Cold drawing technique and apparatus for forming internally grooved tubes

Formation of continuous grooves on the internal surface of a tube shell, in a single continuous cold drawing step, in which the tube shell is first sunk in a die over a reduced diameter cylindrical mandrel portion so that the diameter of the inner surface of the tube shell is reduced to a dimension below the base of the grooves of a grooved plug portion of the mandrel thereby retarding longitudinal movement of a portion of the reduced internal surface of the sunk tube shell at a plurality of circumferentially spaced intervals to effect formation of longitudinally continuous shallow grooves. The mandrel is allowed to rotate if it is desirable to facilitate the formation of spiral grooves on the tube inner surface.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of internally grooved tubes and, 
more particularly, to an improved method of cold drawing tubes for forming 
continuous shallow grooves, narrowly spaced apart in either an axial or 
spiral orientation, on the inside surface of the tubes, and an apparatus 
therefor. 
Known methods have been utilized to place grooves on the internal surfaces 
of tubes for different purposes. Such methods include machining, 
broaching, informing, extruding and drawing techniques. 
Various grooving techniques are described in patent disclosures. 
Hackett (U.S. Pat. No. 2,392,797), for example, discloses a technique for 
imparting rifling, fluting, ridging or the like to an internal tubular 
surface, particularly for a gun barrel or liner, through the use of a die 
and a mandrel arrangement including a mandrel having a surface 
configuration which is converse to that to be imparted to the tube. The 
die compresses the tube onto the mandrel, by relative axial movement of 
the tube and the die, as the tube moves through the die. 
In Harvey, et al (U.S. Pat. No. 2,852,835), an apparatus is disclosed 
wherein metallic tubing is drawn through an annular formed by a stationary 
die and a cooperating rotatable rifling mandrel for simultaneously sizing 
the tubing and forming spiral projections on the interior surface of the 
tubing. The die includes a tapered frusto-conical lead-in portion followed 
by a cylindrical portion which gradually reduces the outside diameter of 
the tube to the desired final outside diameter. The initial contact of the 
internal surface of the tube on a portion of the rifling mandrel and the 
contact of the outer surface of the tube with the tapered lead-in portion 
of die occur concurrently. Hence, the spaced portions of the inside 
surface of the tube are radially forced into the grooves of the rifling 
mandrel simultaneously with a portion of the outer surface diameter 
reduction. No specific type of groove geometry is disclosed although the 
patent indicates that the technique is useful for the production of rifled 
aluminum barrels and the like. 
Drawing techniques similar to that of Harvey, et al (U.S. Pat. No. 
2,852,835) are shown by Nakamura, et al (U.S. Pat. No. 3,830,087), Koch, 
et al (U.S. Pat. Nos. 3,289,451 and 3,088,494), Hill (U.S. Pat. No. 
3,292,408), House (U.S. Pat. No. 3,487,673), Sirois (U.S. Pat. No. 
3,744,290), Stump (U.S. Pat. No. 4,161,112), and Tatsumi (U.S. Pat. No. 
4,373,366). Grover, (U.S. Pat. No. 3,865,184) and Runyan, et al (U.S. Pat. 
No. 3,753,364), for example, both teach a horizontally disposed heat pipe 
as well as a method and apparatus for fabricating the heat pipe. Grover 
(U.S. Pat. No. 3,865,184) is primarily directed towards the actual heat 
pipe apparatus itself, describing, in detail, the very particular 
structure desired. Runyan, et al (U.S. Pat. No. 3,753,364) is primarily 
directed to a method and apparatus for producing capillary grooves on the 
inside tube surface of the heat pipe. The disclosed method and apparatus 
provide a means for fabricating a spiraled capillary groove by cutting the 
metal from the wall of the tube and raising and folding the cut metal over 
to provide a groove having a narrow opening for maximum capillary action. 
The cutting tool has a curved planar edge formed by the intersection of a 
planar surface and a cylindrical surface. The grooves produced thereby may 
have dimensions of a peak to trough depth on the order to 0.014 inches 
(0.3556 mm) and a spacing on the order of 0.007 inches (0.1778 mm) with 
the opening of the grooves narrower than the width of the grooves to 
provide optimum capillary action. The use of separate annular grooves of 
the same geometry is also disclosed. The method of placing the grooves in 
this inner tube wall surface is one of cutting with a cutting tool, and 
not a cold-drawing process. 
When the metal for the inner surface of a tube shell is forced radially 
into grooves of a mandrel, there is a tendency for the metal to elongate 
along the longitudinal direction of the groove rather than radially 
filling the groove. This problem is exasperated as groove depth increases, 
as spacing between the grooves decreases, as drawing speed increases and, 
as well, in the case of hard metal workpieces. 
In practice, no cold drawing method is known to the inventor which has been 
succesfully demonstrated as capable of making continuous shallow grooves 
in a hard metal such as steel, for example, continuous grooves having a 
depth of 0.020 inches (0.508 mm) with 0.040 inches (1.016 mm) between the 
grooves. More particularly, no cold drawing method is known to the 
inventor which is capable of rapidly making, in hard material, shallow 
continuous grooves that exhibit a uniform spiral along the length of the 
tube. Such grooves have particular application to heat pipes which use 
capillary grooves to transfer condensate from a condenser to an evaporator 
as the tubes exhibit increased heat transfer due to the extended surface 
and, accordingly, would be optimum "wicks" when used in thermosyphon-type 
heat pipe applications. 
SUMMARY OF THE INVENTION 
An improved method of cold drawing a tube shell for forming internally 
grooved tubes, according to the invention, includes in a continuous draw, 
the step of first reducing the internal diameter of a tube within a die 
and about a cylindrical mandrel portion prior to contacting the lead end 
of a larger-diameter grooved-mandrel portion, so that the internal 
diameter of the tube is reduced to a dimension not greater than the 
diameter of the grooved mandrel portion at the bottom of the mandrel 
grooves and then contacting the lead end with the reduced diameter tube 
portion to form the grooves. 
An apparatus is provided for cold drawing an elongated tube shell to form a 
cold finished tube having an internal surface with a plurality of 
longitudinally extending grooves. The apparatus includes a die with a die 
land circumscribing a cylindrical bore and a generally conical approach 
zone circumscribing a tapering lead-in portion that forms a continuation 
of the bore. A mandrel is coaxially disposed within the bore and spaced 
from the surfaces of the die to define an annular spacing through which 
the tube shell is to be drawn. In accordance with the invention, the 
mandrel includes a substantially cylindrical grooved plug concentrically 
disposed within the cylindrical bore, a cylindrical bearing section having 
a diameter of smaller dimension than the minor diameter of the grooved 
plug, and a generally conical bearing section interconnecting the 
cylindrical bearing section to the grooved plug, the cylindrical bearing 
section disposed between the tapering lead-in portion and the cylindrical 
bore.

DETAILED DESCRIPTION 
FIG. 1 illustrates a hollow tube shell 10 being drawn from right to left in 
the direction of the arrow through a conventional die 11 by pulling means 
(not shown) such as are well known in the art. The tube shell 10 has 
substantially cylindrical smooth internal and external surfaces prior to 
being drawn through the die 11. 
The die 11 has a die opening including a tapering lead-in portion within a 
generally conical approach zone 12, a cylindrical bore within a 
cylindrical die land 13, and an expanding portion defined within a 
countersunk exit zone 14. The lead-in portion and expanding portion form a 
continuation of the bore at the fore and aft sides of the die 11. 
An internal mandrel 20, preferably of hard or hard-surfaced material such 
as tungsten carbide, is co-axially inserted within the bore and spaced 
from the surfaces of the die to define an annular restraining spacing 
through which the tube shell 10 is to be drawn, as shown, to effectuate 
reduction and grooving of the internal surface of the tube shell 10. The 
mandrel 20 is composed of three working segments--a grooving plug 21 that 
has a working surface comprising a plurality of spiraled or axial grooves 
22, a generally conical bearing section 23, and a cylindrical bearing 
section 24. The generally conical bearing section 23 is connected at its 
larger end to the grooving plug 21 and at its smaller end to the 
cylindrical bearing section 24. The cylindrical bearing section 24, at its 
end opposite the generally conical bearing section 23, is connected to a 
larger diameter cylindrical rod 25. 
The mandrel 20 is oriented within the die 11 such that the cylindrical 
bearing section 24 extends coaxially of the die opening from within the 
generally conical approach zone 12 to within the cylindrical die land 13, 
and both the surface of the zone 12 and the die land 13 are concentrically 
disposed thereabout. 
As the tube shell 10 is drawn through the die, the outer surface of the 
shell 10 first contacts the generally conical approach zone 12. The 
surface of the generally conical approach zone 12 thereby sinks the tube 
shell 10 about mandrel 20 at the smaller diameter mandrel section, i.e. 
cylindrical bearing section 24. 
As shown in FIG. 1, reduction of the diameter of the outer surface of tube 
shell 10 commences in the generally conical approach zone 12 on a portion 
of the tube shell 10 which encircles the cylindrical bearing section 24, 
"before" the grooving occurs. 
As shown in FIG. 1, the diameter of the inner tube wall surface of the tube 
shell 10 is sunk or reduced to a diameter that is equal to or smaller than 
the mandrel diameter at the bottom of the grooves 22 of the grooving plug 
21. This placement overcomes the problem of the inner tube wall surface 
metal taking the easier path of elongating longitudinally rather than 
filling the grooves 22. In effect, this forms grooves in the inner tube 
wall surface with the projections or lands of the grooving plug 21 rather 
than attempting to force the inner tube all surface into the grooves 22 of 
the grooving plug 21. 
The sunk or reduced inner surface of the tube shell 10 is then drawn into 
contact with and expanded over the generally conical bearing section 23 of 
the mandrel 20 and lead into the grooves 22 of the grooving plug 21. The 
projections or lands of the grooved surface of the grooving plug 21 retard 
the longitudinal movement of the reduced internal surface of the sunk tube 
shell at a plurality of circumferentially spaced intervals, thereby 
causing axial flow of the inner tube wall surface material into the 
grooves 22 of the surface of the grooving plug 21 to effect formation of a 
tube having a plurality of longitudinally extending grooves on the 
internal surface thereof. 
The mandrel 20 is allowed to rotate, if it is desirable to facilitate the 
formation of grooves having a spiral orientation on the inside surface of 
the tube shell 10. 
Sinking of the internal diameter of the tube shell 10 prior to contacting 
the groove lead-in portion (generally conical bearing section 23) to a 
dimension in which the internal diameter is no larger than the diameter at 
the bottom of the mandrel grooves 22 has been found to be critical. If 
this is not done, the tube material elongates longitudinally rather than 
entirely filling the grooves 22 radially. 
The generally conical lead-in or bearing section 23 to the flat grooving 
surface of the grooving plug 21 is required to assure that sufficient tube 
material is longitudinally fed to the grooves 22. The groove finish of the 
mandrel grooving plug 21 must be relatively smooth to allow proper 
material flow. Excessive roughness causes misshape and cratered tops on 
the lands placed in the tube shell 10; a surface finish of approximately 3 
microinches has been shown to be effective, and it is estimated that a 30 
microinch or better finish is required. 
During the grooving operation, it is preferable to further sink the outside 
diameter by at least 9% and to achieve a reduction of the tube wall 
thickness of at least 20%. These minimum reductions are requied to yield 
sufficient axial force to cause the tube material to flow into the grooves 
22 rather than over the lands. The tube shell 10 should be annealed prior 
to cold drawing, to allow sufficient tube material ductility to cause 
proper flow. 
In FIG. 2, the reference numerals (one hundred numbers displaced from the 
embodiment of FIG. 1) are used to designate parts which are similar to 
those on the embodiment of FIG. 1. The embodiment of FIG. 2 differs from 
that of FIG. 1 in that the approach zone 112 and bearing section 123, 
while still conical, are curved convexly (as shown) or concavely (not 
shown). 
The present invention has been shown to be capable of providing grooved 
tubes at rates of draw in excess of 34 feet per minute, using the special 
grooving mandrel, a standard tube drawbench and normal equipment to 
prepare tubes for drawing. Variable groove spiral geometries can be made; 
9" to 20" lead spirals have been successfully made with groove fineness 
from 24 per inch to above 35 per inch.