Milling Process – Definition
Milling, a subtractive manufacturing process, is the use of cutting machining tools to perform material removal from a workpiece to form complex geometric patterns. The workpiece usually fixed on a movable worktable is fed into the rotating multipoint machine tools. While the milling process can be performed by a highly skilled technician on a manual milling machine, the process can now be performed on a computer-controlled milling machine called a computer numerical control (CNC) milling machine. The use of CNC milling machines has resulted in higher precision and increased production rates.
CNC Milling Process – Overview
Computer Numerical Control Milling, or CNC Milling, is the use of an automated material removal process by mechanical means to produce the desired product with highly accurate and precise dimensions and complex patterns. The rotating action of cutting tools and the relative motion of the workpiece material are responsible for the conversion of the workpiece into the desired geometry.
The overall stages of material processing are similar to the other conventional machining processes. A workpiece generally progresses through the following stages:
- Preparing a CAD Design
- Conversion of the CAD Design into a CNC program
- Setup of CNC milling machine
- Execution of milling process
The first step in machine processing through CNC milling is the creation of the computer design using CAD software. This design indicates the required dimensions and geometry of the final product. The next step is the conversion of this CAD file into a program that can be executed by the CNC software i.e., CAM. Once the file is ready, CAM then controls the movement of the cutting tool and the feed of the workpiece to produce the final product. Before CAM executes the file, the setting up of the CNC milling machine is done which involves clamping the workpiece in the worktable using a vise or other apparatus. The cutting tool, depending upon the requirement of the job, is fixed in the machine spindle. Once all the preparations are complete, the technician gives the command to the CNC milling machine using a machine interface to execute the milling process.
Depending upon the type of the CNC milling machine being used and the final geometric patterns required, the CNC milling machine performs the material removal process by following actions:
- Feeding the workpiece slowly while keeping the rotatory cutting tool stationary
- Keeping the workpiece stationary while moving the tool across it
- By relative motion between the tool and the workpiece
CNC milling can be performed to create a workpiece from stock material; however, it is usually performed as a secondary machining process to perform finishing operations or to create geometric features like slots or holes in a workpiece. For complex geometrical patterns, CNC milling may be performed multiple times using different orientations and setup of workpiece and tool. Later, the part is transferred to other facilities to perform other post finishing processes.
CNC Milling Machine – Operations
CNC Milling machine provides greater accuracy and tolerance limits; therefore, it is used to machine parts with tolerances ranging between +/- 0.001in and 0.005in. Tolerances of 0.0005in are also achievable with some CNC machines. Hence, the highly precise and accurate machining performed with CNC milling machines makes it useful in the industry for a variety of operations. Some of these widely performed operations are:
Face milling is a machining process generally performed on a vertical or horizontal CNC milling machine to produce flat surfaces or contours on a workpiece. The process involves rotation of the cutting tool with its axis perpendicular to the workpiece. The cutting tool has teeth on the peripheral side as well as on the front. The periphery cutting tool is used to remove material from the workpiece by cutting while the frontal cutting teeth are used to enhance the surface finish of the workpiece.
End milling performed with end milling cutters and side milling carried with side milling cutters is the two types of face milling.
Plain Milling, in contrast to face milling, is performed with the cutting tool rotating parallel to the workpiece. The cutting teeth, that are located on the periphery, are used to cut the material. Depending upon the requirements of the workpiece, the teeth of the cutting tool are altered to get desired cut. For deep cuts, the narrower cutters are used while for cutting larger materials, a wider teethed cutting tool is used. Similarly, varying the speeds of the cutter and the feed of the workpiece can result in a finer or a coarse cut.
Angular milling, or also known as angle milling, is the milling process to produce angular geometric features like chamfers or dovetails. As the name implies angular milling involves the rotation of the cutting tool at an angle relative to the workpiece.
Form milling is the CNC milling operation to produce irregular shapes like semi-hemisphere or round contours. The formed cutting tools are used for these specific shapes and with these job-specific tools some common shapes like curves or beads, as well as complex patterns can also be machined.
CNC Milling Machine – Features
CNC Milling machines offer additional features that make them more productive than conventional milling machines. These features are discussed below:
- Unlike manual milling machines, shapes and geometries that are advanced, and complex can also be machined using CNC milling machines. High control over dimensions and tolerances is also achievable with CNC milling.
- The execution of mathematical models by CNC machine enables it to process parts that are otherwise difficult to machine using conventional or ordinary machine tools.
- With high levels of automation involved in the machining process, the involvement of technicians and operators is reduced. This also reduces the levels of personal errors to a minimum.
- The use of a CNC milling machine to produce parts effectively reduces the production times. The entire process of setting up a CNC machine for operation is straightforward. Hence, changing the position of the workpiece or cutting tool is a time-efficient mechanism and does not waste time.
CNC Milling Machine – Components
Depending upon functionality, the structure of the CNC milling machine differs from each other but almost all CNC milling machines have the same common components which are shown below: