Additional notes:
The direction of hand rotation is usually counter-clockwise, and the shape of the groove of the stones is such that it directs the crushed particles to the outside. After a while, the grooves slow down and you have to re-groove on the previous track to sharpen it.
To see the shape of the grooves in the stone mill, search for the term millstone dressing.
The presence of the central shaft makes the upper stone rotate completely on the lower stone and the device shakes less. Although the absence of this system also has advantages.
If the central shaft is installed, it is better to create a separate hole for the input of the material at a distance from the center of the mill. Because in this way it is ground to the last grain and nothing remains inside the machine. But my mill currently does not have a separate entrance, and at the end of the work, there is always some grain left in the cavity that has not been ground and has not gone under the stone.
In order to increase the pressure of the upper stone, I increased the weight of the stone using concrete, but it is possible to make a system that has adjustable pressure by using screws, springs and ball bearings instead.
The type of grinding stone is important and soft stones should not be used.
Stones with high Mohs hardness, such as granite, wear less than others and are better.
The last point is that the millstones should never be rubbed on each other alone and there should always be something between the stones while working. So before starting work, pour some of the seeds between the stones.
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