12 Types of Milling Operations: A Detail Explanation

The CNC milling process is one of the most prominent methods in manufacturing. This process utilizes various materials and shapes them into desired parts or products. However, a CNC milling machine can perform numerous types of milling operations to handle intricate designs.

Fundamentally, each operation involves removing the materials from a workpiece using milling cutters attached to the movable spindle. However, the tools and spindle movement differ from one type to another.

This article will discuss different milling operations in detail, elaborating their advantages and applications. By the end, you will be able to choose the right milling process for your project.

How CNC Milling Works?

The working of each type of milling in machining involves designing the printed part, converting the design into G and M codes (instructions that the CNC machine understands), tooling & setup, and execution of the process.

Let’s see how the CNC milling machine executes milling operations using its components and milling tools.

Component	Description
Machine Interface	A control panel that takes the instructions for milling as G-code and allows the control of milling parameters, like speed, cutting depth, feed rates, and machining coordinates.
Spindle	The attached chuck or holding device in the spindle holds the machining tool, which can move across the workpiece by connected electrical motor and mechanical bearings.
Work Bed or Table	It is a flat surface with T-slots or other fixtures to mount the workpiece using clamps or vices. The work table provides workpiece stability during the milling.
Column	A sturdy structure that provides vertical support to the spindle mechanism. It makes the spindle stable while moving along the z-axis for machining.
Saddle	It supports the movement of the work bed and provides stability while adjusting the work bed position (workpiece position).
Arbor	An arbor holds one or many cutters simultaneously (extend of spindle).
Cutting Tools	Tools that remove the material from the workpiece with its sharp cutting edges (or points). The tools are generally made with hard materials like tool steel and carbide. For example, end mills, slot drills, ball cutters, etc.

Moreover, choosing the types of CNC milling operations according to 3D design ensures the optimal result. For instance, face operations smooth surfaces, while thread operations precisely cut threads. This ensures that each specific task is addressed with the most suitable technique.

A Brief Overview

The application diversity of CNC milling technology comes from its diversity in the capability of executing the different milling operations to address countless machining requirements, from a slot to intricate undercuts. We will discuss 12 of them later. Before that, let’s see a short overview of those operations.

Milling Operation	Description	Advantages	Applications
Face Milling	It flattens workpiece surfaces.	High removal rate and smooth finish.	Cylinder heads, heat sinks.
Plain Milling	Makes the workpiece flat and creates contours.	Uniform removal; light machining.	Outer layer machining.
Side Milling	Shapes sides of the workpiece.	Forms flat profiles, and grooves.	Suspension mounts and medical implants.
Straddle Milling	Mills two parallel surfaces.	Efficient parallel slotting.	Jigs, fixtures, and gears.
Gang Milling	Uses multiple cutters for complex features.	Multiple operations simultaneously.	Engine blocks and transmission housing.
Angle Milling	Cuts at specific angles.	Precision angular cuts.	Chamfers, T-slots.
Form Milling	Creates irregular contours.	Custom milling for intricate shapes.	Turbine blades and orthopedic implants.
End Milling	Feeds work into end mill.	Intricate profiles, and smooth finishes.	Detailed machining tasks.
Saw Milling	Large cutter for slots and parting-off.	Effective slotting	Slots, dividing workpieces.
Gear Milling	Specialized for gears.	High accuracy on gear teeth.	All gear types.
Thread Milling	Cuts internal and external threads.	Useful for large holes.	Engines, assembly-required products.
CAM Milling	Milling for CAM components.	Precise material removal.	CAMs in mechanical systems.

Types of Milling Operations Based on Workpiece Geometry

This categorization is about the type of geometry the milling can create. It could be a flat surface, angular features, threads, irregular shapes, or specialized geometries like gears or mechanical CAM.

1. Face Milling

This operation flattens the workpiece surface. As part of vertical milling processes, the cutters used in this method have an axis of rotation perpendicular to the surface, aligning the tool’s face parallel to the workpiece. Consequently, the cutter’s sharp teeth remove material, while the end face smooths the surface.

One prime advantage of this method is its high material removal rate, attributed to its tool geometry. This allows for rapid part production with a smooth finish. Face machining is particularly effective for creating flat surfaces in applications such as automotive cylinder heads and heat sinks.

2. Plain Milling

This operation is specifically for machining flat surfaces and contours on the workpiece. Using cylindrical cutters with straight or helical teeth, it removes materials to create flat or stepped surfaces, slots, and recesses. The cutter’s axis aligns parallel to the workpiece surface.

Plain milling is noted for its consistent material removal capabilities among various machining operations. It is ideally suited for light machining or finishing touches. For instance, it first strips material from the outer layer of larger workpieces, setting the stage for further machining processes.

3. Side Milling

As the name says, it refers to machining a side of the workpiece using side milling cutters or end mills. The vertical and horizontal milling machines can perform this task. During the operation, the lateral edges of the rotating cutter (side teeth) remove material to form flat vertical surfaces, edges, grooves, slots, complex contours, fins, and more. The cutter features helical flutes specifically designed to target the sides of the material, in contrast to operations that focus on the top surface, such as plain machining.

This technique is particularly useful in a variety of applications, including creating automotive suspension mounts, machining channels or slots in aircraft components, producing molds for injection or casting, fabricating medical implants, and constructing heat sinks for electrical and electronic appliances.

4. Straddle Milling

This method stands out due to its capacity to simultaneously machine two parallel surfaces on a workpiece, maintaining uniform distances between them.

How does straddle milling manage to machine two faces in a single setup? It employs two or more side cutters mounted on a single arbor that remove chips from the material surface during the feed. As a result, this operation is ideal for creating parallel slots, grooves, and various profiles on a workpiece.

Application Examples: Jigs, fixtures, brackets, levers, gears, sprockets, transmission & axle housings for automotive, etc.

5. Gang Milling

Why is it called gang milling? Because it involves multiple cutters—such as end mills, form cutters, and slab mills—mounted together on a single arbor to create complex features and intricate details. This setup allows for various operations to be performed simultaneously on a workpiece, resulting in reduced machining time and increased efficiency.

Application examples;

• First, any milling application requires multiple features, such as slots, channels, grooves, and flat surfaces.
• Automotive engine blocks and transmission housing.
• Fames, brackets, gears, sprockets, and housings for numerous types of machinery and tools.
• Die prototyping and manufacturing.

6. Angle Milling

Angular features in CNC machined parts are often achieved through angle milling. In this operation, the cutter’s axis is angled relative to the machining surface, while the workpiece is held in the required orientation using an angle plate or fixtures.

The cutters precisely machine the workpiece at specific angles, which can be fixed such as 45°, 60°, or 75°. This method is used for creating features like slight chamfers, bevels, T-slots, dovetail slides, and other intricate geometric profiles.

7. Form Milling

Do you know how CNC machines create irregular contours? This is achieved through form milling, which involves using cutters shaped as the negative of the desired contour. For instance, to machine a turbine blade, a cutter is first made with a geometry that mirrors the blade’s shape. This method allows for precise and custom shaping of complex designs.

When the cutter rotates, it traverses across the workpiece, or the workpiece moves under the cutter. Then, the cutter’s edges cut away the material to create the desired geometry.

Application Examples: Dome-topped pistons, orthopedic implants, turbine blades, die-making, guitar body, custom prototypes, etc.

8. End Milling

It is one of the most popular ones among different types of milling operations. The end mills execute this operation by feeding the work into it in a perpendicular or angled direction. Meanwhile, the multiple cutting edges on the faces and periphery of the end mill remove the material while feeding the workpiece.

The end milling is extremely favorable in the machining conditions where you need: An intricate profile, precise edges, slots, and grooves with varying depths, and a smooth finish on the workpiece surface.

9. Saw Milling

Saw milling is executed by a large cutter with teeth around its circumference. This method is particularly useful for creating narrow slots and dividing the workpiece into two parts. The cutter moves downward, slicing through the material with continuous feeding.

However, Saw milling operations are generally slower than other methods due to the large size of the cutter, which can generate significant heat during rapid material removal, potentially causing thermal damage to both the work material and the cutter.

Moreover, CNC saw operations extend beyond just slots and parting-off tasks typical of traditional machines. They are also capable of producing other geometries, such as slight curves or profiles.

10. Gear Milling

Gear milling is a specialized operation used to refine manufactured gears to precise dimensions and surface roughness, or to create detailed tooth profiles. For instance, it can refine an extruded bevel gear to achieve a surface roughness of Ra 1.2 µm. In some cases, it can also manufacture a complete gear from scratch.

The tools used in gear milling include gear cutters, gear hobbing machines, and form mill cutters. These tools enable highly accurate shaping of gear teeth, surpassing the precision offered by other gear generation techniques.

Moreover, this operation can handle almost every type of gear, regardless of shape and complexity, such as spur gears, bevel gears, helical gears, and rack and pinion setups.

11. Thread Milling

As the operation name, it is for cutting the internal and external threads in the workpiece. For internal threads, an insert tool chips away the material from the existing hole’s surface to form the threads. In contrast, it can easily produce the desired external threads using the right type & size of thread mill cutters or thread mills. Provided that, thread milling is more useful for the internal thread on the large-size holes.

The thread milling parts can be found across industries, from automotive engines to consumer products. Everywhere for assembly and other purposes.

12. CAM Milling

The CAMS are essential components in different mechanical systems and machinery for converting linear motion into rotational or vice versa. Meanwhile, the CAM milling operation produces these components with a diving head tool. This tool facilitates the rotation of the workpiece to position it so the tool can remove the surface materials according to the designed CAM profile.

Different Types of CNC Milling Operations Based On Milling Mechanisms

There are three primary variations of operations on milling machines: manual, CNC, and conventional & climb.

1. Manual Milling

In manual operations, the operator sets the workpiece and tools in the machine by hand. Milling parameters such as depth of cut, RPM, and feed are also manually adjusted.

Control of the cutter movement is manual, allowing the operator to cut the workpiece in the desired direction and orientation to shape it accordingly. Additionally, adjustments to the milling bed can be made using a handwheel. While manual operations are more time-consuming and less precise, they provide flexibility in terms of cost and customization.

2. CNC Milling

CNC milling is the most precise and speedy operation because of automation in the machining process. The computer control of tool movement and workpiece positioning eliminates the chance of errors and reduces the time significantly. Almost every operation is possible with a CNC milling machine, although the right tools or cutters are necessary for any particular operation.

The latest multi-axes machines (3 to 6 typically) enable intricate shapes & details on the workpiece with greater precision. Consequently, the applications are wider than any other milling, from automotive and aerospace to medical components.

3. Conventional Milling and Climb Milling

Fundamentally, these two operations involve different feeding mechanisms. In conventional milling, the tool (or cutter) rotates opposite the feeding direction. In contrast, in climb milling, the tool rotates along the feed direction.

Feature	Conventional Milling	Climb Milling
Surface Finish	Relatively rougher	Smoother and higher quality
Tool Wear	Higher  due to rubbing action	Lower  due to shear cutting
Material Removal	Lower	Higher
Materials	Softer materials	Hard and consistent materials

Furthermore, two other popular techniques or operations are vertical and horizontal milling based on the spindle orientation. For in-depth understanding, you can read Horizontal vs. Vertical Milling here.

How to Choose the Right Milling Operations?

Do you know how many operations are needed to create an automotive gearbox? The answer is many, including face milling, end milling, and gear milling. It is just an example showing that different operations cumulatively create a final part. It applies to almost every project.

Therefore, only correct operations choices according to the 3D model guarantee the project’s success. For that, consider the factors that influence the result of various types of operations performed on a milling machine.

Material Type

First, the work material type significantly impacts the choices of types of cnc milling operations as variations in the properties difference directly impact the material removal rate and tool wear.

Thus, the operation you wish to use should be compatible with properties like hardness, thermal conductivity, and toughness. For example, conventional milling is challenging to shape hard materials like tool steel.

Desired Finish

The surface finish requirements are another consideration in milling operation selection for your Design. This is because each operation produces a surface with different roughness levels. So, select the operation that can achieve the Ra value needed for the functionality and aesthetic of the final product.

Milling Operation	Roughness Ra Value (μm)
Face Milling	0.8 – 3.2
End Milling	0.8 – 6.3
Slot Milling	1.6 – 6.3
Thread Milling	1.6 – 3.2
Gear Milling	1.6 – 3.2

Geometric Complexity

Not all milling techniques can create complex and intricate details on a workpiece. Some operations are suited only for simple profiles and shapes, such as plain, face, and slot techniques. On the other hand, operations like end, thread, and gear are renowned for their ability to produce complex parts and components.

Another thing in design complexity consideration is whether you can optimize the tool path with the chosen operation as needed or not.

Consider Machine Parameters and Settings

The machine parameters such as spindle speed, feed rate, and cutting depth are crucial in selecting the right milling operations. The reason is that these parameters affect the finish, accuracy, and production speed. Additionally, the machine settings like axis movement capabilities, work holding, and rigidity should be also compatible with the milling operation you want to execute.

Select the Right Cutter for Different Milling Operations

Not all cutters are compatible with all milling operations. Each type of operation has its own set of compatible cutters, and using the wrong type can result in subpar results or even damage the tool and workpiece.

Furthermore, specificity is crucial even after selecting the operation. For example, when choosing a cutter for end machining, options include flat end, ball nose, or corner radius, each suited for different applications.

Conclusion

Complex shapes and features in CNC milling are only achievable by selecting the appropriate operations for your project. Each operation offers unique advantages and capabilities for creating specific geometries. Moreover, advanced machines and a suitable set of cutting tools are essential for performing these diverse operations effectively. Only then can the exact desired results be achieved.

At RapidDirect, you can leverage all the latest CNC milling machines and experience handling milling projects. Our CNC Machining and CNC Milling Services encompass design optimization, all milling operations, and even surface finishing for the milled part. Furthermore, a dedicated quality control team ensures you get the parts with the exact specifications you need. So, request a quote and kickstart your CNC milling Project with RapidDirect!

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