ROLLING MILLS
www.machyintl.com
20 Commerce Drive North Branford, CT 06471 USA Tel: +1 203-484-0400 Fax: +1 203-484-4499 sales@machyintl.com
ROLLING MILLS By John Conroy
Most machinery dealers sell equipment designed to shape, fabricate, or cut metal of thin gauge in either sheets or coils. But how do you get from ingots cast as thick as 18” into coil form at very specific thin gauges? The answer is the rolling mill. Rolling mills have one purpose in life - to reduce the thickness of metal by crushing it between power-driven rolls. Rolling mills come in many configurations. They can be hot mills or cold mills. Hot mills are used for the initial breakdown of thick ingots that are preheated bright red to allow for greater reduction per pass. Hot mills can be reversing or consist of a series of multiple rolling mills in tandem. Cold mills, the more common type, take less reduction per pass but can produce metal in the very thin gauges required for finished product in coil form. Rolling mills are designated by their roll size. For example an 8” x 10” 2-hi mill means that the roll diameter is 8” and the roll face width is 10”. A 4-hi mill uses two smaller diameter work rolls supported by two larger diameter backup rolls immediately above and below the work rolls. A 2” x 8” x 10” 4-hi mill means the work roll diameter is 2”, the backup roll diameter is 8”, and the roll face width is 10”. Cluster mills, which utilize even smaller diameter work rolls that are supported in multiple layers of backup rolls, generally produce the thinnest gauges with the greatest tolerance, although advances in roll material and roll bearings today allow for high precision 4-hi mills that can do much the same work as 20-hi Sendzimir cluster mills. The general rule is that the larger the diameter of the work roll the more reduction you can take on the metal. So breakdown mills and hot mills are typically 2-hi mills. 4-hi mills, which have a smaller work roll, are used for more precise gauge control on thinner material. However many other factors come into play such as the horsepower, torque, and roll separation force of the roll housings. That is why some 2-hi rolling mills with polished rolls, for example, are perfectly suitable for creating the shiny very light gauge gold and silver used in the jewelry industry. Another important feature of any rolling mill is the type of screwdown. With each successive pass through the rolling mill the thickness of the material is reduced. The rolls themselves then must be closed tighter for the next pass to roll thinner. This is accomplished by the screwdown. Screwdowns can be manual, such as a large handwheel turned by the operator, or they can be motorized, or they can be hydraulic. Screwdowns can also be tied directly to thickness gauges so that the rolls will be automatically raised and lowered hundreds of times per second on a mill running at 2500 feet per minute with an automatic gauge control (AGC) system. There are many other things to consider in selecting the correct rolling mill for the application, such as the tensile strength of the metal being rolled or the amount of tension between the rolls and coilers which are beyond the scope of this article. Rolling is a complicated business for what appears to be the simple task of reducing the thickness of metal. 4
January 2019
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