Milling differs fundamentally from turning in that the workpiece is held stationary and the cutting tool rotates on a spindle. The workpiece is usually held horizontally in a machine vise, mounted on a table that moves in the X and Y directions. The spindle holds a variety of cutting tools and moves in the X, Y, and Z axes.
There are 3 types of milling machines operational today :
a) 3-axis milling machines: 3-axis milling evolved from the practice of rotary filing and is a milling process that operated on two axes, the X & Y-axis. In three-axis machining, the workpiece remains still while the cutting tool moves along the 3 axes to mill the part. 3-axis machining is still one of the most widely used techniques to create mechanical parts, and can be used for automatic/interactive operation, milling slots, drilling holes, and cutting sharp edges. Because 3-axis machining only operates on the 3 axes, it’s relatively simple and allows the material to be removed in these 3 axes represented by back to front, side to side, and up and down. While it is a more basic machining process, 3-axis machining may be ideal for your machining projects depending on the size of your production run, the workpiece requirements, accuracy and finish constraints, materials used, and your holding capabilities.
b) 4-axis milling machine: 4-axis milling involves the same processes involved in 3-axis machining, where a cutting tool is used to remove material from a piece to create the desired shape and profile. However, in the case of 4-axis machining, milling is performed on an additional axis. A 4-axis CNC machine operates on the X, Y, and Z axes like a 3-axis machine, but it also includes rotation around the X-axis, which is called the A-axis. This is the 4th axis that’s added to our machining process. In most cases, the workpiece will be rotated to allow for cutting to occur around the B-axis. 4-axis milling is useful when holes and cut-outs need to be made on the side of a piece or around a cylinder. They can provide quick and efficient work based on computer numerical inputs for precise results.
c) 5-axis milling machine: 5-axis machining involves all the axes of 4-axis machining, with an additional rotational axis. 5-axis milling machines are the best CNC milling machines available today, capable of creating precise and intricate parts for artificial bones, aerospace products, titanium pieces, oil and gas machine parts, car molds, medical, architectural, and military products. The 5th axis in the 5-axis milling machine is around the B-axis, which rotates around the Y-axis in the X-Y-Z plane. This multidimensional rotation and tool movement allows for B-axis unparalleled precision, finish and speed in the production of a piece. 5-axis machining can create very complex parts, which is why it is so important for high-level uses, such as aerospace applications. However, 5-axis machining is also becoming more popular because it offers the option for single-step machining (reducing lead time), allows better access to part geometry, and improves the tool life and efficiency of the process by tilting the table for the ideal cutting position.
Although a mill can also drill holes and bores, it excels at removing stock from more complex and asymmetrical parts. Mills are used to making square/flat faces, notches, chamfers, channels, profiles, keyways, and other features that depend on precisely cut angles. Together, milling and turning are responsible for the majority of CNC machine tool operations. As with all metal machining operations, cutting fluid is used to cool the workpiece and cutting tool, for lubrication and to flush away metal particles.