I use a belt sander with a backing plate to do the initial bevel on a made-from-scratch slide, but once a slide is made (old or new) I only adjust the angles with a hand file and lots of test fits. I work from the side that enters the machine first and work outwards. This also lets me adjust the tension of the fit. I like to make them so that when in place they are tight for the first 1/4-1/2" of removal then easy slide.