the guide rails 330 moreover a sliding block 350 is engaged with the guide rails 330 to be moved along the longitudinal direction by the worker in order to set the proper position the sliding block 350 includes a sliding frame 352 for assembling the required parts and a slider 354 formed in the lower side of the sliding frame 352 which is mounted to slide in the recessed lateral surfaces 336 the sliding block 350 comprises a pair of sliders 354 located underneath the sliding block 350 for mounting and sliding on the pair of guide rails 330 a pair of right and left pushing taps 362 370 forming a cylindrical pocket 368 for inserting a spring 360 and an integrally formed extended leg 366 with pawls 364 below the pocket 368 for latching or unlatching to the rack gear teeth 334 a pair of right and left compartments 356 358 for installing the right and left pushing taps 362 370 respectively and a controlling plate 380 for enveloping the sliding block 350 to fixedly install the right and left pushing taps 362 370 into the right and the left compartments 356 358 respectively the controlling plate 380 comprises a cutout 382 formed with a rectangular shape at the center of the controlling plate 380 for snapping into a rectangular detent 372 formed rectangular shape in the center the sliding block 350 and a pair of mounting feet 384 formed at the rear of the controlling plate 380 for snapping into the rear of the sliding block 350 thus the right pushing taps 362 and the spring 360 are assembled to the sliding frame 352 and the controlling plate 380 covers the upper side of the sliding frame 352 when the right pushing tap 362 is pushed.