end of swing arm 34 is hinged to a slide 36 as shown in fig3 this hinge connection comprises a bolt 39 which is secured to slide 36 two ball bearings 37 38 held axially by two lock washers 40 41 two spacers 42 43 and a plain washer 44 by means of a ball guide 45 slide 36 is displaceable on a slide rod 46 having its ends fixed in two clamps 47 48 which are secured to table plate parallel to slide rod 46 a channel section guide rail 49 is secured to table plate 2 by one its leg portion the other leg portion designated 50 of guide rail 49 forms two running surfaces for rollers 52 54 which are carried on threaded bolts 51 53 secured to slide 36 and are applied against portion 50 from above and below respectively to the underside of table plate 2 a stepping motor 55 is secured the shaft 56 of motor 55 is passed through table plate 2 and carries a cog wheel or gear 57 for a cog belt or gear belt 58 belt 58 is further trained about a tail wheel 59 with the belt sections between wheels 57 and 59 extending parallel to slide rod 46.