the die, is calculated as follows: Penetration = Blank Dia. - Root Dia. α In accordance with the present invention, the depth of the thread profile at the starting end of the die will equal twice the penetration as indicated on Figure 8. This thread profile or groove depth gradually and smoothly increases from the starting end 12 to the point 18 where it reaches the finish form depth D. Referring to Figures 1 and 7, the die 10 is configured such that the blank will undergo at least 2.5 revolutions between the point 18 and the finish end 14 of the die. As discussed above in the description of Figures 1 - 6, the region of the die between point 18 and finish end 14 may be referred to as a dwell region since the thread profile geometry does not change. This dwell region will typically extend approximately one-third of the length of the die. In accordance with the present invention, the angle of divergence of the facing sides of the grooves in the die varies between the starting angle , i.e., the pointing form angle, and a lesser acute angle at the beginning of the dwell portion of the die. This transition between the pointing form angle  and the finish profile occurs smoothly as the groove depth also undergoes a smooth change. The pointing form angle  will vary as a function of the diameter and pitch of the fastener being formed. Referring again to Figure 3, the pointing angle  may be calculated as follows: Tan. α = thread depth at starting end Pitch - .008 2 Thus, it may be seen that the pointing form angle  is:  = 180°-(2) (α). Figure 9 is a view of a thread rolling die in accordance with the present invention taken in direction A-A of Figure 7, i.e., from the extreme start end. Figure 9 clearly shows that, proceeding from the starting end of the die, the thread groove depth gradually increases from approximately twice the penetration to the finish depth D while the angle of divergence of the groove side walls gradually undergoes a transition from the pointing form angle  to the finish angle which, in the example being disclosed, is 60°. As the angle of divergence decreases, the width of the crest flats increases gradually from an essentially pointed form to the desired thread root width which, as noted, is substantially wider than the crest width in a space type thread. The width of the root of the die thread profile remains constant along the length of the die. It will be understood by those skilled in the art that Figures 7 and 8 constitute a theoretical or idealized depiction. In actual practice, the crest and roots of the die profile are not flat, as depicted in Figures 8 and 9, but are actually rounded as shown in Figure 10. Figure 10, proceeding from bottom to top, depicts on an enlarged scale the actual thread profile at the start and finish ends of the die and at three intermediate points along the die. Curves 10a - 10e may respectively be considered to be cross-sectional views taken along lines A-A, B-B, C-C, D-D and E-E of Figure 7. Figure 10 clearly shows the smooth transition of the thread form between the starting end of the die and the desired final thread form. Figure 10 also shows that, as this transition occurs, the groove profile has a double form, i.e., as the thread depth increases the angle of divergence of the facing side walls of the grooves decreases. While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but rather by the scope of the appended claims. Thus, while the invention has been described as embodied in a flat rolling die, it will be understood by those skilled in the art that the novel thread profile geometry is applicable to cylindrical and planetary dies. Accordingly, the invention has been described above by way of illustration and not limitation.

Claims: A die (10) for use in the formation of a helical thread in a metal blank, the die being provided with substantially parallel thread forming grooves (17) in a face thereof which contacts the blank and having a thread profile defining groove geometry comprising: a dwell portion (D) wherein the depth of the grooves (17) is substantially constant, said dwell portion (D) terminating at the discharge end (14) of the grooves, the facing groove defining side walls diverging at an average acute angle which is commensurate with the finish form of the thread within said dwell portion (D), said dwell portion extending from the discharge end (14) of the grooves toward the starting end (12) of the grooves a distance which corresponds at least to 2.5 revolutions of the blank; and a second portion (18) which extends from the starting end (12) of the grooves to said dwell portion; characterised in that the depth of the grooves in the second portion increases progressively and smoothly between said starting end (12) and dwell portion (D), said side walls diverging at said starting end in accordance with a pointing form angle which exceeds 90°, said side walls undergoing a smooth transition from said pointing form angle to an acute finish form angle along that portion of the grooves (17) where the depth thereof is increasing, said transition progressing outwardly from the base of the grooves toward the crest of the die whereby said side walls have a double form in the portion of the length thereof disposed between said groove starting ends and said dwell portion (D). A die (10) as claimed in claim 1 wherein said die (10) is a flat thread rolling die. A die (10) as claimed in claim 1 or 2 wherein said pointing form angle at said groove starting ends (12) is a function of the diameter and pitch of the thread being formed. A die (10) as claimed in claims 1 to 3 wherein the groove depth at the starting end (12) thereof equals twice the penetration of the metal blank into the die (10) at the said starting end (12). A die (10) as claimed in any preceding claim wherein the die (10) cooperates with a second matching die and each of the dies has a plurality of said grooves (17) the arrangement being such that, in use, rolling motion is imparted to a blank as one of the dies (10) moves linearly with respect to the other die. A die (10) as claimed in claim 1 wherein said die is a flat thread rolling die with plural parallel grooves, and said groove depth varies symmetrically with respect to a pitch line. A die (10) as claimed in claim 1 or 6 wherein said angle commensurate with the thread finish form is approximately 60° and said angle of said pointed form is approximately 98°. A die (10) as claimed in any preceding claim wherein said angle of divergence at said starting end (12) is an obtuse angle. A die (10) as claimed in any preceding claim wherein the depth of each of said grooves (17) varies symmetrically with respect to a pitch line, said pitch lines defining planes, the dies being arranged to move in a direction which is parallel to said planes in order to cause a blank to roll from the start end of the die to the discharge end. A method of forming a threaded fastener by causing deformation of a metal blank, the metal blank being simultaneously subjected to compressive and rotational forces by at least a first die (10), the die (10) having a blank contacting face which is provided with thread profile defining lands (16) and grooves (17), the method being characterised by: progressively increasing the depth of the grooves formed in the surface of the blank between a starting and a finish depth; initially causing the oppositely facing flanks of the grooves to diverge from the root of the grooves at a pointing form angle which is greater than 90°; causing the angle of divergence of said groove flanks to smoothly decrease to an acute angle commensurate with the desired finish thread form during the time the groove depth is increasing; and causing the blank to undergo plural revolutions while remaining in contact with the die (10) after the die thread profile defining groove has evolved to the finish thread depth and form. The method of claim 10 wherein the depth of the thread profile defining grooves (10) is initially twice the depth of penetration of the blank into the die (10). The method of claim 10 or 11 wherein said pointing form angle is a function of the diameter and pitch of the thread to be formed in the blank. The method of any of claims 10 to 12 wherein said pointing form angle is approximately 98° 12′ 48″ and said acute angle is approximately 60°. The method of any of claims 10 to 13 wherein the step of causing the blank to undergo plural revolutions comprises: subjecting the blank to a finish step wherein the groove depth and angle of flank divergence remain constant while the blank undergoes at least two and one-half revolutions.

Inventors: DICKSON JOSEPH F, DICKSON, JOSEPH F.
Applicants: PCC SPEC PROD INC, PCC SPECIALTY PRODUCTS, INC.
Application year: 1992
Main classification: B21H3
IPC codes: B21H3