4 Common Workpiece Defects and Their Solutions in CNC Milling

In recent years, global manufacturing has been undergoing a critical transformation phase, with rapid growth in emerging industries like electric vehicles, aerospace, and high-end electronics. For instance, core components of electric vehicles, such as motors and battery casings, require extremely high machining precision and surface quality, which directly affect vehicle performance and safety.

In this environment, CNC milling, as a critical machining process in manufacturing, has attracted significant attention for its stability and reliability. However, during actual production, CNC milling faces numerous challenges, such as inconsistent surface finish on the same surface, tool marks, workpiece surface burns, and burrs. These problems not only reduce product yield, increase production costs, but also affect competitiveness in the global market.

Here, we will analyze these issues in detail, exploring their manifestations, causes, and effective solutions.

Ⅰ. Inconsistent Surface Finish on the Same Surface

（a） Manifestation

On a single machining surface, there are noticeable differences in surface roughness and texture. Areas with good surface finish may appear smooth and mirror-like, while poorly finished areas may have noticeable tool marks, tiny pits, or granular textures. These irregularities may be randomly distributed or linked to the toolpath in some way.

（b）Causes

Vibration-Induced Surface Issues

Tool wear is a common cause of vibration. A dull tool increases deformation of the workpiece before chips detach, resulting in higher cutting forces. As the cutting force increases, the cutting edge digs deeper into the workpiece, causing the chips to suddenly break off and leaving small holes on the surface. This lowers the surface finish, such as when dull end mills are used to process aluminum alloy parts.

CNC Milling Strategy Influence

CNC milling strategies are mainly divided into climb milling and conventional milling. New machinists might unintentionally use both strategies simultaneously during programming, causing the tool to move back and forth. This results in uneven surface finish on the same surface, where one path has a smoother finish and another appears rougher. For example, improper programming might alternate between climb and conventional milling when processing flat parts, leading to visible surface finish differences.

Chip Removal Issues

Inadequate chip removal can cause chips to accumulate in the cutting zone, leading to re-cutting by the tool, which can scratch the workpiece and cause surface finish inconsistencies. Factors such as improper cooling fluid application, poor chip breaker design, and an unsuitable machining environment can affect chip removal.

（c）Solutions

· Tool Management: Implement regular tool inspection and replacement schedules. Use advanced tool condition monitoring technologies, such as acoustic emission sensors or vibration analysis, to monitor tool wear in real time. Select high-quality tools with appropriate coatings and geometries for specific materials.

· Parameter Optimization: Conduct a series of trial cuts to determine the optimal combination of feed rate, spindle speed, and cutting depth. Use machining simulation software to predict surface finish under different parameter settings. Adjust parameters based on material properties, tool characteristics, and workpiece requirements.

· Improving Chip Removal: Optimize the cooling fluid delivery system to ensure effective cooling and chip removal. Consider cooling fluid types, flow rates, and nozzle placement to improve chip evacuation. Additionally, optimize the machining direction and workpiece clamping to facilitate smooth chip removal.

Ⅱ. Tool Marks Left on the Workpiece After CNC Milling

（a）Manifestation

When numerically controlled milling is used to machine parts made of materials such as brass, copper, bronze, or aluminum alloy, sometimes dents may be found at the clamped parts or other areas of the parts. These dents usually have irregular shapes and vary in size. They may be small dot - shaped dents or larger - area indentations.

（b）Causes

Tool Damage and Chipping

During milling, excessive cutting forces, tool material defects, or improper machining conditions can cause the cutting tool to suddenly break or chip. The tool fragments then strike the workpiece surface, leaving marks.

Tool Tilting and Vibration

High-speed milling operations can produce significant cutting forces that may tilt the tool. If the tool tilts beyond its elastic limit, it can leave permanent depressions on the workpiece upon rebound. Additionally, tool vibrations, caused by improper tool holder assembly, spindle imbalance, or resonance, can also lead to marks.

Excessive Clamping Force

An excessively high clamping force can exceed the material’s ability to withstand pressure, leaving depressions on the surface of the workpiece.

（c）Solutions

· Choose Appropriate Tool Materials and Coatings: Select the right tool material for the workpiece, such as cubic boron nitride (CBN) or ceramic tools for high-hardness materials, and carbide tools for softer materials like aluminum alloys. Tools can be coated with TiN to increase hardness and wear resistance, or TiAlN coatings for better high-temperature stability in high-speed cutting.

· Reduce Cutting Forces: Minimize cutting depth, increase feed rates within reasonable limits, and adopt appropriate cutting strategies to reduce cutting forces. Use toolpath smoothing techniques to reduce sudden changes in cutting direction, which can help reduce forces.

· Distribute Pressure: A cost-effective method to mitigate excessive clamping force is to insert a steel plate between the workpiece and fixture to distribute pressure. Alternatively, use specialized soft materials or uniquely designed fixtures that conform to the workpiece surface and distribute pressure or buffer shocks to protect surface quality.

Ⅲ . Workpiece Surface Burns

（a）Manifestation

Surface burns are characterized by discoloration, usually ranging from light brown to black. These areas may also exhibit changes in hardness, becoming harder and more brittle. Burnt regions may form microcracks that affect the mechanical properties and fatigue life of the workpiece.

（b）Causes

The root cause of burns is excessive heat generation. High cutting speeds, deep cuts, and low feed rates produce excessive heat during milling. Once heat is generated, effective heat dissipation becomes critical. Inadequate cooling fluid flow, improper cooling fluid types, or incorrect cooling fluid delivery methods can severely hinder effective heat dissipation.

（c）Solutions

· Control Heat Generation: Use back-and-forth milling strategies to improve heat dissipation.

· Optimize Cooling Fluid: Use advanced cooling fluid delivery systems, such as through-tool cooling or minimum quantity lubrication (MQL), to enhance cooling fluid effectiveness.

Ⅳ. Burrs

（a）Manifestation

Burrs are small, sharp protrusions of material that extend from the edges or surface of the workpiece.

（b）Causes

Material Factors: The ductility and hardness of the material play a key role in burr formation. Softer, more ductile materials like aluminum and copper tend to form burrs more easily due to plastic deformation during cutting. Hard and brittle materials can form burrs due to material fracture at the cutting edge.

Tool Geometry and Wear: The geometry of the cutting tool, including cutting edge angle, rake angle, and tool nose radius, affects burr formation. Tools with larger cutting edge angles or worn cutting edges are more likely to generate burrs. Tool wear alters the cutting mechanism, increasing burr formation.

（c） Solutions

· Choose materials with a lower tendency for burr formation, if possible. For materials prone to burrs, such as aluminum or copper, consider post-process heat treatment or surface hardening. Regularly check tool wear, and use sharp tools with proper cutting edge geometry to reduce burr formation.

Conclusion

By understanding the manifestations, causes, and solutions to these common defects, you can take proactive steps to optimize machining processes, improve tool management, and enhance overall CNC milling efficiency and quality. If you encounter other challenges during milling operations, feel free to contact MINNUO for expert, free consultations!