of ball bearings it is to be appreciated that a rotation of the spline shaft 58 will impart a rotation to the cylindrical collar 58 as is partially illustrated in fig4 the cylindrical collar 68 is keyed to a casing 90 via a keyway arrangement 92 the casing 90 is more fully depicted in fig3 and is seen to include the pinion gear 56 at its front end the casing 90 and the pinion gear 56 will rotate with the cylindrical collar 68 when the same is rotatably driven by the spline shaft 58 the casing 90 will furthermore rotate relative to the lower portion 60 of the beam structure 22 by virtue of a pair of ball bearing mounts 94 and 96 as has been previously described the pinion gear 56 engages the rack 54 of the carriage 18 a rotation of the pinion gear 56 will move the rack 54 linearly so as to thereby move the carriage along the length of the cylindrical axis member 20 this movement of the carriage may be effected while the lightweight frame is also being displaced along the length of the rotating shaft 58 referring now to fig5 an overall top view of the positioning apparatus is illustrated the y shaped configuration of the brace structure 26 within the lightweight frame is particularly illustrated the relationship of the various positioning elements can also be appreciated in this regard the parallel relationship of the guide 28 the shaft 58 the support 34 and the rack 50 is readily apparent it is to be appreciated from the above that a preferred embodiment of a lightweight rigid and responsive positioning system has been disclosed this.