For a complete gRINDER you will need to print:
1x Top Case
1x Bottom Case - Leaf OR Heart (the Heart is a TM of 420ThreeD)
OR 1xThicker Cases
Print the Case's together on 2xshells 0%infill
Print the Grinder file separately: 3xshells 15%infil (min), top layers: 3
I cannot stress the 3x shells and 15% MIN infil enough, you don't want a holey print.
Although over 25% is EXCESSIVE and it just wastes filament / time / money.
15% is great
Once you've got all the parts out,
Assemble the smaller grinder piece into the bottom, larger, case.
Then slide the larger grinder piece into the top, shorter, case.
Depending on the results from your printer you may need to glue the grinder pieces in to get a good turn.