BACKGROUND OF THE INVENTION This invention relates generally to the field of reconfigurable manufacturing tooling systems and more particularly to a flexible rail system for automatic positioning drill or tool systems which is unlimited in extension length and remains flexible over interlocking extensions. The use of automated positioning tooling systems for fabrication of large structural elements on aircraft and other large systems is prevalent in current manufacturing practice. However, precision flexible rail systems for tool positioning are difficult to make and handle in long lengths. To solve this problem rails are made in short lengths of 1.2 to 2.1 metres (4 to 7 feet). To achieve a long rail the rails are simply spliced together. However, technical challenges arise in the splice. A typical splice tends to be crooked at the joint. While match drilling can make a straight joint, the end result is a non-interchangeable rail set. Non interchangeability is highly undesirable and considered impractical in most manufacturing environments. The key problem characteristics are the difficulty in making straight rail splices that are interchangeable and maintain the flex characteristics of a flexible rail There are other methods that have attempted to hold the rail joint reasonable straight, flexible and interchangeable. Unfortunately they all fall short of that goal. Examples include short and long sandwiching plates that carry stresses across each joint. The stresses are induced by bending the flexible rail to conform to a working surface. Previous devices such as short metal sandwiching clips are too short in length to achieve interchangeability and repeatable accuracy at the same time. Long sandwiching plates can be accurate and interchangeable but add significant local stiffness to the flex-rail. This stiffness can cause significant flat spots when the rail is flexed to a work piece contour. An exemplary prior art system and the tools positioned thereby is disclosed in US Patent number 6,843,328 entitled Flexible Track Drilling Machine, having a common assignee with the present invention. US 5,735,214 discloses a track system for roller-equipped carriers to run along a straight and/or curving guide rail, which includes two pairs of opposed parallel plain lanes, a pair of oppositely directed parallel v edges intermediate the two pairs of plain lanes, and narrow lubricating grooves along each v edge. The rail segments are connected in alignment with each other by means of keys which are put in key grooves provided in one of the set sides. Alternatively, a pair of relatively thin, straight rail segments may be connected to each other by means of a joint plate and set screws. Straight and curving rail segments may be joined where a curving segment is joined together with a base member. The base member extending as a straight extension and being longer than the rail segment provided thereon. The straight rail segment has a rail segment extension extending further than the associated base member. Thus, when the straight and curving segments are connected to each other the extending straight rail segment may be attached to the extending base segment from the curved rail segment to provide a streamline guide rail. US 2004/0084545 discloses a track section that includes a body having a coupling portion. The coupling portion may include a projection which extends outwardly from the coupling portion and recess extending inwardly into the coupling portion. The recess may extend downwardly through the coupling portion or from an upper surface of the body to a lower surface of the body. The coupling portion of a first track section may then therefore be coupled to a coupling portion of a second track section. WO 99/12786 discloses a tracking carriage system for moving a work piece along a flexible track. The system includes a track with at least one track segment, the track segment including a rack assembly formed by a plurality of spaced rack members mounted on the track segment. A carriage assembly is configured to move along the track. The carriage assembly includes a first carriage and a second carriage which may be irremovably mounted on the track. US 5,794,846 discloses a track alignment apparatus for supporting and linking sections of model railway track and/or for receiving a model railroad trestle. Track alignment apparatus may include one or more track alignment members to engage trestles, provide rigidity to curved or straight model railroad tracks, and/or connect model railroad track to a layout base. It is therefore desirable to provide unlimited-length rails for use with automatic position locking manual drills or tool positioning machines for use on new aircraft and other large structural system development programs. It is also desirable to provide a joint for use with such rails that is straight and interchangeable. SUMMARY OF THE INVENTION The present invention provides an interlocking precision flexible rail system for carrying a positionable machine tool employing a first rail element having a first step on a lower surface extending from a first end to a termination at a predetermined length and a second rail element having a mating step on an upper surface extending from a second end to receive the first step. A pair of first clips is removably affixed to a top surface of the first rail element adjacent the first end with a first pin and to the second rail element with a second pin in each clip. A pair of second clips is removably affixed to a bottom surface of the second rail element adjacent the second end with a third pin and to the first rail element with a forth pin in each clip. The first and second pairs of clips and associated pins laterally secure the rail elements in mated engagement. Threaded fasteners secure the first pair of clips to the top surface of the first rail element and second rail element the second pair of clips to the lower surface of the first and second rail elements. BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: - FIGs. 1a and 1b are side section views of the elements of flexible rail elements employed in a track employed with the present invention showing the first end of the track and the second end of the track respectively;
- FIG. 2a is a top view of the assembled elements of the track of FIG. 1;
- FIG. 2b is a top view of the first element of the track without the mated top element; and
- FIG. 3 is an isometric view of the exploded track elements of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION This invention achieves the three most desired traits in a precise-flex-rai splice. First it achieves a straight joint because it is made of relatively long stepped overlaps. Second it achieves interchangeability because the alignment pins are separated by far enough distance that reasonable clearance can be used for assembly. Third, the stepped rail with short splice clips at each end does not compromise rail flexibility at the joint. As shown in FIG. 1 an interlocking feature is provided by a step 10 that is machined into a first end 12 of a first length of precision flexible rail 14 and a mating step 16 in a mating end 18 of a second length 20 of a precise flexible rail. The step is machined to substantially a half thickness of the rail from the underside of the first length and the top side of the second length directly down the center line where no stress occurs during bending of the rail. The rail elements have a high aspect ratio of width 22 to thickness 24 to maintain flexibility perpendicular to the upper surface 26 and lower surface 28 of the rail. In an exemplary embodiment, dimensions are typically rails of 7.62 cm (3 inch) width and 0.424 cm (0.167 inches) thickness. Thickness has less bearing on accuracy, and is more a factor of flexibility and compliance to a complex contour. Wider and thicker rails will translate into a more accurate system if used primarily on a flat surface, especially when positioning cantilevered loads where the flexing of rails is of great concern. An end cap piece 30a, shown in FIG. 1a as a bottom end cap is attached on the machined splice joint at the first end of the second length of rail which constitutes the first end of the track. Similarly, a top end cap 30b is provided to attach to the machined second end of the flexible rail element at the second end of the track as shown in FIG. 1b. As shown in FIGs. 2a and 3, two metal clips 32 are fastened to the rail elements at the ends of each step on the upper and lower surfaces of the rails using pins 34 received in alignment holes 36 and 38 in the upper and lower step portions. Threaded fasteners 40 which are received through a pattern of mating holes 42 in the clips element and threaded holes 44 in the rail elements secure the clips to the rail elements. The clips and pins carry the compression and tension stresses that arise from flexing the rail while the fasteners secure the rail elements in position for use. A small amount of clearance is allowed for the pins to fit in each hole. Similar clearance is provided in the mating holes to receive the fasteners without interference due to tolerance build up. This clearance is relatively small compared to the distance separating the upper clip from the lower clip at opposite ends of the mating splice. In an exemplary embodiment, the step overlap is approximately 15.24 cm (6 inches). This allows tolerancing of the hole/pins to be approximately 0.00254 cm (.001 inch) while maintaining a desired 0.0001 cm/cm (.0001 inch/inch) overall tolerance. In prior art approaches a 2.54 cm (1 inch) overlap between 1.2 meter (4 foot) rails resulted in a 48 to 1 tolerance ratio whereas the present system at 8 to 1 significantly eases the tolerance requirement. For additional tolerance increase, a 30.48 cm (12 inch) overlap would result in a 4 to 1 ratio doubling the available tolerance relief. The resulting package has interchangeability, precision, straightness and virtually uninterrupted flex characteristics. For the embodiment shown in the drawings, the pins are separate elements from the clips which are inserted through concentric holes in the clip and rail element. In alternative embodiments, the pins are integral with a bottom surface of the clip. The flexible rail elements include machined tapered tooth apertures 50 to receive the drive gears of the automatic positioning drill or other tool operating with the track. The apertures are tapered from larger diameter ovaloid shape 52 in the top surface of the track to a smaller diameter ovaloid shape 54 in the bottom surface of the track as shown in FIG. 2b. In the overlapping steps on the track ends, the upper track element, either the first machined step or the top end cap employ the top diameter portion of the taper in the apertures while the second machined step as shown in FIG. 2b without the mating element attached, or the bottom end cap employ the bottom diameter portion of the taper. The engagement of the flexible rail elements by the clips with inserted pins maintains the upper and lower portions of the tapered ovaloid apertures in alignment. For the embodiment shown in the drawings, the rail edges 56 are convex or triangular to accept lateral rollers 58 for positioning and securing the machine tool carriage 60 to the rail elements. The tool is movable on the assembled rail using a gear 62 engaged in the ovaloid tapered apertures in the track. Having now described the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims. |
