Note: Descriptions are shown in the official language in which they were submitted.
DA~
This invention relates to self-tap,ping screws, as well
as methods and equipment for producing ~uch screws,
The fastener manufacturing industry has had the problem
of making threaded metal fasteners possessing at once charac-
S teristics of ductility and toughness, yet having relative threadhardness, The problem is particularly acute in connection with
self-tapping screws, though it may be encountered in different
environments, too,
By being ductile or tough, a metal i9 considered herein
as being pliant yet strong, as having the ability oE being bent
and/or twisted without tearing or br0aking. Antonymously, the
term "hardness" is u~ed herein to connote pronounced suscepti-
bility to breaking, i.e, brittlene~, in addition to resistance
to penetration, In the pertinent industry, it is customary to
use the Rockwell testing devices for measuring hardness. Using
such devices, a metal is penetrated by a probe of known hardness,
such as a diamond, with a predetermined force. The depth of
penetration gives an indication of hardness. One o~ a series of
nur~bers is assigned directly to every penetration depth. The
results of such tests are ordinarily expressed on the scales
designated Rockwell A, Rockwell B and Rockwell C. The higher
the number given as a result of such a test, the harder the
metal, Thus, a metal having a hardness corresponding to a
reading of Rockwell C = 30 or less would be considered ductile
and is used in the manufacture of relatively tough, ductile
fasteners,
A screw made from such tough, ductile material generally
fails as a self-ta~ping ~crew. A self-tapping screw is one
which forrns its own mating thr0ad, or threads (if there are two
parallel threads), in a material into which the screw taps
DA-ll
109~l965
itself, when rotated under the application of an axially oriented
~orce, In one form of self-tapping screw, a previously drilled
hole initially receives the more or less pointed, so-called
piIot end of the screw, and then the thread on the self-tapping
screw forms a mating thread by partially displacing material
around ~he hole, substantially without removing any such
material. This type of screw is frequently referred to as a
thread-forming screw, as di~tinguished from the thread-cutting
screws which, mostly by virtue of axially oriented cutting edges
acro8s the thread, particularly in the pilot end, actually cut
their mating thread by removing material in which the initial
hole has been drilled. As a specific form of thread-forming
screws, the so-called self-drilling variety should also be men-
tioned, This type of self-tapping screw has a pointed pllot end
whose thread drills its own hole, thus avoiding the need for a
previously drilled hole in the material into which the screw taps
itself, Any one of these classes of self-tapping screws i9 fre-
guently subjec~ to fai~ure, inasmuch as such failure occurs when
the ductile or tough thread on the fastener collapses within the
unthreaded hole.
In general, the hole for thread-forming self-tapping
screws should be sized to produce approximately 8~ thread
engagement, This value can vary, however, with the flexibility
of the materials used, Thread-cutting self-tapping screws, on
the other hand, are manufactured to function as a tap to cut
threads into the material being joined, Usually, as a thread-
cutting screw cuts into the mate~ial, chips are generated and
pushed ahead of the screw, For further explanations and illus-
trations, see, for example, "Solutions to Plastic Fastening Prob-
lems" in Assembly Engineerin~, September, 1971.
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DA-ll
~9~ti5
Various methods are presently used to make the fastener
harder only on it~ surface while maintaining toughness by virtue
of the unchanged ductility of the internal part of the body of
the screw, descriptively referred to in pertinent literature as
a core, The most common, presently used process is a three-step
process, First, the fastener is heat-treated to approximately
1700 F or(more, The fastener is then, in a second ~tep, sub-
jected to a treatment commonly and descriptively designated
~Ica~e-hardening~, inasmuch as a hardened shell, i.e~ case or
casing, is produced around the core, which is the interior o
the body of the screw which retains its ductility. In this
second step, the astener's surface i9 thoroughly cleaned, heated
to approximately 1250 F to 1300 F and placed into a carbon-rich
atmosphere, i,e, an atmosphere containing a large proportion of
hydrocarbons. In such an environment, the metaI has a tendency
to attract the carbon. The usual result is a carbon layer of
approximately 0.004 of an inch to 0,006 of an inch in thickness
deposited upon the fastener' 5 surface, The third procedural step
is an induction heating treatment, In this step, the pilot end
of the threaded fastener is placed in a rapidly changing electro-
magnetic field, Upon cooling, it is found that the molecular
structure in a surface layer of at least approximately 0,06 of
an inch in thickness of the pilot end has been changed. The
result of such treatment, unfortunately, hardens the fastener and
makes it more brittle, In fact, such an induction heating pro-
ce~ure with the carbon layer, known as "carburization", has made
test fasteners possess a hardness in the order of Rockwell C = 45
and higher.
To manufacture these relatively hard fasteners in -the
currently known manner, a premium alloy steel containirlg an
10~1965 DA-ll
additional metal component is required, Such additional me-tal
could be manganese, chromium, nickel and the li~se, which makes
these alloys expensive, Furthermore, the three-step procedure
outlined above requires continuous attention and careful handling.
Prior to case-hardening, for example, the fasteners, and parti-
cularly the thread root areas between the threads, must be
thoroughly cleaned to remove the lubricating oils and other grit
of manufacture, so that the carbon will become evenly and finely
deposited upon the surface of the fastener~ The composition of
the carbon-rich atmosphere must be precisely controlled and
require~ constant attention,
Additionally, the induction heating step requires meti-
culous positioning of the fasteners with only the pilot end
exposed to the field, in order to prevent that the entire
fastener be 90 treated. Moreover, the equipment for the induc-
tion heating~step is complex and very costly.
The result of all of these manipulations and attendant
expenses is a Eastener which has substantially lost the desired
ductility and toughness, and in some instances has not even
achieved the desired hardness. For example, a fastener having a
Rockwell C = 45 measure of hardness would be too hard and brittle
in certain applications where predictable shocks require the
fastener to have a certain resiliency, ductility and toughness.
Yet, the thread~ of such a fastener, even after case-hardening
and induction treating, have collapsed when attempts were made to
produce and then use such fasteners in the form of self-tapping
screws.
A self-tapping screw or other threaded fastener made of
comparatively tough, resilient material possessing hardness
characteristics necessa~y for successful use as self-tapping
screws has been long sought but was heretofore unavailable~
--5--
~091965
This invention is based on the recognition that
this long-felt need could be filled, specificallv with
respect to self-tapping screws, by producing them from
comparatively tough, ductile material, and providing a com-
paratively hard substance deposited at, and bonded to,
points of maximum wear, ~.e. on the thread crests exclusively.
To achieve this resuIt, t~e threads ma~ be subjected to an
electrode-depOsitiOn treatment ~h;~ch preferahl~r advantageousl~
immediately folloT.rs, in one s~ngle pass, the thread-rolling
step wherein the threads are rolled onto cylindrical blanks.
S~rews with the resulting thread structure are particularl~
useful in the case of seIf-tapping screws having self-locking
features, as described below, and for high strength fasteners,
norm~ formed o mater~als wh~ch are adversely affected
bv conventional hardening methods, such as case-hardening.
~ ne aspect of a specific method resulting in a
self-tapp~ng screw of the invention involves the eIectrode-
deposition, which is assumed to be associated with impregnation,
upon t~e screw threads or other metal parts as a production
phase o~ the machin~ng, i.e. thread-rolling, process. Unlike
other coating processes which reauire a high degree of clean-
liness, the car~ide deposition, i.e. reinforcing by coating,
process su~gested here~n can tolerate the precence of coolant,
lubricating fluid and machining grit, and thus can be incor-
porated into manufacturing metal-working processes with little
additional expense.
As used herein,~the phrases "eIectrode-deposition"
and "electrode-deposition technology", described and illu.s-
trated further beIow in more detail, are intended to denote
a class of methods, and the relevant technology, wherein the
suhstance of an eIectrode is deposited from the electrode
upon a workpiece under the application of electric power
between the electrode and the workpiece. T~e depositi~n
mb/ )~7`~ 6 -
.. . . .
19~S
effect is. achieved under the formation of an electricalarc, resulting in a strongly adhering deposit constituting
a coating on the workpiece, so th.at re.ference to impreg-
nation is justified, inasmuch as penetration of the elect-
rode material into -the material of the workpiece, as it is
deposited, may safely be assumed to occur. The strong
bo~d, thus achieved, may be explained by the molecular
forces between the mutually penetrati.n~ materials of
deposit and workpiece along the interface.
~hus in accordance with a broad aspect of the
invention, there is provided a se.lf-tapping screw having
a relatively ductile core and carb.on-containing material
substantially harder than the relatively ductile core
deposited on to the crest, or crests, of its thread in at
least the area of the pilot end of the screw, wherein the
relatively ductile core extends to, and is defined by,
surfaces pertaining to grooves consti.tuting the root of
the thread ~etween relatively hard crest areas.
In accordance with specific features of an
embodiment of the invention, a self-tapping screw has
relatively hard sur~ace areas of the crest, or crests, of
the thread only in the area of the pilot end of the screw.
There may be discrete, relatively hard sur~ace areas of
the crest, or crests, disposed along the circumference o
a turn, or turns, of the thread. Alternati~ely, a relative-
ly hard surface area of the crest, or crests, along the
entire circumference of a turn, or turns, of the thread
- may be provided. In the case of self-tapping screws of
the thread-cutting variety, the. relatively hard surface
areas of the crest, or crests, suitably include cutting
edges o-f axial notches in the thread. ~s to useful mater-
ials, generally any metal or alloy having.the des:;re~ ~uctility and
toughness may be selec-ted and gocd results have been.a~e~ed when
the core was made oE lo~-cæbon or a.luminum. With ~ .
-- pg/~ - 7 -
s
DA-ll
respect to the relatively hard surface area, or areas, of the
crest, or crests, a deposit of titanium carbide or tungsten
carbide has been found particularly suitable, as such deposits
lead to successfully tested reinforcement effects.
The invention will become better understood~from the
following detailed description of several emhodiments thereof,
when taken in conjunction with the drawings, wherein:
Figure 1 illustrates, in a side view, one embodiment of
a self-tapping screw in accordance with the
invention'
Figure 2 is a partial, relatively enlarged cross-
sectional view taken al.ong an axial plane
through the pilot end of the screw of Figure l,
Figure 3 schematically illustrates, in.a perspective view,
one emhodiment of a combination of a thread-
rolling machine with electrode-deposition
equipment in accordance with the invention,
Figure 4 is a schematic top view of an al-ternative
embodiment of a combination of a thread-
rolling machine with electrode-deposition
equipment,
Figure 5 is a schematic, partial side view of a different
embodiment of thè electrode-deposition equipment,
Figure 6 is a cross-sectional view, taken along line
6-6 of Figure 4,
Figure 7 is a fragmentary perspective view of yet another
alternative embodiment of the combination of
thread-rolling and electrode-deposition equip-
ment, and
Figure 8 is a cross-sectional view taken along line 8-8
of Figure 7,
1 ~9 ~ ~ ~ S DA-ll
To satisfy the aforementioned, seemingly contradictory
needs in a single threaded fastener, particularly a self-tapping
screw, a new construction for such screw has been developed which
is shown in Figures 1 and 2 of the drawings. As a representative
example of possible applications, a self-tapping screw of the
thread-forming variety having a head 12 and the conventional
pilot end 14, described in more detail below, is shown. The
fastener has a shank 10 with a helical screw thread 22 thereon,
The shank 10, head 12, pilot end 14 and substantially the entire
thread 22 are made from a relatively ductile, possible resilient,
metallic material which is ductile and tough enough to stand up
to the stresses under which the fastener could be -foreseeably
placed. The pilot end 14 of the screw of Figure 1 is shown in
enlarged cross section and more detail in Figure 2,
The thread 22 of the pilot end has applied to its outside
edge, conventionally referred to as the crest 24, a small layer
26 of metal, or other relatively hard substance such as, in
some successfully tested examples, tungsten carbide or titanium
carbide~ A~ such substance is harder than the ductile material
used when initially making the body of the screw, such as the
shank 10 and other parts, the illustrated screw has a relatively
ductile core and relatively hard surface areas of the crest, or
crests, of its thread, and the relatively ductile core extends
to, and is defined by, surfaces pertaining to grooves constitut-
ing the root of the thread between relatively hard crest areas 26A preferred construction i3 ko produce the deposit only upon the
crest 24 of the forward turns of the thread 22, i,e, at the pilot
end 14 of the screw. As shown in Figure 1 and producecl suitably
as described further below, there may be discrete, relatively
hard surface areas 26 of the crest disposed along the circumference
,. .
,. ~.
19~5
of a turn, or turns, of the thread. When the screw is forcibly
introduced into a non-threaded or poorly grooved hole, the
reinforced hardened crest 24 at the pilot end 14 will form a
correctly shaped, mating thread therein. The following non-
treated thread 22 of the shank portion closer to the head 12
will then fit within the mating thread formed by the self-tapping
pilot end 14 so treatedO No undue axial stress will be placed
on the base,'i.e.- ~he root, of the thread 22 where it joins the
shank 10 in such a construction.
Such an impregnated construction of a screw,
wherein the word "impregnated" connotes the penetration of mater-
ials when the layer 26 is deposited upon the crest 24 at the pilot
end 14, is especially useful when employed in combination with the
recently developed self-locking, ribbed screw thread 23 described
in United States Patent No. 3,517,717 and in Canadian Patent
Application Serial No. 157,345 filed November 21, 1972. The
thread in such a screw, known under the trademark ORLO ~, has a
resiliently bendable rib which protrudes outside the boundary of
the flank of a normal thread. The thread 23 in Figure l r'epre-
sents such thread shaped in accordance with this patent, or
patents. The rib of the thread 23 is forced against the remaining
thread when the screw is inserted into a conventional mating
internal thread in a nut or a tapped hole. A ~ery superior grip
is maintained by the outward resilient tension exerted by the rib
of the thread 23. It can be seen that a rela'_ively ductile, tough
and non-brittle material is preferable for the success of such a
self-locking screw thread. Moreover, a precise, correctly formed
mating thread in the nut or hole is highly desirable, in order to
supply the surface against which the resiliently bendable rib will
be forced.
)~
.
~V~ 5 DA-11
A self-tappincJ screw having its thread 22 reinforced by
a hardening deposit ~6 at the pilot end 14 of the shank 10, as
described herein, will self-tap a precisely mating thread, which
then will constitute a nut or a tapped hole. The portion with
thread 23 formed beyond the pilot end 14 and thus beyond the
hardened crest portion will accurately fit into the thus sel~-
tapped nut or hole and the resiliently bendable rib will provide
the desired self-locking ef~ect.
Self-tapping screws o~ the thread-cutting variety, not
shown, frequently have their pilot ends notched, i.e. grooved,
in the axial direction. The axial groove has an axial edge which,
when the screw is forcefully rotated in a hole, cuts its mating
thread into the material around the hole. In the past, such
cutting edge necessarily had to be of material which was harder
than the material around the hole which was being cut. It has
now been found that a hardening, thus reinforcing, deposit upon
the edges provides sufficient hardness to the cutting edges to
permit them to cut the hole itself and to cut the matiny threads
in the material around the hole. Such a satisfactor~ thread is
cut despite the fact that the main body, and therewith the core,
of the screw is made oE a tough, ductile material which may even
be softer than the material surrounding the hole and to be cut.
Other resilient, self-locking screw thread arrangements
have also been proposed in the fastener industry. For example,
one such arrangement involves the displacement, at regular inter-
vals, of a portion of the entire crest along several turns of the
thread of a fastener. The so-called "impregnation", i.e. appli-
cation, of hardening material on these modifications of screw
threads avoids the need ~or sur~ace-hardening by heat treatment
or case-hardeningj thus preventing the otherwise resultant
--11--
DA-ll
degradation and embrittlement of these types of self locking
structures when used on self-tapping screws.
It has been found that the resultant screws possess a
hardness at the hardened crest 24 which is comparable to a metal
having a hardness Rockwell C = 70, or more A screw having its
thread so treated, it has been found, does not require case-
hardening nor induction heating treatment to possess the required
self-tapping capability. Elimination of these steps, as compared
to known processes, allows the use of substantially cheaper
metals in making the screws. By omitting the case-hardening and
induction heating steps, further, the desired ductility of the
screw is kept, because the entire core, reaching the roots and
the major portions oE the Elanks of the thread, reaches, i.e.
extends into, the thread itselE. As clearly illustrated in
Figure 2, only the hardened areas 26 do not pertain to what the
industry calls the core. The remaining, major portion of the
screw forms part of the core. The result is a screw made from
relatively inexpensive material and having a ductility corres-
ponding to a hardness of approximately Rockwell C = 30, or less,
while having sel~-tapping pilot end thread crests with a hard-
ness of approximately Rockwell C = 70, or more.
Experiments with aluminum screws thus described indicate
that such a limited hardening deposit will allow, as presently
believed, for the first time, aluminum and other non-ferrous
metals and low-carbon steels to be used in the pro~uction of
self-tapping screws of any variety.
Having described the self-tapping screws of the invention
in connection with Figures 1 and 2, methods for producing the
screw, as imp].emented by equipment operable in accordance with
the methods, will now be discussed.
-12-
~ 96S DA-ll
The method of producing self-tapping screws of the type
disclosed herein is sub~tantially less difficult and complex
than the method of preparing case-hardened screws requi.ring
induction heating treatments in the presently known methods.
The tungsten or titanium carbide, constituting a particularly
suited hardening substance, can be deposited upon the crest o
the thread, or threads in case of parallel threads, in any of
currently known methods, It is preferred, however, that the
presently known electric arc, vibrating electrode-deposition
.10 technology be used. This procedure is described in detail in
United States Patents Nos. 3,415,970 and 3,524,956, The
electrode-deposition equipment is suitably positioned such that
it operates upon each screw immediately at the end of the thread-
roll forming step performed by a thread-rolling machine, or
equipment, as described in more detail further below, One of
the many advantages gained by combining thread-rolling techno-
logy with electrode-deposition technology resides in the fact
that a thoroughly cleaned thread is not required for the carbide
deposition. Thus, there is no need to remove the lubricating
oil used, nor to remove the grit of manufacture produced in the
thread-rolling process. The high voltage applied to, and the
impact force of, the electrode, or electrodes, during operation,
are sufficient to deposit electrode material upon a screw through
the oils and grit, i.e, in spite of thei.r presence.
During this treatment, the temperature of the core of
the screws, thus produced, remains substantially the same, and
therefore the ductility is not changed, The screws may then be
heat-treated ~or neutral hardness, thereby to increase their
strength. Such a heat treatment up to approximately ]i350 F will
not affect the carbide deposi-t, see Tun~Lsten, 3rd Edition (1955),
~3
DA-ll
by Li and Wang (A~.erican Chemical Society Monograph No. ~4), on
page 39~. It has been found that, under use of electrode-
deposition technology, also descriptively referred to as elec-
tric arc impregnation, some of the thread crest is removed, but
that the deposited carbide substantially compensates therefor.
The carbide so deposited impregnates, and thus anchors within,
the thread crest or other selected parts oE a screw to a depth
of approximately 0.~01 of an inch. Moreover, it forms an addi-
tional buildup of approximately from 0.001 of an inch to 0.0015
of an inch beyond the original surface. The deposit is solidly
implanted upon, and penetrates into, the screw. This explains
why it is highly resistant to impact and breaking, as has been
found.
It is not necessary to deposit the carbide smoothly or
evenly upon the thread crest. Indeed, it is preferred that the
deposition occurs unevenly, resulting in a rough surface condi-
tion which, along the crest line, is comparable to a sawtooth-
type configuration upon the crest 24. Such an irregular appli-
cation is illustrated in Figures 1 and 2 where the hardening
deposit 26 at one location is shown to be somewhat thicker than
at another location. Generally speaking, but particularly when
a screw is intended to remain permanently, once it has been
insexted into another material, it was sufficient to apply the
deposit at only a selected number of discrete, mutually spaced
locations along at least one turn, preferably at the pilot end
of the thread.
It can be seen that a treated body, such as a scre~7 of
the self tapping class, provides a harder and sharper thread
crest for functional forming and cutting its mating thread, yet
the core, ~7hich constitutes the major portion of the whole body,
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DA-ll
~(~9~65
is not brittle and consequently useful where known products
failed. Moreover, it is not susceptible to chipping in ship-
ping and use.
It is contemplated that the deposition of carbide or a
similar substance can be made upon the crest of the screw ~hread
during a single pass by producing the deposit immediately subse-
quent to the thread-rolling step in a slightly modified, otherwise
conventional, thread-rolling machine. In the schematic illus-
tration of Figure 3 is shown a die block 40 and die block 42,
constituting elements of such conventional machine. Die block
40 represents the stationary die, while die block 42 is the
movable die. In the conventlonal and well-known method of manu-
facturing threaded bodies, a cylindrical blank 44 is rolled
between the dies 40, 42 as the die 42 is forcefully moved in the
direction of the arrow. Threads are formed on the blank 44 by
the ridges between mating grooves 46 and ~8 on the respective
working surfaces of the die blocks 40 and 42. The ~acing grooves
46, 48 on these working surfaces are so matched that, when the
blank 44 is rolled between the two dies 40, 42, a continuous
thread, or a set of parallel continuous threads, is, or are,
fo~med on the shank of the blank.
A slightly different thread-rolling arrangement is illus-
trated in Figure 4. As in Figure 3, a stationary die block 50
is shown having grooves 52 on its working surface, and a die
block 54 is movably positioned with its working face opposite
that of die block 50. Die block 54 has grooves 56 so that when
a blank 58 is rolled between the dies 50, 54 when die block 54 is
moved in the direction of the arrow as shown, a conkinuous thread
or a set of parallel continuous threads, is, or are, formed on
the blank 58.
-15-
DA-ll
The die blocks constituting dies 50, 54 differ from the
dies 40, 42 in that the dies of Fi~ure 4 have grooved ridges 5~,
56 which approach each other in such a way as to form, in
accordance with known techniques, a threaded point which will
be the pilot end of the resulting screw. The thread-rolling
machine schematically illustrated in Figure 3 does not form suGh
a pilot end.
The pilot end of a threaded body, such as a screw,
generally is the end of the screw shank opposite the head of the
screw. This end normally has a gradually increasing outside
thread diameter beginning from the shank's end. The inside, or
root, diameter of the thread may or may not be tapered in corres-
pondence with the -thread's outside diameter in order to have a
useful pilot erld. The tapered length may have no thread at all
at the pilot end, although, normally, the thread will extend
over such tapering. The screw produced by the dies of Fig~re 3,
for example, will not have a pilot end.
With a modification of conventional thread-rolling
machines, the deposit causing a qurface-hardening ef~ect only in
limited surface areas, such as a deposit of carbide or metal, can
be made automatically and effectlvely upon the threads of each
screw as it is ejected from the space between the two dies of
the machine. In each of the machines illustrated in Figures 3
and 4, an extension 60 of the stationary die block 40 or 50~
respectively, is shown. This extension 60 is a support for at
least one electrode 62, suitabl~ three such electrodes, of the
type described in United States Patents Nos. 3,524,956 and
3,415,970, mentioned above. Such an electrode 62 of tungsten
carbide or other similar material, during operation, causes a
deposit of which approximately 0.001 of an inch penetrates into,
-16-
DA-ll
and thus impregnates, a metal sur~ace, and additionally forms a
layer of approximately 0.001 of an inch, for example, oE the
carbide, protru~ing from the original surface.
The extension, i.e. support, 6~ is constructed and posi-
tioned such that it makes electrical contact with the newly
formed threads of the screw, thus constituting a guide element
for each screw and, at the same time, closing the electrical
circuit by which power is applied between each screw, as it is
guided by support 60, and one after the other of the electrodes.
The illustrated energizing circuits are believed to be self-
explanatory.
A plurality of electrodes 62 may be placed in the support
60, as is illustrated in Figures 3 and 4. As explained in the
United States patents mentioned above, the electrodes 62 are
caused to vibrate by a mechanism suitable for this purpose and
not shown in the drawings, as corresponding explanations can be
found in the pertinent literature, such as these patents. By
virtue of these vibrations, the electrodes 62 rapidly move into
and out of contact with the newly formed thread crests. The
electrodes 62 are preferably positioned so that only the end,
such as the pilot end 14, will receive deposits 26. Alterna-
tively, the electrodes 62 can be positioned so that the entire
axial length of the thread along the shank will be impregnated
as the result of the electrode-deposition treatment. Also, one
continuous electrode 64 could be used, and positioned as illus-
trated in Figure 5. The continuous electrode 64 would ef~ectively
cause a deposit along the entire circumference oE the thread
cxest, or crests, at the pilot end, during a combined thread-
rolling and electrode-deposition treatment in which the second
immediately follows the first.
-17-
DA-ll
6S
A useful thread-rolling equipment could be constructed
wherein the moving die 42 or S4 could cause energization of the
electrodes 62 at a predetermined moment, namely when the blank
44 or 58 reaches the support 60. A pressure-actuated switch 70,
as schematically shown in Figure 4, or alternatively a lever, a
radiation sensor or any other well-known electric circuit-
actuating device could be employed to caus~e the electrodes 62
to become energi2ed at the desired moment and remain energized
for the desired time period.
It has been found that producing discrete areas of the
hardening deposit 26 upon the tapping portion oE the shan~ of the
screw, which includes the Pilot end approximately every 120,
provides satisfactory sel~-tappinq capabilities in most instances
of later use of the screw. Thus, the three electrocles 62 shown
in Figures 3 and 4 are carefully positioned so that the screws
rolled from blanks 44 or 58 will be caused to perform their
rolling motion past the electrodes such that discrete deposits
are made at precise circumferential distances along the circum-
ference of the thread on which it is desired to produce such
~0 intermittent deposition. By way of illustration, the distance
between the electrodes 62 in Figure 3 will be exactly equal to
that between points which are 120 apart along the clrcumference
of the thread on the screw formed from blank 44.
The electrodes 62 may be triggered into energization by
the moving die 42 or 54 so that they will operate only when the
screw, as it has been roll-formed from a blank 44 or 58, approaches
the guide element, i.e. support, 60, a~d the grooves 48 or 56 of
the moving die have moved beyond the screw. It was found advan-
tageous to provide the moving die 42 or 54 with an extension 68
shown only in Figure 4. Such an extension 68 will not have
-18-
DA-ll
~O~ 5
grooved ridges. Rather, it will serve as a second guide element,
as it assists in the continuation oE the rolling motion of the
screw past the electrodes 62 or 64 after the conventional thread-
rolling operation has been completed,
If there is a pilot end on the screw, such as screw 58
shown in phantom in Figure 6, the electrodles 62 preferably have
a canted electrode tip. Such a canted elec-trode tip may be at
an appropriate angle to fit complementally against the taper o
the diminishing outside diameter of the thread on the pilot end
of the screw 58. Such a complemen-tally cantecl fit is illus-
trated in Figure 6. This type of specially formed electrode
tip also has particular advantages when used to transfer
hardening deposits onto the thread-cutting edges o~ an ax.ially
grooved thread of this type of self-tapping screws. The thread-
cutting edge (not illustrated) will thus be completely treated
along its axial length.
If a self-tapping, thread-cutting type pilot end is
desired, it may prove to be advantageous to cause the deposi~ion
upon the self-tapping, thread-cutting type pilot end of the
shank from the lower side of the thread-rolling mach.ine, when
viewed as illustrated, as opposed to deposit.ion from the side~
as shown in Figures 3 and 4. In Fi.gures 7 and 8, for example, a
basic scheme for electrode-deposi.tion upon a threaded body on its
tip end from the bottom is shown A stationary thread-rolling.
die 80 is shown having an extension 82. The extension 82 is a
guide element, as the support 60, ~igures 3 and 4, but does not
carry an electrode. It extends only so far down from the neck of
the screw 84, illustrated as it has just been formed from a
blank, so as to retain the screw 8~ in its -travel for a short
distance beyond the thread-roll.ing dies.
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A die 86 movable in relation to d.ie 80 is shown having
an extension, namely guide element 88, compa.rable in shape and
purpose to extension 82. The die 86 and guide element 88 move
in the direction of the arrows shown in Figure 7 when the screw
84 is being thread-rolled and the threads thereon are being formed.
An electrode 90 oE the description given above is posi-
tioned in axial parallelism with the screw 84 by a suitable
support 92. The electrode 90 is electrically connected to the
power source, not shown, by a connecting cable 94.
Electrode 90 is shown having a slanted tip 96 which,
during the travel of the screw 84, will come into cngagement
with the selE-tapping thread-forming type pilot end 98 of the
screw 84, when the screw comes into axial alignment with -the
electrode. If the pilot end 9B had axial grooves across the
thread, not shown in Figures 7 and 8, the cutting edges and the
crests of the tip end 98 of the screw 84 will receive, in these
limited surface areas, a deposit of a hard substance, such as
tungsten carbide or titanium ca.rbide, transferred immediately
after the thread-rolling process in a single pass, and thus in
a manner which is econom.ical and efEicient, because the process
is conducted at very high speeds. When the screw 84 is rollecl
in the direction of the arrows in Figure 7, the extensions 82, 88
operate as guide elements, as they cause the roll-formed screw 84
to continue its rolling motion for a short distance beyond the
thread-rolling dies 80, 86. The self-tapping pilot end 9~ of the
screw 84 will then be brought into close proximity to the slanted
tip 96 of the carbide electrode 90. The electrode 90 may be
triggered into energization by an actuating device 100, and l~e
caused to vibrate, thereby to come into rapicl and repeated con~
tact with the pilot end 98. Actuating device 100 may be a switch
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or a sensor, operated by the screw as it leaves the space between
the dies, and electrically connected to cable 94 so as to cause
energization of the electrode 90 for selected, predetermlnecl
periods sufficient to ensure that electrodle 90 will be operating
when the screw 8~ is rolling through successive positions of
axial alignment with the electrode 90.
It is of interest to note that vertical electrodes, as
shown in Figures 7 and 8, and horizontal or angled electrodes,
as shown in ~igures 3, 4, 5 and 6, may be combined in the same
equipment composed of both a thread-rolling machine and an
electrode-deposition arrangement, so that the threads at the
pilot end, as well as threads in other or all areas of the shank,
oE the screw will be subjected to the treatment described. Such
dual deposition treatment could be caused to occur simultaneously
or seriatim.
Self~tapping screws of the thread-forming, thread-cutting
or self-drilling type, when produced as described herein, are
particularly useful when made of aluminum, other non-ferrous
metals or their alloys. The carbide deposit on the crests of
the thread, or threads, of the selE-tapping screws, particularly
a-t the pilot end, allows an aluminum or other non-ferrous snetal
screw to be used in the assembly of parts made Erom light-gauge
steel and aluminum sheets.
It will have become clear that, in accordance with the
combined technology described herein, the hardening deposit 26 is
~ade only in those surface areas of a self tapping screw which
will displace material into which the screw will be caused to
tap itself. The result of this combination of technologies is a
screw having a relatively ductile core and relatively hard surface
areas of the crest, or crests, of its thread, with the specific
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feature that the relatively ductile core extends -to, and is
defined by, surfaces pertalning to grooves constituting the
root of the thread between relatively hard crest areas,
Reviewing the present invention, it may be noted that a
new technique for economically producing high strength self-
tapping screws has been developed~ The body of such screws may
be made. of tough, inexpensive low-carbon steels or of non-
ferrous metals, and are not subject to the embrit-tlement or other
adverse effects of conventional heat treatments for surface
hardening. The hardening deposit, or deposi-ts, may be placed on
two, three or more turns on the lead, i,e, forward, threads of
the self-tapping screws, thus providing self-tapping capabilities
without impai.ring the basic toughness of the body of the screw,
Also, the efficiency of self-locking features provided by
5 resiliently bendable ribs of the threads of screws is preserved,
This application is a divisional of Canadian Patent
Application 196,237 filed March 28, 1974.
a~
