after stringing with this machine two times and playing with the strung racket for a while i've noticed that the tension is not accurate. it is looser than what was intended. i got my machine used and it came with no manuals so if anyone here knows the procedures to accurately calibrate my machine, i'd greatly appreciate some help. btw, i had already attempted to fix this by wrenching the two screws (at the tension adjust knob) to the left by pushing the knob inward first. this helped a little and it seems a little tighter but i don't know if this is the correct way to calibrate my machine. another problem i have is that the gliding bar does not glide too smoothly on the rails. would sanding it down with very high grit sandpapers help. does anyone here not recommend doing that for reasons that it will screw up my machine? tia!