grind together
1. To cause two things to grate or scrape against each other. In this usage, a noun or pronoun is used between "grind" and "together." I wear a mouthguard at night so that I don't grind my teeth together.
2. To grate or scrape against something. I think these parts are grinding together and that's why this side looks so worn.
grind something together
to rub things together. Stop grinding your teeth together. The stones ground together as we drove over them.