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roll mill[′rōl ‚mil]
a machine for milling grain (wheat, rye, and other crops) into flour; it is also used for crushing salt and other kinds of bulk raw materials. The operating members of a roll mill are one or two pairs of rolls that rotate toward each other at different peripheral speeds. The separation between the rollers (gap) is varied from 0.05 to 1.3 mm according to the size of the particles of the product being processed and the degree of fragmentation required. The mill’s feed mechanism continuously and uniformly delivers the product directly into the gap by means of a belt as wide as the whole length of the rolls. Rolling mills are manufactured with an automatic engagement and release for the lower moving roll.
A series of rolls operating at different speeds and used to grind paint or to mill flour. In paint grinding, a paste is fed between two low-speed rolls running toward each other at different speeds. Because the next roll in the mill is turning faster, it develops shear in the paste and draws the paste through the mill. The film is scraped from the last high-speed roll. For grinding flour, rolls are operated in pairs, rolls in each pair running toward each other at different speeds. Grooved rolls crush the grain; smooth rolls mill the flour to the desired fineness. See Grinding mill