I used collision detection and tangency constraints on the parallell surfaces to follow the contour exactly and provide a specific clearance with the blade including a 20 lbs load of grain on top of them. the mixing blade was the key to the success of the system, since I recirculate the mash, it has a tendency to pack down on the screen and create a stuck flow during the sparge. the mixer keeps the grain bed suspended during the mash, throughly mixes the grain so you don't have any dough balls created in the mash, and helps stabilize the temperature of the grain bed from top to bottom, I remove it at the end of the mash, so I can set the grain bed and get a clear flow before exiting to the boiler.