Note: Descriptions are shown in the official language in which they were submitted.
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THREAD ROLLING AND lMPROVED FASTENER
Back~round of the ~lvention
The rol~ing of threads has become accepted as the method for producing
superior threads for externally-threaded fasteners. }-~owever, an unforlunnte
characteristic of rolled threads is the relatively long runout at the inner end of
5 the thread where the thread contour is not complete. During the thread rollingprocess, the fastener blank is rolled between two opposed dies having ridges
complementary to the thread to be produced. These ridges run out at one edge
of the die, which is the location where the inner end of the thread is formed.
Inasmuch as the ridges are at a shallow acute angle with respect to the die edge,
10 a feather edge is produced on these ridges which is susceptible to breakage.
Because of this, the ridges of the die are tapered as the die edge is approached,
becoming more shallow at the edge to provide adequate strength at that portion
of the die. As a result, two or more turns of the thread at its inner end usually
will be incomplete, having less than the full cross-sectional dimension of the
15 remainder of the thread. The incomplete thread can carry no load, and hence
does nothing to enhance the performance of the fastener. However, the fastener
shank must be made sufficiently long to include the runout thread at its inner
end, as well as the portion of the thread that carries the load. Typically, the
transition section where the thread runout occurs has a length axially of the
20 shank corresponding to twice the pitch of the thread.
Manv fasteners include a nut or collar with a counterbore to receive the
transition zone of the bolt or pin that includes the incomplete runout thread. It
is necessary for the counterbore of the nut or collar to have a length sufficient
to accommodate the runout thread. Therefore, the len~th of both the bolt and
25 nut are dictated by the requirement for the incomplete thread at the inner end
of the threaded portion of the bolt.
It has been recognized that making the pin or bolt shorter by reducing the
length of the transition zone with its incomplete threads would result in a saving
in weight of considerable significance in the aircraft and aerospace fie]ds, as
30 well as other areas where minimizing weight is critical. Nevertheless, conven-
tional thread rol:ling will not permit this.
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One aE~proach to reddcing the length of the thread runout has becn to first
form a groove in the transition zone to approximately the minor d;ametcr of the
thread to be produced. This is accomplished prior to rolling the thread, eithcr by
cutting to the required geometry or by cutting~ to ~ess than ful] dimcnsion,
S followed by cold rolling to produce a groove of the desired depth. After this, the
thread is rolled, with the runout extending into the grooved portion. The resultis a thread runout transition zone shorter than that of a conventional fastcner
with a rolled thread. A drawback to this system is the increased expcnse
incurred in the extra operation of forming the groove in the shank prior to the
10 thread-rolling operation.
Summary of the Invention
The present invention provides a f~stener having a significantly shortened
transition thread runout zone, yet without the necessity for producing a groove
in the blank prior to the thread-rolling operation. The thread on the bolt has a15 relatively abrupt beginning at its inner end, achieving its full cross-sectional
dimension much more rapidly than with conventional thread-rolling techniques.
The inner end of the thread terminates in a wall, preferably rounded concavely,
which is generally symmetrical about the longitudinal axis of the thread. ~he
end wall is relatively short, extending lengthwise of the thread preferably no
20 more than one-fourth the circumference of the thread at its pitch diameter.
This permits the transition ~one at the inner end of the thread to extend axially
of the shank a distance no more than a length corresponding to the pitch of the
thread. In other words, the transition zone is about half the length that it is in
fasteners with conventional rolled threads. This permits the counterbore in the
25 nut to be made shorter because it needs to accommodate only the shorter
transition zone of the bolt. Therefore, there is a weight saving for both the nut
and the bolt.
The ridges on the thread-rolling dies of this invention do not come to a
feather edge as they run out on the edge of the die where the inner end of the
30 thread is formed. Instead, they come to relatively abrupt ends which are
generslly symmetrical about longitudinal axes of the ridges rather than being
elongated and extremely asymmetrical RS in the conventional thread-rolling dies.The ends walls of the ridges are convexly rounded to produce the rounded inner
ends of the threads.
As viewed in elevation, the end walls of the ridges that run out to the die
ed~e increase very slightly in elcvation from the end of the die uhere the bLqnk
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enters to the end where i~ ]eaves. This is to flssllre that each ridge end rcachcs
the inner end of the thread groove being produced in the blank. The increased
elevation causes each die end to engage the blank slightly beyond the groove endas the blank turns between the dics. This is done because it is impossible to
5 a~sure that each ridge end will strike the blank in exactly the same place flS the
blank is rolled between the dies. Therefore, the ridge ends are caused to cngagethe b]ank progressively further into the groove to ensure that the inner cnd of
the groove is formed properly. The increment of added engagement at the end
of the groove is kept small in order to protect the dies against breakage.
Fasteners in accordance with this invention are advantageous even where
weight saving is not important. This is because the reduced length of the threadrunout results in a longer grip length for the bolt. This means that a bolt willhave increased versatility by being able to secure together articles of a greater
range of thicknesses. This allows bolt inventory to be reduced because fewer
15 sizes are necessary to accompl~sh a full range of fastening requirements.
Brief Description of the Drawings
Fig. ~ is a longitudinal sectional view of a prior art fastener having rolled
threads;
Fig. 2 is a longitudinal sectional view of a fastener made in accordance
20 with the present invention;
Fig. 3 is an enlarged fragmentary elevational view, showing the inner end
portion of the thread of the fastener pin of Fig. 2;
Fig. 4 is a perspective view of the dies performing the thread rolling
operation;
Fig. 5 is an enlarged fragmentary perspective view of one of the thread
rolling dies;
Fig. 6 is an elevational view of one of the thread rolling dies and the screw
blank in position for the thread rolling operation;
Fig. 7 is an enlarged fragmentary sectional view, taken along line 7-7 of
30 Fig. 6;
Fig. 8 is an enlarged fragmentary elevational view of a portion of one of
the thread rolling dies; and
Fig. 9 is an enlarged fragmentary view illustrating a typical contour of the
end of the thread-forming ridge of the die.
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Detailed Descri~tion of th~ Invention
Illustrated in Fig. 1 is a prior art shear fastener 10, commonly used in the
aircraft industry, shown here holding together panels 11 and 12. The fastcner 10includes a pin 13 having a flush head 14 from which projects a silank hnving thrce
5 sections. The first section 15 of thc shank adjacent the hcad 14 has a straight
cylindrical exterior surface of a relatively large diameter. The outer end portion
16 of the shank includes a rolled thread with a major diameter less than the
diameter of the section 15. Between the inner and outer portions of the shank isa shorter transition section 17 where the shank tapers from the diamcter of the
10 unthreaded section 15 to the smaller diameter outer portion 16 The pin 13 is
capable of accommodating panels or other workpieces whose combined thickness
does not exceed the grip length G, which is the distance from the outer plane ofthe flush head 14 to the transverse grip plane P at the end of the shank section15.
The thread extends for the full length of the outer portion 16 of the shank
and includes a gradual runout at its inner end portion 18 which extends into thetransition section 17 of the shank. In accordance with conventional practices,
the thread is incomplete at the runout portion 18, not having its full cross-
sectional dimension in that zone. The portion of the pin 13 occupied by the
20 runout of the thread, therefore, carries no load and cannot perform any useful
work as the fastener is used. The length T1 of the transition section 17 that
includes the incomplete threads typically is around twice the pitch of the thre~d
in order to accommodate the runout portion 18 of the thread.
Engaging the pin 13 is a collar 20 having an intermediate internally
25 threaded portion 21 that meshes with the thread on the outer end 16 of the pin.
The base part 22 of the nut 20, which bears against the panel 12, flares
outwardly and includes a counterbore 23 dimensioned to receive the transition
section 17 of the pin. At the other end, the collar 20 is unthreaded and includes
external wrenching surfaces 24, inwardly of which is an external peripheral
30 groove 25. The latter provides a frangible portion where the end of the nut can
break off upon the exertion of a predetermined torque as the collar is tightenedagainst the panel.
Illustrated in Fig. 2 is a fastener 27 of the same type as thnt of Fig. 1, but
made in accordance with the present invention. The fastener 27 of this invention35 includes a pin 28 and a collar 29 used in securing together panels 30 and 31,which are of the same thickness as the panels 11 and 12. The pin 28 includes a
flush head 32 from which projects R shank having an unthreaded, rclativcly large-
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diameter portion 33 adjac~nt the head, a transition section 34 and an outer end
portion 35 of reduced diameter on which is a rolled thread. The grip length G ofthe fastener pin 28, which is the length of the unthreaded shank portion 33 fromthe head 32 to the grip plane P at the beginning of the transition scction 34, is
5 the same as the grip length G of the fastener pin 13. The threadcd outer end
portion 35 also is of a ]ength cqual to that of the threaded end 16 of the pin 13.
The thread on the pin 28 does not have the conventional runout of
incomplete cross section at its inner end characteristic of the thread on the
shank end 16. Instead, the inner end portion 36 of the thread has its full
10 dimension almost to the point where it terminates. This enables the transition
section 34 to be made shorter than the transition sec$ion 17 of the conventionalfastener pin 13, because it does not have to accommodate a long thread runout.
The length T2 of the transition section 34 may be approximately equal to the
pitch of the thread on the shank end 35. This contrasts with the transition
15 section 17 of the conventional fastener which has a length equal to twice thethread pitch. Thus, although the grip length of the fastener pin is the same as
before, and the threaded section is of equal length to that of the conventional
fastener, the overall length of the pin has been reduced by the distance of one
thread pitch. This means that at a longitudinal distance from the grip plane P
20 corresponding to around one pitch of the thread, the thread will have its full
dimension.
The collar 29 also is made shorter than the collar 20 of the conventional
fastener. The collar 29 is similar in most respects to the collar 20, including an
outwardly-flaring base portion 38, an intermediate, internally-threaded por-
25 tion 39, and wrenching surfaces 40 on its outer end. A breakaway peripheralgroove 41 also is included. However, the counterbore 42 of collar 2~ is shorter
than that of the collar 20. This is because the pin transition section 34 is of
reduced length and a shorter counterbore will accommodate it. Accordingly,
both components of the fastener are of reduced length, and a significant weight
savings is realized.
Although the inner end of the thread begins abruptly, it is preferred to
avoid a flat inner end wall at the terminus of the thread, and instead to provide a
concave, rounded wall 43 of compound curvature. The re~sulting trnnsverse end
wall; which is generally symmetrical about the longitudinal axis of the thread,
35 will provide some taper at the inner end part of the thrend. This is to improve
the life of the thread-rolling dies and to avoid stress risers which could result
from sharp corners in the complcted fastener. The resulting runout preferably is
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no greater in length thar~ one-fourth the circumierence of the pin at the pitch
diameter, and frequently is less.
The thread on the pin 28 is produced by a pair of thread-rollin~ dies 46 and
4~, as illustrated in Fig. 4. One of these, the die 46, is movable, while the die 47
5 is stationary. These dies are flat, but cylindrical dies also can be constructed
embodying the principles of this invention. As shown in the drawing, the dies 46and 47 are used to roll threads on the outer end part 48 of the shank of a blank49, which is used to produce the completed fastener pin 28. The shank of the
bLqnk 49 has a portion 50 of larger diameter adjacent its head 51, and Q short
10 tapered transition surface 52 (be$ter seen in Fig. 6) between the end part 48 and
the portion 50.
The movable die 46, illustrated in enlarged detail in Figs. 5, 6 and 7 is
identical in configuration to the stationary die 47. The die ~6 includes flat
parallel longitudinal top and bottom edges 53 and 54, and a vertical face 55
15 which is used in producing the thread. Formed on the face 55 is a series of
parallel ridges 56 which are complementary to the thread to be produced, and
therefore generally V-shaped in end elevation. In accordance with standard
practice, these ridges include flanks 57 and 58 with a 60 included angle betweenthem. The ridges 56 are at an acute angle relative to the top and bottom edges
20 53 and 54, appropriate for producing a thread helix when the blank 49 is rolled
between the dies.
Between the top edge surface 53 of the die and the face 55 is a stepped,
beveled surface 59 which forms a part of the upper die edge. This surface is at
an angle of 25 relative to the top edge surface 53. Accordingly, the stepped
25 surface 59 is at only at a 5 differential with respect to the upwardly-fscing
flanks 57 of the ridges 56 that run out at the surface 59. In practice, the
surface 59 is made to blend with the flanks 57 that it intersects. The vertical
dimension of the surface 59 should be equal to at least twice the pitch of the
thread to be produced to assure adequate clearance as the thread is formed.
The ridges 56 that intersect the stepped surface 59 have relatively abrupt
ends 60 which preferably are convexly rounded, with compound curvature. As a
result, the ridges 56 have their full cross-sectional dimension, symmetrical on
either side of their longitudinal axes, at a location close to where they
terminate. The only runout of the ridges is provided by the transverse rounded
35 ends 60, which are generally symmetrical about the ridge axes. Preferably, this
runout does not exceed one-fourth of the circurnference of the thread to be
produced at its pitch diameter.
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The rounded ends 60 of the ridges 56 that extend to the surfncc 59 b~end
smoothly into flat, narrow surfaces 61 which extend to the top edge 53 surface
of the die. This results in the stepped configuration of the beveled surface 59,dividing it into segments, each of which connects to the flank 57 of one of the
ridges 56. The surfaces 61 are transverse with rcspect to the ridgcs 56, bcing
perpendicular to the longitudinal a~es of the ridges.
Lengthwise of the die, the ridge ends 60 are spaced apart a distance E that
is approximately the same as the circumference of the thread to be produced at
its pitch diameter. This encompasses some variation from the precise circum-
]Q ferential distance. For example, the distance E for a die to produce a fastener
pin of titanium may fall within the range of around ~T x 0.8 x pitch diameter ofthe thread to 1r x 1.0 x pitch diameter of the thread.
The spacing between the ridge ends 60 is made such that there is an
increase in height from one to the next from the die end 62, where the thread
15 rolling begins, to the opposite end 63. This effect is illustrated in Fig. 8, and can
be seen in Figs. 6 and 7, as well. As shown in Fig. 8, the ridge end 60 on the
right (toward the die end 63) is closer to the top die edge surface 53 by a small
distance D than is the ridge end 60 on the left. The same height differential D
of adjacent ridge ends applies throughout the length of the die.
The dies of this invention are operated as conventional dies, with one die
46 being moved longitudinally relative to the other die 4~. The screw b]ank 49 is
positioned prior to the threading operation as shown in Fi~s. 4 and 6, which
locates it adjacent the end 62 of the die 46 and perpendicular to the die edge
surface 53. Therefore, the inner end 43 of the screw thread that is produced as
25 the dies are actuated is formed by the rounded ends 60 of the ridges 56. The
result is the relatively abrupt termination of the screw thrcad at its inner end, as
described above.
The gradual increase in elevation of the ends 60 of the ridges 56 from the
die end 62 to the die end 63 is to make certain that each of the ends 60 will
30 strike the screw blank at the inner end of the groove being produced to form the
thread. This is because it is impossible, as a practical matter, to hnve each
end 60 engage the screw blank at precisely the same location. Thercforc, the
gradual increase in height of the ridge ends 60 assures that each succcssivc ridge
engages the blank at a position slightly beyond where the preceding ridge had
35 engaged it. At the same time, the increase in height is small from onc ridge to
the next, so that only a small increment of the unformed r~ortion of tllc screw
blank is engaged by the end of the ridge of the die. This avoids breaknr~c of the
ends of the die ridges.
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The beveled surface-59 is beneficial in providing clearAnce flt thc inner cnd
of the thread being produced, again protecting the ridges of the die to avoid
breakage.
As illustrated, the end surface 60 of each ridge 56 is symmetrical about the
5 longitudinal axis of the ridge in order to produce the symmetrical inner end ~3 of
the thread. In actual practice, if the ends of the ridges are polished by hand to
produce the curved end walls 60, they will not achieve a precise gcome~ric
symmetry, as the exposed side of the ridge at the flank 57 normally may be cut
away a small amount more than on the side of the flank 58. As used herein, the
10 term "generally symmetrical" as applied to the end of the ridge and the end of
the thread is intended to include such deviations.
Although illustrated with respect to a fastener having three shank sections,
the invention can be applied to fasteners having a uniform shank diameter. The
inner end of the rolled thread then will terminate abruptly, as in the embodiment
15 described, but in a straight shank section rather than a tapered transition zone.
The foregoing detailed description is to be clearly understood as given by
way of illustration and example only, the spirit and scope of this invention being
limited solely by the appended claims.
What is claimed is:
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