Patent Abstract:
A collet for holding a workpiece having an axis includes a collet base defining a collet axis. A plurality of contact points that are not integral with the collet base are supported on the collet base for movement relative to the collet base in directions generally toward and away from the collet axis. The collet is self-aligning whereby the contact points help to align the workpiece coaxially with the collet base.

Full Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/481,103 filed Jul. 17, 2003, the entire disclosure of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   Commercially available pipe typically is manufactured to a nominal outer diameter which varies plus and minus depending on the tolerance range. Collets are used to hold or clamp pipes during, for example, welding of the pipes. Welding can be accomplished with, for example, an orbital weld head of the type shown generally in U.S. Pat. No. 4,379,215, the entire disclosure of which is hereby incorporated by reference. A collet for a given pipe size needs to provide enough compliance to securely hold pipes that have a size anywhere within the given tolerance range for that pipe size. The collet typically must fit certain space constraints. Some prior art pipe collet designs use integrally machined components to provide spring-like compliance, but these designs can be inconsistent and relatively expensive, and can require mating collet components that also are relatively expensive. 
   The use of rigid collets for holding the pipe during the welding operation is much preferred as compared to split and/or adjustable collets or similar holding devices. The reason for this is that during the welding operation, thermal stresses tend to cause the pipe to move creating misalignment between the two sections. The movement is greater and/or more likely to happen with the split collets and the adjustable collets or holders. With respect to the solid or rigid collets, however, there are problems in assuring that pipe throughout the range of standard commercial tolerances can be held properly. That is, a typical commercially available pipe used for fluid systems and the like, has, for example, a nominal outside diameter of 3.000 inches which varies ±0.030 inches. It has been difficult to compensate for the diameter variations which result from the tolerance variations and, at times, it has been difficult to properly hold the pipe during the welding operation. Also it has at times been difficult to hold out of round pipes or tubes in the proper position relative to the weld head. 
   Some prior art pipe collets, designed to accommodate a significant amount of size variation, use a series of slots that are cut into the base collet material to form resilient fingers. The slots can be cut either radially outward from the theoretical center of the object to be held, or they can be cut tangentially with respect to that theoretical center. The geometry of these slots enable the collet fingers to flex, or bend, in response to the geometry of the pipe that is being clamped within the collet. Where a large amount of compliance is not required, the collets can be left solid, that is, without machined slots. Some manufacturers cut the inside diameters of these collets on the true center of the collet, and some manufacturers offset the ID cut. 
   SUMMARY OF THE INVENTION 
   The present invention relates to work holders and, more particularly, to a collet for holding cylindrical workpieces in alignment with an axis of the collet. The invention is especially suited for use in a pipe clamp for association with an orbital welder and will be described with particular reference thereto. The invention is, however, capable of broader application and could be incorporated in a wide variety of work holders and clamping units for different types of work pieces and tools. For example, the invention may be used with a facing or other finishing tool. The invention may also be used with tubes in addition to pipes, although commercially available tube collets are often sufficient to accommodate the tolerance variation in tube sections. 
   In one embodiment, the invention relates to a collet for holding a workpiece having an axis. The collet includes a collet base defining a collet axis. A plurality of contact points that are not integral with the collet base are supported on the collet base for movement relative to the collet base in directions generally toward and away from the collet axis. The collet is self-aligning whereby the contact points help to align the workpiece coaxially with the collet base. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the invention will become apparent from the following description when read in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a pictorial view of a pipe clamping fixture including two collets that are a first embodiment of the invention; 
       FIG. 2  is an elevational view of a portion of the fixture of  FIG. 1  showing the two collets in a closed condition; 
       FIGS. 3-5  are views similar to  FIG. 2  showing the two collets in a condition clamping pipes of different sizes; 
       FIG. 6  is an exploded sectional view of parts of one plunger; 
       FIG. 7  is a sectional view of a portion of one collet with a plunger shown in a first position; 
       FIG. 8  is a view similar to  FIG. 7  showing the plunger shown in a second position; 
       FIG. 9  is an exploded perspective view of one collet; 
       FIG. 10  is a schematic view of a collet that is another embodiment of the invention; 
       FIGS. 11-13  are a series of views of a collet that is another embodiment of the invention; and 
       FIGS. 14-15  illustrate another clamping fixture in accordance with the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a pipe clamp fixture  10  which is intended to be used with an orbital welding head. The fixture  10  is designed to hold two pipe sections in aligned relationship with their facing ends abutting so that an electrode of an orbital welding head (for example as shown schematically at  11 ) can rotate about the mating pipe ends to perform a butt weld operation. 
   The pipe clamp fixture  10  includes a pair of clamping units  12  and  14  which are joined to opposite sides of an intermediate spacer  16 . The various components are formed from stainless steel, aluminum, or the like and the clamp units  12  and  14  are removably joined to the spacer  16  in any convenient manner such as through the use of machine screws. The clamp units  12  and  14  are joined to the spacer  16  to form a somewhat U-shaped structure with an open welding space  20  adapted to receive the orbital welding head  11 . The fixture  10  may if desired be secured to the welding head  11  by screws, for example. 
   In the illustrated embodiment, the clamping units  12  and  14  are of identical construction except that they are mirror images of one another. Therefore, only the clamp unit  14  is described herein. 
   As best shown in  FIGS. 2 and 3 , the clamp unit  14  comprises a pair of opposed clamp halves  22  and  24 . The clamp halves  22  and  24  are connected with each other by a hinge or hinge mechanism which permits the clamp halves to be moved toward and away from one another between a closed position as shown in  FIGS. 1 and 2  and an open position (not shown). The two clamp halves  22  and  24  are releasably connected in their engaged or clamping position by a releasable latch or lock fixture such as the one shown at  28 . 
   Each one of the clamp halves  22  and  24  carries a collet  30 . The collets  30  cooperate to define a circular work piece clamping opening  32  ( FIG. 2 ) that is centered on an axis  34 . The collets  30  of the two clamp halves  22  and  24  are identical to each other. The clamp halves  22  and  24  together form a collet holder that holds the two collets  30 . 
   Each collet  30  includes a collet base  40 . The collet base  40  as illustrated is a rigid metal member that is secured to the clamp half  22  by a pair of mounting screws (not shown). The collet base  40  has a circumferential extent of about 180 degrees about the axis  34 . The collet base  40  has a cylindrical inner surface or base surface  44  centered on the axis  34 . 
   The collet base  40  has one or more plunger openings  46 . The plunger openings are spaces or cavities or chambers or recesses that receive and guide contact points  60  and springs  80 , as described below. The contact points  60  are the portions of the collet  30  that actually contact the workpiece. The contact points  60  are formed separately from and are not integral with the collet base  40 . 
   In the illustrated embodiment, the collet base  40  has two plunger openings in the form of recesses  46 . The plunger openings  46  are formed in the base surface  44  of the collet base  40  and extend radially outward. 
   Each plunger opening  46  is defined by a cylindrical side surface  48  and a circular bottom end surface  50  both centered on a radially extending plunger axis  52 . A threaded bore  54  extends from the plunger opening  46 . Associated with each plunger opening  46  is a locking screw opening  56  that extends transverse to the bore  54  and that intersects the bore near its bottom end. In the illustrated embodiment, two plunger openings  46  are provided, spaced ninety degrees apart about the axis  34 . A different number of plunger openings  46  could be provided, and different spacing could be provided between the openings. The plunger openings could alternatively be formed or configured in another manner—for example, they might not need to be recessed from the base surface  44 . 
   Disposed within each one of the plunger openings  46  is a plunger assembly  58  ( FIG. 6 ). The plunger assembly  58  includes a contact point or plunger  60 ; a mounting screw  70 ; and spring  80 . 
   The plunger  60  is a movable member or contact point that) (together with the other plungers  60 ) forms that portion of the collet  30  which the workpiece contacts. The plunger  60  is movable relative to the collet base and to the axis  34 , as described below. The plunger  60  is formed separately from and is not integral with the collet base  40 . 
   The plunger  60  in the illustrated embodiment is an open-ended, hollow cylindrical member adapted to fit closely within the plunger opening  46 . The plunger  60  is preferably made from 416 stainless steel, and has a hardness of 36 to 42 on the Rockwell C scale. Parts that are of this hardness have acceptable wear characteristics without being so hard that they would be brittle. The invention is not limited, however, to any particular material for the plungers. 
   The plunger  60  has a cylindrical side wall  62  that defines a central opening  64  centered on the axis. The side wall  64  has an annular top end surface  66  and an annular bottom end surface  68 . The plunger  60  has a bottom flange  69  that extends radially inward from the side wall  62  to narrow the central opening  64  at the bottom (radially outer) end of the plunger. 
   The mounting screw  70  is an element or assembly that retains the plunger  60  in the plunger opening  46 , that is, that keeps the plunger from moving radially inward past a certain position. In the illustrated embodiment, the retaining screw  70  is a socket head screw with a head  72  and a threaded shank  74 . The head  72  of the retaining screw  70  is smaller in diameter than the central opening  64  of the plunger  60 , but larger in diameter than the bottom flange  69  of the plunger. 
   The spring  80  is an element or assembly that biases the plunger  60  radially inward in a direction along the plunger axis  52 . In the illustrated embodiment, the spring  60  is a stack  82  of Belleville washers  84 . Other types of springs  80  could be used, for example, a compression spring. Thus, the term “spring” when used herein could refer to a single member that provides a biasing force or to a plurality of members or elements that act together to provide a biasing force. For example, a coil spring  80  could provide adequate compliance and loading, but not necessarily within the same small space as the Belleville washers  84 . Polymer (as opposed to metal) springs  80  might also be used, but might not be able to withstand the temperatures commonly encountered when welding. 
   Each one of the Belleville washers  84  is dished or cupped. In the illustrated embodiment, the washers  84  are stacked in a particular manner so as to increase both load (resistance) and travel (total available deflection). A single Belleville spring  84  has a specific load and deflection. Belleville springs in stacked arrangements provide increased load and/or deflection. Specifically, two springs stacked in parallel (in the same direction or orientation) provide double the load or resistance of the single spring, with no increase in total deflection available. Two springs stacked in series (in the opposite direction or orientation) provide the same load or resistance as the single spring, with double the total deflection available. A parallel-series combination results in the load or resistance of two springs and the total available deflection of two springs. 
   In the illustrated embodiment, the washers  84  are stacked in a pattern that repeats three times. The pattern includes two washers  84  cupped down (in parallel with each other) and the next two washers cupped up (in parallel with each other but in series with the first pair). This arrangement provides a total of twelve washers  84 . 
   The number and pattern of washers  84  illustrated herein is only exemplary. Different numbers of washers  84  could be provided, and they could be stacked in a different order or pattern. Different types of individual spring elements could be used, also, in the spring  80 . 
   When the collet  30  is assembled, the stack  82  of washers  84  is disposed loosely in the plunger opening  46  in the collet base  40 . The side wall surface  48  of the collet base  40  locates the washers  84  and keeps them centered in the plunger opening  46 . The plunger  60  is located over the stack  82  of washers  84 , resting on the uppermost washer. The shank  74  of the retaining screw  70  extends through the plunger  60  and through the stack  82  of washers  84  and is screwed into the threaded lower portion  54  of the plunger opening  46 . The retaining screw  70  is preferably screwed in only until the head  72  of the retaining screw engages the bottom flange  69  of the plunger  60 , taking the play or looseness out of the stack  82  of washers  84 . Attempted further threading in of the retaining screw  70  would begin to compress the washers  84 , which would resist such motion strongly and rapidly. The retaining screw  70  is left in this position where the spring  80  is not compressed by any significant amount. 
   A locking screw  88 , which could be a socket set screw, is screwed into the locking screw opening  56  in the collet base  40  and engages the shank  74  of the retaining screw  70 . The engagement of the locking screw  88  with the retaining screw shank  74  helps to hold the retaining screw  70  in the desired position relative to the collet base  40 . 
   In the desired position, a portion but not all of the plunger  60  projects from the base surface  44  of the collet base  40 . Up to one third to one half the length of the plunger  60  might project from the base surface  44 . Preferably, no more than about twenty per cent of the plunger  60  projects from the base surface  44 . The side wall surface  48  of the plunger opening  46  helps to guide the plunger  60  and keep it aligned, to minimize skewing. The head  72  of the retaining screw  70  is recessed below the base surface  44 , as are all of the washers  84 . The plungers  60  are the only portions of the collet  30  that contact the workpiece; the collet base  40 , itself, does not. 
   When the fixture  10  is fully assembled, it includes four of the collet assemblies  30 , two on each one of the clamping units  12  and  14 . Each one of the clamping units  12  and  14  includes one collet  30  on its upper clamp half and one collet  30  on its lower clamp half. 
     FIG. 3  illustrates schematically the clamping unit  12  in use clamping a pipe  90 . The four plungers that are included in the two collets  30  are in engagement with the pipe  90 . Specifically, the top end surface  66  of each plunger  60  is in engagement with the outer side surface  92  of the pipe  90 . Because the plungers  60  are located on opposite sides of the pipe  90 , their combined resistance is averaged out to approximately center the pipe between them. This works for round pipe  90 , as well as for out-of-round pipe, which often, from handling, has an oval outside profile. In this respect, the collet  30  (or a set of collets  30 ) can be considered to be self-aligning or self-centering. 
     FIG. 4  shows the fixture  10  in use in clamping a pipe  90  having a relatively larger diameter.  FIG. 5  shows the fixture in use in clamping a pipe  90  having a relatively smaller diameter. 
   The clamping unit  14  is capable of rigidly and tightly engaging the outer diameter of a pipe of a particular size depending on the diameter of the collet base surface  44 . By changing collet bases  40 , the clamping unit  14  can be made to accommodate tubing or piping of different size ranges. Additionally, by providing different size collets in one clamping unit  14  relative to those in the other clamping unit  12 , it is possible to bring into alignment two workpieces of different sizes such that it is possible to weld various pipe and fitting combinations. 
   The upper collet  30  and the lower collet  30  of a clamping unit  12  or  14  are the same and are interchangeable. Some other collet designs are sold as matched sets—if one collet becomes damaged to the point where it can no longer be used, the other collet half must be scrapped, because the new collets must be ordered as a set. 
   Another advantage of the collet  30  is cost-effectiveness. A collet in accordance with the present invention can be relatively inexpensive to make, because the majority of machined surfaces are clearance surfaces. Also, wear components of the collet  30 , such as the plungers  60  and the washers  84 , can be replaced very easily, while retaining the collet base  40  itself. This can provide the collet  30  with a very long service life. For example, if the spring  80  begins at some point to lose its temper, it can be easily and inexpensively replaced—in comparison to a collet with integral spring fingers in which case the entire collet must be replaced. 
     FIG. 10  illustrates schematically a collet  30   a  that is an alternative embodiment of the invention. The collet  30   a  functions similarly to a camera aperture. The collet includes a plurality of contact points in the form of plates  92  that slide about a center point. The plates  92  are biased radially inward by springs shown schematically at  94 . The plates  92  open and close equally so as automatically to center a pipe that is captured between them. In this respect, the collet  30   a  can be considered to be self-aligning or self-centering. 
     FIGS. 11-13  illustrate schematically a collet  30   b  that is another alternative embodiment of the invention. The collet  30   b  includes a base ring  100  on which are rotatably mounted four contact points in the form of eccentric cams  102 . Rotation of the cams  102  is controlled by a slider ring  104  that engages the cams. Relative rotation of the slider ring  104  about the base ring  100  causes the cams  102  to pivot. 
     FIG. 11  shows the eccentric cams  102  fully open. A pipe section  110  is disposed within the collet  30   b  in a position not centered in the collet.  FIG. 12  shows the cams  102  rotating as the slider ring  104  is rotated relative to the base ring  100 , in a counter-clockwise direction as viewed in  FIG. 12 . The cams  102  move radially inward, two of the cams contact the pipe  110 , and the pipe begins to move towards the center of the collet  30   b .  FIG. 13  shows all four cams  102  in contact with the pipe section  110 . The pipe section  110  is centered within the collet  30   b . In this respect, the collet  30   b  can be considered to be self-aligning or self-centering. 
     FIGS. 14 and 15  illustrate still another embodiment of the invention. In this embodiment, a collet includes contact points in the form of plunger assemblies at two axially spaced locations (that is, spaced apart in a direction along the axis of the workpiece being clamped) to help improve clamping accuracy and squareness. 
     FIG. 14  shows a clamping fixture  120  with an attached or inserted weld head  122 . The clamping fixture  122  includes four collets (numbered  30   c  in  FIG. 14 ) like the collet  10 . Each one of the collets is modified by the addition of two additional plunger assemblies. 
   In the collet  30   c , the collet base  40   c  has an outer side surface  124  to which are attached two cantilever arms  126 . The cantilever arms  126  are attached to the collet base  40   c  by mounting screws  128  and pins  130 , or other mounting structure. The arms  126  may be attached to the collet base  40   c  at locations that are spaced apart circumferentially from the locations of the plunger assemblies  58 . The arms  126  extend axially away from the collet base  40   c  (that is, in a direction along the axis of the workpiece being clamped). 
   At the outer end of each arm  126  is a contact point in the form or a plunger assembly  58   c  that is preferably similar to or identical to the plunger assemblies  58 . The plunger assembly  58   c  is by virtue of its location on the cantilever arm  126  spaced apart axially from the collet base  40   c . As a result, the plunger assembly  58   c  is spaced apart axially from the other plunger assemblies  58  that are in the collet base  40   c . The pipe section is thereby clamped at two locations along its length. This can help to increase the clamping accuracy (squareness) of the fixture  120  by an order of magnitude or more. This can be useful when welding on a vertical run of pipe, for example. 
   Collets formed in accordance with the subject invention can be used in a variety of structures and clamping assemblies. For example, the collet could be used in a fixture for holding a pipe to be end faced (squared). Also, the collet could be used in a welding fixture for welding a fitting to a pipe, with the two clamp units being of different sizes. Accordingly, applicant intends to include all such modifications and alterations as part of the invention insofar as they come within the scope of the appended claims.

Technology Classification (CPC): 1