In thread rolling dies to which the invention relates, workpieces are transformed into finished screws by a rolling process as the work- pieces pass between a pair of elongated generally planar dies. One of the dies is stationary, and the other die is displaced relative to the other to produce a surface material flow on the workpiece to thereby form a continuous helical thread path on the screw. In the thread rolling die machines for which the invention has particular applicability, a shorter die of a pair of dies is held in stationary relationship while the longer die is moved in a direction generally parallel to a longitudinal reference plane. The axis of rotation of the body of the workpiece travels longitudinally as the workpiece rolls between the pair of dies. The diameter of the finished thread is controlled by the diameter of the workpiece and the distance between the dies at the finished end of the stroke. The dies are configured so that as the workpiece rolls across the dies, the desired threading is formed on the workpiece.

The rolling process is applicable for gimlet point type screws and for self-piercing and self-tapping screws which require a sharp screw point. For example, Dixon U. S. Patent 3,789,643 discloses a double angle cut-off die and method for rolling screws which is employed in connection with gimlet point screws. The gimlet point is produced by an extruding cut-off ramp. The ramp has a high rate of penetration initially followed by a more gradual penetration which removes the extrusion slug formed at the end of the point in the rolling process.

In conventional flat thread rolling dies to which the invention relates, the opposed dies cooperate in a symmetrical bilateral configuration relative to the rolling center at the final stage of the screw thread rolling to produce the final pointed screw. For such conventional

processes and apparatus, the-production of a consistent and precisely formed tip, such as, for example, may be required in self-piercing and self-tapping screws, becomes problematical due to difficulties in replicating the slug removal in the final stages of screw formation. For applications wherein the pointed tips are configured at a relative severe angle, achievement of consistent screw formation is difficult and may require die modification by skilled technicians in the field to obtain a consistent finished screw configuration.

Summary of the Invention Briefly stated, the invention in a preferred form comprises a pair of thread rolling dies which are specifically adaptable for manufacturing self-piercing, self-tapping or gimlet-type screws at a high rate in a flat thread rolling process. The dies depart from the conventional substantially symmetrical die configurations by implementation of an off center rise and an asymmetrical angle configuration at the tip formation portions. Such a configuration produces a very consistent point on the finished screw and enhances the replication of the slug removal at the tip of the screw.

One of the dies has a die rise at the rolling center and defines a double or enlarged angle point at the finish end of the die. The second die which cooperates with the first die to produce the fastener thread in the rolling process has a die rise which is spaced from the rolling center of the workpiece while maintaining a single or normal pre- established production angle to the point at the finish end of the workpiece.

An object of the invention is to provide a new and improved die for forming the points of self-piercing screws.

Another object of the invention is to provide a more consistent sharp screw point for high rate thread forming manufacturing processes.

A further object of the invention is to provide a pair of flat thread rolling dies which produce a consistent pointed screw at a high rate of manufacture in an efficient and cost effective manner.

A further object of the invention is to provide a new and improved flat roll thread die pair which produces a consistent overall screw length as a consequence of a highly consistent point formation.

A yet further object of the invention is to provide a new and improved flat roll thread die pair which produces a consistent self- piercing type screw without requiring significant adjustment to the dies by the screw manufacturer in the field.

Other objects and advantages of the invention will become apparent from the detailed description and the drawings.

Brief Description of the Drawings Figure 1 is a side elevational view of a pair of thread rolling dies in accordance with the invention comprising a stationary short die and a displaceable long die in a matched position with a screw blank therebetween; Figure 2 is an enlarged end sectional view, partly in diagrammatic form, of the short stationary die of Figure 1 at the dwell portion thereof; Figure 3 is an enlarged end sectional view, partly in diagrammatic form, of the long moveable die of Figure 1 at the dwell portion thereof; Figure 4 is an enlarged transverse sectional view taken along the line 4-4 of Figure 1 wherein the lines on the dies are in vertical alignment prior to movement of the long die as viewed to the left as viewed in the Figure 1 position and with the diameter of the workpiece aligned with the section lines with the threads omitted to illustrate the point formation; Figure 5 is an enlarged transverse sectional view taken along the line 5-5 of Figure 1 wherein the lines on the dies are in vertical alignment prior to movement of the long die as viewed to the left as viewed in the Figure 1 position and with the diameter of the workpiece

aligned with the section lines-with the threads omitted to illustrate the point formation; Figure 6 is an enlarged transverse sectional view taken along the line 6-6 of Figure 1 wherein the lines on the dies are in vertical alignment prior to movement of the long die as viewed to the left as viewed in the Figure 1 position and with the diameter of the workpiece aligned with the section lines with the threads omitted to illustrate the point formation; Figure 7 is an enlarged transverse sectional view taken along the line 7-7 of Figure 1 wherein the lines on the dies are in vertical alignment prior to movement of the long die as viewed to the left as viewed in the Figure 1 position and with the diameter of the workpiece aligned with the section lines with the threads omitted to illustrate the point formation; Figure 8 is an enlarged transverse sectional view taken along the line 8-8 of Figure 1 wherein the lines on the dies are in vertical alignment prior to movement of the long die as viewed to the left as viewed in the Figure 1 position and with the diameter of the workpiece aligned with the section lines with the threads omitted to illustrate the point formation; Figure 9 is a top plan view of the short stationary die of Figure 1; Figure 10 is a top plan view of the short stationary die of Figure 9 with ridges and grooves removed to more clearly illustrate the planar faces and schematically divided to illustrate various regions; Figure 11 is an enlarged schematic view illustrating the fade point for the dies of Figure 2 and 3; Figure 12 is a side elevation view of a representative screw manufactured by the dies of the present invention; and Figure 13 is an enlarged schematic view illustrating the fade point for alternate embodiments to the dies of Figures 2 and 3.

Detailed Description of the Preferred Embodiment With reference to the drawings, wherein like numerals represent like parts throughout the figures, a stationary die 10 and a moving die 12 are employed to roll a thread and form a pointed tip on a workpiece 20 to produce a finished self-piercing or self-tapping screw 21 by a reciprocating flat die method. Various stages in the workpiece transformation to the finished screw are illustrated as 20a, 20b, 20c, 20d and 20e in Figures 4-8. The reciprocating flat die 12 moves relative to the stationary die 10 in the direction of the Figure 1 arrow to define a rolling cycle. The workpiece as initially engaged by the die has a head 22 and a generally cylindrical shank 24 (Figure 4). As the workpiece rolls longitudinally between the initial and final ends of the stationary die, a thread 26 and pointed tip 28 are formed on the workpiece.

As illustrated in Figure 1 and in Figure 9 for die 10, the dies each have opposing faces 14,16 configured with ridges and grooves which form the threads and define, for example, the pitch, major diameter, minor diameter and thread type of the finished screw. The dies during each rolling cycle cooperatively gradually penetrate the workpiece to form the finished screw. The thread 26 and pointed tip 28 are fully developed just prior to parting the dies. The dies 10 and 12 in accordance with the invention are specifically configured for manufacturing the space-threaded, self-tapping, gimlet, cone-pointed or self-piercing screws having a relatively severe point as illustrated by screw 21 in Figure 12. Thus, the dies have three functions--thread formation, point formation and removal of excess material.

A starter finger (not illustrated) engages the workpiece blank to ensure that the moving die picks up the blank and starts the rolling process. For most applications, as the workpiece starts at the starting end 32,42 of each die, the threads are deep and sharp. In the dwell sections 34,44, the threads are flat and shallower. The starting end threads are sharp for easier penetration as the screw starts to roll and

get progressively wider along the die length until they ultimately reach width and depth equal to the desired finished thread form. The dies are aligned or"matched"to produce the proper optimum thread continuum.

The final die form is termed the dwell section 34,44 and extends along the die for distance long enough to properly dimension the screw. The dies are configured so that the least amount of rolling work as possible is done in the dwell section to maximize the life of the die. Typically, a cut-off edge is provided along the edge of the die to finish the pointed portion of the screw. As the dies are configured with surfaces which cooperatively form the thread, and in addition the tip portion of the screw, region C of the dies inclues a cut-off or extruding ramp plane which penetrates the cylindrical portion of the workpiece at a high rate.

In the subsequent region, the angle of incidence in the cut-off ramp 36, 46 is altered, and the cut-off ramp continues to engage the workpiece at a lower rate at which time the extruded slug is fully formed but not yet severed from the cylindrical portion of the workpiece, as illustrated in Figure 7.

In the final region, the point threading surface which started in region C continues to fully form the threads on the gimlet point which has been fully formed at the end of region D. A planar surface is parallel to the base of the short die to provide a final severance of the extrusion slug as the corresponding surface from the long die bears against and increases in width as the workpiece cylindrical portion passes therebetween. A back taper portion 38 is provided to urge the extrusion away from a pair of dies. The back cut surface is cut away to provide an area for the extrusional slug to drop away, as best illustrated in Figure 8.

With reference to Figures 2 and 3, the stationary die 10 and the moving die 12 are especially configured at the respective dwell portions of the dies to provide an off-center enlarged or double angle configuration which enhances the ability of the dies to remove the slug portion 23 in a highly precise and replicatable fashion. As illustrated in

Figure 11, the centerline of the rolling workpiece is represented by axis Z. One die, for example, die 10, in the die pair has a die rise T, o which extends to a plane through the rolling center Z of the screw (perpendicular to the Figure 11 view). The second die 12 has a smaller die rise T12 which extends to a position spaced from the centerline Z.

This off-center rise configuration on the die faces is implemented so that a uniform gap G is defined by the die rises. The die root thread paths 50 and 60 for respective dies 10 and 12 are illustrated by broken lines in Figures 11 and 13, and are selected to achieve a given screw design objective.

Representative dimensions for a No. 9-12 self-piercing screw, as depicted in Figure 11, are set forth in Table 1.

Table 1 Angle w 10° Angle x 20° Angle y 10° Gap G. 003 ins.

LT. 016 ins.

Lp. 160 ins.

Rlo. 0495 ins.

R, 2.0465 ins.

Tao. 0825 ins.

T. 2.0795 ins.

Rao and R12 are representative root formation dimensions for dies 10 and 12, respectively. Tao and T, 2 are representative thread formation dimensions (die rises) for dies 10 and 12, respectively. LT is the axial length of the asymmetrical tip formation portions of dies 10 and 12. Lp is the axial length of the point formation portions of dies 10 and 12.

Corresponding representative die rises To and T, 2 are such that To > Tl 2'

In the illustrated embodiment, the die crest angle x is twice the initial die angle y. In practice, when the dies are assemble to the flat die machine, the off-center design and special die crest angle cooperate to produce a constantly sharp screw point as the screw makes its final approximately 1.5-2 revolutions in the point area of the die. The 10° and 20° angles illustrated in the point zone achieve the shallow thread depth for starting the screw point. Naturally, other angular relationships are also possible such as illustrated in Figure 13.

Die 10 has point formation surface 52 which terminates in a tapered tip formation surface 54. Die 12 has a point formation surface 62 which terminates in a tapered tip formation surface 64. The formation surfaces 52 and 62 are generally symmetric to the centerline Z except for the tip formation surfaces 54 and 64 which are asymmetric. Surface 54 defines an enlarged tip cavity portion formed by surfaces 56 and 58 which intersect and extend at different angles w and x. Surfaces 58 and 64 extend at respective die crest angles x and y from a conceptual vertex point 70. Angle x is greater than angle y. After an axial length LT, the surface 56 extends at angle w relative to the centerline Z which is equal to angle x. The magnitudes of angle x and length LT are designed to achieve substantially equal Lp lengths on both dies 10 and 12.

Representative dimensions for a No. 8-9 self-piercing screw, as depicted in Figure 13, are set forth in Table 2.

-Table 2 Angle w 13 ° Angle x 23° Angle y 13 ° Gap G. 003 ins.

LT. 0155 ins.

Lp. 221 ins.

Rao. 0540 ins.

Ra2.0510 ins.

Trio. 0865 ins.

T. 2. 0835 ins.

Rao and R, 2 are representative root formation dimensions for dies 10 and 12, respectively. To and T, 2 are representative thread formation dimensions (die rises) for dies 10 and 12, respectively. LT is the axial length of the asymmetrical tip formation portions of dies 10 and 12. Lp is the axial length of the point formation portions of dies 10 and 12. It is again noted that the die rises of dies 10 and 12 are such that T, o > T12 It will be appreciated that the foregoing design, including the enlarged die crest angle x, is especially adaptable for self-piercing screws. The dies as described are employed to provide a relative efficient setup technique and dispense with any requirement for on site customer adjustments, such as stoning, notching and grinding, and are especially adaptable for single face cut-off dies. By moving the center point of the die off center relative to the screw, the slug is fractured at a well-defined point and a much more consistent severing of the slug from the workpiece is achieved.

While preferred embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.