I rode out to Kalkar Mill on Saturday afternoon to check out the stones. Kalkarermühle is an operating windmill in Kalkar, NRW, Germany, and home to a diverse bunch of people who have decided to keep the windmilling trade alive as volunteer millers. One of the millers is a friend and fellow rider, and introduced me to this neat old technology last fall.
The key to the mill is the stone set. The lower stone, shown here, is fixed and does not move. The upper stone is supported on a pintle that is driven by the familiar sails that catch the wind and power the operation. The entire rig runs at around 120rpm, which is quite speedy, considering that the stones are about 1.6m in diameter. That comes to an edge speed of 24m/s! When the season for milling is low (winter), the millers open the stones for cleaning, resurfacing, and rhynd repairs. On this stone, the darker areas are the wear surfaces, and the grooves are the feeders that feed the grain in.
In the photo, the piece of wood is a gauge used to measure the feeder depth across the stone. For the stones to function, the feeds must smoothly transition from deep to shallow, otherwise pockets of dead space will develop. The pattern of the lands and feeds on this stone is relatively modern, spirals did not develop until around the turn of the 20th century due to the difficulty in tracing them out and maintaining the even spacing required. Earlier stones were cut using a herringbone pattern, however, this is somewhat less efficient than the spiral pattern seen here.
Milling is a great curiosity for me, due to my tribology background. Inside of the stones, one finds what amounts to a fluidized bed – the small grains and eventual flour are in constant motion due to the stone speed. The stones are set to a roughly 2mm separation, although this can change for several reasons. Typically, the miller will run the stones closer for a finer flour, and farther apart for a coarser flour. Different grains can require different set-offs, as can changes in the moisture content of the grain. The high speed of the stones means balance is critical – above 120rpm, the running (upper) stone will begin to float and bounce, destroying itself and the grain in the process. I am fascinated at by the engineering rules that apply – today, I can calculate and measure, optimize and control, all using science developed in the last 30 years, what millers have been doing by hand, eyes, and ears for some three centuries.