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Metal milling is a sub-category in the process of metal working, and involves the shaping, cutting. and punching of metal parts in a milling machine. Earlier metal milling machine equipment was manually controlled, but the latest metal milling technology often relies on computer numerical control (CNC) processes to more finely shape complex parts and components. The most common type of metal milling machine is the vertical spindle, which operates similarly to an industrial power drill to bore holes and slots in metal or cut complex shapes, such as gears.
The purpose metal milling machinery serves in modern industry cannot be understated, as it is vast and difficult to quantify. Most moving metal parts cannot be die cast to the fine level of detail that they can be shaped and cut to with various types of milling equipment. It is estimated that as of 1996, at least 49% of added value to products in manufacturing was provided by the metal industry overall. The industry accounted for 11% of total gross domestic product (GDP) worldwide in the same year.
Though metal milling is only one subset of the overall metal industry, it is a crucial component of metal products production. Estimates of employment in the metal industry worldwide as of 1996 were in the range of 70 million people. Global export value for metal products was estimated at $2.2 trillion US Dollars (USD) in 1997 alone, with the leading export nations being the US, Japan, and Germany.
The types of milling machine range from standard vertical cutters to horizontal mills built to cut key hole shapes, end mills for cutting slots and pockets in metal, two fluted end mills for plunge work, and ball end mills for fillet cutting. Though most metal milling machine work involves flat surfaces, a mill can also be set to produce a defined irregular surface, or even complex three-dimensional shapes. The typical design involves a multi-toothed cutting bit of various shapes which is fed into a workpiece fastened to a computer-controlled moving table.
Milling tends to be an expensive manufacturing process because the bits and cutters operate at high temperatures and burn coolant that is sprayed on them in the process to keep them from overheating. The cutting bits themselves are also very expensive, as they must be hardened to a degree where they can repeatedly cut into metals, such as aluminum and stainless steel, for hours on end. Hard metal milling, also known as high speed machining (HSM), is milling of hardened steel, titanium, and cobalt alloys, and is considered the most profitable and challenging sector of the industry for milling aerospace components and more.