Mill machining

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Definition

Milling is the machining process of using rotary cutters to remove material from a work piece advancing (or feeding) in a direction at an angle with the axis of the tool. It covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes in industry and machine shops today for machining parts to precise sizes and shapes.

History

Milling machines evolved from the practice of rotary filing—that is, running a circular cutter with file-like teeth in the headstock of a lathe. Rotary filing and, later, true milling were developed to reduce time and effort spent hand-filing. The full story of milling machine development may never be known, because much early development took place in individual shops where few records were kept for posterity. However, the broad outlines are known, as summarized below. From a history-of-technology viewpoint, it is clear that the naming of this new type of machining with the term "milling" was an extension from that word's earlier senses of processing materials by abrading them in some way (cutting, grinding, crushing, etc.).

Grist Mill

The early mills had horizontal paddle wheels, an arrangement which later became known as the "Norse wheel", as many were found in Scandinavia. The paddle wheel was attached to a shaft which was, in turn, attached to the center of the millstone called the "runner stone". The turning force produced by the water on the paddles was transferred directly to the runner stone, causing it to grind against a stationary "bed", a stone of a similar size and shape. This simple arrangement required no gears, but had the disadvantage that the speed of rotation of the stone was dependent on the volume and flow of water available and was, therefore, only suitable for use in mountainous regions with fast-flowing streams. This dependence on the volume and speed of flow of the water also meant that the speed of rotation of the stone was.

Grist mill.jpg

Hammer Mill

The basic principle is straightforward. A hammermill is essentially a steel drum containing a vertical or horizontal rotating shaft or drum on which hammers are mounted. The hammers are free to swing on the ends of the cross, or fixed to the central rotor. The rotor is spun at a high speed inside the drum while material is fed into a feed hopper. The material is impacted by the hammer bars and is thereby shredded and expelled through screens in the drum of a selected size. Hammer mill apple shredder for juicing. The hammermill can be used as a primary, secondary, or tertiary crusher. Small grain hammermills can be operated on household current. Large hammer mills used in automobile shredders may be driven by diesel or electric motors ranging from 2000 to over 5000 horsepower (1.5 - 3.7MW). The screenless hammer mill uses air flow to separate small particles from larger ones. It is designed to be more reliable, and is also claimed to be much cheaper and more energy efficient than regular hammermills.

Pulverizers.jpg

Ball Mill

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on the principle of critical speed. Picture with labels of the ball mill.

Ball-mill-diagram1.jpg

Disc Mill

A disc mill, is a type of crusher that can be used to grind, cut, shear, shred, fiberize, pulverize, granulate, crack, rub, curl, fluff, twist, hull, blend, or refine. It works in a similar manner to the ancient Buhrstone mill in that the feedstock is fed between opposing discs or plates. The discs may be grooved, serrated, or spiked. Substances are crushed between the edge of a thick, spinning disk and something else. Some mills cover the edge of the disk in blades to chop up incoming matter rather than crush it.

Saw Mill

A sawmills basic operation is much like those of hundreds of years ago; a log enters on one end and dimensional lumber exits on the other end. After trees are selected for harvest, the next step in logging is felling the trees, and bucking them to length. Branches are cut off the trunk. This is known as limbing. Logs are taken by logging truck, rail or a log drive to the sawmill. Logs are scaled either on the way to the mill or upon arrival at the mill. Debarking removes bark from the logs. Decking is the process for sorting the logs by species, size and end use (lumber, plywood, chips). The head saw, head rig or primary saw, breaks the log into cants (unfinished logs to be further processed) and flitches (unfinished planks) with a smooth edge. The impact of the saw mill affects the environment because they can wipe out entire forests by logging.

Sawmill.jpg

Steel Mill

A steel mill or steelworks is an industrial plant for the manufacture of steel. An integrated steel mill has all the functions for primary steel production: Iron making (conversion of ore to liquid iron), Steelmaking (conversion of pig iron to liquid steel), Casting (solidification of the liquid steel), Roughing rolling/billet rolling (reducing size of blocks) Product rolling (finished shapes). Steel mills affect the environment because they release excess carbon, sulphur and phosphorous.

Steel mills.jpg

References

Wikipedia, (2014). Disc mill. [online] Available at: http://en.wikipedia.org/wiki/Disc_mill [Accessed 29 May. 2014].

Wikipedia, (2014). Gristmill. [online] Available at: http://en.wikipedia.org/wiki/Gristmill [Accessed 29 May. 2014].

Wikipedia, (2014). Hammermill. [online] Available at: http://en.wikipedia.org/wiki/Hammermill [Accessed 29 May. 2014].

Wikipedia, (2014). Milling (machining). [online] Available at: http://en.wikipedia.org/wiki/Milling_machine [Accessed 29 May. 2014].

Wikipedia, (2014). Sawmill. [online] Available at: http://en.wikipedia.org/wiki/Sawmill [Accessed 29 May. 2014].

Wikipedia, (2014). Steel mill. [online] Available at: http://en.wikipedia.org/wiki/Steel_mill [Accessed 29 May. 2014].