CNC Milling: A Detailed Explanation of the Process

An Overview of the CNC Milling Process

CNC milling is a computer-controlled process and a specific type of CNC machining that involves cutting a block of material mounted on a machine bed with different rotating multi-point cutters to achieve the desired shape and feature. Since the CNC mills use rotary tool cutters, this CNC machining process is compatible with various CNC materials, including metal, plastic, and wood. The automated CNC controls dictate the sequence of CNC tools, the spindle RPM, and the feed rate to the toolpath of each cutter.

Machinists use different types of CNC milling machines in machining processes since they offer varying levels of complexity. Various tools are compatible with the dynamic nature of the milling process, ranging from standard end mills to special face milling and profile milling tools. Consequently, product engineers and machinists can achieve almost any design or shape with a CNC milling machine since there are many types of CNC mills and cutters.

Common CNC Milling Methods

Various milling methods have distinct procedures and applications that make them ideal for different manufacturing tasks. Here are standard CNC milling operations:

Angular Milling

The angular milling method cuts a workpiece at an angle relative to its surface. It can create angled features such as grooves, dovetails, chamfers, V-blocks, and other angular shapes. Machinists often employ angular milling when machining components for gears and machinery that demand precise angles.

Face Milling

The face milling method creates large, flat surfaces with high-quality finish on the workpiece. It offers a superior surface finish than other milling methods. The milling cutter engages the workpiece’s surface perpendicularly, moving across it to cut material. These face milling cutters have teeth on the tool face and periphery. Machinists use face milling to prepare the workpiece’s surface for further machining.

Form Milling

This standard milling operation involves using CNC milling tools with specific profiles to cut curved or irregular surfaces. Machinists often use fly cutters or milling cutters for specific applications. Concave cutters or corner rounding cutters are other machining tools used to cut intricate designs like hemispherical or semi-circular designs. However, the cutter’s shape determines the workpiece’s final profile. Form milling is ideal for machining complex shapes and profiles like screw threads, cam shapes, and gears.

Plain Milling

This is a milling operation where the cutting tool’s axis of rotation is parallel to the workpiece’s surface. The plain milling cutters have sharp edges on the periphery that execute the cutting operation. Machinists often use narrow and wide cutters, which allow for deeper cuts and larger surface areas to be machined. Also, fine-toothed and coarse cutters are suitable for plain milling. Fast cutting speeds and slow feed rates are used for fine-toothed cutters, while slow cutting speeds and fast rates are ideal for coarse cutters, producing a more detailed final part.

Horizontal Milling

Conversely to vertical milling, the spindle engages the workpiece horizontally in horizontal milling. However, all other horizontal CNC milling machine components are similar to those of vertical mills. Horizontal mills are typically ideal for handling heavier or longer workpieces including gears and splines.. A horizontal machining center is best suited for heavy-duty manufacturing processes where considerable material removal rates is necessary.

Vertical Milling

With vertical milling, the work surface is a 3-axis milling table below the arm to which a spindle is fastened. When vertical turret mills are used, the spindle can be stationary. However, the table only moves along the X-axis when using a bed vertical milling machine. The spindle moves along the arm’s length toward the Y-axis.

Other common milling operations are:

• Straddle Milling: This method uses two milling cutters to mill parallel surfaces of a workpiece in a single cut.
• Gang Milling: This cutting process is the fastest because it involves using two or more cutting tools mounted on a machine arbor simultaneously to execute the same operations on a single workpiece. Although the shape and sizes of milling cutters may vary, gang milling increases the production rate.
• Slot Milling: Slot milling makes a slot or hole in a workpiece. Also, the width of the cutter used in this milling method is less than that of the workpiece.
• Side Milling: Side milling creates a flat vertical surface in machined parts. You can rotate the vertical feed screw on the worktable to control the depth of the cuts.

Stepwise Guide to the CNC Milling Process

Various steps are involved in the CNC milling process, from transforming a CAD model into a physical part. These include:

Making a CAD Model

Designing a 3D CAD model for the intended part is the first step in this production process. The CAD model comprises details pertaining to the shape, geometry, and size of the part to be fabricated. Product designers need to make CAD models capable of being cut by the CNC milling machines. Using the CAD software, designers create a 2D or 3D model blueprint of their design idea.

Converting CAD to CNC Program (G-code)

Once you have developed a compatible 3D CAD model for a specific part, you must create a CNC program to control its fabrications on the CNC milling machine. With CAM software, the product designer can generate program instructions for the CNC machine to direct the cutting tool’s movement during manufacturing.

These programs consist of G-code or M-code. The G-code aspect of the CNC program governs the machine tool’s operating parameters like the direction of movement, cut depth, and spindle speed. M-code dictates miscellaneous tasks, including powering the CNC machine on and off, making tool changes, and performing other auxiliary functions. CAM software has a simulator that lets users confirm their CNC programs’ capability to fabricate the intended part successfully.

Setting up the Milling Machine

The CNC milling machine is ready to fabricate the designed CNC parts once the CAD model and CNC program are ready. Before the CNC milling begins, a machine operator must import the CNC program into the machine and insert a pre-prepared blank with the proper pre-machining dimensions into the machine’s work-holding device. The specific CNC tools, fixtures, vices, and spindles are programmed in the machine.

Executing the CNC Program

The CNC milling process can be executed once you have developed the CAD model, the designed CNC program, and prepared the CNC machine. Human intervention is seldom required when a CNC program is running. The CNC mill will follow the program line by line, executing all specified machining operations on the raw material. Once the full program has been completed, the part can move on to the next planned manufacturing phase.

What Is a CNC Milling Machine?

A CNC milling machine creates precise components with superior quality and a high level of detail. These CNC mills use rotary tools guided by instructions from a CAD file to remove material from a blank to achieve the desired product design. The process often requires minimal human input since the milling machine’s computer controls the machine’s function and tool cutter’s coordinates. Milling machines can handle the production of many components from different materials.

CNC milling machines fabricate intricate final parts with 3, 4, or 5 axes. The number of axes a CNC machine has determines its ability to handle complex, intricate parts and features. Manufacturers often employ a more complex machine like universal milling machine due to its greater flexibility. This machine has both vertical and horizontal spindle perfect for handling projects requiring high precision, like aerospace and medical components, to achieve the desired final design, aesthetics, and surface finish.

• 3-axis Machines: The 3-axis milling machines are the most common CNC mills typical for handling simple geometries. They are easy to program and operate, resulting in higher accuracy at a low cost. A 3-axis CNC mill can cut material vertically (Z-axis) in the X and Y directions. However, undercuts are intricate features that cannot be achieved with a standard end mill like the 3-axis CNC mills.
• 4-axis Milling Machines: These CNC mills are more sophisticated than the standard 3-axis mills due to the allowed workpiece rotation on the A-axis. Hence, 4-axis CNC mills are ideal for machining parts that need cutting around a cylinder or the side of a piece.
• 5-axis Milling Machines: These are the most sophisticated milling machines best suited for handling highly complex parts designs such as titanium pieces, aerospace components, medical devices, and implants. 5-axis milling machines support movement along the three linear axes, the machine head, and the tool head rotation. The multidimensional rotation of 5-axis mills eliminates the need for multiple machine setups, allowing single-step machining for faster and higher production.

Basic Parts of CNC Milling Machines

Although the parts of different CNC mills differ based on the type, capacity, and manufacturer. These parts help to understand how the CNC milling processes work.  A standard CNC mill comprises these parts:

• Knee: The knee is an adjustable component attached to the column. It can be raised or lowered on the Z-axis. Its primary purpose is to provide support for the saddle and the worktable.
• Column: The column is a principal support structure for the milling machine. It reinforces other parts within the milling machine.
• The Saddle: This feature is placed over the knee and beneath the worktable. It can travel parallel to the spindle’s axis and move the workpiece horizontally if necessary.
• The Worktable: This is the section above the saddle where the workpiece is mounted. Depending on the type of milling machine, the worktable of a milling machine can be adjusted and used horizontally or vertically.
• Spindle: The machine spindle is a rotating component of the milling machine that holds either the arbor or the machine tool. An electric motor drives the spindle on a milling machine.
• Arbour: This is found in horizontal mills. It is passed into the spindle and functions as a shaft that can be used to attach different CNC machine tools.
• Ram: The ram is often used in angular milling or vertical milling machines. It is attached to the column’s top, where the spindle receives support.
• Machine Tool: This includes cutting tools like end mills that execute the milling operation. The spindle holds the machine tool and cuts material from the solid block. There are different machine tools used for milling operations.
• Automatic Tool Changers: An automated tool dispensing system that improves efficiency during machining.
• Machine Interface: This is where the machine operator communicates with the computer. It usually comprises a display screen and keyboard.

Types of a Milling Machine

Milling machines are of various types, each with distinctive features and functions. Here are some of the common types of milling machines:

• Ram-type: The knee-type milling machine permits maneuvering the machine tool on the XY axes through a spindle fastened to a moveable arm on the column. Horizontal milling machines are typical ram-type machines.
• Bed-type: Bed-type machines have the workpiece clamped below the cutting tool. The machine tool can move along the XYZ axes.
• Knee-type: These milling machines adjust the workpiece vertically instead of the spindle. The knee raises and descends the worktable along the column to approach the machine tool. Machinists often perform plain milling on a knee-type milling machine.
• Planner-type: The planner-type milling machines fix the worktable along the YZ axes with spindles that can move along the XYZ axes. Also, these milling machines can accommodate about four machine tools.

Advantages of CNC milling

The CNC milling process is a robust, subtractive manufacturing process with many benefits. Here are some of the expected benefits of the CNC milling method:

High Production Output

Unlike manual machines, CNC mills operate at higher levels, enabling greater production levels. CNC milling is a reliable and standard method for fabricating parts in mass volume, with each part having the same level of quality and finish. Moreover, CNC mills, especially 3-axis milling machines, are often easy to program and operate and achieve high accuracy at a reduced cost.

Guaranteed High Quality and Precision

A high level of accuracy and precision is the inherent nature of CNC milling, leaving little to no room for error. Since the process operates on a CNC program, it inputs 3D designs developed with CAD (computer-aided design) software, allowing the milling machine to execute these instructions that the machining program supplies without requiring manual input. These automated CNC milling machines offer tight tolerances required in medical, automotive, and aerospace fields. Therefore, machine operators can technically handle the most finite and complex geometry.

Reduced Labor

The milling process is less labor intensive since CNC machines are computer-controlled. A CNC milling tool can spin at thousands of RPM at total capacity, increasing production output and time-saving. Moreover, the automation of the milling process reduces human errors as in manual processes, and there is no need for specially skilled machinists to manage the machines.

Scalability

The CNC milling process allows businesses of every size to scale production depending on consumer demand. They can either increase production for large-scale runs or decrease production when handling one-off custom projects or low-volume productions. CNC milling is suitable for mass-producing identical parts since machining costs significantly reduce as the number of units increases.

Extensive Material Compatibility

CNC milling is a versatile mechanical machining process applicable to various materials, including wood, glass, plastics, and metals. Irrespective of the intended application, CNC milling capabilities can match the chosen material’s needs. The process exhibits minimal effect on raw material properties.

Challenges of CNC Milling Technology

Manufacturers usually encounter various difficulties when fabricating parts with CNC milling. Some of the typical limitations of CNC milling processes include:

• Purchasing and setting up CNC machines can be a significantly high initial investment. The higher the sophistication of CNC mills, the more expensive they are.
• CNC milling is not entirely free from human error since machine operators can input incorrect programs or set up wrong machine tools, resulting in bad parts.
• CNC mills require a high level of maintenance for optimum performance. Although these machines are expensive, operational costs increase due to the regular maintenance and calibration to ensure their precision and peak functionality.
• Limitation in the size and geometry of milled parts is another CNC milling challenge. While the size of the CNC machine limits the CNC part’s size, the tool’s ability to physically make the desired cut limits the geometry. Undercut, and other features recessed beneath a CNC part’s external surface can only be machined with unique tools or multi-axis machines.

Contact AT-Machining for Reliable CNC Milling Service

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Conclusion

Thanks to the advancement in digital technology, CNC milling is a reliable and efficient method that can fabricate very complex parts with tighter tolerances from different materials. Manufacturers across electronics, medical, robotics, automotive, and defense industries utilize milling machining to create custom-designed components with superior finish and quality.