What materials Cannot be CNC machined?

Before we discuss what materials cannot be CNC machined, we first need to understand the principles and limitations of CNC machining, and then see what materials are not suitable for CNC machining. CNC machining has become one of the core technologies of modern manufacturing due to its high precision, high efficiency and wide applicability. However, not all materials are suitable for machining by conventional CNC processes. The physical or chemical properties   of some materials can cause excessive tool wear, poor machining quality, and even damage to machine tools. This article will explore which materials are difficult or impossible to use CNC machining, and analyze the reasons behind it and possible alternatives.

CNC machining is usually achieved by cutting materials with tools, which mainly depends on two factors. One is the hardness of the tool. The tool must be harder than the material being machined, otherwise it will wear out quickly. The second is material stability: the material must have a certain strength to avoid deformation or fragmentation due to cutting force. If the material cannot meet the above conditions, conventional CNC machining will be difficult to carry out.

1. Superhard materials:

Typical materials: natural diamond, cubic boron nitride (CBN)

Processing difficulties: The hardness of diamond is as high as Mohs 10, and CBN is close to 9, which is much higher than conventional carbide tools (hardness is about 8-9). The tool will quickly become blunt or even break during cutting.

Alternative solutions:

Use electrical discharge machining (EDM) or laser cutting to remove materials using heat or discharge principles.

2. Highly brittle materials

Typical materials: ordinary glass, unsintered ceramics, high-purity graphite

Processing difficulties:

The material is prone to breakage during cutting, and burrs or cracks are generated on the edges, making it difficult to ensure accuracy. For example, glass may shatter instantly under tool pressure.

Alternative solutions:

Use grinding technology (such as precision grinders) or water jet cutting to reduce direct impact.

3. Highly viscous or soft materials

Typical materials: rubber, silicone, soft plastics (such as PE, PU)

Processing difficulties:

The material is highly elastic, and the tool is prone to "pulling" rather than cutting, resulting in deformation or rough surface. Soft plastics may also melt due to frictional heat and stick to the tool.

Alternatives:

Freeze curing: Processing after hardening the material at low temperature (such as cooling rubber with liquid nitrogen).

Use high-speed spindles (above 20,000 RPM) to reduce heat accumulation.

4. High-temperature sensitive materials

Typical materials: lead, tin, PVC plastics

Difficulties in processing:

Low-melting-point metals (such as tin melting point 232 ° C) are easy to soften and stick to the tool during high-speed cutting; PVC releases toxic chlorine gas at high temperatures, which endangers operators.

Alternatives:

Control cutting temperature (such as using coolant), or use 3D printing or injection molding instead.

5. Heterogeneous composite materials

Typical materials: Carbon fiber reinforced composite materials (CFRP), glass fiber

Difficulties in processing:

Material anisotropy leads to uneven cutting force and extremely fast tool wear; carbon fiber may also delaminate, affecting structural strength.

Alternatives:

Use diamond-coated tools and optimize cutting parameters (low feed, high speed).

It is worth noting that although many materials are difficult to process with conventional CNC, they are not absolutely "unprocessable". The following methods can be used to break through the limitations:

Special tools: such as polycrystalline diamond (PCD) tools for processing carbon fiber.

Process improvement: combined with cooling technology (such as cryogenic processing), vibration suppression, etc.

Hybrid process: CNC rough processing first, then EDM or laser finishing.

For example, although nickel-based high-temperature alloys (such as Inconel) have extremely short tool life due to high strength and high thermal conductivity, they can still be processed with special ceramic tools and extremely low feed speeds, but the cost is extremely high.

خاتمة

Although CNC processing is powerful, its limitations also remind us that material properties and processing technology need to be highly matched. When facing special materials, engineers need to comprehensively consider cost, efficiency and quality, and flexibly choose traditional cutting, special processing or additive manufacturing solutions. In the future, with the development of technologies such as superhard tools and intelligent cooling systems, the boundaries of CNC may be further broadened, but "applying technology according to materials" will always be the core logic of the manufacturing industry.