U.S. patent number 3,889,516 [Application Number 05/421,055] was granted by the patent office on 1975-06-17 for hardening coating for thread rolling dies.
This patent grant is currently assigned to Colt Industries Operating Corp.. Invention is credited to Robert J. Hlavaty, Herbert L. Yankee.
United States Patent |
3,889,516 |
Yankee , et al. |
June 17, 1975 |
Hardening coating for thread rolling dies
Abstract
A thread rolling die is disclosed for threading a generally
cylindrical bolt or screw blank. Such die is characterized by a
thin electric arc-applied coating of a hard metal or a carbide of a
hard metal which coating is applied only to the gripping surfaces
of the die. The thread forming or extrusion surfaces remain
substantially free of such coating. The present invention relates
to the die and to the method for increasing the useful life
thereof.
Inventors: |
Yankee; Herbert L. (North
Royalton, OH), Hlavaty; Robert J. (Avon Lake, OH) |
Assignee: |
Colt Industries Operating Corp.
(W. Hartford, CT)
|
Family
ID: |
23669001 |
Appl.
No.: |
05/421,055 |
Filed: |
December 3, 1973 |
Current U.S.
Class: |
72/469;
72/88 |
Current CPC
Class: |
B21H
3/06 (20130101) |
Current International
Class: |
B21H
3/00 (20060101); B21H 3/06 (20060101); B21d
017/00 () |
Field of
Search: |
;72/469,204,88,90 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lanham; C. W.
Assistant Examiner: Rogers; Robert M.
Attorney, Agent or Firm: McNenny, Farrington, Pearne &
Gordon
Claims
We claim:
1. A thread rolling die for use with a corresponding thread rolling
die in forming a threaded screw from a blank, said die including
thread-forming means comprising a tool steel body formed with a
projecting flattened thread root-forming portion, a recessed thread
crest-forming portion, and thread side-forming portions joining the
root and crest-forming portions, said projecting flattened
root-forming portion having a nodularized surface of droplet-shaped
nodules in which the nodules are formed of a material in a group
including hard metal and metal carbides, and the surface of said
crest and thread side-forming portions being substantially free of
nodularized surface.
2. A thread rolling die in accordance with claim 1 in which the
metal carbide is tungsten carbide.
3. A thread rolling die as set forth in claim 1 wherein said die is
a cut-off die having a slug-forming surface, and said slug-forming
surface is provided with said nodularized surface.
4. A thread rolling die as set forth in claim 1 wherein said die is
provided with lateral serrations and said serrations are
substantially free of said nodularized surface.
5. A thread rolling die as set forth in claim 1 wherein said
nodularized surface is not provided along the finish end part of
said die.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to thread rolling dies and more
particularly to thread rolling dies which are provided with a
nodularized carbide coating on selected blank engaging surfaces. In
general, the coating is applied by a sputtering technique utilizing
a rotating electrode formed of a hardened metal carbide, e.g.,
tungsten or silicon or chromium carbide.
PRIOR ART
Thread rolling dies of various types are known. Such dies fall
generally into two categories; flat dies which reciprocate with
respect to each other and rotary dies which may either rotate or
oscillate with respect to each other.
In the past, various types of steel have been used to form thread
rolling dies and such steel has been provided with various types of
heat treatment or the like to improve the die life by reducing the
rate of wear. In addition, in such dies various arrangements have
been utilized to provide improved blank control. For example,
serrations are often provided on dies to reduce or eliminate
slippage during the rolling operation. Proper control of the blank
tends to produce an improved product and also tends to improve the
die life.
The dies in accordance with the present invention may be of any of
the usual types. However, the present invention as illustrated is
particularly suited to cut-off type thread rolling dies used to
form gimlet pointed screws, such as sheet metal screws. Typical
examples of such dies of the prior art are illustrated in the
patent to Yankee, U.S. Pat. No. 3,654,800; the patents to
Orlomosky, U.S. Pat. Nos. 3,538,739 and 3,538,740; the patent to
Mau et al., U.S. Pat. NO. 3,176,491, reissued as U.S. Pat. No. Re.
26,518.
SUMMARY OF THE INVENTION
It has now been found that by providing a nodularized hard metal or
metal carbide surface on the blank-gripping surfaces of the thread
rolling dies, tool life may be improved by from 50% to 100%.
Additionally, and most unexpectedly, it has been found that the
range of materials which can be handled by thread rolling
techniques has been extended. For example, aircraft bolts may now
be formed by thread rolling techniques whereas heretofore the
nature of the materials and the quality required has normally
necessitated their manufacture by other means.
Briefly stated, the present invention is in a thread rolling die
for use in forming a threaded screw from a blank having a generally
cylindrical shank. The thread forming means comprise a projecting
root-forming portion which in the illustrated embodiment is
flattened, a recessed crest-forming portion, and thread
side-forming portions joining the root and crest-forming portions.
The projecting flattened root-forming portions are provided with a
nodularized hard metal or hard metal carbide surface, and the
surface of the crest and thread side-forming portions are
maintained substantially free of the nodularized metal or metal
carbide. In the preferred embodiment, the metal carbide is tungsten
carbide.
The method of the present invention contemplates connecting the die
to one side of a source of electric current, preferably A.C., and a
rotating electrically charged electrode material to the other side
of such electrical energy source. At a voltage of from about 15 to
40 volts and an amperage of from about 15 to 20 amps, an electric
arc between the die surface and the tip of the rotating electrode
is struck, and deposition of electrode material onto the die
surface occurs as a rough or nodularized thin coating adhered as in
the case of any weldment. No coating is deposited in the groove in
the die surface. The rotating electrode is conveniently cooled with
a suitable cooling medium, i.e., air, during the application of the
coating. Deposition of the coating material occurs only where the
die surface is confronted by the electrode, and there is no
throwing or flowing of the molten electrode material into the
grooves in such a way as to cause adhesion to the die metal in
those regions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of one die of a pair of self-pointing
or cut-off thread rolling dies incorporating this invention.
FIG. 2 is a perspective fragmentary view at an enlarged scale
showing the nodularized coating on the flattened root-forming
portions, and the crest and side-forming portions being
substantially free of nodularized metallic carbide.
FIG. 3 is an enlarged fragmentary perspective view of a second
embodiment of the invention in which the die is formed with
serrations.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now more particularly to FIGS. 1 and 2, the die structure
illustrated is very similar to the die structure shown in U.S. Pat.
No. 3,654,800 in FIG. 1 thereof with the exception that serrations
are not provided and a nodularized coating of a metallic carbide
has been provided on the root-forming portions. It should be
understood that even though the present invention is illustrated in
conjunction with a cut-off die similar to the die disclosed in the
aforesaid U.S. Pat. No. 3,654,800, other thread rolling dies may be
provided with the nodularized coatings of the present invention to
the achievement of the advantages noted herein.
FIG. 1 is a perspective view of a long die of a pair of cut-off
dies such as those shown in U.S. Pat. No. 3,654,800. The die 41
includes an elongated body 10 with a thread and point-forming
surface 11 thereon. The die is also provided with a slug-forming
surface 12. These surfaces perform the operating functions on the
blank. The die also includes a back surface 13 which is normally
formed at a sufficiently steep angle to prevent it from engaging
the slug with sufficient force to cause any forming functions.
Similarly the die is formed with a clearance surface 14 and a
curved section 16 which deflects the slugs clear of the dies if it
does not fall free by gravity at a fast enough rate.
The thread and point-forming surface 11 includes two cooperating
surfaces, both of which are provided with thread-forming grooves
27. The first of these surfaces 17 engages and forms the threads on
the main shank portion. The second surface 18 is the point-forming
portion and it is this surface of the die which in cooperation with
the slug-forming surface forms the point on the finished screw. It
is recognized that the surfaces 17 and 18 are not planar surfaces
since the thread grooves 17 are formed therein. However, in the
manufacture of dies, it is customary to form the die with planar
surfaces and then to cut the grooves 27. Therefore, for purposes of
description the term "surface" will be used as though planar
surfaces were referred to with the understanding that such surface
may have thread-forming grooves therein.
Referring to FIG. 2 which is a fragmentary perspective view on an
enlarged scale of the die shown in FIG. 1, there is shown the
thread-forming means comprising a projecting root-forming portion
40, a recessed crest-forming portion 42 and thread side-forming
portions 44 and 46 joining the root and crest-forming portions,
respectively. As shown in FIG. 2, the flattened root-forming
portion 40 is provided with a thin nodularized carbide coating 48,
e.g., tungsten carbide, applied by any suitable means such as that
described below. It should be noted that the surfaces of the
crest-forming portion 42 and the thread side-forming portions 44
and 46 are substantially free of nodularized tungsten carbide. The
thickness of the coating is usually less than about 0.001 inch. The
coating is also provided on the slug-forming surface 12 as
illustrated in FIG. 1.
The method by which the carbide coating is applied is quite simple
and contemplates the use of a rotating electrode which is connected
to one side of an alternating current source, the metallic die body
itself either being directly connected to the other side of an
alternating current source or supported upon a base which in turn
is connected to an alternating current source. The rotating
electrode may be formed of a suitable hard coating applying
material, such as tungsten carbide. Such material is very hard and,
due to its nodularized character, it provides a rough surface. The
hardness provides wear resistance while the roughness resists
slippage between the die and the blank.
In order to prevent overheating of the rotating electrode, the
electrode may be provided with a hollow bore extending therethrough
and communicating with a source of pressurized air or inert gas.
Alternatively, the application of the coating may occur in the
presence of a static cooling atmosphere, e.g., air or inert gas
such as nitrogen, helium, carbon dioxide, or the like. The current
at which the coating is applied is approximately 30 volts A.C. at
an amperage of from 15 to 20 amps. Any suitable source of electric
power may be utilized so long as the voltage is in the range of
from 15 to 50 volts, and the amperage drawn at the time of arcing
is from 15 to 30 amps. In general, these levels of current are
below those normally used in arc welding. Arcing between the
rotating electrode and the planar surface of the die is quite
evident and is sufficient to cause deposition of metal from the
rotating cooled electrode. The coating which is applied is
extremely thin and rough and I use the term "nodularized" to
connote the roughened arc-deposited surface coating adhered to the
surface as by welding. When viewed through a microscope the coating
appears as a multitude of separate droplet-shaped nodules. The
roughened surface is readily visible under a microscrope. The
crest-forming portion 42 and the sloping side portions 44 and 46
are seen under a microscope to be free of deposited metal.
Deposition occurs only on the die surface where contacted by the
rotating electrode. The electrode develops a pencil point
appearance from use.
The resulting surface is quite hard, and die-life ranging from 50%
to 100% longer than heretofore obtainable has been experienced.
While the entire planar surface of the die may be coated in the
manner indicated above, it has been found preferable to coat with a
very rough coating applied at a higher amperage over that portion
from the starting end 52 to about the center of the die length.
The balance of the die can be either uncoated or coated with
electrode material at a lower amperage to deposit a much smoother
albeit quality hard coating to the surface. The portions of the die
contacting the blank at the ends of the stroke experience lower
pressures and therefore less wear than the starting portions of the
die. Consequently, it is often not necessary to coat the die along
its entire length. Also the surfaces within the groove are not
subjected to as high a pressure as the surfaces 40 so it is
desirable to provide a structure in which the rough coating is not
present in the grooves themselves since such a rough coating would
tend to cause a roughening of the thread portion of the finished
screw.
In the past, it has been common to provide serrations such as the
serration 54 illustrated in FIG. 3 to limit or resist slipping of
the blank during the initial forming process. Such serrations
although they reduce slippage weakened the die and also tended to
cause a rough surface in the finished screw. With the present
invention, the hard coating is usually sufficiently rough to
prevent the slippage as well as to improve die life without the
need for serrations. However, if serrations are required even when
the nodularized coating is provided, they may be used to further
resist slippage. FIG. 3 illustrates such a die where lateral
serrations are provided across the die in combination with a hard
nodularized coating in accordance with the present invention. Here
again, the nodularized coating exists only along the flat upper
surface of the die and does not extend down into the serrations to
any appreciable extent.
With a die manufactured in accordance with the present invention,
greatly improved die life is provided and greater control of the
blank is achieved. Consequently, in many instances, it is not
necessary to utilize the serrations generally used in the past, and
it is possible to manufacture higher quality threaded devices.
Although preferred embodiments of this invention are illustrated,
it should be understood that various modifications and
rearrangements of parts may be resorted to without departing from
the scope of the invention disclosed and claimed herein.
* * * * *