Corn grinders have two primary purposes: one, to make ground meal for human consumption, the other, to crack corn for animal fodder. Its intended purpose will dictate the type and scale of grinder you construct.
Stone Hand Grinder
Make a stone hand grinder with two stones: a flat or slightly hollowed stone and a rounded stone that fits the surface of the first stone. Grind the corn by placing it on the flat stone and then rolling or rubbing the grain with the top stone. Use granite rocks to make your grinding stones; other types have historically been used, but sand or limestone may add bits of rock to the cornmeal.
Hollow Log and Stick Grinder
According to Gourmet Sleuth, the hollowed log and stick grinder is a large version of a wooden mortar and pestle. To make one, you hollow out a log and round a stick at least 3 inches in diameter on the end. The cook then places the grain or other substance in the bottom of the hollowed log and pounds it with the stick until it is broken into fine bits.
The Biblical grist mill comprised a large stone placed in the hollowed bowl of another large stone, reports Theodore Hazen in "The History of Flour Milling in Early America." The grinders hitched their cattle to sticks protruding from the top stone; they then drove the cattle in a circle, which caused the stone to turn. This process grinds the grain placed in the bowl stone. The process was refined as stone cutters learned to create round millstones and inventors created gear works driven by wind or water to turn the giant stones.