The muller framework and drive train have been built at this point and this photo is a mock- up of the muller bowl. The axle- end of the rear- end assembly protrudes through the bowl bottom and the steel plate floor that the wheels run against. The dome at the center of the muller floor is the top of a small propane tank... It fills an area that would otherwise collect "dead" sand.
Note that the wheels ride on axles which carry through the center of the muller via a box section. An automotive wheel rim was cut and formed so that when installed on the axle hub, a slot aligned with each axle. The axles, riding in the slots are turned by the rotating wheel rim, but are free to ride up and down as needed. The wheels were weighted with barbell weights, and the hollow tubes of the carry- through were filled with lead. The sand is "squished" by the weight of the wheels, axles, and carry-through.
In this photo, the wheels and carry-through are still visible, but are covered by the wheel rim that drives them. It is difficult to see clearly, but the axles protrude through slots in the wheel rim, and that the plows are attached to arms that project from the whel rim. Near the center of the muller bowl is the sand exit. In use, this area is blocked by a small cover during operation, and the cover is removed to dump the sand through the hole, down a pipe, and into a 5 gallon pail. Yes, a dump chute on the outside perimeter of the muller bowl would have worked better!
This muller is happy working loads of about 5 gallons... 50 lbs?