Major diameter correcting re-roll thread rolling die

Abstract

A thread profile is first rolled into a cylindrical work piece with a first set of thread dies, wherein the metal free-flows into the grooves of the first set of dies to form a rounded (or other shape) crest. Then the thread is re-rolled in a second set of dies having a reduced groove depth such that the rounded crest is re-formed and flattened. A thread profile may also be formed in a cylindrical work piece using a set of dies having surfaces containing two or more distinct groove shapes. Diagonally cut grooves encountered by the work piece would have a shape substantially identical to a “normal” die groove profile with a depth of D+H. Grooves in contact with the work piece as it exits the die would have a shape with a reduced groove depth of approximately “D.”

Description

RELATED PATENT APPLICATION

This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 60/648,325; filed Jan. 28, 2005; entitled “Major Diameter Correcting Re-Roll Thread Rolling Die,” by Michael A. Garver; which is hereby incorporated by reference herein for all purposes.

TECHNICAL FIELD

The present disclosure relates to fastener thread manufacturing dies, more particularly, to a major diameter correcting re-roll thread rolling die.

BACKGROUND

Referring to FIG. 1, in thread manufacturing today, threads are normally applied to externally threaded products such as bolts and machine screws by a process where the threads are formed by pressing dies 102, onto which the shape of the finished thread 104 has been impregnated into a cylindrical member 106.

Referring to now FIG. 2, the metal of the cylindrical member 106 is progressively displaced by the pressing dies 102 as the cylindrical member 106 is rotated in a direction 208 between opposing faces of the dies 102a and 102b having proper shapes for forming the threads on the cylindrical member 106, similar to the action of rolling a pencil between one's hands as they are pressed together.

Referring to now FIGS. 3A, 3B and 3C, in order to accomplish this, the shape machined into the dies 102 is a series of parallel grooves 310, cut diagonally across the face of the die 102. Each groove 310 has two flat sides 312a and 312b, each of these sides angled at approximately thirty degrees, such that the included angle between the two sides 312a and 312b of the grooves 310 is approximately sixty degrees. At the bottom of each of the grooves 310 is a flat surface 314, perpendicular to the plane forming the center of the groove 310, and intersecting each angled side 312a and 312b at approximately a thirty degree angle.

Referring now also to FIGS. 4A, 4B and 5, when the metal of the cylindrical member 106, e.g., “work piece,” to be threaded is pressed into the grooves 310, the cylindrical member 106 metal is displaced so as to fill the grooves 310 in the die 102. Depending on the pressure exerted, the cylindrical member 106, e.g., bolt or machine screw, may either partially fill the die groove 310 (represented by 416 of FIG. 4A) or completely fill the die groove (represented by 418 of FIG. 4B). If the pressure is great enough, the resulting shape of the work piece threads, e.g., 418, may have a shape substantially matching the shape of the groove 310 cut in the die 102. This may result in an external thread profile having two flanks 520a and 520b (FIG. 5) and a flat peak or “crest” 522 (FIG. 5).

Referring to FIG. 6, as pressure is reduced, the sharp corners of the die 102 tend not to fill completely with metal. Rather than being “packed” into a sharp corner, the metal of the work piece 106 may flow naturally in an area where the corners had been. The thread profile may then appear to have a radius where the sharp corners had been, e.g., 624 shown in FIG. 6. As pressure continues to be reduced this natural flow eventually forms what appears to be a completely curved surface on the peak of the thread 418, linking both flanks and forming the “crest” 624 of the thread 418. Further reducing pressure may result in a curved surface degrading into a “ragged” appearing crest (726 of FIG. 7), then eventually, with lower pressure and because of lack of metal, the flanks 520a and 520b may themselves begin to degrade. Typically, standard externally threaded fasteners may tend to have some variant of these shapes on the crest of the threads.

In fastener manufacture, production processes for both dies and threads have significant inherent variations. Because of these variations, it is very uncommon in the thread rolling process to completely fill the corners of the die groove as first discussed above and shown in FIG. 4B. What is more common, are various degrees of corner fill depending upon the tolerances of the cylindrical work piece 106 and dies 102, and the amount of pressure applied during actual production thereof.

Due to this phenomenon, die manufacturing specifications are such that it is assumed the that the normal result of the rolling process will not be a thread profile with sharp corners, e.g., crest 522 of FIG. 5, but some variant of the crest shape discussed above and shown in FIGS. 6 and 7. The most common shape seen in commerce today is the “fully radiused” crest 624 shown in FIG. 6.

In order to allow this condition, commonly known as “underfill” of the die, the depth of the flat bottom of the groove in the die is normally made deeper than expected. Thus, there is “extra” room in the die, so that any metal forming the curved crest seen on the resulting thread profile 828 of the work piece shown in FIG. 8, exceeds the metal required to obtain the minimum flank height of the thread profile represented by 830 of FIG. 8. This, therefore, insures that the thread flanks 520a and 520b are high enough to meet minimum thread strength characteristics while allowing for excess metal flow.

Thread rolling die specifications, therefore, are set up not to match the desired shape, but to exceed the specified envelope of the minimum height of the actual thread profile, as designated in ISO and other thread standard specifications, incorporated by reference herein for all purposes. Minimum flank height and thread strength required by the specification thereby may be maintained.

Referring to now to FIG. 9, depicted is a comparison of an M8×1.25 6 g external thread tolerance envelope (on the right) and the accompanying die dimension envelope (on the left). Note that the die envelope shows the desired die tolerances 932, shown solid black, along with the permissible die movement allowed by thread pitch diameter tolerances 934. The thread envelop is shown with its entire permissible major, minor and pitch diameter variations shaded and represented as 936. Note that the peak of the die specification, when adjusted to maximum pitch diameter may significantly exceed the maximum permissible major diameter of the thread.

The threads formed thusly, therefore tend to have a major diameter (for practicality, measured across the rounded crests of the threads) that is larger than expected for a thread of that pitch diameter. These threads also have smaller flank height than expected for such a major diameter. It, in fact, means that external threads produced with dies made to present day die specifications can, within the permissible range of pitch diameters, exceed the permissible major diameter of the thread, while having flank heights that are within specification. This potentially leads to internal/external thread interference in use. This often leads manufacturers to make their fasteners to the low end of the pitch diameter range, in order that the measured major diameter does not exceed the maximum specification. It also means that threads produced deliberately to the low end of the pitch range to allow for thick plating on the surface of the thread, potentially have flank height below that required for minimum strength requirements.

SUMMARY

Therefore, there is a need for a method of and apparatus for manufacturing threads that do not ultimately result in an undesirable rounded crest, thus allowing thread major diameters to be made to the specified diameter, with sharp flank-crest interfaces and no excess metal beyond said diameter, while not significantly affecting the current thread manufacturing process. Such a method of and apparatus for manufacturing threads would guarantee intended thread strength characteristics across the entire range of possible thread and die dimensional envelopes, thus allowing threads to be deliberately manufactured to the lower end of the pitch diameter range to permit painting and/or plating of the fastener and also allow interfaces with painted internal threads.

According to a specific example embodiment of the present disclosure, an apparatus for rolling threads on a cylindrical work piece and having major diameter thread crest correction may comprise a first die set for rolling threads on a cylindrical work piece, wherein the first die set has thread forming grooves at a first depth; and a second die set for re-rolling the threads on the cylindrical work piece, wherein the second die set has thread forming grooves at a second depth, wherein the second depth is less than the first depth. The second die set thread forming grooves may reshape a crest of the threads on the cylindrical work piece to a desired height. The second die set thread forming grooves may reshape a crest of the threads on the cylindrical work piece to a desire shape. The second die set thread forming grooves may reshape a crest of the threads on the cylindrical work piece to a substantially flat peak.

According to another specific example embodiment of the present disclosure, an apparatus for rolling threads on a cylindrical work piece and having major diameter thread crest correction may comprise a die set for rolling threads on a cylindrical work piece, wherein the die set has a first portion with thread forming grooves at a first depth, and a second portion with thread forming grooves at a second depth, wherein the second depth is less than the first depth; wherein during thread rolling on the cylindrical work piece in the first portion of the die set the thread crests are at substantially the first depth, and during thread rolling on the cylindrical work piece in the second portion of the die set the thread crests are at substantially the second depth. The second portion with thread forming grooves at the second depth may reshape a crest of the threads on the cylindrical work piece to a desired height. The second portion with thread forming grooves at the second depth may reshape a crest of the threads on the cylindrical work piece to a desired shape. The second portion with thread forming grooves at the second depth may reshape a crest of the threads on the cylindrical work piece to a substantially flat peak.

According to yet another specific example embodiment of the present disclosure, an apparatus for rolling threads on a cylindrical work piece and having major diameter thread crest correction may comprise a die set for rolling threads on a cylindrical work piece, wherein the die set comprises a plurality of portions having progressively smaller groove depths, whereby during thread rolling the cylindrical work piece has thread crest heights at substantially the groove depths of each of the plurality of portions of the die set and at the end of thread rolling has a thread crest height at substantially a groove depth of a last one of the plurality of portions of the die set. The thread forming grooves of the last one of the plurality of portions of the die set are at a depth and shape whereby the crest of the threads on the cylindrical work piece are reshaped to a desired height. The thread forming grooves of the last one of the plurality of portions of the die set are at a depth and shape whereby the crest of the threads on the cylindrical work piece are reshaped to a desired shape. The thread forming grooves of the last one of the plurality of portions of the die set are at a depth and shape whereby the crest of the threads on the cylindrical work piece are reshaped to a substantially flat peak.

According to still another specific example embodiment of the present disclosure, a method for rolling threads on a cylindrical work piece and for correcting a major diameter thread crest may comprise the steps of rolling threads on a cylindrical work piece with a first die set, wherein the first die set has thread forming grooves at a first depth; and re-rolling the threads on the cylindrical work piece with a second die set, wherein the second die set has thread forming grooves at a second depth, wherein the second depth is less than the first depth. The step of re-rolling the threads on the cylindrical work piece with the second die set may include the step of reshaping a crest of the threads on the cylindrical work piece to a desired height. The step of re-rolling the threads on the cylindrical work piece with the second die set may include the step of reshaping a crest of the threads on the cylindrical work piece to a desired shape. The step of re-rolling the threads on the cylindrical work piece with the second die set may include the step of reshaping a crest of the threads on the cylindrical work piece to a substantially flat peak.

According to another specific example embodiment of the present disclosure, a method for rolling threads on a cylindrical work piece and for correcting a major diameter thread crest may comprise rolling threads on a cylindrical work piece with a die set, wherein the die set comprises a first portion with thread forming grooves at a first depth, and a second portion with thread forming grooves at a second depth, wherein the second depth is less than the first depth; wherein during the step of rolling threads in the first portion of the die set the cylindrical work piece has thread crests at substantially the first depth and during the step of rolling threads in the second portion of the die set the cylindrical work piece has thread crests at substantially the second depth. The step of rolling threads in the second portion of the die set may include the step of reshaping a crest of the threads on the cylindrical work piece to a desired height. The step of rolling threads in the second portion of the die set may include the step of reshaping a crest of the threads on the cylindrical work piece to a desired shape. The step of rolling threads in the second portion of the die set may include the step of reshaping a crest of the threads on the cylindrical work piece to a substantially flat peak.

According to yet another specific example embodiment of the present disclosure, a method for rolling threads on a cylindrical work piece and for correcting major diameter thread crests may comprise the steps of rolling threads on a cylindrical work piece with a die set, wherein the die set comprises a plurality of portions with thread forming grooves at progressively smaller depths; wherein during the step of rolling threads on the cylindrical work piece the threads are formed to a desired shape. The step of rolling threads may include the step of reshaping a crest of the threads to a desired height. The step of rolling threads may include the step of reshaping a crest of the threads to a substantially flat peak.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic cross-section view of pressing dies impregnating finished threads into a cylindrical member, according to prior technology;

FIG. 2 is schematic elevational view of pressing dies impregnating finished threads into a cylindrical member, according to prior technology;

FIG. 3A is a front view of a die face showing the machined series of parallel groves;

FIG. 3B is a cross-section view A-A of the groves in the die face shown in FIG. 3A;

FIG. 3C is a magnified cross-section view B of a portion of the groves in the die face shown in FIG. 3B;

FIG. 4A is a cross-section view of a die face grove and thread forming in the cylindrical member when low pressure is exerted on the work piece;

FIG. 4B is a cross-section view of a die face grove and thread forming in the cylindrical member when high pressure is exerted on the work piece;

FIG. 5 is a cross-section view of a thread formed when high pressure is exerted on the work piece;

FIG. 6 is a cross-section view of a thread formed when reduced pressure is exerted on the work piece;

FIG. 7 is a cross-section view of a thread formed when even less pressure is exerted on the work piece;

FIG. 8 is a cross-section view of an curved crest formed in excess on a thread in relation to a desired thread profile;

FIG. 9 shows a die profile envelope in comparison to a thread profile envelope;

FIG. 10 shows a two step process for forming threads in a work piece, according to a specific example embodiment of the present disclosure;

FIG. 11 shows a single step process for forming threads in a work piece using a set of dies having surfaces containing two or more distinct groove shapes, according to another specific example embodiment of the present disclosure; and

FIG. 12 shows a comparison of a desired normal crest height and forming crest heights necessary to equalize the total area so as to obtain substantially the desired normal crest height.

While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims.

DETAILED DESCRIPTION

Referring now to the drawings, the details of example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix.

Referring to FIG. 10, depicted is a two step process for forming threads in a work piece, according to a specific example embodiment of the present disclosure. A thread profile is first rolled into a cylindrical member 106 work piece with a first set of thread dies 1038 as is done today (as shown in the top section details of FIG. 10), allowing the metal to free-flow into the corners of the grooves of the first set of dies 1038, resulting in an under-filled, rounded, or other shape crest. Then the thread is re-rolled in a second set of dies 1040 having a reduced groove depth (as shown in the bottom section details of FIG. 10), such that the rounded (or other shape) crest is re-formed and flattened, thus forcing its height down to a desired height and into the corners of the grooves of the second set of thread dies 1040 (as shown in the bottom section details of FIG. 10).

Referring to FIG. 11, depicted is a single step process for forming threads in a work piece using a set of dies having surfaces containing two or more distinct groove shapes, according to another specific example embodiment of the present disclosure. Diagonally cut grooves encountered by the work piece cylinder, as it enters the threading die set at the feed end 1144 would have a shape substantially identical to the “normal” die groove profile, e.g., as shown in FIG. 3C, with a depth of D+H (FIG. 10), or such other shape as may be efficiently re-rolled, per the process described below. Grooves in contact with the work piece as it exits the die at the exit end 1146, would have a shape similar to the shape shown in FIG. 3C, but with a reduced groove depth of approximately “D” (FIG. 10).

Depth “D” of FIG. 10 is the nominal design height of the thread flank profile intended to be formed, shown as 1242 in FIG. 12. Depth “H” may be defined by computing the height necessary to equalize the total area of the shaded sections 1244 and 1246 (FIG. 12), on either side of 1242. “H” is then considered an approximate basis for the reduction in die groove depth, required to re-roll the crest profile to achieve a final flank height of “H” on the work piece. The “H” dimension may be varied slightly to obtain the best results in actual practice, such that the reduction produces the desired thread shape and dimensional characteristics.

According to the teachings of this disclosure, such crest shape change may be accomplished by either altering the shape of each groove as it progresses toward the exit end 1146 of the die 1142, or, by maintaining a constant shape in each groove, but, giving grooves at the entrance end 1144 of the die a deeper shape and those at the exit end 1146 of the die 1142 a shallower shape. The later of these may require die lengths longer than those normally associated with standard fastener manufacturing.

It is contemplated and within the scope of the teachings of this disclosure that the change in depth of the die groove may be accomplished in one or more steps such that the revolving work piece may encounter different depth and/or shape grooves every time it touches a die, as it progresses through the die set, or groove depth may be changed in one step. Such a die might provide one step, intermediate stepped grooves, or a constantly changing groove depth, so long as the desired change in crest height and shape ultimately results.

Intermediate groove shapes may also be present throughout the die, so long as the desired change in crest height and shape ultimately results. Inadvertent and/or deliberately created crest shape variations are anticipated during the rolling process, as the work piece progresses from the beginning to end of the die. Grooves not in contact with the work piece at the entrance or exit ends of the die, may have either of the shapes in question, or both, or any other shape which allows for the re-rolling to be accomplished. Re-rolling as described herein may be accomplished by one or both of the dies in the rolling process, and that either of the dies may be a “normal” die, or itself be a specialize die for another purpose. Each die may have a different progression of different groove depths, as discussed above, or may differ from the opposing die in any of the variants discussed hereinabove. The re-rolling process may be accomplished in a variety of other styles of thread rolling dies, such as, but not limited to, cylindrical dies, and/or segmented dies. Either or both of the such dies may also be combined with other thread shapes, such as, but not limited to anti-cross-thread type dies, which may or may not be re-rolled by a similar process.

While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.

Claims

1. An apparatus for rolling threads on a cylindrical work piece and having major diameter thread crest correction, comprising:

a first die set for rolling threads on a cylindrical work piece, wherein the first die set has thread forming grooves at a first depth; and

a second die set for re-rolling the threads on the cylindrical work piece, wherein the second die set has thread forming grooves at a second depth, wherein the second depth is less than the first depth.

2. The apparatus according to claim 1, wherein the second die set thread forming grooves reshapes a crest of the threads on the cylindrical work piece to a desired height.

3. The apparatus according to claim 1, wherein the second die set thread forming grooves reshapes a crest of the threads on the cylindrical work piece to a desire shape.

4. The apparatus according to claim 1, wherein the second die set thread forming grooves reshapes a crest of the threads on the cylindrical work piece to a substantially flat peak.

5. An apparatus for rolling threads on a cylindrical work piece and having major diameter thread crest correction, comprising:

a die set for rolling threads on a cylindrical work piece, wherein the die set has a first portion with thread forming grooves at a first depth, and a second portion with thread forming grooves at a second depth, wherein the second depth is less than the first depth;

wherein

during thread rolling on the cylindrical work piece in the first portion of the die set the thread crests are at substantially the first depth, and

during thread rolling on the cylindrical work piece in the second portion of the die set the thread crests are at substantially the second depth.

6. The apparatus according to claim 5, wherein the second portion with thread forming grooves at the second depth reshapes a crest of the threads on the cylindrical work piece to a desired height.

7. The apparatus according to claim 5, wherein the second portion with thread forming grooves at the second depth reshapes a crest of the threads on the cylindrical work piece to a desired shape.

8. The apparatus according to claim 5, wherein the second portion with thread forming grooves at the second depth reshapes a crest of the threads on the cylindrical work piece to a substantially flat peak.

9. An apparatus for rolling threads on a cylindrical work piece and having major diameter thread crest correction, comprising:

a die set for rolling threads on a cylindrical work piece, wherein the die set comprises a plurality of portions having progressively smaller groove depths, whereby during thread rolling the cylindrical work piece has thread crest heights at substantially the groove depths of each of the plurality of portions of the die set and at the end of thread rolling has a thread crest height at substantially a groove depth of a last one of the plurality of portions of the die set.

10. The apparatus according to claim 9, wherein the thread forming grooves of the last one of the plurality of portions of the die set are at a depth and shape whereby the crest of the threads on the cylindrical work piece are reshaped to a desired height.

11. The apparatus according to claim 9, wherein the thread forming grooves of the last one of the plurality of portions of the die set are at a depth and shape whereby the crest of the threads on the cylindrical work piece are reshaped to a desired shape.

12. The apparatus according to claim 9, wherein the thread forming grooves of the last one of the plurality of portions of the die set are at a depth and shape whereby the crest of the threads on the cylindrical work piece are reshaped to a substantially flat peak.

13. A method for rolling threads on a cylindrical work piece and for correcting a major diameter thread crest, said method comprising the steps of:

rolling threads on a cylindrical work piece with a first die set, wherein the first die set has thread forming grooves at a first depth; and

re-rolling the threads on the cylindrical work piece with a second die set, wherein the second die set has thread forming grooves at a second depth, wherein the second depth is less than the first depth.

14. The method according to claim 13, wherein the step of re-rolling the threads on the cylindrical work piece with the second die set includes the step of reshaping a crest of the threads on the cylindrical work piece to a desired height.

15. The method according to claim 13, wherein the step of re-rolling the threads on the cylindrical work piece with the second die set includes the step of reshaping a crest of the threads on the cylindrical work piece to a desired shape.

16. The method according to claim 13, wherein the step of re-rolling the threads on the cylindrical work piece with the second die set includes the step of reshaping a crest of the threads on the cylindrical work piece to a substantially flat peak.

17. A method for rolling threads on a cylindrical work piece and for correcting a major diameter thread crest, said method comprising the steps of:

rolling threads on a cylindrical work piece with a die set, wherein the die set comprises a first portion with thread forming grooves at a first depth, and a second portion with thread forming grooves at a second depth, wherein the second depth is less than the first depth;

wherein during the step of rolling threads in the first portion of the die set the cylindrical work piece has thread crests at substantially the first depth and during the step of rolling threads in the second portion of the die set the cylindrical work piece has thread crests at substantially the second depth.

18. The method according to claim 17, wherein the step of rolling threads in the second portion of the die set includes the step of reshaping a crest of the threads on the cylindrical work piece to a desired height.

19. The method according to claim 17, wherein the step of rolling threads in the second portion of the die set includes the step of reshaping a crest of the threads on the cylindrical work piece to a desired shape.

20. The method according to claim 17, wherein the step of rolling threads in the second portion of the die set includes the step of reshaping a crest of the threads on the cylindrical work piece to a substantially flat peak.

21. A method for rolling threads on a cylindrical work piece and for correcting major diameter thread crests, said method comprising the steps of:

rolling threads on a cylindrical work piece with a die set, wherein the die set comprises a plurality of portions with thread forming grooves at progressively smaller depths;

wherein during the step of rolling threads on the cylindrical work piece the threads are formed to a desired shape.

22. The method according to claim 21, wherein the step of rolling threads includes the step of reshaping a crest of the threads to a desired height.

23. The method according to claim 21, wherein the step of rolling threads includes the step of reshaping a crest of the threads to a substantially flat peak.