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Milling machines come in sizes ranging from small to those requiring warehouse space to operate. Using a wide range of tools, milling machines carve and drill into raw products to create shapes and nearly finished products. They can drill holes, add threads for bolts or make precise shapes out of any solid material. Older machines relied on the machinist for precision. Modern machines are controlled by computers to make very precise parts.
Milling Machine Parts
Milling machines have two main parts. The head holds the collets, which hold the tools used to mill the materials. The head moves up and down on the "Z" axis and the tools are spun by a motor. The motor is either a variable-speed motor, or a system of pulleys that is used to vary to the speed of the tool. The table moves on two axes, the X and the Y. The table is moved manually by hand-wheels across the X-Y plane to position the material under the head or move it against the cutting tool.
Various tools are used for different types of cutting and for different materials. The drill is what most people would refer to as a drill bit. It is held in the collet and the head lowered while the drill is spinning to drill a hole in the material being milled. A tap is a tool for cutting threads like those found in a nut. Once a tap is used on a hole, a bolt may be threaded into it. A mill is used to cut material through and around the workpiece. Different sizes and types are used for different purposes.
Computer Numerical Control
A computer numerically controlled mill, or CNC Mill, makes precise or production parts with little intervention by the operator. Precise measuring instruments and special motors called stepping motors move the head and the table, select tools and machine the parts to precise dimensions that are difficult or too time consuming to do manually. The operator should still be a trained machinist to ensure that the machine operates correctly, is set up properly and has the necessary tools to perform the task. Programming may be done manually at a computer terminal, at the machine itself, or come from computer-aided manufacturing (CAM) software in conjunction with computer-aided design (CAD) software.
A machinist follows a blueprint to make parts or program the computer. By selecting tools and performing various operations, he machines a blank piece of material into a precision part within the tolerances allowed. Precision operations with tolerances of .005 inches are common, while .0001 or .0002 inches are also possible. Knowing which tools will best perform each task is a large part of being a good machinist.
Commonly Milled Items
Car engine blocks are first cast from steel or aluminum, then precisely machined on a mill. The engine head is also milled before assembly, as are the valves, camshaft and pistons. Some of the customized accessories used on milling machines are milled by the machinist himself for a specific task. Parts don't have to be large and heavy to be milled; many small, precision parts are manufactured on milling machines.