US2812794A - Method and machine for manufacturing helical fin structures - Google Patents

Description

E. CHAPMAN Nov. 12, 1957 METHOD AND MACHINE FOR MANUFACTURING HELICAL FIN STRUCTURES Filed Jan. 13,

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METHOD AND momma FOR MANUFACTURING HELICAL FIN STRUCTURES Filed Jan. 1:, 1954 5 Sheets-Sheet 2 j H 8 i w wu is: \D 7 H E a; (Q g, E

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E. CHAPMAN Nov. 12, 1957 METHOD AND MACHINE FOR MANUFACTURING HELZICAL FIN STRUCTURES 5 Sheets-Sheet 4 Filed Jan. 13, 1954 FIG. 5

Inventor:

Nov. 12, 1957 E. CHAPMAN 2,812,794 7 METHOD AND MACHINE FOR MANUFACTURING HELICAL FIN STRUCTURES Filed Jan. 13, 1954 5 Sheets-Sheet 5' g0 2 LICK, yg

2/4 genera/0r [rave-afar Evereii' Chqvman flizar I w 56mm United States v Patent METHOD AND MAHIN E FQR MANUFACTURING HELICAL FIN STRUCTURES Everett Chapman, West Chester, Pa. Application January 13, 1954-, Serial No. 403,769 7 Claims. (Cl. 153--7) A principal object of this invention istcprovide a practical method of producing an improved form of helical fin for thermal exchange or transfer elements, said fin being characterized by a corrugated or ruffled formation in the outer peripheral area and a plane or fiat formation, substantially free from irregularities, in the inner peripheral area of the helix.

In accordance with the invention, the aforesaid fin may be formed as a separate unit on a suitable mandrel, or it maybe formed and be simultaneously applied to a tubular member functioning as a mandrel and constituting an element of the thermal transfer device of which the fin also constitutes a functional part. In either case, a further object is to provide a method and mechanism for producing a fin for thermal exchange purposes in a novel form calculated to set up a desirable turbulent scrubbing action of the circulating air over the finned surfaces of the element, without, however, appreciably increased pressure drop, thereby materially improving the thermal transfer characteristics.

In a the attached drawings:

Fig.1 is an elevational view of a machine for applying a helical fin to a tubular thermal exchange element;

Fig. 2 is a fragmentary enlargement, partly sectioned, of the structure of Fig. 1 showing details of the drive and loading mechanisms;

Fig. 3 is a schematic view in perspective, illustrating the manner of feeding the strip material of the fins to the roller and anvil;

Fig. 4 is an enlarged detail of the roller and anvil elements of the machine;

' Fig. 5 is a detail view showing a section of the helical fin material with its radial corrugations and tapered cross-section;

Fig. 6 is similar to Fig. 5 except that the corrugations have less amplitude;

Figs. 7 and 8 are wiring diagrams illustrating an electrical control system affording uniform torque characteristics in the operation of the machine.

The invention contemplates fabrication of helical fin, or of tubularthermal exchange element including such fin, which is corrugated radially with the corrugations having a maximum depth or amplitude at the outer edge of the fin and becoming progressively shallower and narrower to vanish short of the inner edge.

The invention contemplates also a machine for ferning said corrugations in strip material and for simultaneously wrapping the corrugated strip on a tube, or other mandrel with the undistorted, uncorrugated edge of the strip tightly engaged with and describing a smooth helix on the outer cylindrical surface of the tube, frcin which surface the strip projects radially. The corrugating mechanism is adjustable for controlling the magnitude of the corrugations and the number thereof in a unit length of fin. The machine is also adjustable to control the pitch of the helix.

As will hereinafter appear, the working of the base or blank strip material from which the fin is produced, to

the form of the finished article, particularly where the helix defined by the latter is of relatively small internal diameters, may involve distortions and internal stresses beyond the normal strength of the strip, and the novel method of the invention effectively conditions the strip for the excessive strains and stresses to which it is subjected during the forming operation.

Referring more particularly to Figs. 1 and 2 of the drawings, the machine for fabricating the fin or thermal exchange elements described above in general terms comprises a hollow anvil 25 rotatably mounted on a bracket 26 of the main frame 27 through the medium of an annular thrust bearing 28. The upper surface 29 of the anvil 25 is bevelled and coacts with the tapered nose 31 of a roller 32, as hereinafter described, to produce the aforesaid corrugations in the strip material 30, which forms the fin of the tubular thermal exchange element. Under normal operating conditions the base of the nose 31 reacts with the peripheral edge of the surface 29 at point 33 and the geometry of this portionof the mechanism is such that the relative peripheral speeds of the nose 31 and the surface 29 arethe same at any radius. The roller 32 is supported in a cylindrical housing34 by means of two antifriction bearings 35 and 36 which take the radial load, and the bearing 35 also functions as a thrust bearing to take the axial load imposed upon the roller. The end of the roller 32 opposite the nose 31 is extended to form a recessed shank 37 which is internally splined for connection with a driving shaft 38. I

An electric motor 39 serves to drive both the roller 32 and anvil 25. The roller is driven through a universal joint 41 and shaft 38; the anvil through a pinion gear 40 on the motor shaft, a ring gear 43, shaft 44, sprockets 45 and 47, and a sprocket chain 46. The ratio of'the pitch diameter of pinion 40 to that of the ring gear 43 is the same as the ratio of the diameter of the roller nose 31 at the point 33 to the diameter of the anvil face 29 at point 33, thus affording a common peripheral speed for the roller and anvil at that point, it being noted that the sprockets 45 and 47 are, in the present instance, of the same pitch diameter. Also, the taper of the nose '31 is such that the imaginary apex of the truncated cone lies on the rotary axis of the anvil which axis, of course, contains the imaginary apex of the conoidal surface 29 of the anvil.

The roller housing 34 is supported at three points, one of which is provided by a set screw 48 located in the frame 27 in a position near the rear end of the housing, i. e., that end through which the shank 37 of the roller projects. A second point of support is provided by a set screw 49 also in the frame 27, this screw being arranged for adjustment of the housing 34 in the axial direction. The third point of support is the reaction point 33 between the nose and the anvil, the nose being pressed against the anvil at this point by forces applied to the housing 34 through a pair of rods 51, see Figs. 1 and 2, at points directly over the bearing 36. With the aforedescribed arrangement adjustment of the screw 43 serves to vary the effective working area of the roll and anvil by adjusting the rolling angle, designated in Pig. 4 by the reference number 50, while manipulation of the screw 49 serves to adjust the roll nose axially with respect to the rotary axis of the anvil so that the apex of the cone defined by the tapered sides of the nose will lie in said axis. A

The loading of the roller and anvil forms an important part of the invention since the form and radial length of the corrugations depend upon the compressive force exerted upon the strip 30. I have found that deadweight loading affords highly satisfactory resultsand for this purpose I employ a weight 52 (see Fig. l) which is suspended in a hanger 53 from an adjusting screw 54 at the top of a hydraulic bellows 55. A second bellows or 'equivalent'expandable element 56 is connected with the bellows 55 through a duct 57 and is supported on a platform 58 for-minga part of the frame 27. The bellows '56 coacts with one end'of'a lever 59 which is fulcrumed against a screw 60 in the frame so'that when the bellows 56 is expanded theresulting force applied'to the lever 59 will urge the outer end of the lever downwardly and will, through the rods 51 to which the said outer end ofthe lever is connected, exert a resultant pressure upon the housing 34. The hydraulic pressure tending to expand the bellows 56 is a function of the weight 52 acting through the bellows 55 so that the effective pressure upon the housing 34 tending to press the roller nose31 against the peripheral edge of the anvil 25 is determined by the magnitude of the weight 52, it being noted that the transmission system through the bellows 55 and 56 and the lever 59 is of a character to'fmagnify the pressure by a ratio of approximately 80 to 1. In practice the screw 60 will be adjusted so that the lever 59 will occupy a substantially. horizontal position as illustrated. The device provides for application of a constant pressure to the strip between the roller and anvil.

In operation and with the strip 30 passing between the anvil and the roller, the dead-weight is applied directly to the strip since under these conditions the bearing of the roll against the strip constitutes one reaction point for the dead weight, the other reaction point being the screw 48, the adjustment of which then determines the "nip profile. Obviously, the rolling pressure upon the strip 30 will be constant regardless of possible variations in the thickness of the strip. The working of the outer edge of the strip to corrugate and bend the latter is under control while the machine is running, as to magnitude and as to the type and degree of deformation of the strip.

The tube 61 to which in the present instance the strip 30 is to be applied in the form of a helical fin extends upwardly through the anvil 25 in axial alignment with the latter, and in the present instance said tube and anvil are relatively proportioned so that the outer diameter of the tube closely approximates the diameter of the bore of the anvil. The lower end of the tube 61 is secured in a collet 62 at the upper end of a threaded shaft 63 having a longitudinal keyway 64. The thread of the shaft 63 is engaged with an internal thread (not shown) in a part 66 of the frame structure 27, said shaft being in axial alignment with the anvil 25. A sprocket 68 is operatively connected to the shaft 63 by way of a key 67 which slides in the keyway 64 of the shaft so that when the sprocket 68 is rotated the shaft 63 will be advanced axially in the frame part 66 by reason of the threaded engagement of the shaft with the frame. Rotation of the sprocket 68 is effected in the present instance by a constant torque motor 69 operating through a sprocket 69a on the motor shaft and a sprocket chain 70 which connects the sprocket 69a with the sprocket 68. Axial advancement of the screw 63 effects a corresponding axial movement of the tube 61 through the anvil 25, the rate of axial movement of the tube being a function of the pitch of the thread of the shaft 63 and the speed of rotation of said shaft. It will be apparent that the axial rate of speed of the tube 61 (as determined by the pitch of the screw 63) with respect to the anvil 25 will control the pitch of the winding of the strip 30 upon the tube, it being noted that in operation the leading end of the strip is' attached by suitable means to the outer surface of the tube as indicated at 86. Thus, the tension under which the strip is wound upon the tube is determined by the torque at which the motor 69 is set.

' The manner in which the strip 30 is directed to, the roller 32 and anvil 25 is illustrated in detail in Fig. 3. As therein illustrated a guide or drag block 80, through which in the present instance the strip is drawn under considerable tension, controls the I angular relation of 7 while producing the wrinkle or corrugation in the unor v the strip to said roller and anvil. The position of the guide with respect to the axis of the anvil Z5 is such that the longitudinal axis of the block forms with a plane passing through the anvil axis at right angles to the axis of the roller 32, an angle which in Fig. 3 is termed the pre-wrap angle. The amount of the prewrap angle will vary somewhat in accordance with the speed and the materials used, but this angle insures that the edge of the strip 30 shall be forced against the periphery of the tube during the winding operation. The pre-wrap angle causes a gathering or bunching of the strip in front of the anvil-roller nip, indicated in broken lines at 118 in Fig. 4, and this bunching is maintained in a stable manner by skewing the guide block 80 at an angle, designated in Fig. 3 the biasing skew angle. The effect of this angle is to offset a tendency of the gather to oscillate and to insure a smooth flow of the strip material to the anvil and roller.

Referring now to Fig. 4, the frusto-conical roller nose 31 is arranged so that the imaginary apex 83 of the cone lies in the axis of the anvil 25 as shown in broken lines. The nose is hardened by heat treating and the base is provided with an annular concave recess 84 for the reduction of stress concentration. With the apices of the two conical surfaces thus located on the same axis, the two surfaces will have the same relative peripheral speeds at any given radius, and by then adjusting the driven speeds of the cones so that at any operatively related point, such for example as the primary reaction point 33 previously referred to, the surfaces have the same linear velocity, assurance is had that the conical surfaces in engagement with the strip Will roll on the latter thereby avoiding the scufiing that would occur if such rolling action were not achieved.

In order to bend a strip edgewise around a tube or mandrel, either the metal at the inside radius must be contracted, or the metal at the outer radius must be elongated. The conventional method is to contract the strip by corrugating the edge which is to form the inside of the helix which places the corrugations at the inner edge of the helical fin where they are undesirable. In the present instance the strip is bent to the desired helical form by the latter method, the edge which is to form the outer edge of the helix being contracted by compression through the medium of the roll and anvil, the resulting curvature of the strip varying in accordance with the amount of the contraction or thinning. If the outside edge is compressed and thinned to a greater extent than is necessary for bending the strip to the required radius-in the present instance the radius corresponding to the outer cylindrical surface of the tubethe metal in the outer edge portion of the strip being longer than necessary for the desired radius, will be forced to assume the wrinkled or corrugated form. In other words, the overstretching of the metallic fibers is taken up by the increased circumferential length of the corrugated outside edge of the strip.

In accordance with the invention, this excessive stretching and working of the metal at the edge of the strip, which result in the bending of the strip away from the worked edge, are made possible by subjecting the strip si-' multaneously with the lateral compression of the strip by the roll and anvil to material longitudinal tension, through the medium in the present instance of the motor 69 and, drag block 80. Without this combination of compression and tension the metal of the strip would be subject to rupture. Metal, under a single force, exhibits ductility or crackless stretchability of, say, 25% for steel. Ifa tension force accompanies a lateral compression, the crackless stretchability can go as high as 200%. 'It is this effect of the bi-axial forces applied simultaneously which makes it possible to get the strip around the tube thodox location of the outer edge. I This system of forces makes available crackless. stretchability in excess of the normal ductility suflicient not only to form the strip to the shape of the tight helix, butin addition to upset the outer edge.

It is apparent that with this device the character and magnitude of the waved or corrugated outer edge formation may be closely controlled. By adjusting the machine either before or during operation the overcompression at the outer edge of the strip may be increased or reduced, as may be desired, so that the distortion of the outer edge of the strip may be varied between the slight ripple as shownin Fig. 6 and the substantial corrugation illustrated in Fig. 5. As illustrated, the corrugations or waves decrease progressively in both width and depth toward the inner edge of the strip and terminate short of said inner edge so that the latter defines a smooth helical curve, this smoothly curved inner edge, which by reason of the characteristic tapered cross-sectional form of the strip is of substantial thickness, being pressed firmly against the surface of the tube. This tapered form is desirable in that it constitutes the most efficient shape for thermal transfer and is, therefore, more economical in that for an given requirement a fin with this formation will use less metal than would be necessary with another shape.

In setting up the machine the strip 30 is threaded through the block .80 and the end of the strip is pressed into the nip between the roller and anvil. The motor 39 is then energized with the result that the roller and anvil act to feed the strip longitudinally, bending the strip edgewise to a radius corresponding to that of the outer surface of the tube, and simulaneously waving or corrugating the outer edge of the strip. After a few turns have been formed, the motor 39 is stopped and the end of the strip is fastened to the tube with screw 86. The motor 69, which is of constant torque type, is now started, with the result that the tube 61 is rotated and simultaneously longitudinally advanced, the pre-set torque of the motor being sufficient to take up the slack and to tighten the strip on the tube, but without buckling or rupturing the strip. The motor 39 is then restarted at slow speed and is gradually brought to the full running speed, the motor 69 exerting a constant torque to keep the newly wound strip tight on the tube. The speed at which the motor 69 operates is entirely dependent upon the rate at which the strip is payed out by the motor 39. The torque of motor 69 is maintained constant by the electrical control system described hereinafter, which constitutes a feature of the invention. It is important that the torque of motor 69 remain substantially constant in order that the finished product shall be uniform. The amount of the torque and the resulting tension in the strip, which may vary widely with the materials involved and the characteristics desired in the finished product, should not exceed the yield point of the metal in the strip; and since best results are obtainable when the function of the roll and anvil is confined entirely to the exertion on the strip of the lateral compressive force, the longitudinal tensions on the strip at the opposite sides of the nip are preferably balanced. To this end the block 80 may have adjustable tensioning means as illustrated in Fig. 3. As shown the block contains a friction plate 215 which is pressed against the strip 30 by springs 216, and the pressure of the springs may be adjusted by screws 217 through an intervening press plate 218. The pitch of the screw 63, which controls the rate of axial feed of the tube 61, determines the pitch of the helically wound fin.

For some purposes, the finned tube may be used without brazing or otherwise securing the fins to the tube, the fins being held in place upon the tube by the tight fit effected by the substantial wrapping torque described above. Where additional rigidity and higher heat conduction are required, the fin may be brazed to the tube to form a solid, mechanically strong joint affording a materially improved thermal conduction between the tube and the fin. In accordance with this phase of the invention a suitable solder or brazing wire 87 is wound around the tube at the base of the fin, as shown in Fig. 2. This may be done simutaneously with the fin-wr-apping operation, as indicated in Fig. 2, wherein the wire 87, shown in broken lines, is drawn onto the tube from a suitable source (not shown) by rotation of the tube in the process of applying the fin as described above, the leading end of the wire being secured to the tube by the same screw 86 which fastens the end of the strip 30. The tube assembly is then subjected to a heating operation by means of suitable apparatus (not shown).

The method of corrugating and applying the fin described above possesses material advantages both in the characteristics of the finished article and in the production thereof. The gentle corrugation near the outer edge of the fin produces in the use of the finished article a turbulent scrubbing action of the air without appreciably higher pressure drop that accounts for additional thermal transfer for the same equivalent area of an additional twelve or more percent over a straight fin or one corrugated at the inner edge. The method and apparatus for producing the edgewise bent strip and applying the formed strip to the base member provides a close control of the magnitude and character of the corrugation and insures that the fin shall bear tightly against the surface of the base member. The apparatus also atfords wide latitude in the pitch of the helically Wound fin and is capable of producing the fin and base member assembly at economically high rates of speed.

While the foregoing description is directed primarily to the formation of a finned tubular element, it is obvious that the invention is not limited to this character of end product. The tubular element might, for example, be constituted by the cylinder of a combustion engine; or might be replaced by a solid cylindrical base member. It is to be noted also that the tubular or other base member constitutes in effect a mandrel for the strip-forming operation, and that following application of the formed strip to this mandrel the resulting helical structure may be removed bodily from the mandrel to be sold as a unit for subsequent use where a fin structure of this type may be required. The helical strip structure thus constitutes in itself a new article of manufacture.

It is important, as previously set forth, that the motor 69, see Fig. 1, shall exert a constant torque on the shaft 61 during the forming and winding operations. Fig. 7 shows an electrical control system for maintaining the torque of motor 69 substantially constant, as hereinafter described.

The D. C. motor 69 is driven by the D. C. generator 202 whose field excitation is supplied by an exciter 233. Motor 39 is driven by generator 204. The field excitation for motor 39 and generator 204 is supplied by an exciter 205. Voltage divider 206 serves to furnish very fine gradations of the exciting voltage applied to the field of generator 204. The ex-citer 205 also furnishes the field excitation for motor 69 and for a D. C. generator 267, whose shaft is mechanically coupled to the shaft of motor 69 as indicated by the dotted line connection. The purpose of the generator 207 will presently appear. Control of the torque of motor 69 is effected through a vacuum tube 298, whose space discharge path is included in the energizing circuit for the field of generator 292. The vacuum tube constitutes a controllable resistance in series with said field. Plate voltage for tube 208 is supplied by exciter 203.

An adjustable resistor 209 is serially included in the circuit between the armature of generator 202 and the armature of motor 69. Armature current Ia flowing through this resistor produces a voltage thereacross having the value IaRa and having the polarity indicated, Ru. being the resistance value of the effective portion of the resistor. A C-battery 210 has its positive terminal connected to the negative side of resistor 209, and has its negative terminal connected to the control grid of tube 208. Connected across the armature of D. C. generator 207 is potentiometer 211. The polarity of the voltage generated by generator 207 is as indicated, and this voltage appears across the potentiometer 211 with the indicated polarity. It Will seen, therefore, that the resistor 209 and the potentiometer 211 serve to provide adjustable voltages in series in the grid-cathode circuit of tube 208 and the battery 210 provides a fixed voltage in saidcircuit. Two of the voltages are negative as regards the grid of tube 208, while the third voltage is positive, as indicated.

Assuming given adjustments of resistor 209 and potentiometer 211, the voltage of generator 202 will be automatically controlled, through tube 208, by the voltages across the effective portions of the resistor and the potentiometer. The efiective voltage on the grid of tube 208 will determine the resistance of the tube and thus determine the magnitude of the current flowing through the field winding of generator 202. An increase in such current will cause an increase in the generated voltage E; On the other hand, a decrease in field'current will cause a decrease in the generated voltage. As the effective voltage on the grid of the tube 208 becomes more negative, it will increase the resistance of the tube and thereby cause a decrease in the field current of generator 202. On the other hand, if the grid of the tube 208 becomes less negative, it will decrease the resistance of the tube and thus produce an increase in the field current of generator 202. As described hereinafter, the torque of motor 69 is controlled through tube 208.

A preferred form of the control system is shown in Fig. 8 in which compensation is effected for torque components due to acceleration and deceleration. Except for the additional devices or elements now to be described, the sys tem is exactly the same as that of Fig. 7.

The additional elements comprise a generator 212, a potentiometer 213 and a condenser 214 electrically connected as shown. The armature of generator 212 is mechanically coupled to the armature of motor 69 and the field of said generator is supplied from the exciter 205. With this arrangement the voltage across potentiometer 213 varies in magnitude according to acceleration or deceleration of motor 69, and said voltage has a positive or negative polarity according to whether the motor 69 is accelerating or decelerating; For a given speed of motor 69, the condenser 214 will be charged to the voltage of generator 212. Then, if the motor 69 accelerates, a charging current will flow producing a positive voltage across the effective portion of potentiometer 213, thereby producing a transient increase in the torque of motor 69 to compensate for the increased torque losses due to acceleration. On the other hand, if the motor 69 should decelerate, the condenser 214 will discharge and produce a negative voltage across the effective portion of potentiometer 213, thus producing a decrease in the total torque of motor 69 to compensate for the decrease of such losses.

Considering the operation of the preferred form of the control system as shown in Fig. 8, it should be noted that when the motor 69 is held at standstill there are no voltages across the potentiometers 211 and 213, and therefore the voltage across resistor 209 and voltage of battery 210 drive the grid of tube 209 negative, and hence the resistance of tube 208 in series with the field of generator 202 is high. Under such condition, the voltage across resistor 209 and the voltage of battery 210 determine the blocked torque of the motor 69 and thus determine the initial tension on the strip 135. It is desired to keep this tension constant, no matter at what speed the motor 39 is operated to pay out the strip. Motor 69 must follow the lead of motor 39 and maintain the blocked torque value.

It should be noted that ordinarily the torque of a motorwill decrease with increasing speed thereof, be-' cause the generated voltage or back E. M. F. is in opposition to the applied E. M. F. and reduces the armature current. 'However, in this instance, as described below, the control action compensates for increasing back E. M. F. of motor 69.

With potentiometers 211 and 213 adjusted to maintain constant torque, suppose that the motor 69 is permitted to rotate by paying out the strip 30 by means of motor 39; As soon as motor 69 starts, the generators 207 and 212 start to generate voltages which aifect the potential on the grid of tube 208. The voltage across the effective portion of potentiometer 211 drives the grid of tube 203 in the positive direction in proportion to the speed of motor 69. The voltage across the effective portion of potentiometer 213 will produce transient changes in the potential on the grid of tube 208 whenever the speed of motor 69 tends to change rapidly.

As the grid of tube 208 is driven in the positive direction the resistance of the tube decreases, more current flows in the field of generator 202, and more voltage is generated by the generator to overcome the normal back E. M. F. Hence, the current through the armature of motor 69 stays constant. If the field of that motor re mains constant, then its torque remains constant, no matter what speed variations are forced on it under the paying out influence of motor 39.

From the foregoing description, it will be seen that the torque of motor 69 is dependent upon the efiective grid voltage of tube 208, which in turn is dependent upon the voltages produced across potentiometers 211 and 213. While it is desired to maintain the torque of motor 69 constant in the apparatus illustrated and described, the control system is highly flexible as to adjustment and may be utilized to cause increase or decrease of the torque of a motor with increasing speed. Thus a particular setting of potentiometers 211 and 213 will effect exact compensation to keep the torque of motor 69 constant as described above, but a setting on either side of the first mentioned setting willcause overcompensation or undercompensation.

It will be apparent, therefore, that the system provides a novel arrangement for control of the torque of a motor irrespective of the function of the motor, which arrangement may be utilized to maintain constant torque or to cause desired torque variations in response to speed, acceleration or deceleration of the motor.

In 'a particular embodiment for use in apparatus of the character described, the field of generator 202 has a resistance of about 400 ohms, and two 6A3 tubes are used in parallel. The series resistance in the armature circuit of motor 69 is variable between 25 ohms and 35 ohms. The C-battery has a voltage of 16.5 volts. The speed responsive generator 207 generates about .08 volt per R. P. M. The potentiometer 211 has a total resistance of 7500 ohms. In such embodiment the voltage E generated by generator 202 is equal to +3.f7e where e is the voltage on the grids of the tubes.

It is to be understood that there may be further substantial modification in the details of both apparatus and method without departure from the invention.

I claim: i q

-l. The method of working a metal strip edgewise to helical form with corrugations extending inwardly from the outer peripheral edge and terminating short of the inner edge of the helical strip, said method comprising applying a transverse compressive force to an edge portion of the strip progressively along its length so as to reduce the thickness and increase the length of said edge portion to an extent exceeding that required for production of an uncorrugated helix of desired curvature, limit ing'the curvature of the strip resulting from said' progressive compression so as to cause the said edge portion of the strip to buckle With production 'of said corrugations, and applyinga second force to the strip to maintain the strip continuously in longitudinal tension at the point of application of said compressive force.

2. The method of working a metal strip to helical form with corrugations extending inwardly from the outer peripheral edge and terminating short of the inner edge of the helical strip, said method comprising transversely compressing an edge portion of the strip progressively along its length while simultaneously subjecting the strip at the point of said compression to a longitudinal tensile force so as to eflect a progressive elongation of said edge portion and consequent deformation of the strip edgewise to the desired helical form with the elongated edge at the outer periphery of the helix, the said elongation exceeding that required to produce the curvature at the inner edge of the helix and thereby causing the outer edge of the helix to buckle with formation of said corrugations.

3. The method of forming a helical fin structure from metal strip, said method comprising progressively rolling an edge portion of the strip under a driven pressure roller so as to elongate said edge portion and to thereby cause the strip to bend edgewise, winding the strip continuously about a mandrel, said mandrel limiting the bending of the strip resulting from the rolling operation to thereby produce an excess elongation of the said edge portion and resultant corrugation of said portion, rotating the mandrel about its axis to exert a constant positive longitudinal tensioning force upon the strip at the said roller, and simultaneously axially advancing the mandrel to form thereon a helical fin structure of desired pitch.

4. In a machine of the character described, an annular rotary anvil having a working surface defining the truncation of a cone whose apex lies in the rotary axis of the anvil, a roller coactive with the anvil and having a working surface defining the truncation of a second cone whose apex lines in the said axis at a point removed from the apex of the anvil whereby the nip between the roller and anvil tapers toward the outer edges of the anvil, means for driving the roller and anvil so that the operatively related portions of the two working surfaces have substantially the same linear velocity, means for guiding a strip of material into the tapered nip of the collar and anvil, a mandrel extending through and coaxial with the anvil, and means for rotating and simultaneously advancing the mandrel axially in timed relation to the rotary movements of the roll and anvil.

5. A machine according to claim 4, including means for maintaining the strip under constant longitudinal tension at the said nip.

6. In a machine of the character described, an annular rotary anvil having a working surface defining the tuncation of a cone Whose apex lies in the rotary axis of the anvil, a roller coactive with the anvil and having a working surface defining the truncation of a second cone Whose apex lies in the said axis at a point removed from the apex of the anvil whereby the nip between the roller and anvil tapers toward the outer edge of the anvil, means for driving the roller and anvil so that the operatively related portions of the two working surfaces have substantially the same linear velocity, means for guiding a strip of material into the tapered nip of the roller and anvil, a mandrel extending through and coaxial with the anvil, means for attaching the leading end of the strip to the mandrel, and means for rotating and simultaneously advancing the mandrel axially in timed relation to the rotary movements of the roll and anvil.

7. A machine according to claim 6, comprising a constant-torque motor constituting a prime mover for the mandrel rotating and advancing means and exerting a constant predetermined longitudinal tension in the strip between the said nip and the mandrel.

References Cited in the file of this patent UNITED STATES PATENTS 760,448 Gustavsen May 24, 1904 1,850,886 Lane Mar. 22, 1932 1,850,936 Lane Mar. 22, 1932 1,851,760 Ellis Mar. 29, 1932 1,884,203 Pickhard Oct. 25, 1932 1,896,350 Bundy Feb. 7, 1933 2,063,810 Iversen Dec. 8, 1936 2,094,204 Carter Sept. 28, 1937 FOREIGN PATENTS 117,992 Germany Feb. 28, 1901 322,494 Germany June 30, 1920 677,026 France Dec. 7, 1929

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