CNC milling is the most popular type of CNC machining process used by manufacturers around the world to produce three-dimensional parts with complex geometries and shapes. CNC milling is a subtractive manufacturing process that uses a rotating spindle where a multi-tooth cutting tool is attached to remove material when it comes into contact with the workpiece.
CNC milling machines are automatic cutting machines that create precise cuts, features and thin geometries in a part by following a pre-programmed path generated from a CAM software.
While there are many aspects of CNC milling, the most important is understanding Computer Numerical Control (CNC) and how CNC machined parts are produced using different CNC milling processes. In this blog, you’ll learn about CNC milling from a machining perspective and at the end, you’ll find a helpful tip about selecting the right milling process for your part.
An Overview of CNC Machining
CNC milling is part of CNC machining processes like CNC turning, CNC drilling, and CNC EDM machines, which are all part of the subtractive manufacturing process. CNC machining uses computer numerical control (CNC) to control the movement of the cutting tool.
Modern CNCs are capable of adjusting the cutting speed, spindle speed, direction of tool rotation, feed rates, and performing tool changes. All of this information is stored in the CNC instructions which are in the form of a G-code or M-code.
Key features of CNC Machining:
High precision
Close tolerances
Repeatability
Multi-axis machining
CAD and CAM optimized
Low to high volume production parts
What Is a CNC Milling Machine
A CNC Milling machine is an advanced manufacturing machine that uses computer numerical control to control a rotating spindle attached to a milling cutter. CNC milling machines consist of several key components
Spindle: straight rod with a taper that holds the cutting tool
Worktable: Made of fixtures and jigs with clamps to securely hold the workpiece
Axes: 3 or more axes, including the spindle axis, enable movement of the tool in X, Y, and Z axes.
Control Unit: A small microprocessor built into the machine that can interpret the machine code
Tool Changer: A Module that can change the cutting tool
Bed: The Milling machine bed is made of cast iron or structural steel to support the weight of the machine and the rotating and moving parts.
Probe: Advanced CNC milling machines have probes that can measure the workpiece dimensions or calibrate the machine before cutting the workpiece.
How Does CNC Milling Work?
The CNC milling process starts with workpiece preparation. The raw material stock is secured on the worktable using jigs and fixtures or standard clamps to ensure no vibrations or loose movements.
Tool selection: There are different types of CNC milling cutters that are suitable for different types of cuts, like pockets, cavities, and contours. Additionally, the type of cutting tool also affects the overall finish and tool marks on the part. You can select from a wide variety of end mills, drills, or face mills based on your design requirements.
The CNC milling machine’s onboard controller interprets the uploaded G-code to control the movement of the cutting tool. It can direct the cutting tool and control the worktable movements, and in advanced CNC milling machines, it can either rotate the worktable or the cutting tool around the spindle axis. These machines are called 5-axis milling machines and have additional rotating axes called A and B.
Material Removal
The material stock is cut in layers through the rotating cutting tool controlled by the machining program. Tougher materials and high cutting speeds require a coolant spray or a coolant flow over the workpiece to keep the cutting tool cool and prolong its life.
The CNC milling machine executes the G-code, which controls the spindle RPM, tool movement, feed rate, depth of cut, and number of cutting passes. Once the complete G-code is executed, the part comes with “as-machined” surface finish with a roughness value of Ra 3.2. Post processing techniques like deburring, polishing and grinding can further improve the surface roughness and meet tight tolerances.
CNC milling is a highly repeatable process that can produce identical parts to a large degree, and the automatic tool changers in milling machines reduce human intervention, manual labor, and downtime.
Different Types of CNC Milling Processes
CNC milling can achieve various shapes and complex geometries, like flat surfaces, curved contours, and round corners, through various CNC milling processes.
Peripheral (Slab) Milling
In peripheral milling, the cutting tool removes material from the outer edge of the workpiece, creating flat surfaces and flat features like slabs or contours. Peripheral or slab milling is one of the starting processes in milling and can be used to prepare the workpiece for the subsequent milling process.
The axis of rotation of the cutting tool is parallel to the workpiece surface and the periphery (outer edge) of the tool contacts the material. The depth of cut is small and the cutting width is no more than the tool’s diameter.
Cutting tools for peripheral milling: slab mills, side and face milling cutters, and end mills with outer cutting tool geometry.
Applications of Peripheral Milling
Flat surfaces
Shoulders
Slots
Planar areas
Plain Milling
Plain milling is a surface machining operation where a cylindrical cutting tool removes material from the workpiece surface to create a smooth plane on the workpiece. Plain milling can also be thought of as a type of peripheral milling.
The cutting tool rotates parallel to the workpiece cutting surface and produces a flat surface. Plain milling also involves shallow depths of cut, and the cutting width is the same as the tool diameter.
There are two ways to do plain milling: climb milling and conventional milling. Both processes are capable of achieving the required tolerances, but climb milling produces better surface finishes.
Plain milling has many advantages, specifically, it can produce flat surfaces efficientlys
Applications
Machining metal blocks
New metal castings
Machining larger parts and surfaces
Face Milling
Face milling is a CNC milling process where the milling cutter removes material in layers to create a flat surface. The direction of cut is perpendicular to the axis of rotation of the tool. The tool’s bottom interacts with the workpiece, chipping away material from the workpiece. In some cutting processes, even the side edge engages the workpiece.
The material is removed parallel to the workpiece surface, creating a uniform surface. The CNC milling tool used for face milling is a flat end mill with multiple teeth on the bottom and sides. Tools like shell mills, fly cutters and face mills produce smooth finishes in face milling.
Face milling can produce parts with tight tolerances (up to +/- 0.001″) and is a versatile milling CNC machining service that uses CNC program to automatically adjust the cutting depth, feed and speed.
Applications
Flat surfaces
Fine surface finishes
End Milling
End milling is a specific milling operation based on a type of tool called an end mill with round, tapered, or straight end configurations. End milling uses the tool’s side and bottom, depending on the operation (profiling or slotting). Common end mill tools include flat end mills for slots and pockets, ball nose end mills for curved surfaces, and corner radius end mills.
Applications
Slots
Pockets
Contours
3D features in metals and plastics
End milling can perform operations like slotting, pocketing, profiling, and plunging. End mill cutters come in multiple flute designs that can optimize cutting and performance.
Pocket Milling
Pocket milling defines the milling zone within certain parameters and removes material in that enclosed area within a workpiece. It is a type of end milling that can create cavities and is used to quickly clear material from the workpiece.
Pocket milling produces precise and exact features like recesses and mold cavities for industrial production parts. The tool used in pocket milling is generally an end mill and the material is removed in multiple passes in a combination of 3-axis machining to achieve the desired depth and shape.
Applications
Creating pockets in stainless steel or aluminum blocks
Mold making
Creating recesses
Angular Milling
Angular milling is simmiing to other types of CNC milling, except the cutting tool engages the workpiece from an angle relative to the workpiece surface. This means, if the workpiece has a flat plane, the tool can come into contact at an angle between the vertical and the horizontal axis.
Angular milling is used to make angled features on parts and is one of the few processes that can be consistently and repeatedly done on a CNC milling machine, and not a manual milling machine.
Angular milling utilizes several types of milll cutters: single-angle cutters, double angle cutters, and end mills.
Applications
Machining angled surfaces
Machining grooves
Beveled edges
The angled part features in the gears
Molds and dies
3D Contour Milling
3D contour milling can produce complex 3D features in a workpiece. 3D contours follow intricate, precise tool paths generated from computer-aided design and manufacturing processes, which, unlike conventional milling processes, focus on freeform cutting and contoured geometries like curves, slopes, or organic shapes.
3D contour milling removes tiny material and gradually shapes the workpiece. It typically uses a ball nose end mill, which is ideal for smooth, curved surfaces.
In 3D contour milling, the key is the type of toolpath strategy.
Z level contouring: Tool moves in vertical depth layer following 3D shape.
Flowline milling: The tool follows natural curves of the surface for a smooth transition
Helical paths: The tool follows a spiral for a continuous, curved surface.
What Are The Different Types of CNC Milling Machines
There are different types of CNC milling machines, each designed to efficiently perform repeated tasks with accuracy. CNC milling machines, like 3 or 5-axis milling machines, are capable of performing almost all milling operations, even the ones done on vertical and horizontal milling machines.
Vertical Milling Machines
Vertical milling machines get their name from the orientation of the cutting tool. The spindle axis is perpendicular to the workpiece, and the tool moves up and down to remove material from the workpiece.
Vertical milling machines have limited access to the workpiece for cutting complex geometries and intricate designs. Most vertical milling machines are bed mills, where the table moves only in the X and Y axes, while the spindle moves along the Z-axis.
Horizontal milling machines
Horizontal milling machines are similar to vertical milling machines, but the spindle axis and the cutting tool are oriented parallel to the worktable. Horizontal milling machines have a rotary table that makes it possible to achieve multi-sided machining.
Horizontal mills can perform milling operation on large parts and heavy-duty materials like stainless steel and cast iron.
Horizontal mills are large CNC machines that have higher costs and more complex setups.
3- Axis Milling Machines
3-axis CNC milling machines are the standard mills capable of machining small to medium and in some cases large parts. The machine tool operates in 3 linear axes, while the workpiece is fixed. 3-axis milling machines have a user-friendly machine interface and many milling machines can be controlled by non-technical people due to the automatic nature of the machines.
3-axis milling machine is easy to program, affordable and widely used throughout the industry. The only downside to 3-axis CNC milling machines is angular complexity in parts, which the machines cannot cut.
5- Axis Milling Machines
5-axis CNC milling machines can move their machine tool in 3 linear axes (X, Y, and Z) and have the capability to rotate the tool around 2 linear axes (A + B axes), giving the machine tool a total of 5 axes to maneuver.
The tilting machine tool allows the creation of complex geometries like turbines and medical implants. Overall, 5-axis CNC milling machines are a complex machine set that is reserved for specialized tooling. It has high costs, complex programming, and requires a technical expert to operate.
Planer-Type Milling Machines
Planer-type milling machine features a bridge with a rail where the machine tool is suspended. Planer type milling machines can have more than one spindle. The machine tool, in most planer-type mills, can move in the Z-axis, and the worktable can move the workpiece in the X and Y axes. In some planer-type milling machines the tool can move in 3 or more axes.
The use of more than one milling spindle makes planer-type milling efficient to remove material from long workpieces with automatic tool changes.
Universal Milling Machine
A universal milling machine combines the capabilities of both vertical and horizontal milling machines and provides CNC machining for toolrooms, workshops, and small to medium manufacturing processes.
The structure of a universal milling machine has a large base and spindle that can be oriented vertically or horizontally. A universal milling machine has a swiveling table, which can rotate, providing angular machining possibilities.
Most universal milling machines are used to make tool and die geometry, gear cutting and prototyping large parts.
Gantry Milling Machines
Gantry milling machines are large CNC milling machines with a bridge structure and the machine tool and spindle are mounted on a rail that suspends from left to right on the bridge.
Gantry mills are specialized CNC milling machines for large metal structures and parts like ship components, aircraft wings and molds. Gantry mills are expensive to operate but are suitable for heavy-duty machining.
Advantages of CNC Milling
Precision and Repeatability
CNC milling machines can produce identical parts to a certain degree of tolerance. It is capable of achieving tight tolerances (within +/- 0.0001″), which ensures consistent and high-quality parts. CNC machining capabilities of milling operation enable almost any shape to be made with minimal human intervention and chances of error.
High Production Runs
CNC milling machines rely on computer-aided manufacturing to generate tool paths and sequence the operations down to the number of tool passes. This allows designers to create parts exactly how they want by controlling milling cutter movement and parameters like RPM, cutting speed, depth of cut, approach angle, etc.
CNC milling work can be done 24/7 without breaks, which increases production efficiency and scalability of machining operations.
Complex Geometry
CNC milling can produce intricate features and part designs for your workpiece. It is also possible to change part design and adjust the part model at any stage, which allows rapid prototyping and design adaptation.
Capabilities
Modern CNC machining uses CNC milling and a combination of milling and CNC turning called a turn-mill that can produce complex parts and undercuts in rectangular blocks. The capabilities of CNC milling are endless, but the cost of CNC milling increases with the CNC technology and part complexity.
Machines Various Materials
Manual machines for cutting and shaping are bound by material. For example, many manual machines are specifically operated to cut just stainless steel or softer metals like bronze.
A CNC milling machine only requires a tool change to machine different materials, from acrylic to plastics to titanium. For harder materials, you can choose a milling machine capable of high and low RPMs, which gives you the advantage of CNC machining both soft and hard materials.
What Is the Cost of CNC Milling?
The cost of CNC milling depends on the type of CNC machining (3 or 5 axis), material, and scale of production. For larger quantities, the price is averaged over parts, resulting in a smaller price per part.
CNC milling costs also depend on the tolerance required and the complexity of the part.
The hourly machining rates start from a minimum of $40 for 3-axis CNC machining services, and this does not include material, programming, or any special tooling.
Conclusion
CNC milling has changed the CNC machining industry by offering design versatility and flexibility. From CNC routers that can engrave intricate designs to turret mills enabling different machining operations and tool changes, these machines excel in creating parts from stainless steel, titanium, aluminum with excellent corrosion resistance for industries like aerospace and automobiles.
The machine spindle is a key component in the CNC milling machine, powering tools like corner rounding cutters, end mills, flat mills, and many other multi-flute cutters for a smooth finish. Whether producing prototypes of molds for injection molding, CNC milling is ideal for small to medium batch production parts.
Frequently Asked Questions
Q: What is meant by CNC milling?
A: CNC milling uses computer-controlled machines to cut and shape materials like metal or plastic, creating precise parts with complex designs.
Q: Is CNC milling easy?
A: CNC milling requires learning programming fundamentals, CAD and CAM skills and machine setup. Become a CNC machining operator can take months to years.