What is CNC Milling?
CNC milling, full name is Computer Numerical Control Milling, is a machining process that employs computer-controlled and rotating multi-point cutting tools to gradually remove material from a workpiece and produce a custom-designed part or product. The process is suitable for machining a variety of materials, such as metal, plastic, wood, and producing a variety of custom-designed parts and products.
Multiple capabilities are available under the umbrella of precision CNC machining services, including mechanical, chemical, electrical and thermal machining. CNC milling is a mechanical process that includes drilling, turning, and various other machining processes, which means that material is removed from a workpiece by mechanical means, such as the action of a milling machine’s cutting tool.
This article focuses on the CNC milling process, outlining the basics of the process, as well as the components and tools of a CNC milling machine. Additionally, this article explores various milling operations and provides alternatives to the CNC milling process.
What is milling? This is a type of machining that uses a tool to shape the workpiece on a table that is usually movable, although some milling machines also have movable tools. Milling was originally a manual operation performed by workers, but today, most milling is done by CNC milling machines, which utilize computers to oversee the milling process. CNC milling can provide greater precision, accuracy, and productivity, but there are situations in which manual milling can be useful. Manual milling requires a lot of technical skills and experience, thus reducing turnaround time. It also has the added benefit that manual mills are cheaper and users don’t need to worry about programming the machine.
Overview of CNC Milling
Like most traditional mechanical CNC machining processes, CNC milling processes utilize computer controls to operate and manipulate machine tools that cut and form blanks. Additionally, the process follows the same basic production stages as all CNC machining processes, including:
- Design CAD models
- Convert CAD models to CNC programs
- Setting up the CNC milling machine
- Perform a milling operation
What is CNC Milling?
The CNC milling process begins with the creation of a 2D or 3D CAD part design. The complete design is then exported to a CNC-compatible file format and converted by CAM software into a CNC machine program that dictates the motion of the machine and the movement of the tool across the workpiece. Before the operator runs the CNC program, they prepare the CNC milling machine by fixing the workpiece to the work surface of the machine tool (i.e. the table) or to the workpiece holder (such as a vise) and mounting the milling tool to the machine tool spindle. The CNC milling process employs horizontal or vertical CNC powerful milling machines – depending on the specifications and requirements of the milling application – as well as rotating multi-point (i.e., multi-tooth) cutting tools such as milling cutters and drills. When the machine is ready, the operator starts a program through the machine interface that prompts the machine to perform a milling operation.
Once the CNC milling process is initiated, the machine tool begins to rotate the cutting tool at speeds of up to several thousand revolutions per minute. Depending on the type of milling machine used and the requirements of the milling application, when the tool plunges into the workpiece, the machine will do one of the following to make the necessary cuts on the workpiece:
- Feed the workpiece slowly into the stationary rotating tool
- Moving the tool on a stationary workpiece
- Relative movement of tool and workpiece
Contrary to the manual milling process, in CNC milling, the machine tool usually feeds the movable workpiece by the rotation of the cutting tool rather than by the rotation of the cutting tool. Milling operations that adhere to this convention are called climb milling operations, while the opposite operation is called conventional milling operations.
In general, milling is best suited as an adjunct or finishing process to a machined workpiece to provide the definition of part features such as holes, slots, and threads, or to produce part features. However, the process can also be used to shape stock material from start to finish. In both cases, the milling process gradually removes material to create the desired shape and part form. First, the tool cuts small pieces, or chips, from the workpiece to create an approximate shape. The workpiece is then milled at a higher schedule and with greater precision to finish the part with its precise features and specifications. Often, the finished part needs to be machined several times to achieve the desired accuracy and tolerances. For parts with more complex geometries, once the milling operation is complete and the part is produced to custom-designed specifications, the milled part goes into the finishing and post-processing stages of production.
CNC Milling Operation
CNC milling is a machining process suitable for producing high-precision, high-tolerance parts in prototype, one-off, and small to medium production runs. While parts are typically manufactured to tolerances ranging from +/- 2 filaments to +/- 10 filaments, some milling machines can achieve tolerances as high as +/- 1 filament or even higher. The versatility of the milling process allows it to be used in a wide range of industries and for a variety of part features and designs, including slots, chamfers, threads and pockets. The most common CNC milling operations include:
- Face milling
- Flat milling
- Angle milling
- Form milling
Face milling in which the axis of rotation of the cutting tool is perpendicular to the workpiece surface. This method uses a face mill that has teeth on both the perimeter and the tool face, where the perimeter teeth are primarily used for cutting and the face teeth are used for finishing applications. Typically, face milling is used to create flat surfaces and contours on the finished part and is capable of producing a higher quality finish than other milling processes. Both vertical and horizontal milling machines support this process.
Types of face milling include end mills and side mills, which use end mills and side mills, respectively.
Face milling, also known as face milling or slab milling, in which the axis of rotation of the cutting tool is parallel to the surface of the workpiece. The process uses ordinary milling cutter teeth to perform cutting operations at the periphery. Depending on the specifications of the milling application, such as depth of cut and workpiece size, narrow and wide cutters are available. Narrow knives allow for deeper cuts, while wide knives can be used to cut larger surface areas. If a face milling application requires the removal of a large amount of material from a workpiece, the operator begins by using a coarse-tooth cutter, slow cutting speeds and fast feed rates to produce the approximate geometry of the custom-designed part. Operators then introduce finer-toothed cutters, faster cutting speeds and slower feed rates to produce the details of the finished part.
Angle milling is where the axis of rotation of the cutting tool is at an angle relative to the workpiece surface. The process uses single-angle milling cutters (angled according to the specific design being machined) to create angular features such as chamfers, serrations, and grooves. A common application for angle milling is the production of dovetails, which use 45°, 50°, 55° or 60° dovetail cutters depending on the design of the dovetail.
Profile milling refers to milling operations involving irregular surfaces, contours and profiles, such as parts with curved and flat surfaces or fully curved surfaces. The process employs profile or fly cutters designed for specific applications, such as convex, concave and corner fillet cutters. Some common applications for form milling include the production of hemispherical and semicircular cavities, beads and profiles, as well as complex designs and intricate parts from a single machine setup.
Other milling operations
In addition to the above operations, milling machines can be used to perform other specialized milling and machining operations. Examples of other types of milling operations available include:
Step Milling: Step milling refers to a milling operation in which a machine tool machines two or more parallel workpiece surfaces in a single cut. The process uses two cutters on the same machine spindle, arranged so that the cutters are on either side of the workpiece and can mill both sides simultaneously
Combined Milling: What is Combined Milling? Combined milling is a milling operation performed with two or more tools (usually of different sizes, shapes or widths) on the same machine arbor. Each cutter can perform the same cutting operation at the same time, or different cutting operations at the same time, allowing more complex parts to be produced in shorter production times.
Contour milling: Contour milling is where a machine tool creates a cutting path on a workpiece along a vertical or inclined surface. The process uses profile milling equipment and cutting tools, which can be parallel or perpendicular to the workpiece surface.
Gear Cutting: Gear cutting is a milling operation that uses an involute gear cutter to create gear teeth. These cutters are a type of profile milling cutter and are available in a variety of shapes and pitch sizes depending on the number of teeth required for a particular gear design. In this process, special turning tool bits can also be used to produce gear teeth.
Other Machining Processes: Since milling machines support the use of other machine tools than milling tools, they can be used for machining processes other than milling, such as drilling, boring, reaming, and tapping.
Like most CNC machining processes, the CNC milling process uses CAD software to generate the initial part design and CAM software to generate the CNC program that provides the machining instructions to produce the part. The CNC program is then loaded onto the CNC machine of choice to initiate and execute the milling process.
Milling Machine Precautions
Generally, milling machines are divided into horizontal and vertical machine configurations and are differentiated by the number of axis of motion.
On a vertical milling machine, the machine spindle is vertically oriented, while in a horizontal orientation the milling machine spindle is positioned horizontally. Horizontal machines also incorporate spindles during milling for additional support and stability, as well as the ability to support a wide variety of cutting tools, such as in wheel and straddle milling.
The control of both vertical and horizontal milling machines depends on the type of machine used. For example, some machines can raise and lower the spindle and move the table laterally, while other machines have a fixed spindle and table that can move horizontally, vertically, and rotationally. When choosing vertical and horizontal milling machines, manufacturers and shops must consider the requirements of the milling application, such as the number of surfaces that need to be milled and the size and shape of the part. For example, heavy workpieces are better suited for horizontal milling operations, while sinker applications are better suited for vertical milling operations. Auxiliary equipment is also available which can retrofit vertical or horizontal machines to support the opposite process.
Most CNC mills can use 3 to 5 axis – typically providing performance along the XYZ axis and (if applicable) around the rotational axis. The X and Y axis represent horizontal movement (moving left and right and back and forth on the plane, respectively), while the Z axis represents vertical movement (moving up and down), and W represents horizontal movement. – The axis represents the diagonal movement in the vertical plane. In basic CNC milling machines, horizontal movement is possible in two axis (XY), while newer models allow additional axis of motion, such as 3-, 4-, and 5-axis CNC machines. Below is an overview of some of the characteristics of milling machines classified by the number of axis of motion.
- Capable of meeting most machining needs
- Machine setup is simple.
- Only one workstation is required
- Higher knowledge requirements for operators
- lower efficiency and quality
- Better capabilities than 3-axis machines
- Higher precision and accuracy than 3-axis machines
- Machine setup is more complex than 3-axis machines
- More expensive than a three-axis machine
- Can be configured with multiple axis (e.g. 4+1, 3+2 or 5)
- more powerful
- Depending on the configuration, quicker keys are easier to operate than three- and four-axis machines
- Higher level of quality and precision
- Depending on configuration, it runs slower than 3-axis and 4-axis machining
- More expensive than 3-axis and 4-axis machines
Depending on the type of milling machine used, the machine, the machine table or both components can be dynamic. Typically, dynamic tables move along the XY axis, but they can also move up and down to adjust the depth of cut, and rotate along the vertical or horizontal axis to expand the cut. For milling applications requiring dynamic tools, in addition to its inherent rotational motion, the machine tool moves vertically along multiple axis, allowing the circumference of the tool (rather than just its tip) to cut into the workpiece. CNC milling machines with greater degrees of freedom allow for greater versatility and complexity of the milled parts produced.
Type of milling machine
Several different types of milling machines are available for a variety of machining applications. In addition to being classified based solely on machine configuration or number of axis of motion, milling machines can also be classified according to their specific characteristics. Some of the most common types of milling machines include:
- knee type
- Ram type
- Bed type (or manufacturing type)
Knee type: Knee type milling machines feature a fixed spindle and a vertically adjustable table that rests on a saddle supported by the knee. Depending on the position of the machine, the knee can be lowered and raised on the column. Some examples of knee mills include floor-standing and table-top horizontal mills.
Ram Type: Ram type milling machines use a spindle that is fixed to a column, which allows the machine to move along the XY axis with a movable housing (ie, RAM). The two most common types of vertical milling machines include floor-standing general purpose horizontal and rotary cutter head milling machines.
Bed type: A bed type milling machine uses a table directly fixed on the machine to prevent the workpiece from moving along the Y and Z axis. The workpiece is located under the cutting tool, which, depending on the machine, can move along the XYZ axis. Some of the bed-type milling machines available include single-sided, double-sided and triple-sided milling machines. Single-sided machines use a single spindle that moves along the X or Y axis, while double-sided machines use two spindles, while triple-sided machines use three spindles (two horizontal and one vertical) to machine along the XY and XYZ axis, respectively.
Planer Mills: Planer mills are similar to bed mills in that they have a table that is fixed along the Y and Z axis and a spindle that can move along the XYZ axis. However, planers can simultaneously support multiple machines (usually up to four), reducing lead times for complex parts.
Some of the special types of milling machines that are available include rotary table, drum and planetary milling machines. Rotary table milling machines have a circular table that rotates around a vertical axis and uses machines located at different heights for roughing and finishing. A drum milling machine is similar to a rotary table machine, except that the table is called a “drum” and it rotates around a horizontal axis. In a planetary machine, the table is fixed and the workpiece is cylindrical. A rotating machine moves across the workpiece surface, cutting both internal and external features such as threads.
The CNC milling process is best used as a secondary machining process to provide finishing capabilities for custom-designed parts, but can also be used to produce custom-designed and specialty parts from start to finish. CNC milling technology allows the process to machine parts in a variety of materials, including:
- Metals (including alloys, special metals, heavy metals, etc.)
- Plastics (including thermosets and thermoplastics)
- composite material
As with all machining processes, several factors must be considered when selecting a material for milling applications, such as the material’s properties (i.e. hardness, tensile and shear strength, and chemical and high temperature resistance) and the cost-effectiveness of the material . processing material. These criteria determine, respectively, whether the material is suitable for the milling process and the budgetary constraints of the milling application. The material chosen determines the type of machine used and its design, as well as the optimal machine settings including cutting speed, feed rate and depth of cut.
So what is CNC milling?
CNC milling is a machining process suitable for machining a variety of materials and producing a variety of custom-designed parts. While this process may show advantages over other machining processes, it may not be suitable for every manufacturing application, and other processes may prove more suitable and cost-effective.
Some other more conventional machining processes include drilling and turning. Like milling, drilling is usually done with a multi-point tool (i.e. a drill), while turning is done with a single-point tool. However, while rotating, the workpiece can move and rotate like some milling applications, while during drilling, the workpiece is stationary throughout the drilling process.
Some unconventional machining processes (i.e., without the use of machine tools, but still with mechanical material removal processes) include ultrasonic machining, water jet cutting, and abrasive jet machining. Unconventional, non-machining processes (i.e. chemical, electrical, and thermal machining processes) provide other alternatives to removing material from workpieces that do not use machine tools or mechanical material removal processes, including chemical milling, electrochemical deburring, laser Cutting and plasma arc cutting. These unconventional machining methods support the production of more complex, more demanding and specialized parts that are often not possible with conventional machining.