What Materials Cannot Be CNC Machined? The Most Engineers Miss

CNC machining is a subtractive manufacturing process widely used to produce complex parts. From automotive-grade aluminum and titanium alloy components to the plastic parts we use every day, CNC-machined parts are an integral part of our lives. However, CNC machining is not a one-size-fits-all solution. This article will explore the materials that are not suitable for CNC machining.

Limitations of CNC Machining in Terms of Materials

CNC stands for "Computer Numerical Control" and involves using high-speed rotating cutting tools to mill and drill metal. This subtractive process places fundamental demands on the material’s rigidity and heat resistance.

For example, if a material is too rigid, it can accelerate tool wear and compromise cutting stability. This not only drives up costs rapidly but also affects the machining accuracy of the parts.

During CNC milling, friction between the workpiece and the cutting tool generates heat. If the workpiece itself lacks sufficient heat resistance, it may deform due to thermal expansion. Since CNC machining parts typically have strict precision requirements, this outcome is unacceptable.

Materials not suitable for CNC machining

Materials that are not suitable for CNC machining are primarily limited by their physical properties, such as high elasticity and composite nature. In this section, I will introduce several types of materials and explain why they are not suitable for CNC machining.

Highly Elastic Non-metallic Materials

These materials offer excellent flexibility and resilience and are commonly used in industrial seals, vibration isolation, and soft consumer goods. Typical examples include silicone, soft polyurethane, and various thermoplastic elastomers (TPE).

LVMA CNC processes these materials using die-cutting or injection molding.

Cause: During CNC machining, these materials tend to stretch outward or deform when exposed to rapidly rotating cutting tools, resulting in rough, uneven machined surfaces and making it difficult to achieve the desired tolerances.

High-Hardness Brittle Materials (Ceramics and Glass)

Although these materials possess excellent heat resistance and chemical stability, their crystalline structures are extremely brittle and lack the ductility found in metals. Common examples include various industrial ceramics, quartz, decorative porcelain, and tempered glass that has been heat-treated to increase its strength. Compared to CNC machining, these materials are better suited for processes such as waterjet cutting and ultrasonic machining.

Cause: Once mechanical cutting forces from CNC machining are applied, microscopic cracks can easily form on the surface, leading to chipping or complete fragmentation of the entire piece. Additionally, the material’s high hardness can cause the cutting tool to wear out prematurely.

Carbon Fiber Composites

This is a new type of lightweight structural material created by bonding high-stiffness carbon fibers with a resin matrix. It is frequently used in aviation, racing, and high-end sports equipment where weight and strength are critical. This material is more commonly processed through mold pressing and laser cutting.

Cause: Carbon fiber exhibits extreme abrasion resistance when CNC milling, causing tools to dull rapidly. Furthermore, machining often results in delamination and tearing between layers, and the resulting fine conductive dust poses a health hazard to operators.

Extremely Soft and Sticky Metals

Due to their internal atomic structure, these metals exhibit extremely high ductility and very low compressive hardness. Because they have good fluidity in their molten state, we typically use investment casting. Well-known examples include lead, tin, and high-purity solid gold. If the goal is to produce thin-walled products, stamping can be used.

Cause: When CNC tools perform high-speed cutting, these metals not only fail to form clean chips but, due to heat and pressure, become sticky and adhere to the cutting edge like a thin paste. They may even deform immediately when clamped in vise grips or fixtures because they cannot withstand the clamping force.

Low-Density Porous Foams

The defining characteristic of these materials is that their interiors are filled with numerous air pockets. Although they have a large volume, their overall structural rigidity is extremely low, making them highly compressible. Common examples include foam sponge and polystyrene foam. We typically recommend wire EDM, laser cutting, or custom die-cutting for higher cutting efficiency.

Cause: When a CNC tool attempts to cut this material, the compression causes it to collapse inward continuously. Once the tool is withdrawn, the material rebounds, resulting in completely uncontrollable cut dimensions. In severe cases, localized melting may occur due to frictional heat generated by high-speed rotation.

Non-homogeneous Materials

These materials typically exhibit significant variations in physical density and hardness across different sections due to natural growth patterns, rough processing methods, or contamination from recycled materials. Examples include natural logs with knots, recycled plastics containing unknown foreign matter, and laminated panels with unevenly applied adhesive layers. For such materials, it is generally recommended to use traditional manual woodworking machinery for slow-speed processing. However, wood materials with uniform texture can be surface-machined using CNC.

Cause: When a CNC spindle is moving at high speed, if it suddenly cuts from a low-density area into a high-hardness knot or foreign object, the cutting force will surge instantly. This unpredictable impact can easily cause severe machine vibration, or even directly result in tool breakage or equipment damage.

Conclusion

Selecting the appropriate material is critical for CNC machining. If the material is unsuitable for CNC machining, it can lead to a series of issues, including excessive tool wear, prolonged machining times, personal injury, and failure to achieve the intended design results. Choosing the right material for your part is the first step in CNC machining.

If you have an ongoing project and are unsure which material to choose, click the button below to submit an inquiry. LVMA CNC's engineers will immediately analyze your part and provide you with appropriate material recommendations.

FAQ

What are the limitations of CNC machining?