Milling is a cutting or chipping manufacturing process for the production of flat surfaces or of components with very specific geometric contours. Milling is counted as «cutting with a geometrically defined edge» as the geometry of the cutting edges on the respective milling tools is given. As in all machining processes, material is removed from a blank in the form of chips and removed by means of rinsing with oil or a water-oil emulsion or due to the milling tool’s high-speed rotation. The tool torques around its axes while either the tool traverses the contour to be produced or the workpiece is moved. The feed motion can be perpendicular or oblique to the axis of tool rotation.
Thanks to high efficiency, productivity, and precision, milling is very popular – especially in metalworking production technology. In milling, a distinction is made between 2D milling, 2.5D milling, and 3D milling The D stands for dimension and the prefixed figure stands for the number of dimensions.
What are the basic principles of milling? What processes are feasible? What kind of materials can be machined?
Milling is a machining process with geometrically defined cutting edges. In contrast to other manufacturing processes such as grinding, honing, or lapping, the geometric contour of the milling cutter is clearly defined. The main component is the milling cutter. It usually has several cutting edges and can be used either perpendicular or oblique to the axis of rotation. Both manual milling machines and complete machining centers are possible.
In milling, we distinct between climb milling and up-cut milling. In climb milling, the cutter enters the material with maximum tension thickness and exits with minimum thickness. Like that, particularly tight tolerances of surface quality can be achieved. Wood and plastics can be machined but almost all metals as well. Some parameters must be considered in order to achieve the desired contours and surface qualities:
- Cutting speed
The speed has a decisive influence on the process’ economy and depends on the material and the cutting material. - Cutting width
The cutting width defines how far the tool’s cutting edge enters the material. Normally, the ratio of cutting width to tool width is approx. 2/3. - Feed motion
It has a significant influence on the surface finish. The faster the feed motion, the higher the wear of tool and stress thickness, especially for alloyed and high-alloy steels. - Pressure angle
The pressure angle is the angle between the cutting-edge entry and its exit. It also determines the number of required cutting edges. The higher the figure of cutting edges, the smoother and more uniform becomes the milling process.
What milling processes are there?
There are different methods, depending on the feed motion and surface machining. The most important milling processes are:
- Face milling: with this method, milling heads achieve a very precise flat surface within a short time. The process is characterized by a straight feed and a rotary movement perpendicular to the tool.
- Circular milling: this process is mainly used to produce constant radius surfaces. Depending on the desired shape, a further distinction is made between internal circular and external circular milling.
- Shape milling: with a controlled feed motion, workpieces can be given very specific shapes, e. g. edges, protrusions, or flat and spatial surfaces.
- Hobbing: this involves the use of a profiled cutter that simultaneously performs both a feed motion and a hobbing motion. Hobbing is mainly used for the production of gears.
- Profile milling: in this process, a specific tool profile is formed on the workpiece. Guides can be machined as well.
- Screw milling or thread milling: by helical feed movements, screws, threads, and spindles can be manufactured or machined.
- Corner milling: this milling process is used to machine heels and large surfaces.
- Groove milling: need grooves of specific depth and width? No problem with this procedure.
What is 2D milling?
In 2D milling, a workpiece is machined in the two spatial directions x and y. The machining depth z has to be determined in advance, fixed, and won’t be changed. During machining, all tool paths are in one plane.
What is 2.5D milling?
In this procedure, a workpiece is machined in all three spatial directions x, y and z. The infeed depth z varies during the ongoing machining process without the workpiece having to be re-clamped. The variation of the infeed z is also called «line-by-line milling» since each z plane is traversed line by line. Like this, holes, countersinks, pockets with vertical sides, curves, chamfers and also feather key grooves, studs, threads, and even hemispheres can be milled.
What is 3D milling?
In 3D milling, workpieces can be machined in all three spatial directions x, y and z by means of a 5-axis CNC milling machine. By dynamically tilting the milling spindle or clamping table, the tool can be brought to the workpiece at any angle. This milling process can easily produce 3D contours, true free-form surfaces, spherical surfaces, or eccentrics.
What machines and tools are used for milling?
Milling requires milling machines with at least three axes. These machines can be manual or CNC-controlled. CNC-controlled machines are much more advantageous and have almost completely replaced conventional systems. Thanks to customer-oriented programming systems and intelligent tool changers with tool life monitoring function, even the most demanding milling jobs are feasible at economical prices.
Milling machines require special milling tools with one or more cutting edges. Rotating movements remove material from the surface of the workpiece by cutting. The strategy used, the type of entrainment, as well as the cutting shape do greatly depend on the workpiece to be produced.
What cutting sizes should be considered when milling?
High-precision molds can only be achieved with optimum parameters. The following sizes should be given special attention:
- Cutting width: it indicates how wide the milling tool engages in the workpiece. The cutting width should ideally be 2/3 of the diameter of the tool.
- Cutting depth: A distinction is made between radial (radial adjustment of the tool for end milling or disc milling) and axial depth of cut (adjustment depth for face and end milling, determines metal removal rate).
- Cutting speed: it depends both on the material and cutting material. The higher the cutting speed, the more economical the production.
- Feed motion: it determines the achievable surface finish as well as the cutting load. The higher the feed rate, the higher the chip thickness, cutting force and tool wear.
- Chip thickness: in face milling, the cutting-edge load depends on the center chip thickness. In circumferential milling, it is the depth of cut, milling diameter, and feed per tooth that determine the chip thickness.
- Removal rate: this is an important criterion for the milling process’ production economy as it indicates the volume of workpiece removed per minute.
- Pressure angle: this is the angle between the entry and exit of the cutting edges. The number of cutting edges required also depends on this angle. The more cutting edges in use, the smoother the milling.
Which materials can be milled?
Milling is considered a very efficient machining production process that can achieve high surface finishes. The most important milling materials are steel and cast steel, hardened steels, aluminum and cast aluminum, brass, titanium, gold, silver, bronze, copper, tungsten copper, wood, and plastic. Milling is especially popular in the metalworking industry. Milled parts can be used in a wide variety of industries as electrical engineering, medical technology, aerospace and automotive industry, mechanical engineering, and tool-mold making.
What is computerized milling?
Milling machines can be operated manually as well as fully automatically and computer-assisted. The exact shape of the workpiece can be programmed on the computer in advance, after that the milling machine works autonomous. Like that, there can be produced many and exactly identical parts very quickly and automatically for e. g. the industrial sector.
What is the difference between turning and milling?
Turning and milling are cutting manufacturing processes: metals, plastics, or wood can be shaped according to a predefined form. Distinguishing between the two processes may seem difficult but it is quite simple: in milling, the workpiece lies still, and the milling tool moves. In turning, it is exactly the opposite: the workpiece moves around its own axis and cuts; the fixed turning tool is guided along the workpiece on a carriage. Exceptions are CNC controlled lathes with additionally controllable axes. In this type of turning, the tools are additionally driven.
What is a milling?
Milling belongs to the cutting processes with geometrically determined cutting edges (DIN 8589). Milling can be used therefore to produce flat surfaces and rotationally symmetrical parts if an absolutely precise shaping of metal, plastic, or even wood is required. So, among other things, correct cutting sizes and cutting speed are crucial for milling with exact tolerances. During this process, the workpiece is firmly clamped, the milling tool moves along the predetermined contour and removes material through rotational movements.
How does face milling work?
Face milling can produce flat surfaces with chatter-free toolpath and excellent surface finishes as circumferential cutters chip the material while face cutters finish the surfaces in the same procedure. With a symmetrical position of the milling tool to the workpiece, the force effects of climb milling and up-cut milling balance each other out.
What is a finishing cutter?
Finishing cutters made of HSS steel or carbide belong to the group of end mill cutters. They can be used universally and are available in various geometric designs. Thus, any requirement can be executed professionally and with absolute precision. With appropriate carrier systems, surface finishes of up to Ra = 0.5 μm can be achieved in finish milling.
What does CAD mean?
CAD is the abbreviation of Computer Aided Design and refers to software applications that enable the computer-based design of components. Thanks to special CAD programs, detailed 3D models of desired components can be designed and generated. This helps to avoid errors in the run-up of design processes and guarantees optimum functionality of the manufactured milled parts.