Rigid Collet Assembly for Cylindrical Workpieces

The subject invention is directed toward the art of work holders and, more particularly, to a rigid collet device for holding cylindrical workpieces.

The invention is especially suited for use as a tube clamp unit in an orbiting head tube welder and will be described with particular reference thereto; however, the invention is capable of broader application and could be incorporated in a wide variety of work holders and clamping units for different types of cylindrical work pieces.

In U.S. Pat. No. 4,379,215, issued Apr. 5, 1983, for "Orbital Cassette Welding Head", there is disclosed an apparatus for welding abutting tube sections. The apparatus includes an orbital welding head unit and a separate U-shaped tube clamp unit which holds the tube sections to be welded in abutting aligned relationship during the orbiting of the weld head about the tube ends. The tube clamp unit uses rigid collets to hold the tube sections in alignment during the welding operation.

The use of rigid collets for holding the tubing 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 tubing 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 tubing throughout the range of standard commercial tolerances can be held properly. That is, a typical commercially available tubing used for fluid systems and the like, has, for example, a nominal outside diameter of 0.50 inches which varies .+-.0.005 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 tubing during the welding operation.

As can be appreciated from the foregoing, the need persists for a collet design which allows the maintenance of the rigid collet form while providing compensation for the outer diameter variations which result from tolerance variations.

The subject invention provides a collet design and clamping unit which overcomes the noted problems and provides ready compensation for tubing tolerance variations. With the use of the subject invention, the rigid collet construction can be used to eliminate tubing movement and/or misalignment and overcome the internal stresses generated from the heat of the welding operation.

In particular, the subject invention provides a clamping device particularly intended for clamping cylindrical tubes having a nominal diameter D which varies from a minimum of (D-T.sub.1) to a maximum of (D+T.sub.2). The device comprises a pair of opposed clamp elements mounted for movement toward and away from one another. Each clamp element has a clamp face carrying a rigid collet element with each collet element having a semi-cylindrical clamping surface extending inwardly from the clamp face of the respective clamp element. The diameter D.sub.c of each semicylindrical clamping surface is substantially equal to (D+T.sub.2) with the center of the diameter D.sub.c of the semicylindrical clamping surface being offset outwardly of their respective clamping face a combined amount of at least 1/2(T.sub.1 +T.sub.2).

By forming the fixed collets as described, compensation takes place for the outer diameter size variation due to tolerance variations in the tubing being held. That is, the collet will function to hold the tubing rigidly from the minimum tubing diameter encountered to the maximum diameter. Full closing and firm tubing engagement is assured throughout the full tolerance variation of tubing.

In accordance with a more limited aspect of the invention, the diameter D.sub.c is preferably offset outwardly of the clamp face of the respective clamp half by an amount at least substantially equal to one half of (T.sub.1 +T.sub.2 +T.sub.3) where T.sub.3 is the tolerance deviation for the diameter D.sub.c. This takes into account the tolerance to which the clamping faces can be bored or machined. Thus, slight tolerance variations in the actual forming of the respective clamp halves does not adversely affect the overall functioning of the clamp halves.

In accordance with a still further aspect of the invention, each of the collet elements is removably connected to its clamp half and includes an end cavity to compensate for any length tolerance variation or to accept any forging bosses or the like which may appear on the tubing or tube fitting being held by the collet. This design variation allows the clamping halves and collets to function for a wide variety of tube fittings in addition to tube sections per se.

Accordingly, a primary object of the invention is the provision of a tube clamping assembly which compensates for tubing outer diameter changes that result from tolerance variations.

A still further object of the invention is the provision of an assembly of the type described which can hold tube and fitting elements in precision face-to-face aligned relationship for butt welding.

Yet a further object of the invention is the provision of an apparatus of the type described wherein the tubing or fitting elements are held by solid collets which are designed to compensate for outer diameter variations in the tubing or tube fittings.

The above and other objects and advantages 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 tube clamping assembly formed in accordance with a preferred embodiment of the subject invention;

FIG. 2 is an elevational view taken on line 2--2 of FIG. 1;

FIG. 3 is a perspective view of the clamp component shown in FIG. 2;

FIG. 4 is a partial end view of one half of a fixed collet element used to illustrate the manner in which the collect clamping surface is formed;

FIG. 5 is a partial elevational view, somewhat diagrammatic, showing the clamp or fixed collet elements in engagement with a tubing whose outer diameter is at a maximum within its tolerance range;

FIG. 6 is a view similar to FIG. 5 but showing the collet elements in engagement with a tubing of minimum diameter within its tolerance range; and,

FIG. 7 is a view taken on line 7--7 of FIG. 3.

Referring more particularly to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting same, FIG. 1 shows a tube clamp assembly 10 which is intended to be used with an orbital cassette welding head assembly of the type generally shown and described in U.S. Pat. No. 4,379,215 of Apr. 5, 1983. The assembly 10 is designed to hold two tubing sections in aligned relationship with their facing ends generally abutting so that an orbital welding head can rotate about the mating tube ends to perform a butt weld operation. The disclosure of U.S. Pat. No. 4,379,215 is incorporated herein by reference and reference should be had thereto for a more complete description of the welding cassette and its relationship to the assembly 10.

The tube clamp assembly 10 broadly includes a pair of clamping units 12 and 14 which are joined to the opposite sides of an intermediate spacer member 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 block 16 in any convenient manner such as through the use of machine screws 18. The clamp units 12 and 14 are joined to the spacer member 16 to form a somewhat U-shaped structure with the open center section 20 adapted to receive the orbital welding cassette which carries and drives the welding electrode in a manner more fully described in the aforementioned U.S. patent.

In the embodiment under consideration, the clamping units 12 and 14 are of generally identical construction except that they are mirror images of one another. Accordingly, a description of the clamp unit 14 is to be taken as equally applicable to unit 12 unless otherwise noted. As best shown in FIGS. 2 and 3, the clamp unit 14 comprises a pair of opposed clamp halves 22 and 24. Each of the clamp halves 22 and 24 are preferably of rigid, solid construction and are joined by hinge means which permit the clamp halves to be moved toward and away from one another. Specifically, in the subject embodiment, the clamp halves 22 and 24 are joined by a hinge plate 26 which is fixed to the upper clamp half 24 by a pair of pins or the like 28. The lower end of the hinge plate 26 is suitably received in a slot in the lower half 22 and is pivotally connected thereto by a hinge pin 30.

The two clamp halves 22 and 24 are releasably connected in their engaged or clamping position by a releasable latch or lock assembly 34. Many different types of lock assemblies could be provided; however, according to the preferred embodiment, the lock assembly 34 includes a swinging hook element 36 which is pivoted to the upper clamp half 24 and has a hook portion 38 which engages a transverse pin 40 as shown in FIG. 2. An eccentric pivot pin 42 extends through the upper end of the hook member 36 and is rotated through an external lever 44. By rotating the hook plate 36 to the dotted line position shown in FIG. 2 and rotating the eccentric 42, the hook portion 38 is caused to tightly engage the pin 40 and pull the two clamp halves tightly together.

Each of the clamp halves 22 and 24 carries a rigid collet element with the collet elements in each half cooperating to define opposed semi-cylindrical work piece clamping or engaging surfaces. More particularly, as shown in FIGS. 2 and 3, separate, identical rigid collet elements 48 and 50 are releasably carried in the clamp halves 22, 24 respectively. As illustrated, the collet elements are releasably connected to their respective clamp half by socket head machine screws or the like 52. As more particularly illustrated in FIG. 7, the collet elements comprise a generally semi-cylindrical rigid body 54 having a semi-cylindrical clamp surface 56 formed therein. Preferably, at the entrance end to the semi-cylindrical clamping surface 56, there is a somewhat conical or tapered mouth portion 58 which is significantly wider and allows fitting bodies or forging bosses to be accommodated while the tubing section is clamped and extends through the semicylindrical clamping surface 56.

The apparatus thus far described is capable of rigidly and tightly engaging the outer diameter of tubing of a particular size depending on the diameter of the clamping surface 56. By changing the collet elements 48, 50, the clamp assembly can be made to accommodate a variety of different tubing or fitting sizes. Additionally, by changing the collets in one unit relative to those in the mating unit, it is possible to bring into alignment tubings or fittings of different sizes such that it is possible to weld various tube and fitting combinations.

Commercially available tubing typically is manufactured to a nominal outer diameter which varies plus and minus depending on the tolerance range. The design of the subject rigid collets is such as to compensate for these tolerance variations and permit the clamping units to firmly engage and positively hold the tubing sections irrespective of their particular variation within the accepted tolerance range.

Referring in particular to FIG. 4 (which is a greatly enlarged elevational view of the collet element 50), the subject invention can best be explained. According to the subject invention, the semi-cylindrical tube gripping surface 56 is formed with a diameter which is equal to the maximum tubing diameter which is expected to be encountered for the nominal tubing size for which the collet is designed. That is, the surface 56 is bored to a diameter equal to the nominal tubing diameter plus the tolerance variation for that particular nominal diameter. For example, with one half inch nominal tubing diameter, a normal tolerance range is from .+-.0.005 inches. Thus, the clamping surface is bored to a diameter of 0.505. In addition, the center line of the bore which defines the surface 56 is offset outwardly from the clamping face of the respective clamp half. As illustrated in FIG. 4, the center of the bore diameter D.sub.c is offset from the respective clamp face 22a an amount equal to the total of the tubing tolerance plus the collet outside diameter fit tolerance divided by two. That is, the total tubing tolerance plus the total collet outside diameter fit tolerance divided by two assures that each clamp half will be able to properly clamp even the smallest tubing. This is illustrated more satisfactorily in FIGS. 5 and 6.

Referring specifically to FIG. 5, this figure illustrates a tube 60 which is clamped between the collet elements 48 and 50. Tube 60 in this illustration has the maximum outer diameter encountered for this particular nominal tubing size. That is, the tubing is at its maximum tolerance variation. In this instance, when the clamp halves 22 and 24 come together there is significant spacing between the opposed clamp faces and the faces of the respective collets 48 and 50.

FIG. 6 illustrates the clamping function when the tubing 60 has a minimum outer diameter as dictated by the maximum tolerance variations in the negative direction. Specifically, as illustrated in FIG. 6, the gap between the collet elements 48 and 50 is at a minimum. However, the collets firmly engage the outer diameter of the tube at the diametrically opposed faces as illustrated. By offsetting the center of the semi-cylindrical clamp surface 56 in the manner shown in FIG. 4 and as described earlier, the collet has the proper design to properly engage the minimum tube outer diameter.

Obviously, rigid collets formed in accordance with the subject invention could be used in a variety of structures and clamping assemblies. Accordingly, it is intended to include all such modifications and alterations as part of the invention in so far as they come within the scope of the appended claims.