Patent Publication Number: US-2003230127-A1

Title: Perforated spiral pipe

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to an improved apparatus for producing spirally formed pipe, particularly spiral pipes having a diameter of approximately one inch or more.  
       BACKGROUND  
       [0002] A large potential for small diameter spiral pipes exists in the filter market, such as automobile oil filters. These filters typically have a perforated inner metal cylinder that is approximately one inch in diameter. Because pipes such as those used in oil filters need to be accurately and cleanly cut in large quantities, a pipe forming and cutting apparatus capable of fast and accurate cuts is necessary. There are several known ways to form and cut a pipe. A pipe may be formed by spirally or helically winding a continuous strip of metal, and joining adjacent edges of the wound strip to form a spiral lockseam in the pipe. In some pipe forming and cutting machines, the spirally formed pipe is cut by moving a knife outside the pipe into an overlapping position with a knife inside the pipe. Other types of spiral pipe forming and cutting machines use multiple knives or rotate the knives around the pipe to cut the pipe into sections.  
       [0003] The performance of the oil filter depends on the performance of the spiral pipe, typically an oil outlet at the center of the filter, where a strong flow of oil must be maintained for engine performance, and a strong filter must be maintained to resist pressure and insure functioning of the filter. Oil inflow is typically achieved by perforating the spiral formed pipe, that is, by punching holes in the inlet pipe so that oil can flow from the pipe into a downstream filter element. The filter element is typically paper, but need not be, and may be made from any of a number of other materials. The perforations in the center pipe necessary for the filter to function may be achieved in one or more of several ways. The strip or coil used for the central pipe may be perforated off-line, that is, in a separate operation.  
       [0004] Prior art perforations and machinery for perforating sheet metal typically included punches and dies that performed a unit operation off-line before the strip was fed into the spiral tube forming machine. A typical prior art punch  105  is depicted in FIG. 11 b . The punch  105  is V-shaped, having a low portion  104  and two high portions  114 . The punch forces strip or sheet metal into a die, such as the die  115  depicted in FIG. 11 a . As shown in FIG. 11 c , these tools produce perforated sheet metal  107 . The surface of the sheet metal  108  is pierced by V-shaped perforations  109 . These perforations allow communication between one side of the metal and the opposite side, necessary for a filter. Typically, surface  108  becomes the outer surface of the filter pipe which is later formed, while the pierced portions  108  remain on the inside.  
       [0005] There are several drawbacks to the prior system and method. For example, the V-shaped punch is prone to chipping and breaking. This method results in very high tool wear, when chipping  106  or other damage occurs on the punch  105 . Also, the configuration of the perforation, with two “high points”  114  and their separation from sheet metal surface  108 , does not strengthen the pipe, but instead may form weak points, where the metal is stressed and may tear. In addition, to maintain flow of oil or other fluid medium, there must be sufficient distance between the pipe itself and the “tips”  114  of the V. The tips can be no further away from the sheet metal surface  108  than the tensile strength of the metal will allow, or there will be tears in the resulting pipe. This could lead to uncontrolled flow of oil or other medium.  
       [0006] What is needed is an improved way to perforate and form sheet metal for filter pipe with a pattern that yields longer tool life and does not require an off-line operation. What is needed is a better way to form perforated filter pipe.  
       BRIEF SUMMARY  
       [0007] One aspect of the invention is a filter pipe. The filter pipe comprises sheet metal having a first edge and a second edge, and having a first end and a second end. The pipe also comprises a lock seam formed by locking the first edge to the second edge in forming a spiral-shaped pipe. The filter pipe also has a plurality of perforations or flutes, the perforations or flutes allowing communication between an inside of the filter pipe and an outside of the filter pipe. At least one side of the perforations forms an angle of from about ten to about forty degrees to a plane of the sheet metal. In one embodiment, the perforations or flutes are generally in the shape of a rectangle or a square, joined to the sheet metal with two sloping sides and two open sides, the open sides allowing communication between an inside of the filter pipe and an outside of the filter pipe.  
       [0008] Another aspect of the invention is a method for forming a filter pipe. The method comprises drawing a strip of sheet metal into a forming machine, and forming seal forms on a first edge and a second edge of the sheet metal. The method also includes drawing the metal and forming perforations or flutes in the sheet metal, after the edges are formed. The method then forms the strip into a spiral. The first and second edges of the sheet metal are sealed to each other to form a lockseam. The pipe is then cut to a desired length.  
       [0009] Another aspect of the invention is an apparatus for forming a spiral pipe from a metal strip. The apparatus comprises rollers for forming lateral edge portions of the metal strip. The apparatus comprises a forming head having a lateral bore for guiding for the metal strip into a spiral pipe. The apparatus further comprises a forming tool extending into the lateral bore of the forming head. The apparatus also comprises a closing assembly for coupling and compressing lateral edge portions of the metal strip into a lockseam of a pipe. The apparatus also has rollers for perforating and driving the metal strip around the forming tool, against an interior surface of the forming head, and in an axial direction, wherein the lateral edge portions of the strip mate and are coupled and compressed by the closing assembly to form a lockseam and provide a continuous spiral pipe.  
       [0010] Another aspect of the invention is a punch for perforating sheet metal. The punch comprises a roller having a plurality of protrusions radially extending from the roller, the protrusions arranged in at least two circumferential rows on the roller, the at least two rows staggered on a circumference of the roller, wherein the protrusions cause at least one of openings, perforations and corrugations in the sheet metal, but do not cause portions of the sheet metal to separate from the sheet metal. There are many ways to practice the invention. These and many other aspects of the invention will become apparent from the drawings below and the detailed descriptions of the preferred embodiments. 
     
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
     [0011]FIG. 1 is a perspective view of the back and right sides of a preferred embodiment of the present invention.  
     [0012]FIG. 2 is an elevation view of the back side of FIG. 1.  
     [0013]FIG. 3 is a plan view of FIG. 1.  
     [0014]FIG. 4 is an elevation view of the right side of FIG. 1.  
     [0015]FIG. 5 is a left side, elevational view of the forming head assembly of FIG. 1.  
     [0016]FIG. 6 is an front, elevational view of the forming head assembly of FIG. 1.  
     [0017]FIG. 7 is a plan view of the top side of the forming head assembly of FIG. 1.  
     [0018]FIGS. 8 and 14 are side elevational views of a part of a spiral pipe forming machine forming a part of the present invention.  
     [0019]FIGS. 9 a - 9   d  are sectional views of the edge forming and corrugation rollers that are used in the spiral pipe forming machine, with the strip edge configuration illustrated between the rollers.  
     [0020]FIG. 10 is a sectional view of the guide plates and clamping members that are used in the spiral pipe forming machine.  
     [0021]FIGS. 11 a - 11   c  depict a prior art punch and die set useful for making perforations in sheet metal.  
     [0022]FIGS. 12 a - 12   g  depict perforated coilstock and filter pipe according to a variety of embodiments of perforations or flutes.  
     [0023]FIGS. 13 a - 13   e  depict upper and lower rollers useful in the present invention for driving and perforating coil stock.  
     [0024]FIG. 15 is a front, sectional view of an upper bracket and pipe support assembly for use in the apparatus of FIG. 1.  
     [0025]FIG. 16 is a cross-sectional view taken along line  12 - 12  of FIG. 15.  
     [0026]FIG. 17 is a fragmentary side view of the upper bracket and pipe support assembly of FIG. 15 with the outer knife in a cutting position.  
     [0027]FIG. 18 is a fragmentary side view of the upper bracket and pipe support assembly of FIG. 15 with the outer knife in a stand-by position.  
     [0028]FIG. 19 is a front sectional view of a support assembly for use in the apparatus of FIG. 1.  
     [0029]FIG. 20 is a magnified view of inset A of FIG. 19.  
     [0030]FIG. 21 is an alternate embodiment of a pipe-making machine.  
     [0031]FIG. 22 depicts a method for making a perforated spiral pipe. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS  
     [0032] Referring now to the drawings, FIGS. 1 and 2 depict the spiral pipe forming apparatus  10  useful in making pipes according to the present invention. Many elements of the pipe forming machine  10  are conventional, and are described in greater detail in U.S. Pat. No. 5,636,541, issued Jun. 10, 1997. The disclosure in that patent is incorporated by reference herein, and made a part hereof. Many of the parts disclosed therein can be used in the present pipe forming machine  10  with some adaptation to accommodate the one and one-half inch wide strip  15  and its particular edge and the flutes or corrugation/perforation configurations that are used in the present pipe forming machine  10 .  
     [0033]FIGS. 8 and 14 depict some of the elements of pipe forming machines  10 ,  100 . The machine  10 ,  100  includes a frame  11  and a control cabinet  12 . A control panel  13  contains a plurality of control elements  14 , such as knobs, gauges and dials, for controlling and monitoring the operation of the pipe forming machine  10 ,  100  and the slitter  75 . The functions of the various control elements are described in U.S. Pat. No. 4,706,481, issued Nov. 17, 1987. The disclosures contained in that patent are hereby incorporated by reference in their entirety.  
     [0034] A continuous metal strip  15  is fed into the frame  11  of the pipe forming machine  10 ,  100 . To make one inch diameter filter pipe, the strip  15  is preferably 1.5 inches wide. If the pipe diameter increases, a wider strip  15  can be used and is preferred. The metal strip  15  passes through a roller housing  16  that contains a plurality of rollers that bend the edges of the strip  15   a  into a predetermined shape for forming a lockseam. FIGS. 9 a - 9   d  show the upper edge forming rollers  16 - u  and the lower edge forming rollers  16 - l  that are preferably used for forming the strip edges in forming filter pipe. The strip  15  first passes through the rollers shown in FIG. 9 a , and successively through the rollers shown in FIG. 9 b  through FIG. 9 d . Further information about the function and operation of the edge forming rollers is disclosed in U.S. Pat. No. 4,567,742, which is hereby incorporated by reference in its entirety and made a part hereof.  
     [0035] Returning to FIGS. 8 and 14, a lower drive roller  17  and an upper drive roller  18  are rotatably mounted in the frame  11 . The drive rollers cooperate to pull the metal strip  15  into the frame  11  and through the roller housing  16 . The two drive rollers  17 ,  18  then push the metal strip  15  between the upper guide plates  19  and lower guide plates  20 . The width of the drive rollers  17 ,  18  and the guide plates should be adapted to conform to the width of the strip  15 . As shown in FIG. 10, the lower guide plates  20  are secured to the frame  11  by bolts  20   a . The lower guide plate  20  also contains grooves to accommodate the edges formed in the strip  15 . Clamps  20   b  are pivotally connected to a base  20   c  that is attached to the frame  11 . The clamps  20   b  hold the upper guide plates  19  against the lower guide plates  20 .  
     [0036] The strip  15  can be perforated before entering the pipe forming machine  10 , or in the pipe forming machine  10  with perforating drive rollers  17 ,  18 . In FIG. 8, lower drive roller  17  and upper drive roller  18  are perforating rollers that also drive the strip through the pipe-forming machine. In FIG. 14, there are two sets of rollers, first set  17  and  18 , and second set  17   b  and  18   b . Either configuration may be used to drive the strip through the pipe forming machine and will also perforate the rollers as desired.  
     [0037] An improved perforation extends the metal no more than necessary and also strengthens the metal by adding “corrugations.” FIG. 12 a  depicts strip  15  with “square” flutes or perforations  15   b , each having two sloping sides  15   c  and two openings  15   d . The sloping sides  15   c  provide continuity of metal and insure that the ribs will continue to form a continuum with the strip, that is, a continuous piece of sheet metal, with no welds or other means for attaching. The openings  15   d  will allow oil or other medium being filtered to communicate from one side to the other when the spiral pipe is used as the central pipe in a filter.  
     [0038] In a preferred embodiment, the flutes or perforations  15   b  are substantially in the form of a square or a rectangle. The angle formed by the plane of the sheet metal and the sloping sides  15   c  may be any angle consistent with good tooling and manufacturing practices. The perforations may be formed by drawing or punching the metal to a depth of from about two times to about four times the thickness of the metal, with a depth of about three times preferred. This technique is sufficient to enable perforations that are large enough for good medium flow through a filter made from the filter pipe. The resulting flutes or perforations are also sufficient to “corrugate” or “reinforce” the filter pipe with added dimensional stability. By limiting the draw or deformation of metal, this process also prevents or at least limits tearing the metal at corners of the perforations.  
     [0039] Using perforations or flutes  15   b  as shown in FIG. 12 a  may result in a strengthened filter pipe section  23   a  because of the reinforcing nature of the apertures  15   b , as shown in the filter pipe section  23   a  in FIG. 12 b . Rather than being arranged in rows perpendicular to a longitudinal axis of the metal strip, perforations may be formed at an angle C to a horizontal axis of the sheet metal. That is, each row of perforations may be staggered from the adjacent rows by a portion of a perforation, thus forming an “angle” or circumferential distance from the adjacent rows. Thus, each row of perforations is offset from the other rows, preventing stress concentrations in the metal and in the resulting pipe. The pipe section  23   a  has been formed into a spiral shape with lock seams  23   b . A cross sectional view of pipe section  23   a  is shown in FIG. 12 c , in which the filter pipe section  23   a  has an outside surface  23   c , an inside surface  23   d , and the perforations  15   b  preferably reside on the inside of the pipe section  23   a . In other embodiments, the perforations may be on the outside. Flutes or perforations formed as a rectangle or square are preferred.  
     [0040] In one embodiment, the indentations or perforations have a pitch from about ¼ inch to about ½ inch, as shown in FIG. 12 a . A pitch of about 0.365 inch to 0.375 inch is particularly preferred. Indentations from about 0.25 inches square to about 0.375 inches square are preferred, as are rectangular indentations having sides of about this size. The axial spacing of the indentations may vary, as does the pitch. There should be sufficient space between the indentations or perforations to insure the strength of the resulting filter pipe. Areas between indentations are preferably from at least about 0.100 inches to about 0.200 inches. The sheet metal resulting from this processing desirably uses a considerable extent of its available surface area for these flutes, indentations, apertures, or perforations. The perforations preferably occupy from about thirty to about seventy percent of the surface area of the sheet metal, not including the edges of the sheet metal devoted to the lock seams or the forms on the edges that are later made into lock seams. Calculations to determine the area devoted to these features may include the transitions, such as sloping sides  15   c  as part of the area of the perforations or indentations  15   b . Although the embodiment of FIG. 12 a  illustrates uniformly spaced perforations of a uniform shape, a mixture of perforation shapes fabricated in the same pipe with any of a number of spacings or patterns between perforations is also contemplated.  
     [0041] The preferred embodiments described above cause a separation or kerf between surfaces of the sheet metal strip that is perforated, but no metal is actually severed from the metal strip and no pieces of scrap are generated. In addition to the indentations described above, other perforations and indentations may be used in embodiments of the present invention. Fluid communication between the outside and inside of the filter pipe may be achieved by processes that result in complete separation of pieces or metal from the metal strip, that is, processes that generate scrap from the sheet metal, and thus from the resulting pipe. Therefore, for purposes of this disclosure, perforation means a penetration or an opening in sheet metal whether scrap is generated or not. Besides the square and rectangular retained perforations, other openings in the sheet metal, and therefore the resulting pipe, are meant to be included in the present disclosure. For instance, FIG. 12 d  shows a triangular perforation in sheet metal  15 , the perforation retained on the sheet metal. FIG. 12 e  shows a polygonal perforation in sheet metal, again retained on the sheet metal. FIG. 12 f  depicts a perforation in which the scrap  15   e  is severed from the sheet metal  15 ; such a configuration will work but is less preferred and may also be in the prior art. FIG. 12 g  includes another configuration in which the perforation is raised from sheet metal  15  and scrap  15   e  is severed from the sheet metal. The configuration shown in FIG. 12 g  may require a two-step operation, preferably a first punching step to sever the scrap and a second drawing step to form the raised portion or dimple in the sheet metal. Two sets of perforating rollers, as shown in FIG. 14, may be used for these operations. The punches should register within 0.001-0.002 inches of each other. This configuration allows a “dimpling” effect on the resulting pipe, less than a “corrugating effect,” but still useful in lending dimensional stability. All the above configurations, and their equivalents, are meant to be included in the embodiments of the invention  
     [0042] Machinery to create these flutes or perforations is also revealed herein. The drive rollers  17 ,  18  may be used as a punch and die to perforate the strip  15  as it enters the pipe forming machine  10 . In a preferred embodiment, the lower drive roll is the punch and the upper drive roll acts as a die, although in other embodiments, the upper and lower drive rolls may be reversed. FIGS. 13 a  and  13   b  depict drive rollers  17  and  18  formed as a punch and die, respectively. In FIG. 13 a , the upper drive roller  18  has 6 lands or raised surfaces  18   a  with 5 grooves or lower surfaces between the lands, for receiving and forming sheet metal pushed into the die by punch  17  with raised surfaces  17   a.    
     [0043]FIG. 13 c  depicts one such raised surface  17   a , with corner radii R on two sides and forming an angle B with the surface of the punch. Corner radii R and angle B will be reproduced in the sheet metal or strip that is perforated by drive rollers  17 ,  18 . In one embodiment, angle B is from about 15 degrees to about 30 thirty degrees, but other angles may also be used. The radius used in the corner is preferably 0.040 inches, but may also take on other values as desired, and radii from about 0.010 inches to about 0.060 inches may also be used. The raised surface of FIG. 13 c  will produce the perforation or corrugation depicted in FIG. 12 a . The other two sides S of raised surface  17   a  have sharp edges for piercing and separating portions of a strip of sheet metal passing between rollers  13   a  and  13   b . The raised surface  17   a  will preferably have a depth (or height) of about two to three times the thickness of the metal strip to be processed through the rollers. The raised surfaces of the punch of FIG. 13 a  and the die of FIG. 13 b  cause the sheet metal to deform on radiused edges and cause separation or kerf on sharp edges, between the plane of the sheet metal and the corrugation formed. In preferred embodiments, there is no severing of the metal from the metal strip and thus no pieces of scrap produced.  
     [0044] In a preferred embodiment, as shown in FIG. 13 c , the raised surface  17   a  is configured with rounded corners having radii R so that sheet metal will deform but not separate where the surface comes in contact with each corner radius R and will only separate where sharp edges S of the raised surface come in contact with the metal strip. An advantage of this configuration is that wear on the roller is reduced and there are no narrow, sharp regions on the teeth or raised surfaces that might chip or dull quickly. Further, the resulting square or rectangular perforations produced by the rollers of FIGS. 13 a  and  13   b  have parallel openings or separations on two opposite sides while leaving the sheet metal connected on the other two opposite sides, shown in FIG. 12 a . The invention is meant to include any technique consistent with sheet metal arts and good manufacturing practices.  
     [0045] Particulars about the flutes or perforations may be varied as desired. In one preferred embodiment, the raised surfaces  17   a  are staggered circumferentially as shown in FIG. 13 b , and also shown earlier in FIG. 12 b , at an angle C to an axis of the drive roller. In one embodiment, the angle may be about fifteen degrees. Other punches or perforating drive rollers may use other angles. This design staggers the impact and the load of the sheet metal formation on the drive rollers and may help “average” the effective instantaneous load over each turn or partial turn of the drive rollers.  
     [0046] The drive rollers will typically be part of a larger set of tools used within the pipe-forming machinery. As shown in FIGS. 13 d  and  13   e , the drive rollers are part of an assembly or combination. FIG. 13 d  depicts a die assembly  130 , which is preferably used in an upper roller assembly. The die assembly  130  comprises a roller  131  and a die  132 , the die having at least two grooves  138  for mating with punches from another roller assembly. The die  132  is mated to the roller  131  with a key  137  and a lock ring  133 . There may also be a spacer ring  136  to space the assembly  130  in the pipe forming machine. There may preferably also be a gear  134  for driving the assembly and a second spacing ring  135 . The assembly  130  is preferably mounted on a shaft through the center (not shown) for driving or being driven by conventional driving components within the pipe-forming machine  10 .  
     [0047] A mating roller assembly  140  shown in FIG. 13 e  provides punches for the die of FIG. 13 d . Assembly  140  is preferably used in a lower roller assembly. The assembly is similar, except for the protrusions that this assembly provides. There is a roller  141  and a punch  142 , mated by key  149  and lock ring  143 . The punch  142  has at least two rows of protrusions  142 ′. There is a spacer ring  146  for proper spacing of the assembly  140  within the pipe-forming machine. There is preferably a gear  144 , mating with gear  134  for driving the assembly, and a second spacing ring  145 . The gears and their drivetrain should be designed and assembled to be backlash-free. The assembly  140  is preferably mounted on a shaft through the center (not shown) for driving or being driven by conventional driving components.  
     [0048] The sheet metal used for filter pipe is preferably tin-plated or cold-rolled steel, although the invention is not limited to this embodiment. Typical alloys that may be suitable include 1010 and 1020, in thicknesses from about 0.010 inches to about 0.030 inches. Galvanized steel, stainless steel, and other alloys may also be used. Coilstock thicker or thinner may also be used consistent with the strength and performance desired from the finished pipe. The flutes or perforations used are preferably drawn to a depth or height less than about five times the thickness of the sheet metal used, and preferably about two or three times the thickness of the sheet metal. For example, if SAE 1020 strip 0.020 inches thick is being perforated, the gap between the bottom surface of the sheet metal and the bottom surface of the perforation is desirably about 0.040 inches (twice the metal thickness) and the perforation itself is also displaced an additional 0.020 inches (the thickness of the metal). The gap and the thickness of the metal combine for a total displacement of three times the thickness of the metal. Other perforation dimensions are also possible, and may further depend on whether the user desires to remove or retain the perforation or aperture.  
     [0049] Referring now to FIGS. 1 through 5, a forming head assembly  21  and a mandrel assembly  22  with cover plates  65 ,  67  cooperate to form the perforated metal strip  15  into a spiral pipe  23 . The forming head assembly  21  includes a base  27  which is detachably secured to a forming head table  28 . A clamp  26  is used to secure the forming head base  27  to the forming head table  28 . As best shown in FIGS. 5 and 6, the forming head assembly  21  also includes a forming head  29  which is bolted to the forming head base  27 . The forming head  29  is enclosed around a lateral bore  30 . In one embodiment, the metal strip  15  is formed inside of the lateral bore  30  into a spiral pipe having a diameter of approximately one inch to about five inches. Helical grooves  32  are provided for the formed edges  15   a  of the strip  15  and the resulting lockseam  24 . Grooves  32  help guide the helically-wound strip  15  and spiral pipe  23  through the forming head  29 . The inner diameter of the lateral bore  30  determines the outer diameter of the spiral pipe  23 . If the diameter of the spiral pipe is to be varied, a forming head  29  with a different diameter lateral bore  30  should be used. Interchangeable forming heads with different diameter lateral bores can be used in a preferred embodiment of the present invention. In one embodiment, the pipe forming apparatus of the present invention may be used to make spiral filter pipe one to two inches in diameter from a one and one-half inch wide perforated metal strip  15 . It is expected that spiral pipe as small as one-half inch (½ inch) in diameter can be made with the pipe forming apparatus  10  of the present invention. The present invention is not limited to making perforated filter pipe and may also be modified to produce larger or smaller pipe diameters using wider strip.  
     [0050] The forming head  29  mates with a removable inset  33 . The inset  33  is held in place by pins (not shown). The radius of curvature of the removable inset  33  is smaller than the radius of curvature of the lateral bore  30 . The inner surface of the removable inset  33  can be coated with a friction reducing material. The removable inset  33  is intended to prevent the strip  15  from locking up as it is driven around the lateral bore  30  of the forming head  29 .  
     [0051] Referring to FIGS.  5 - 7 , a lock seam closing roller assembly  50  is positioned on top of the forming head  29 . The rotational axis of the lock seam roller head  52  is oriented in a laterally angled position, as shown in FIGS.  5 - 7 . The lock seam closing roller head  52  protrudes through an opening in the top of the forming head  29  and contacts the folded helically wound strip edges. The roller head  52  is rotationally attached to an end of a shaft  51 . Bearings inside the roller head  52  allow the roller head to be passively rotatable. The shaft  51  passes through an upper roller holder  53  that is attached to the top of the forming head  29  by threaded bolts  54 . The roller shaft  51  is also eccentric and has a hexagonal end  51   a  that can be accessed through an opening in the upper roller holder  53  (see FIG. 7). The lock seam roller head  52  can be adjusted vertically relative to the helically wound strip  15  by turning the hexagonal end  51   a  of the shaft  51 . A set screw  56  adjusts the lock seam roller head  52  axially with respect to the folded, helically-wound strip edges. A nut  55  holds the set screw  56  in place. In this top view of the forming head, bore  45  is shown in phantom.  
     [0052] Referring to FIGS. 1, 2 and  5 , the spiral pipe  23  is not only formed inside the enclosed forming head  29 , but at the same time is formed around a completely cylindrical mandrel  60 . The clearance between the mandrel  60  and the surface of the lateral bore  30  in the forming head  29  is approximately twice the thickness of the metal strip, plus 0.006-0.003 inches each side. The closely controlled clearance between the mandrel  60  and enclosed forming head  29  provides greater accuracy in producing pipe having a consistent diameter. If there is too much clearance, the strip  15  will buckle in the forming head. If there is too little clearance, the strip  15  will lock up inside the forming head. For a more detailed discussion of a suitable pipe forming apparatus, reference is made to U.S. Pat. No. 4,924,684 issued May 15, 1990. The entire disclosure of U.S. Pat. No. 4,924,684 is incorporated by reference. The preferred embodiment of the present invention also includes an apparatus for slitting the spiral pipe made with the pipe forming apparatus  10 . The present slitting apparatus  75  includes many elements of the slitting apparatus disclosed in U.S. Pat. Nos. 4,706,481 and 4,924,684. The descriptions of the slitting apparatus contained in these patents, as well as the disclosure in their entirety, are hereby incorporated by reference.  
     [0053] Referring now to FIGS.  1 - 4  and  15 - 18 , an inner knife  80  is attached to a boom  81  with a bolt  82 . A washer  83  is positioned between the bolt  82  and the inner knife  80 . The inner knife  80  has an oversized central opening  84  (not shown), which permits the position of the inner knife to be adjusted in any radial direction relative to the inner surface of the spiral pipe  23 . In general, the knife  80  will be centered within the spiral pipe  23 . It is preferred that the inner knife can be centered within the pipe without an oversized opening  84 .  
     [0054] The boom  81  passes through the mandrel  60 , and is free floating within the mandrel  60 . Thus, the boom  81  does not necessarily rotate with the mandrel, but is designed to rotate only during this slitting process. The boom is preferably passively rotatable, i.e., it is rotationally driven by the overlapping inner knife  80  and outer knife  110  during the slitting process. Other embodiments are possible, however, wherein the boom is drivably rotatable. To provide the passive rotation, the end of the boom  81  opposite the inner knife  80  is surrounded by combination needle/thrust bearings (not shown). These needle/thrust bearings can be obtained from IKO Bearings, of Arlington Heights, Ill. The bearings are held in a boom holder assembly  86  by an annular support member  87 , a lock washer  88 , and a lock nut  89 .  
     [0055] The boom holder assembly  86  has an upper section  90  and a lower section  91 . Each section has a central semi-cylindrical cavity which abuts the annular support member  87 . The upper section  90  and the lower section  91  are clamped to each other by a plurality of alien bolts  92 . The lower section  91  is mounted on an attachment block  93 , and fixed thereto by alien bolts  94 . The attachment block  93  passes between guide shafts  95 , and is secured to a shaft connector  96  by alien bolts (not shown). A plurality of alien bolts  97  squeezed together the ends of the shaft connector  96  around the guide shafts  95 , so that the shaft connector  96  slides axially with the guide shafts  95 . The guide shafts  95  pass through openings in the forming head table  28 , and slide through the bearing housings  98 , which preferably include THK Slide Bearing SC 30 assemblies. There are four such bearing housings  98 , each of which is attached to the top of a mounting leg  99  by alien bolts  101 . The four mounting legs  99  are provided to support the mandrel assembly  22  and the slitting apparatus  75  at the correct height with respect to the forming head table  28  and the pipe  23 . The mounting legs  99  are attached to the base plate  102  by alien bolts  103 . The base plate  102  is attached to the pipe forming machine  10 . Oval pivot slots (not shown) are provided in the base plate  102 , so that the pipe cutting apparatus can be pivoted about the center of the inner knife  80 . Most of the bolts that connect the various components of the boom assembly  86  pass through oval slots so that the position of the components can be adjusted relative to each other.  
     [0056] As can be seen in FIG. 1- 4  and in FIGS.  15 - 18 , an outer knife  110  is generally positioned below the inner knife  80  and outside of the pipe  23 . The outer knife is held in a vertical holder  111  by a lock washer and lock nut  114  that are connected to the shaft of the knife. Bearings (not shown) permit the outer knife  110  to be passively rotatable, that is, rotationally driven by contact with the rotating pipe  23 . The vertical holder  111  is attached to a slide bearing assembly  111   a  (not shown) (e.g., THK Roller Table Type VRM 3105A). The slide bearing assembly  111   a  is also attached to the central portion of a knife slide block  112 . The vertical holder  111  and outer knife  110  can thus slide up and down relative to the knife slide block  112 . The knife slide block  112  has two cylindrical openings through which the guide shafts  95  pass. A plurality of allen bolts  113  squeeze together the sides of these openings around the shafts  95 , so that the knife slide block  112  is also affixed to and slides axially with the guide shafts  95 .  
     [0057] During the pipe forming process, the outer knife  110  should be maintained in a standby position, where it will not interfere with the spirally moving pipe  23 . When it is time to cut the pipe, the outer knife blade is moved to a cutting position, where it punctures the spiral pipe  23  and overlaps the inner knife  80  (see, e.g., FIG. 17).  
     [0058] The outer knife blade  110  is moved into and out of its cutting position by the pneumatic cylinder assembly  116 . This assembly includes a pneumatic cylinder  117  that controls a piston  118 . A lower clevis  119  is attached to the piston  118  and a set of links  120 ,  121 . The lower links  120  are pivotally connected to the clevis  119  and an arm  122  which is integral with and extends from the central portion of the knife slide block  112 . The upper toggle links  121  are pivotally connected to the clevis  119  and the bottom of vertical holder  111 . Thus, when the piston  118  is fully extended, the vertical holder  111  and outer knife  110  will be raised vertically into the cutting position where the cutting edges of the inner and outer knives overlap and puncture the pipe  23 . See FIGS. 2, 4, and  17 . When the piston  118  is retracted into cylinder  117 , toggle links  120  and  121  will collapse and pull down the vertical holder  111  and the outer knife  110  to the standby position. See FIG. 1.  
     [0059] An upper clevis  123  is attached to the top of the cylinder  117 . The upper clevis  123  is pivotally connected to a threaded shaft  124 . Nuts  125  secure the threaded shaft  124  to one end of a cylinder support bracket  126 . The other end of the cylinder support bracket  126  is attached to the central position of the knife slide block  112 . The vertical holder  111  and slide bearing assembly  111   a  (not shown) are connected to the opposite side of the knife slide block  112 . As a result of its connection to the knife slide block  112 , the cylinder support bracket  126  and other components of the pneumatic cylinder assembly  116  move axially with the guide shafts  95 . The threaded shaft  124  of the pneumatic cylinder assembly  116  permits adjustment of the standby and cutting positions of the lower knife  110 .  
     [0060] As shown in FIG. 1, and also in FIGS.  15 - 18 , the slitting apparatus  75  of the present invention also includes a pipe support assembly  230 . The support assembly  230  includes a support sleeve  231  mounted on a sleeve holder  232 . The sleeve is removably affixed to the sleeve holder  232  by bolts  235 . The sleeve holder  232  is secured to an upper bracket  233  by bolts  234  extending vertically through the sleeve holder  232  and upper bracket  233  such that it is fixed in a radial direction with respect to the pipe  23 .  
     [0061] During the pipe forming process, the support assembly  230  is maintained in a fixed position where it does not interfere with the spirally moving pipe  23  (see, e.g., FIGS. 16 and 18). When it is time to cut the pipe and the outer knife  110  is moved to its cutting position, the support sleeve  231  will keep the pipe from being deflected where it contacts the spiral pipe (see, e.g., FIG. 17). The support sleeve  231  is positioned at the end of the pipe and preferably surrounds the pipe. The support sleeve  231  thus operates to prevent the boom  81  from deflecting upward or laterally in response to the force exerted by the outer knife  110 . Additionally, the sleeve prevents the pipe material at the cut end from flaring out as it is being cut.  
     [0062] With small diameter pipes (i.e., approximately 1 inch), it is difficult to keep the boom  81  rigid when the outer knife  110  engages the pipe  23 . If the boom  81  deflects away from the outer knife  110  during the cutting process, the inner and outer knives will move apart and will not overlap to cut the pipe. The support sleeve  231 , by limiting deflection of the pipe, maintains the inner and outer knives in an overlapping relationship during the slitting process. The support sleeve  231  is preferably constructed from a heat-treated steel such as AlSI A2 steel. It may be noted that many of the components of the pipe forming apparatus  10  and slitter apparatus  75  are made of tool steel (58°-62° HRc), CRS or Mehanite.  
     [0063] Referring to FIG. 15, the sleeve  231  substantially surrounds the pipe material to be cut. The sleeve is configured such that a predetermined distance, preferably 0.005 inch, is maintained between the outer diameter of the pipe and the inner diameter of the sleeve. The sleeve therefore preferably has an inner diameter  241  that is about 0.010 inches greater than the predetermined outer diameter of the pipe being cut. Different size support sleeves  231  may be constructed to accommodate specific pipe diameter requirements. In one embodiment, the sleeve  231  surrounds the entire circumference of the pipe where the pipe enters the sleeve and surrounds approximately 270 degrees of the pipe&#39;s circumference leaving a gap where the outer knife engages the pipe. In another embodiment, the sleeve may surround the entire circumference of the pipe and not have a gap. When the embodiment of a sleeve with no gap is used, the outer knife engages the pipe outside of the sleeve.  
     [0064] FIGS.  16 - 18  illustrate the extended upper portion  236  of the sleeve. The upper portion of the sleeve  232 , opposite the outer knife  110 , preferably has a width of 1.5 inches extending axially from approximately 1.3 inches behind the cutting edge of the inner knife  80  toward the pipe forming portion of the apparatus to approximately 0.2 inches past the cutting edge of the inner knife. The sleeve also includes a beveled or tapered receiving edge  237 . The tapered receiving edge preferably extends around the entire inner diameter of the sleeve and aids in guiding the pipe through the sleeve. The recessed lower portion  238  of the sleeve  231  permits necessary clearance for the outer knife  110  to engage the pipe  23 .  
     [0065] As best shown in FIGS. 19 and 20, the support sleeve  231  also preferably includes an angled edge  244 . The angled edge  244  is positioned on one side of the recessed lower portion  238  of the sleeve  231 . When a section of pipe rotates between the inner and outer knives during a cut, sharp or jagged edges may develop at the newly formed pipe edges. The pipe forming and cutting apparatus  10  may jam due to the sharp edges catching on the sleeve  231  as the newly cut edges are rotated in the sleeve. A more gradual transition to the preferred 0.005 inch clearance between the sleeve  231  and pipe is accomplished with the angled edge  244 . In one embodiment, the angled edge  244  forms approximately a 15 degree angle with the vertical axis. In other preferred embodiments, this angle may be optimized for the type of material to be cut or for different cutting applications. By providing a gradual transition for a freshly cut edge of a pipe rotating in the sleeve, the angled edge minimizes potential jamming difficulties.  
     [0066] The description so far has focused on pipe forming machines using a mandrel inside the forming head, the metal strip being wrapped around the mandrel by the forming head to form the filter pipe. Other embodiments of pipe forming machines may also be used to form pipes, especially those for pipes having larger diameter, such as 2″ to about 5″ in diameter (from about 50 mm to about 125 mm). Certain embodiments are further described in U.S. Pat. No. 4,706,481, the contents of which are hereby incorporated by reference. It is easier for large diameter pipes to utilize a boom and an inside roller and an outside roller, rather than a mandrel and forming head.  
     [0067] One embodiment of a machine with these features is depicted in FIG. 21. A pipe forming machine  200  for forming larger diameter pipe includes a boom  240  and a forming head  241 . The forming head  241  is mounted to the forming head base  242  by clamping bars  249 ,  251  and bolts  253 . In a preferred embodiment, the forming head base is movable on guide rails  255 . The forming head  241  curls the metal strip  15  into a cylindrical spiral, whereby the opposing preformed edges of the strip  15  mesh. The meshed edges are then compressed between a support roller  243  and a clinching roller  245  to form a lockseam. The metal strip, as described above for other pipe forming machine embodiments, is continuously pushed by the drive rollers so that a hollow cylindrical metal pipe is continuously produced with a spiral lockseam. The clinching roller  245  is moved into and out of its clinching position by a conventional hydraulic cylinder assembly  247 . The hydraulic cylinder assembly  247  includes a yoke  257  which holds the clinching roller  245 . The yoke is appended to a piston rod  263  which slides in and out of cylinder head  261 . The cylinder head  261  is attached to the cylinder barrel  259  by bolts  265 . The hydraulic cylinder assembly  247  provides the pressure on clinching roller  245  to close the lockseam on the filter pipe.  
     [0068] Referring again to FIG. 4, the support assembly  230  preferably attaches to the upper bracket  233  which is connected to opposite ends of the knife slide block  112 . The upper bracket  233  includes an overhead member  233 -U bolted to the tops of two vertical members  233 -F and  233 -B. The bolts pass through oval slots in the overhead member  233 -U, which permit angular adjustment of the support sleeves position. The support sleeve is connected to the overhead member  233 -U via the sleeve holder  232  and support bolts  234 . Each vertical member of the upper bracket  233  includes oval slots  233 -S. These oval slots  233 -S permit the height of the overhead member  233 -U, and hence the position of the support assembly  230 , to be adjusted. Although fixed in a radial direction with respect to the pipe  23 , the support sleeve  231  moves in the axial direction of the pipe during the cutting operation because the upper bracket  233  is connected to the guide shafts  95  via the knife slide block  112 . A slide  147  is provided to catch pipe sections  23   a  that have been severed by the slitter apparatus  75 . The slide  147  has a vertical flange  148  that is connected to the cylinder support bracket  126 . Thus, the slide  147  also moves in unison with the cutting knives  80 ,  110  and support sleeve  231  during the cutting operation.  
     [0069] When the outer knife  110  punctures the pipe  23  and overlaps the inner knife  80 , the guide shaft system allows the axially moving pipe to push the overlapping knives and the support sleeve, and their connected components, in unison with the pipe. An axial motion cylinder assembly  150  is provided to assist the axial movement of the pipe cutting apparatus  75 . As best shown in FIG. 2, this assembly  150  includes a pneumatic cylinder  151  which is supported by a piece of flat stock  152 , and held in place by a nut  153 . The flat stock  152  is attached to a mounting leg  99 . The piston  154  is secured to a second piece of flat stock  155  by a pair of nuts  156 . The second piece of flat stock  155  is bolted to the central inner portion of the shaft connector  96 . When air is supplied to the cylinder  151  in one direction, the piston  154  extends out of the cylinder, and pushes the shaft connector  96 , and its connected components, in the axial direction of the pipe  23 . When the air to the cylinder  151  is reversed, the piston  154  retracts and pulls the inner and outer knives  80 ,  110 , back to their starting or “begin-cut” position.  
     [0070] A stop/shock-absorber mechanism  160  is provided to fix the begin-cut position of the inner and outer knives. (See FIG. 2.) This mechanism comprises a mounting plate  161  which is attached to the forming head table  28 . A commercially available hydraulically-dampened plunger  162  extends through the mounting plate  161  in the axial direction of the pipe. The plunger  162  is held in place by nuts  163 , which mate with the threaded portions of the plunger  162 . A plastic tip  164  is mounted on the piston (not shown) of the plunger  162 . The stop/shock-absorber assembly  160  serves two functions. First, it serves as a stop, which sets the begin-cut position of the pipe slitting apparatus  75 . When the axial motion of the piston  154  fully retracts, a strip of flat stock  165  attached to the upper bracket  233  comes to rest again the plastic tip  164  of the fully retracted plunger  162  as shown in FIG. 2. Thus, the nuts and threaded portions of the plunger  162  can be adjusted to set the begin-cut position. In the second function, when the piston  154  extends and pushes the upper bracket  233  and flat strip  165  away from the stop/shock-absorber mechanism  160 , a spring (not shown) in the plunger  162  pushes its piston (not shown) and plastic tip  164  out of the plunger  162 . When the upper bracket  233  and flat strip  165  return to the begin-cut operation, they will push the plastic tip  164  and its piston into the plunger  162 , until the upper bracket  233  returns to the begin-cut position. While the piston is pushed back into the plunger  162 , it provides a hydraulic cushion or shock-absorber effect on the upper bracket  233  and its connected components.  
     [0071] A proximity sensor  170  is also mounted in the mounting plate  161  adjacent to the stop/shock absorber mechanism  160 . The sensor  170  is connected to the slitter control circuit, and prevents the slitting process from beginning if the slitter is not completely in its begin-cut operation. If the slitter is not in its begin-cut position and the slitting process begins, the axial motion piston  154  will reach its end of travel before the pipe  23  is fully severed.  
     [0072] In the pipe forming machine  10  a perforated strip of metal  15  is pulled into the roller housing  16  by the drive rollers  17  and  18 . In the roller housing  16 , the strip is corrugated and the edges of the strip are formed in the shapes desired to produce a spiral lockseam. Drive rollers  17  and  18 , or drive rollers  17 ,  17   b ,  18 ,  18   b  then perforate the strip and push the perforated, corrugated and edge-formed strip through the guide plates  19  and  20  and into the forming head assembly  21 . The strip is driven around the rotatable mandrel  60  and inside the lateral bore  30  of the forming head  29 . The metal strip is driven between the mandrel  60  and inside the lateral bore  30  of the forming head  29 . The metal strip is driven between the mandrel  60  and forming head  29  in a helical manner, so that the outer edges of the strip are positioned adjacent to each other in helical fashion. The folding rollers  36  and  37  cooperate to fold the adjacent, mated edges of the helically wound strip. The lockseam closing roller  52  compresses the folded strip edges against the mandrel  60  to form a tight lockseam  24 . During the pipe forming operation, the mandrel  60  is passively rotatable and pivotable, thereby eliminating friction that might otherwise cause the helically wound strip and pipe to lock up between the mandrel  60  and forming head  29 .  
     [0073] As the spiral pipe production continues, the pipe  23  moves out of the forming head block  29  in a helical fashion. That is, the pipe  23  moves in its axial direction while it rotates. During the pipe production process, the outer knife  110  is in its standby position, as well as in the begin-cut position. Thus, the pneumatic cylinder assemblies  116 ,  150  have their respective pistons fully retracted. When the pipe  23  reaches its desired length, air is sent to both of these pneumatic cylinder assemblies to fully extend their respective positions. Thus, the pneumatic cylinder assembly  116  pushes the outer knife  110  upward so that it punctures the pipe  23  and overlaps the inner knife  80 . The pneumatic cylinder assembly  150  extends its piston, which pushes all of the components and outer knives and the support sleeve, in the axial direction of the pipe. As the pipe forming machine  10  continues to produce the pipe  23  in a spiral manner, the pipe moves axially with, and rotates between the overlapping inner knife  80  and the outer knife  110 . After the pipe completes one revolution, the section of the pipe  23   a  in front of the overlapping knives will be completely severed and will fall into slide  147 .  
     [0074] Once the cutting process is complete, the air supplied to the pneumatic cylinder assemblies  116 ,  150  will be reversed. As a result, the outer knife  110  will be returned to its standby position, and the piston  154  will pull all the components connected to the guide shafts  95 , including both knives and the support sleeve, back to the begin-cut position.  
     [0075] Referring now to FIGS. 2, 8 and  14 , to operate the cutting apparatus  75  in an automatic mode, an electrical encoder  27 ′ is coupled to the lower drive roller  17 , or lower drive rollers  17  and  17   b , of the pipe producing machine  10  by a pulley belt  28 ′. The encoder  27 ′ is adapted to generate pulses corresponding to the number of rotations of the lower drive roller  17  or  17   b . These pulses are transmitted over a cable  29 ′ to a control box  44 ′. The control box  44 ′ is programmed to check for a first pulse count corresponding to the desired length of the pipe, a second pulse count corresponding to a slow-down point for pipe production, and a third pulse count corresponding to the amount of axial travel of the pipe required for the pipe to be completely cut by the cutting apparatus  75 . Three counters  45 ′,  46 ′ and  47 ′ are included in the control box  44 ′. These counters can be incremented or decremented, one pulse at a time. The first pulse count (i.e., pipe length) is set with the first counter  45 ′, the second pulse count (i.e., slow-down point) is set with the second counter  46 ′, and the third pulse count (i.e., cut length) is set with the third counter  47 ′. The control box  44 ′ sends pneumatic signals to the pneumatic cylinders  117 , 151  over line  48 ′ in response to the first, second and third pulse counts.  
     [0076] The control box  44 ′ also has four control switches  147 ′,  148 ′,  149 ′, and  150 ′. A first control switch  147 ′ selects manual or automatic control of the pipe cutting apparatus  75 . In the manual mode, the second, third and fourth control switches  148 ′,  149 ′ and  150 ′ are operable to manually actuate the pneumatic cylinders  117 ,  151 . That is, the second control switch  148 ′ may be used to move the piston rod  118  in and out of its cylinder  117 , and thereby move the outer knife  110  into and out of its cutting position. The third control switch  149 ′ may be used to move the piston  154  in and out of its cylinder  151 , and hence slide the inner knife  80 , outer knife  110  and support sleeve  231  in the axial direction of the pipe  23 . When the first control switch  147 ′ is put into automatic mode, the second, third and fourth control switches  148 ′,  149 ′ and  150 ′ are deactivated, and all three counters  45 ′,  46 ′ and  47 ′ are reset to zero. The pipe cutting apparatus  75  will automatically cut the pipe  23  into sections  23   a  as pipe is produced on pipe forming apparatus  10 .  
     [0077] The control panel  13  is provided with an on/off switch for the pipe cutting machine  75  and three speed adjustment knobs  135 ′,  136 ′ and  137 ′. The first speed adjustment knob  135 ′ controls the production speed of the pipe as it is formed with the pipe forming machine  10 . The second speed adjustment knob  136 ′ controls the speed at which the pipe is formed prior to the outer knife  110  moving from its standby position to its cutting position. In order to consistently obtain pipe sections that are cut to the same length, it is important that the pipe  23  travels at a constant, relatively slow speed while the outer knife  110  moves from its standby position to the cutting position. A relatively low speed minimizes the effect of any pulse count errors on the length of the pipe sections  23   a . Thus, prior to moving the outer knife  110  to the cutting position, it is preferred that the pipe production is slowed from its fastest, most efficient production speed to a transitional, “slow-down speed.” The second speed adjustment knob  136 ′ controls this speed. The third speed adjustment knob  137 ′ controls the speed of the pipe production while the outer knife  110  moves to, and is in, the cutting position where it cooperates with the inner knife  80  and support sleeve  231  to cut the pipe  23 . The cutting speed is usually set at one-half the production speed, or whatever speed is convenient. The speed control knobs  135 ′,  136 ′,  137 ′ can be used to adjust the production speed, slow-down speed and cutting speed of the pipe cutting apparatus  75  during both manual and automatic modes of operation.  
     [0078] The cutting apparatus  75  operates in conjunction with the pipe producing machine  10  in automatic mode in the following manner. The spiral pipe forming process is initiated with the pipe forming machine  10  in a known way. When the leading edge of the pipe  23  begins to leave the forming head  29 , the pipe producing machine is temporarily halted, and the pipe cutting apparatus  75  is energized by turning on the on/off switch on the control panel  13 . The pneumatic cylinder assemblies  116 , 150  are initialized to be in their standby positions, so that the outer knife  110  does not overlap the inner knife  80 . The first counter  45 ′, the second counter  46 ′, and the third counter  47 ′ are set to zero. Air is sent to the axial motion cylinder  151  to fully retract piston  154 , so that the inner and outer knives are in the begin-cut position.  
     [0079] Typically, the pipe cutting apparatus  75  will be initially operated in its manual mode to cut a few sections of pipe to determine the optimum positional adjustments for the inner knife  80 , outer knife  110  and support sleeve  231 . The pipe cutting apparatus  75  then run in and out of automatic mode a few times to find the optimum settings for the production speed, slow-down speed, cutting speed, and the pulse counts for the pipe length, slow-down point, and cut length. Once these variables are determined, the pipe cutting apparatus  75  is ready for continuous automatic operation.  
     [0080] In a typical example of automatic operation, the first counter  45 ′ may be set to 1250 pulses for pipe length, the second counter  46 ′ may be set to 1100 pulses for the slow-down point, and the third counter  47 ′ may be set to 375 pulses for the cut length. A cutting cycle begins by resetting all three counters  45 ′,  46 ′,  47 ′ to zero, and by cutting the part of the pipe  23  that extends past the inner and outer knives will in the begin-cut position. This part of the pipe will be referred to as the “leading section”.  
     [0081] When the pulse count is at zero in all three counters, the control box  44 ′ sends a first pneumatic pulse signal, via line  48 ′, to the pneumatic cylinder assembly  116 . The piston  118  is thereby energized and pushed downward to its extended position. The outer knife  110  is thereby moved to the cutting position where the cutting edges of the inner and outer knives puncture the pipe  23 . The first pneumatic pulse signal also reverses the direction of air supplied to the axial motion cylinder  151 , so that the piston  154  pushes the shaft connector  96 , and all components connected to the guide shafts  95 , axially with the pipe. The pipe forming machine  10  continues to produce the pipe  23  in a spiral manner. The pipe  23  thus moves axially with, and rotates between, the overlapping inner knife  80  and the outer knife  110 . The encoder  27 ′ generates a train of pulses that correspond to the length of the next section of pipe being formed, which has its leading edge at the overlapping knives. This section of pipe will be referred to as the “new section.” All three counters  45 ′,  46 ′,  47 ′ count in unison as the new section of pipe is formed and the leading section of pipe is severed.  
     [0082] The guide shafts  95  allow the inner and outer knives to move in the axial direction of the pipe under the forces provided by the new section of pipe pushing on the overlapping knives and the extension of the axial motion piston  154 . Thus, the inner knife  80 , outer knife  110  and support sleeve  231  cooperate to cut the complete circumference of the leading section of pipe as the pipe moves axially and rotates between the inner and outer knives. The third pulse count will be set at the number of pulses corresponding to the axial travel corresponding to slightly more than one pipe rotation. It is generally preferred to have a little overlap in the cut to assure that the leading pipe section is completely severed.  
     [0083] When the third pulse count is reached, the third counter  47 ′ stops counting, but the first and second counters  45 ′ and  46 ′ continue to count as the new section of pipe continues to be produced. Also, the control box  44 ′ sends a second pneumatic pulse signal to the pneumatic cylinder assemblies  116 , 150  along line  48 ′. This second pneumatic signal indicates that the cutting process is completed, and thus operates the pneumatic cylinders  117 ,  151  to fully retract their respective pistons. The outer knife  110  is then moved to its standby position. The air supplied to the cylinder  151  is also reversed, so that the piston  154  pulls the cutting assembly  75  mounted on the guide shafts  95  back to its begin-cut position. When the third pulse count is reached, the new section of pipe also stops being produced at the cutting speed, and begins to be formed at the production speed.  
     [0084] The new section of pipe will continue to be produced at the production speed, and the first and second counters  45 ′,  46 ′ will continue to count pulses, until the second pulse count is achieved. At that time, the slowdown point will be reached. The second counter  45 ′ will stop counting, and the new section of pipe will be formed at the slow-down speed. The new section of pipe will continue to be formed at the slow-down speed, and the first counter  45 ′ will continue to count pulses, until the first pulse count is reached. The first pulse count indicates that the new section of pipe has reached its desired length. When the first pulse count is reached, all three counters are reset to zero, and cutting process just described is repeated for the new section of pipe. The same cutting process will continue to be repeated as additional sections of pipe are produced.  
     [0085] The control scheme just described for the pipe cutting machine  75  is preferred because the pulse counts for the pipe length, slowdown point and cut length can be set independently, and the cut length is automatically accounted for in the pipe length. Moreover, the foregoing control scheme is preferred for cutting the spiral pipe into sections up to approximately five feet in length. For longer sections of pipe, the first pulse counter can be eliminated, and a limit switch connected to the pipe runoff table can be used to indicate that the desired pipe length has been reached. Other control schemes and considerations are disclosed in U.S. Pat. No. 4,823,579 issued Apr. 25, 1989. The control schemes disclosed in that patent, as well as all other disclosures in that patent, are hereby incorporated by reference.  
     [0086] The invention may be practiced in many ways. One embodiment of the invention is a method of forming a perforated spiral pipe. One step of the method is to draw coilstock or a strip of sheet metal into a machine for forming spiral pipe. A second step is to form indentations or perforations in the strip, which may be formed by the same drive rollers that propel the strip into the machine. The method includes one or more steps of forming seal forms on the edges of the strip, and then forming the strip into a spiral. The edges of the strip are then sealed to form a lockseam, and a desired length of pipe is cut and released from the machine. The method may include other steps as desired for further finishing the pipe so produced.  
     [0087] It should be understood that changes and modifications to the preferred embodiment described above will be apparent to those skilled in the art. It is intended that the foregoing description be regarded as illustrative rather than limiting, and that it is the following claims, including all equivalents, which are intended to define the scope of the invention.