A fanning mill is a winnowing machine. Winnowing is the step of cleaning the grain to separate the considerable chaff, dust, weed seeds and whatnot from what you want to eat. Winnowing is done after the thresher knocks the grains loose from the head.
This is a page from the collection "Bicentennial Reflections: History of Des Moines Public Schools, 1876-1976" by Dr. Robert R. Denny, published by the Des Moines Public Schools in Des Moines, Iowa in 1976.
John F. McGuire, M.A., Notre Dame
University, Associate Professor, English
Dr. Matthew M. McMahon, Ph.D.,
Catholic University, Chn., Political
Dr. Bertrand J. Miller, Ph.D., Iowa
University Chairman & Professor,