The round ceiling fixture boxes supposedly take regular 8-32 machine screws to mount fixture crossbars... I can't get them to take them, even when running starter threads with an 8-32 tap. I have been to their website, and they say use the 8-32 screws that come with the fixtures to secure the crossbars to the boxes. I must be doing something wrong, as I don't think NEC/UL permits self-tap screws (like sheetrock screws), but only screws with machine threads.... What am I doing wrong? I have seen plenty of houses using self-tap screws into the blue boxes, apparently for the same reason.