COLD DRAWING TECHNIQUE AND APPARATUS FOR FORMING INTERNALLY GROOVED TUBES

Abstract: Formation of continuous grooves in the internal surface of a tube shell (10) is effected in a single continuous cold drawing step, by first sinking the tube shell (10) in a die over a reduced diameter cylindrical mandrel portion (20) so that the diameter of the inner surface of the tube shell (10) is reduced to a dimension below the base of grooves (22) of a grooved plug portion (21) of the mandrel (20) thereby retarding longitudinal movement of a portion of the reduced internal surface of the sunk tube shell (10) at a plurality of circumferentially spaced intervals to effect formation of longitudinally continuous shallow grooves. The mandrel (20) is allowed to rotate if it is desirable to facilitate the formation of spiral grooves on the tube inner surface.

Description: The invention relates to the manufacture of internally grooved tubes. When a tube sheet is cold drawn through a die, the resulting inner and outer tube surfaces formed may be pitted or rough. US-A- 4.232.541 discloses a method and apparatus for improving the interior or exterior surface smoothness of a tube shell during cold drawing. A tube shell is drawn into a die and the interior of the tube shell makes contact with a mandrel within the die opening. In one embodiment, the die has a die land including a first cylindrical section, and a second cylindrical section of smaller diamater. In another embodiment, the mandrel has a large diameter cylindrical working section joined to a smaller diameter working section by a frusto-conical section. Various methods have been utilised to form grooves in 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 specifications. US-A-2,392,797 discloses a technique to impart rifling, fluting or ridging 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. US-A-2,852,835 discloses apparatus wherein metallic tubing is drawn through an annulus formed by a stationary die and a cooperating rotatable rifling mandrel simultaneously to size the tubing and form 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 the 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. The specification indicates that the technique is useful for the production of rifled aluminium barrels and the like. Drawing techniques similar to that of US-A-2,852,835 are shown in US-A-3,088,494, 3,289,451, 3,292,408, 3,487,673, 3,744,290, 3,830,087, 4,161,112 and 4,373,366. US-A-3,865,184 and 3,753,364 both teach a horizontally disposed heat pipe as well as a method and apparatus for fabricating the heat pipe. US-A-3,865,184 is primarily directed towards the actual heat pipe apparatus itself, describing, in detail, the very particular structure desired. US-A-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 spiralled 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 of 0.356 mm (0.007 inches) 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 the 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 to fill the groove. This problem is exacerbated as groove depth increases, as spacing between the grooves decreases, as drawing speed increases and as the hardness of metal workpieces increases. In practice, no cold drawing method is known which has been successfully demonstrated as capable of making continuous shallow grooves in a hard metal such as steel, for example, continuous grooves having a depth of 0.508 mm (0.020 inches) with 1.016 mm (0.040 inches) between the grooves. More particularly, no cold drawing method is known 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. According to one aspect of the invention there is provided a method of cold drawing an elongate tube shell to form a cold finished tube having an internal surface with a plurality of longitudinally extending grooves and comprising longitudinally drawing the tube shell along a mandrel, comprising the steps of sequentially, and in a single continuous draw pass, sinking the tube shell through a die having a tapering lead-in portion to reduce the diameter of the internal surface of the tube shell to a dimension less than the minor diameter of the grooves to be formed, progressively enlarging the reduced internal surface of the tube shell over a bearing section of the mandrel, and longitudinally retarding the longitudinal movement of a portion of the reduced internal surface of the tube shell at a plurality of circumferentially spaced intervals by the tube shell being longitudinally drawn along the mandrel to effect formation of the grooves. According to another aspect of the invention there is provided an apparatus for cold drawing an elongate tube shell, to form a cold finished tube having an internal surface with a plurality of longitudinally extending grooves, comprising a die with a die land circumscribing a cylindrical bore and a generally conical approach zone circumscribing a tapering lead-in portion forming a continuation of the bore, and a mandrel coaxially disposed within the bore and spaced from the surfaces of the die to define a spacing through which the tube shell is to be drawn, characterised in that the mandrel includes a substantially cylindrical grooved plug having a groove surface finish of approximately 76nm (3 µ inches) concentrically disposed in 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 being disposed partly within the tapering lead-in portion and the cylindrical bore. Thus the internal diameters of the tube within the die and about the cylindrical mandrel portion is reduced prior to contacting the lead end of the 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 before the lead end contacts with the reduced diameter tube portion to form the grooves. The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which:- Figure 1 is a side view, partly in section, of a tube shell being drawn relative to a die in a method of cold drawing according to the invention; and Figure 2 is a partial view, similar to Figure 1, showing a die and a mandrel of apparatus for cold drawing according to the invention. Figure 1 illustrates a hollow tube shell 10 being drawn from right to left in the direction of the arrow through a 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 the 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 coaxially 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 effect 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 spiralled 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 the mandrel 20 at the smaller diameter mandrel section, i . e . the cylindrical bearing section 24. As shown in Figure 1, reduction of the diameter of the outer surface of the 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 Figure 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 wall 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 in 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 of 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 (the 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 ensure 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 mis-shapen and cratered tops on the lead placed in the tube shell 10; a surface finish of approximately 76 nm (3 microinches) has been shown to be effective, and it is estimated that a 760 nm (30 microinches) or better finish is required. During the grooving operation it is preferable further to 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 required 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 Figure 2, the reference numerals (one hundred numbers displaced from the embodiment of Figure 1) are used to designated parts which are similar to those on the embodiment of Figure 1. The embodiment of Figure 2 differs from that of Figure 1 in that an approach zone 112 and a bearing section 123, while still conical, are curved convexly (as shown) or concavely (not shown). The invention can provide grooved tubes at rates of draw in excess of 10.4 metres (34 feed) 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; 230 mm to 508 mm (9 inches to 20 inches) lead spirals have been successfully made with groove fineness from 0.95 per mm to above 1.38 per mm (24 per inch to above 35 per inch).

Claims: 1. A method of cold drawing an elongate tube shell (10) to form a cold finished tube having an internal surface with a plurality of longitudinally extending grooves and comprising longitudinally drawing the tube shell (10) along a mandrel (20), comprising the steps of sequentially, and in a single continuous draw pass, sinking the tube shell (10) through a die (11) having a tapering lead-in portion (12) to reduce the diameter of the internal surface of the tube shell to a dimension less than the minor diameter of the grooves to be formed, progressively enlarging the reduced internal surface of the tube shell over a bearing section (23) of the mandrel (20), and longitudinally retarding the longitudinal movement of a portion of the reduced internal surface of the tube shell (10) at a plurality of circumferentially spaced intervals by the tube shell (10) being longitudinally drawn along the mandrel (20) to effect formation of the grooves. 2. A method according to claim 1, further comprising the step of providing the mandrel (20) to be freely rotatable and including a spirally grooved plug (21) uniformly to spiral the grooves along the length of the tube. 3. A method according to claim 1, further comprising the steps of concurrently, with the formation of the grooves, reducing the outer diameter of the tube shell by at least 9% and reducing the wall thickness of the tube shell by at least 20% along the same portion of the reduced internal surface. 4. Apparatus for cold drawing an elongate tube shell (10), to form a cold finished tube having an internal surface with a plurality of longitudinally extending grooves, comprising a die with a die land (13) circumscribing a cylindrical bore and a generally conical approach zone (12) circumscribing a tapering lead-in portion forming a continuation of the bore, and a mandrel (20) coaxially disposed within the bore and spaced from the surfaces of the die (11) to define a spacing through which the tube shell (10) is to be drawn, characterised in that the mandrel (20) includes a substantially cylindrical grooved plug (21) having a groove surface finish of approximately 76nm (3 µ inches) concentrically disposed in the cylindrical bore, a cylindrical bearing section (24) having a diameter of smaller dimension than the minor diameter of the grooved plug (21), and a generally conical bearing section (23) interconnecting the cylindrical bearing section (24) to the grooved plug (21), the cylindrical bearing section (24) being disposed partly within the tapering lead-in portion and the cylindrical bore.

Inventors: MAYER, DEAN LOWELL
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1988
Main classification: B21C37
IPC codes: B21C3, B21C37, B21C1