The hand-operated grinding stone, the earliest grinding device used on the farm, some-whst resembled a mortar and pestle in form. It consisted of two pieces of stone roughly cut in the shape of flat discs. The top disc was rotated by hand over the stationary bottom disc, and the material to be ground was fed through the space between the two discs. The old-time grist mill driven by water, wind, or mule power is a good example of a mechanized version of the grinding wheel.
The modern burr-stone mill is a further development of the grinding wheel and is often referred to as an attrition mill, meaning a mill which grinds by friction. The burr mill consists of two disc-shaped stones with burrs or grooves cut into their grinding faces. The grain is fed at the center of the stones and is cracked without crushing as it passes toward the periphery. The stone discs have generally been replaced by chilled, cast-iron plates (hence the name plate mill) with corrugations or projecting cutting edges cast into the rubbing surfaces.
The spate between the plates is controlled by a spring-loaded, spacing screw, and the plates are usually corrugated on both sides to make them reversible. When starting a plate mill, the grain teed is blocked off and the spring pressure relaxed. leaving the plates at their coarsest setting. Once the mill is rotating at normal speed, the grain is fed into the mill. The plates are gradually forced together by tightening the spring until the desired texture is obtained.
This kind of mill is best suited for grinding meal and flour for household use since it grinds the finest and loses none of the whole grain. When set correctly, the tight hulls on barley and oats will come off in stone grinding. The ground material comes out as a Hour and crushed hull mixture. The hulls, which now resemble flakes, can be sifted away. Steel-burr plates grind the entire kernel, making separation of hulls and grain much more difficult. Small, hand-operated barley pearlers are available to shatter the hull
away from the kernel before the grain is millet!.
The main advantages of a burr 01 plate mill are relatively low speeds of operation (around 1,200 rpm), belter adaptability for grinding texture than hammer mills, uniform grade of grinding, and fairly low power requirement. This is the farm mill best suited for kitchen use, since it grinds the finest and loses none of the whole grain. Disadvantages of a burr mill include: fast-wearing plates, the possibility of equipment being damaged by foreign objects like stone or metal fragments, a relatively high power requirement for finer grinding, and the possibility of damaging the plates when operating the mill empty, unless the pressure between the plates has been released.
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