What is the milling process? What tools, techniques and technologies are needed to achieve this type of machining? What materials can be milled?
By reading this article you will find the answers to these questions and much more, to perform perfectly milling machining, obtaining multiple advantages.
Definition of milling
Milling is an operation that is carried out by removing material, known as chips, using specific tools and machinery called milling machines.
The milling machine, through the rotary motion of the cutter, which is the tool installed on it, removes the chips, thereby working the material. It consists of a base on which the piece to be milled is placed, and a column on which the spindle is fixed. The spindle is the device that drives the cutter through an electric motor. The orientation of the spindle determines the type of milling machine, which can be either vertical or horizontal. The cutter is circular in shape and, depending on the arrangement of the cutting edges, can be helical, face, flat, double-angle, staggered teeth, face, chamfer.
The history of milling
The history of milling is a fascinating journey that spans centuries, evolving from simple hand tools to sophisticated machines used in modern manufacturing. The concept of milling can be traced back to ancient times when people used hand tools like chisels and hammers to shape materials. Early milling was mostly done by hand, with simple tools to cut, shape, and smooth wood and stone. During the Middle Ages, the development of water wheels led to the creation of water-powered mills. These mills used the power of flowing water to drive large, circular stones that ground grain into flour. The Industrial Revolution in the late 18th and early 19th centuries brought about significant advancements in milling technology. The invention of steam engines and the development of metalworking led to the creation of more sophisticated milling machines.
One of the most important inventions was the milling machine designed by Eli Whitney in 1818. Whitney's machine uses a rotating cutter to remove material from a workpiece, allowing for precise shaping of metal parts. This innovation was crucial for the mass production of interchangeable parts, particularly in the arms industry. During the early 20th Century, the development of electric motors and advancements in metallurgy led to the creation of more precise and powerful milling machines. During this period, milling machines became a staple in manufacturing industries, producing everything from car parts to aircraft components. The 1950s saw the introduction of numerical control (NC) technology, where milling machines were controlled by punched tape or digital instructions. This innovation laid the groundwork for computer numerical control (CNC) milling, which emerged in the 1970s. CNC milling machines use computer programming to control the movement of the milling cutter, allowing for extremely precise and complex operations.
Today, milling machines are often integrated with CAD and CAM software, enabling the design and manufacturing process to be highly automated and efficient and can operate with multiple axes (up to five or more). This allows to produce intricate and high-precision components used in industries like aerospace, automotive, and medical devices.
How does milling work?
Milling is a machining process that removes material from a workpiece using a rotating cutting tool called a milling cutter. It is widely used in manufacturing to shape and create parts from various materials, including metal, plastic, wood, and composites. There are two primary types of milling: Face Milling where the cutter is mounted on a spindle that is perpendicular to the workpiece (this type is used to create flat surfaces on the material) and Peripheral Milling (or Slab Milling) where the cutter is mounted parallel to the workpiece (tt’s used for cutting deep slots, grooves, and other features on the sides of the workpiece).
Stages of the milling process
The milling process involves several stages, each of which is critical to achieving the desired shape, dimensions, and surface finish of the workpiece. Here are the key stages of the milling process:
Preparation and Planning: the process begins with the design of the part, typically using Computer-Aided Design (CAD) software. The specifications include dimensions, tolerances, material type, and surface finish and tool selection.
Setting Up the Machine: the workpiece is securely clamped to the machine’s table, and the appropriate cutter is attached to the spindle. The milling machine’s parameters (such as spindle speed, feed rate, and depth of cut) are set based on the material and desired operation.
Cutter Movement: The cutter rotates at high speed, which is powered by the spindle. The rotation is crucial for the cutting action, where the edges of the cutter remove material from the workpiece. The table moves the workpiece in relation to the rotating cutter. This movement is typically automated and controlled by a program in CNC (Computer Numerical Control) machines, or manually controlled in simpler machines.
Material Removal: as the rotating cutter meets the workpiece, it cuts away small pieces of material, known as chips or swarf. The cutter’s shape, speed, and feed rate determine the amount of material removed and the final shape of the workpiece. The process can involve multiple passes of the cutter over the workpiece, each pass removing more material to achieve the desired shape.
Cooling and Lubrication: during milling, friction generates heat, which can affect the tool and workpiece. Coolant fluids are often used to cool the cutting area, lubricate the tool, and wash away chips. This helps maintain tool life and ensures a smooth finish on the workpiece.
Completion: once the desired shape, size, and surface finish are achieved, the milling operation is complete. The workpiece is then removed from the machine for inspection and further processing, if necessary.
Types of milling operations
Milling operations allow the material to be shaped according to specific needs and requirements, making it more rounded, for example by removing edges, or profiling it into a wide range of shapes, creating corners, recesses, joints, and guides. Through different milling techniques, it is possible to achieve surfaces that are not only functional but also aesthetically decorated with a high degree of precision. Milling can also be classified into two types: manual milling, achieved using portable manual routers or those used on a workbench, and numerical control milling, achieved using numerically controlled machines on which the cutting tool, cylindrical and circular in section, called a mill, is mounted. These machines are known as CNC milling machines. Milling operations can be classified into various types based on the orientation of the tool and the features being machined. Each type of milling operation is suited to producing specific shapes and surface finishes on the workpiece. Among the main types of milling operations, we can mention: Face Milling (the cutting tool’s axis is perpendicular to the surface of the workpiece and the tool removes material primarily from the face or the flat surface of the workpiece); Peripheral Milling (Slab Milling) (The cutting tool’s axis is parallel to the surface of the workpiece, and material is removed from the sides of the workpiece); End Milling (the tool has cutting edges on both the end and the sides, allowing it to perform operations on the workpiece’s face as well as the sides); Angular Milling (The cutting tool is set at an angle relative to the surface of the workpiece, and the material is removed at an inclined angle); Profile milling that involves cutting the outer contour of a part. It can be used for machining the outside or inside profiles of parts.
Types of milling machines
Mill and drill machines come in various types, each designed for specific applications and tasks. They differ in terms of their structure, capabilities, and the types of milling operations they can perform. Here are the main types of milling and drilling machines:
Vertical Milling Machine: the spindle axis is oriented vertically, and the cutting tool moves up and down. The workpiece is usually mounted on a movable table that can move in multiple directions. It is commonly used to mill and drill the material.
Horizontal Milling Machine: the spindle axis is horizontal, and the cutting tool is mounted on an arbor that extends from the machine’s side. The workpiece is also mounted on a movable table. It is suitable for heavy-duty operations such as slab milling, face milling, and forming slots or grooves.
CNC Milling Machine: a Computer Numerical Control (CNC) machine automates the milling process using computer programming. The machine operates along multiple axes (typically 3, 4, or 5) to achieve precise and complex milling tasks. It is suitable for high-precision and complex part manufacturing, such as in aerospace, automotive, and electronics industries.
Universal Milling Machine: a versatile machine that can perform both vertical and horizontal milling operations. The table can rotate along the horizontal axis, allowing for angular cuts. It is ideal for complex and multi-angle milling tasks, such as helical milling or gear cutting.
Equipment used in milling
In milling operations, various types of equipment and tools are used to achieve precise and effective material removal. Let’s discover the most common equipment and accessories used in milling.
Milling machine
The primary machine tool used for milling operations, equipped with a spindle that holds and rotates the cutting tool is the milling machine that can come in different types, including vertical, horizontal, CNC, and universal.
Milling cutter
A milling cutter is a tool used in milling machines to remove material from the workpiece. Milling cutters come in various shapes, sizes, and types, each designed for specific cutting operations. They are used for a variety of tasks, including creating flat surfaces, slots, grooves, contours, and complex 3D shapes. The choice of cutter depends on the specific requirements of the milling operation.
Horizontal milling machine
A horizontal milling machine has a spindle that is oriented horizontally. The cutting tool is mounted on an arbor that extends from the side of the machine. The workpiece is mounted on a table that can move in multiple directions.
Vertical milling machine
A vertical milling machine has a spindle that is oriented vertically. The cutting tool is mounted directly in the spindle, and the workpiece is mounted on a movable table.
Types of cutting fluids
We can distinguish coolants (fluids used to cool and lubricate the cutting tool and workpiece during milling. They help reduce friction, dissipate heat, and extend tool life) and lubricants (fluids or oils that reduce friction and wear on the cutting tool and workpiece, improving surface finish and cutting efficiency).
Key parameters in milling
When performing milling operations, several key parameters influence the efficiency, quality, and outcome of the machining process. Understanding and optimizing these parameters is crucial for achieving desired results. We can list:
Cutting Speed (Vc): Affects surface finish and tool life.
Spindle Speed (N): Determines the cutting speed of the tool.
Feed Rate (F): Influences material removal rate and surface quality.
Depth of Cut (ap): Affects cutting forces and productivity.
Width of Cut (ae): Influences material removal rate and tool load.
Tool Material: Determines hardness and wear resistance.
Tool Geometry: Affects cutting efficiency and performance.
Coolant/Lubrication: Reduces heat and improves tool life.
Tool Wear: Impacts cut quality and tool performance.
Workpiece Material: Dictates machining parameters and strategies.
Advantages of milling
Milling offers several advantages in terms of versatility, efficiency, and speed; these benefits make milling a fundamental activity in the manufacturing industry. Let’s explore these benefits in more detail:
Versatility
The main advantage of milling lies in its versatility, as it allows the machining of any shape, performing a set of complex and very different operations with a single program, all without operator intervention, who only needs to input instructions into the program and monitor the machining process. Additionally, it can be used to work with various materials beyond wood, such as metals, plastics, and composites.
Efficiency
Milling machines ensure efficiency in terms of cost and time reduction, thus improving productivity. The optimization of cutting parameters and tools allows for maximizing material removal with the least possible energy consumption, offering benefits in terms of sustainability as well.
Speed
The milling process offers advantages in terms of production cycle speed while maintaining a high standard of precision and quality in the finished piece.
Common milling materials
Milling can be used to machine a wide variety of materials, each with its own unique properties and machining characteristics. Below are some of the most common materials used in milling.
Metals
Different metals have unique properties that influence how they are machined. Here is an overview of common metals used in milling, their characteristics, and considerations for milling them: aluminum has favorable properties, such as its lightweight, excellent machinability (with good chip formation and low cutting forces), and resistance to corrosion. Aluminum dissipates heat well, which helps in preventing excessive heat buildup during milling and it forms a protective oxide layer that prevents further oxidation, making it resistant to corrosion. Other metals that can be milled are steel and stainless steel: the first one is generally tough and strong, with a range of hardness depending on the specific alloy, the second one is corrosion-resistant and tough but tends to work harden, making it more challenging to machine. We can also list titanium, strong, lightweight, and highly resistant to corrosion and heat, but it is challenging to machine, and brass, a copper-zinc alloy that is easy to machine, with good corrosion resistance and a bright appearance.
Plastics
Milling plastics is a common practice in various industries, from prototyping to producing finished parts for consumer products, medical devices, and electronics. Plastics vary widely in their properties, so it's important to tailor your milling approach to the specific type of plastic you are working with. Among the common plastics we can mention Acrylic (PMMA), Polycarbonate (PC), Nylon (Polyamide, PA), Polyethylene (PE), Delrin (POM or Acetal), Polypropylene (PP), PVC (Polyvinyl Chloride), ABS (Acrylonitrile Butadiene Styrene).
Composites
Composites are engineered materials made from two or more constituent materials with different physical or chemical properties. These materials are combined to create a composite that has superior characteristics compared to the individual components. Many composites can be milled such as Carbon Fiber Reinforced Polymer (CFRP), Glass Fiber Reinforced Polymer (GFRP), Kevlar Reinforced Polymer, Fiber-Reinforced Thermoplastics (FRTP), Metal Matrix Composites (MMC), Laminated Composites, Wood-Plastic Composites (WPC), etc.
Woods
Milling wood is a common practice in woodworking and manufacturing industries, involving the shaping, cutting, and carving of wood materials to create various products. Different types of wood require different milling techniques, depending on their hardness, grain structure, and moisture content. Softwoods (pine, cedar, fir) are generally easier to mill, with a lighter weight and less dense structure. They are more prone to splintering and tear-out, especially when milling across the grain. Hardwoods (Oak, Maple, Walnut and Cherry) are denser and harder than softwoods, providing greater durability and resistance to wear. They can be more challenging to mill, requiring sharper tools and slower feed rates. Exotic woods (Ebony, Teak, Mahogany) often have unique grain patterns, colors, and densities. They can be more difficult to mill due to their hardness, oiliness, or interlocking grain structure.
Ceramics
Milling ceramics requires specialized tools, careful selection of cutting parameters, and an understanding of the material's unique properties. Ceramics (alumna, zirconia, silicon carbide, silicon nitride…) are typically hard, brittle, and resistant to heat and wear, making them useful in many high-performance applications, such as aerospace, medical devices, electronics, and industrial components. Diamond-coated tools or polycrystalline diamond (PCD) tools are essential for milling ceramics due to their extreme hardness and abrasion resistance. Carbide tools can be used for less abrasive ceramics but wear out quickly.
Others
Beyond metals, plastics, composites, woods, and ceramics, several other materials can be milled. We can list Stone and Minerals (Marble, Granite, Limestone, Soapstone, Slate), Specialty materials (Bakelite, Thermosetting Plastics, nanomaterials (Nanocomposites), biomaterials (for medical applications), Photopolymers (used in 3D printing and rapid prototyping), other Advanced Materials (Shape Memory Alloys (SMA), Amorphous Metals (Metallic Glasses), High-Entropy Alloys (HEA), Biodegradable Polymers, Piezoelectric Ceramics).
Unsuitable materials for milling
Milling is a versatile machining process, but not all materials are suitable for this type of operation. Some materials present challenges due to their properties, which can affect tool performance, machine efficiency, and part quality. We can mention brittle materials (like glass) that are prone to chipping, cracking, or breaking during milling. Their lack of ductility makes them difficult to cut without causing damage to the workpiece; Soft materials that can deform easily under cutting forces, leading to poor surface finish and dimensional inaccuracies; highly Elastic or Stretchable Materials (like rubber) that can stretch or deform during milling, leading to inaccuracies and poor surface finish. They often require different machining strategies or equipment.
Milling process: hazards and safety
Maintaining safety during milling operations involves understanding and addressing the various hazards associated with the process. The hazards can be mechanical (milling machines have various moving parts, including spindles, tables, and cutting tools, which can pose a risk of entanglement or crushing) or electrical (because of electrical systems that can pose shock hazards if not properly maintained or if there is a malfunction). There are also noise hazards and ergonomic hazards if the operators don’t adopt awkward postures or work in confined spaces.
By implementing safety measures such as machine guards (that ensure that all moving parts and dangerous areas are equipped with appropriate guards to prevent accidental contact), proper training, personal protective equipment (such as safety glasses or face shields and earplugs or earmuffs), regular maintenance, and safe handling practices, you can help create a safer working environment and reduce the risk of accidents and injuries.
Common problems in milling
Some common issues encountered in milling can include excessive wear or breakage of the cutting tool because of poor tool material, improper cutting parameters, high cutting speeds, insufficient coolant, and improper tool selection; undesirable vibration that leads to poor surface finish and potential damage to the tool or machine due to worn or incorrect cutting tools, incorrect cutting parameters, chatter, or poor machine alignment. We can also mention inaccurate dimensions, improper chip formation that can lead to poor surface finish, tool wear, and even machine damage (due to incorrect cutting parameters, unsuitable tool geometry, or poor material selection), warping or distortion of the workpiece after machining, material accumulation on the tool, workpiece, or machine bed, causing interruptions or poor finishes, and tool runout due to poor tool holder quality, improper tool installation, or machine spindle wear.