slot is formed with the slot 76a of the spindle 30 and the slot 78a of the guiding bar 38a the bearing unit 100a is arranged in the quadrangular slot a longitudinal rectangular slot 76b is provided on the other side surface of the spindle 30 a longitudinal rectangular slot 78b is provided on the side surface of the guide bar 38b another longitudinal quadrangular slot is formed with the slots 76b and 78b where the bearing unit 100b is arranged therein the bearing units 100a and 100b include roller means 84 and a supporting frame 80 as is best shown in fig5 each of the bearing units 100 100a and 100b in fig4 includes a supporting frame 80 having a quadrangular aperture 82 and roller means 84 including a plurality of tubular rollers 84a 84b and 84c the number of the roller means 84 fig4 can be altered as required each of the rollers is positioned so as to be orthogonally directed with respect to other adjacent rollers as shown in more detail in fig4 and 5 a pair of supporting brackets 88 are provided at the supporting frame 80 of which one bracket 88 is opposed to the other a plurality of pairs of the supporting brackets 88 is arranged on the supporting frame 80 spaced apart at a predetermined distance as shown in fig5 each cylindrical shaft 86 is supported by a corresponding supporting bracket 88 each tubular roll 90 is rotatably mounted on the supporting bracket 88 and is formed integrally with shaft 86 as described above.