15 Types of Milling Operations
Milling has several meanings, most of which relate to circular tool cutting of materials. In particular, the most important definition of milling is the use of generally multi-tooth, rotating cutting tools to “extract” a finished part from a material blank by end and side cutting processes.
Milling includes functionally different methods for achieving particular aspects of the extraction and material removal process, and these are described below:
Face milling is a machining procedure with two processes. The first refers to the flat end face of a net-circular section cutting tool, called a face mill, removing material from the surface of a workpiece by plunge cutting the tool along its axis, to a predetermined depth within the material. The second occurs when the cylindrical face of a tool is traversed across the face of a material piece to cut the entire surface away to the required depth.
These methods are used to produce flat surfaces, which can be angled to the horizontal norm by tilting the arbor that drives the tool. Face milling is widely used in manufacturing for achieving precise and smooth finishes on large, flat workpieces, such as engine blocks and metal plates.
To learn more, see our article on Face Milling.
Slot milling is the form of milling process used to create slots or channels in a workpiece. A specialized cutting tool, such as an end mill or slotting cutter, removes material to form slots of various shapes and sizes. A slot cutter is constructed such that its cylindrical face is suited to side cutting. An end mill is capable of both plunge cutting (cutting along its axis using the end face) and a degree of side cutting, to initiate blind slots that must begin by drilling.
Slot milling is the manufacturing technique for producing keyways, grooves, and any other slot format features.
End milling is the milling process in which the net-flat end of an end mill removes material from the workpiece's surface on the tool axis by plunging. This method is used to create or initiate various features, including slots, pockets, and contours.
To learn more, see our article on What is an End Mill.
Thread milling is the milling process that uses a specialized cutting tool with multiple cutting edges, essentially identical to a tap (internal threads) or die (external threads) to cut threads in a workpiece. These very precise tools can deliver complex profiles and offer long tool life, as their cutting function is limited. Thread milling is widely used in manufacturing applications for integrating thread cutting into a workpiece.
To learn more, see our article on Thread Milling.
Shoulder milling is the machining process that removes material from the shoulder or sidewall of a workpiece, generally using a side cutter. A milling cutter with multiple inserts or cutting edges is used to create flat or contoured surfaces. Shoulder milling is used in manufacturing to produce precise shoulders and step features in components.
Side milling is a machining process in which a cutting tool, typically an end mill or side milling cutter, removes material from the side of a workpiece. It is used to create flat or contoured surfaces on the side of the workpiece, such as slots, grooves, and pockets.
Profile milling is an intricate milling process used to create complex shapes, contours, or profiles on the surface of a workpiece. A specialized cutting tool, generally with multiple cutting edges and often with a “ball” (hemispherical) tip, follows a programmed path to remove material, forming the desired profile. The ball-ended tool assists in smoothing the aliasing that results from a sharp-cornered tool being used for profile work. This cutter path will usually, but not always, involve the cooperative movement of multiple axes to deliver the required shape.
It is commonly used in the production of molds, dies, and components with intricate shapes. Profile milling offers precision and versatility for manufacturing parts with unique and detailed surface geometries. CNC machines are often employed for accurate profile milling operations, as axis cooperation in manual milling is essentially impossible, preventing these machines from generating complex profiles.
Saw milling is an uncommonly used term for the straight, slot parting of a workpiece using a narrow section, large diameter cutting tool with many teeth at its circumference—similar in appearance to a circular-saw blade. These tools can operate in a plunge mode, cutting downwards along a vertical radial line or, more commonly traversing the workpiece in a conventional or climb milling mode, depending on a variety of factors.
Such tools are often termed slitting wheels, and the process is more commonly referred to as slitting.
CAM milling is the process of computer-aided manufacture involving the milling of parts or products.
Gear milling is the process used to produce gears or gear teeth on workpieces. It involves using specialized cutting tools, such as gear hobbing cutters or gear milling cutters, to remove material and form the gear teeth.
Gear milling is crucial in industries like automotive and machinery manufacturing, in which precise gearing and high-precision tooth profiles are essential for smooth and low-wear power transmission and motion control. CNC and manual machines are employed for high-precision gear milling/hobbing, to create spur, helical, bevel, and other gear types.
Angle milling involves cutting material at an angle to the workpiece's surface. It is used to create beveled edges, chamfers, or angled features on components. Angle milling can be performed either with specialist tapered milling cutters or with parallel-faced cutters in a tilting arbor or 4+ axis machine.
To learn more, see our guide on Angle Milling.
Form milling creates intricate and contoured shapes or forms on the surfaces of workpieces. It can involve using a specialized milling cutter with the desired shape or profile, or it can be a more repetitive process that resembles profile milling, using more generic cutters.
Form milling is common in industries such as: aerospace, automotive, and mold-making to produce complex components and molds with precise shapes. CNC machines are often used for form milling operations due to their ability to precisely control the tool's movement, enabling the creation of intricate and irregular forms.
Straddle milling creates parallel slots, grooves, or surfaces on a workpiece. Two side-by-side milling cutters are mounted on an arbor, allowing them to machine two sympathetic and parallel features simultaneously. This process doubles productivity and is often used in applications like keyway milling and machining flats on opposing sides of shafts.
Plain milling is a basic machining process in which a flat, horizontal cutting tool removes material from the surface of a workpiece. It creates flat surfaces, slots, or grooves. Plain milling is commonly used for producing square or rectangular features and is one of the fundamental operations in milling.
Gang milling is a milling operation where multiple cutters are mounted on a single arbor. They are utilized simultaneously to machine multiple surfaces or features on a workpiece in a single pass. This process increases productivity and ensures accuracy by reducing setup and handling times. Gang milling is commonly used in the mass production of parts with multiple identical features. The setup cost and complexity make it impractical for low-volume or one-off production.
Metal milling is machining that involves the removal of material from a metal workpiece using a rotary cutting tool. Either the tool or the workpiece or both are moved to create profiles and complex shapes in the cut path.
In metal milling, the workpiece is typically secured to a worktable or fixture, and a cutting tool, such as an end mill, face mill, or drill, rotates at high speeds. The tool makes contact with the workpiece and, as it rotates, its cutting edges remove material, creating the desired features, profiles, and forms. The cutting tool, or part, or both can move along multiple axes, allowing for the creation of flat surfaces, contours, holes, threads, and intricate shapes.
Metal milling is widely used in industries such as: aerospace, automotive, manufacturing, and engineering. It offers the advantages of high precision, versatility, and the ability to work with a variety of metals, including steel, aluminum, brass, and titanium.
Machines used for milling are either multi-purpose and adaptable manual or CNC mills but can be special-purpose fixtures that are integrated into a production line.
Milling plays a central and irreplaceable role in most manufacturing sectors due to its adaptability and expanding capability set. It allows for the production of precise and repeatable components, meeting tight tolerances and quality standards that can adapt to cost and functional needs. Milling offers the flexibility to create an almost unlimited range of parts with divergent shapes, sizes, and complexities, with few material limitations. It is efficient in reducing material waste, energy use, and processing time when compared with other manufacturing methods. The process can be used with virtually all metals, plastics, composites, ceramics, and many natural materials such as stone, wood, and bone. Milling enables the customization of parts, meeting specific design and engineering needs. Small design alterations during a production run generally involve minor programming alterations and can allow design tuning on the fly. CNC milling machines allow for automated and repeatable production, increasing productivity and consistency. CNC was the earliest form of digital manufacturing automation and remains central to the process. Milling is used for prototyping and product development, allowing for testing and refinement of designs. The benefit of “real” material components at prototype is that full-force and long-duration testing is easier and more precisely reflects the mass-produced product.
Milling machining is central to the delivery of single, moderate-volume, and mass-production components across essentially all sectors of industry. For example, milling is central to the creation of critical components like: aircraft frames, engine parts, landing gear, engine blocks, transmission components, and chassis parts. It allows fast processing and is extensively used for finishing operations on sand, die, and investment cast components. Milling is used in the precision manufacture of implants, prostheses, and surgical instruments, often using exotic materials and always requiring high precision. The high-volume manufacturing and precision machining of printed circuit boards (PCBs) and connector components is also generally performed on special classes of CNC mills.
Selecting the appropriate type of milling process for machining components depends on various factors. For example, for simple components with flat surfaces or straight features, plain milling will often serve well. Complex components with intricate shapes may require more specialized processes like profile milling or form milling. Tight tolerances and high-grade surface finishes are best achieved with precision milling processes such as finish milling or surface milling. This involves fine cuts and low feed rates and results in longer processing times and higher costs.
Consideration of the type of material greatly affects equipment, cutting tool selection, and cutting parameters. Harder materials generally benefit from climb milling for reduced tool wear. For low-volume or prototype production, flexibility and setup simplicity may be prioritized, favoring manual or CNC milling.
The most prevalent type of milling for machining parts is by 3+ axis CNC processing. The overwhelming majority of tasks only require 3 or 3 ½ axis machines, which are the most common type. CNC milling has become dominant in fourth-generation manufacturing, due to its numerous advantages.
While CNC milling is prevalent, it's important to note that manual milling for small-scale or specialized applications is still widely used. Also, more specialized milling techniques like gear milling or thread milling, are still used when the specific requirements of a part call for them.
The cheapest milling operation for making parts typically involves manual milling using conventional milling machines. Manual milling relies on skilled operators to control the machine's movements and tool engagement.
It is cost-effective because it requires a relatively low initial investment compared to CNC milling machines, which are considerably more expensive. This approach is suitable for small-scale production, prototyping, and repair tasks. The choice between manual and CNC milling depends on factors like: part complexity, production volume, and budget constraints.
The most expensive milling operation for making parts typically involves processes that require advanced equipment, tooling, and operator/programmer expertise. For example, utilizing high-spindle-speed machining centers with advanced tooling and cutting strategies for precision and speed can increase machine charges. It is utilized by 30 to 50% but increases throughput to a lesser degree, increasing overall costs. A job that costs $100 manufactured with normal spindle speeds is liable to be around 10-30% more expensive when performed on equipment that supports higher spindle speeds, as throughput is barely altered but equipment costs are increased.
On the other hand, increasingly complex applications in aerospace and medical industries commonly require machines with five or more degrees of freedom for complex, multi-sided machining. Multi-axis machines typically require charge-out at two to three times the rate, but the reduction in additional setups will generally more than compensate for this. Overall, a complex task will generally cost 20 to 70% less when performed on higher-capability equipment, despite higher machine costs. Simple tasks are similarly higher by 2 or more times, through poor exploitation of machine capabilities.
Utilizing the most advanced types of machine and exotic techniques in the tool path can also deliver extremely tight tolerances and high-quality surface finishes. This is common for optical and semiconductor applications. This adds a multiplier of 2 to 3 times, compared with more ordinary precision-level machining of an otherwise identical part. Finally, working with exotic or optical materials, superalloys, ceramics, and composites raises the cost of tooling and expertise, making the process expensive. Exotic materials can increase costs by multiples of 10 to 100 times and more, as material costs can completely overwhelm machine costs even after additional machining difficulty is factored in.
The most accurate general milling operation for making parts is typically precision CNC milling, particularly when combined with advanced machining techniques and equipment. The use of higher-precision equipment, more advanced and esoteric tool path planning, and extremely precise cutters can elevate precision and repeatability considerably.
For industries like: aerospace, medical devices, and optics, in which tight tolerances are critical, precision CNC milling is the only practical choice.
Helical flute cutters have flutes (cutting edges) that spiral around the tool's axis in a helix form. Helical end mills and side cutters efficiently evacuate chips from the cutting area and are particularly useful in applications in which chip removal and improved surface finish are essential. In particular, the helical form reduces the adhesion and buildup of cuttings behind the tool edge. It also encourages a degree of chip flow along the cutting edge, making them self-clear. The recruitment of cuttings onto the tool has the greatest and most immediate impact on the surface finish, as the cuttings score the freshly cut face.
Helical end mills are available in various configurations, including: square end mills, ball nose end mills, and corner radius end mills, making them versatile for varied milling operations and materials.
Not specifically. CNC has come to mean a broad family of machining processes, any of which can be controlled by a computer numerical control system and a G-code program. The overwhelming majority of broad capability machining centers are 3+ axis CNC mills.
This article presented types of milling in machining, explained each type, and discussed when to best use each type. To learn more about the types of milling in machining, contact a Xometry representative.
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