How Does a Post Mill Work?
The Spocott Windmill, like its predecessor, is a post mill in which the entire structure rests on a post, allowing it to rotate into the wind. These mills were primarily European, dating back to the 12th century. The tower mill largely replaced them in the 19th century; in these, only the cap with the blades rotates. Tower mills could be constructed taller with longer blades, enabling them to operate in lighter winds. Dorchester also had smock mills, a variation of the tower mill in which the base is more tapered, allowing it to be built on less stable ground. George L. Radcliffe recalled that his father’s mill had been a post mill, and the reconstruction you see today is very similar to that post mill that blew down in 1888. All of these were primarily used for grinding grain into meal.
In the post mill, the entire building has a main structural crosspiece that rests on a post, which is supported by four quarter bars. Extending out from the mill on the side opposite the blades is a tailpole attached to a wheel on the far end. One person can move the wheel, turning the entire mill building into the wind. The steps act as an anchor but can be raised by a lever to allow the mill to rotate.
The four blades are attached to a main shaft, which connects to a gear wheel in the mill. When turning, the teeth on the gear wheel turn a spindle attached to the top stone. This allows the rotation of the blades and shaft to be transferred to the top stone. All of this is located on the second floor of the building.
A wheel on the first floor raises and lowers the bottom stone, allowing one to change the stone separation to adjust the coarseness of the meal.
Halyards can raise and lower the sails on each blade, much like raising the sails on a sailboat. In stronger winds, the mill can operate with two sails.
We try to operate the mill twice a year, and there is no substitute for being in the mill when it is running. One can see the amazing power generated, sufficient to rotate the 750-pound top stone.
