Manufacturing a weed grinder

Hi, a friend and i want to make our own grinder. We have some ideas but we’re not sure how to manufacture it, I was thinking in steel or aluminum. We would buy a 3” round stock, not sure how long, and then give the contour and remove inside material with a lathe. What I’m not sure how we could achieve are the grinding parts, a friend suggested a molding process, how could we achieve this with the metal parts I mentioned? Is my idea too far fetched?