While modern CNC (Computer Numerical Control) machining is a versatile and powerful manufacturing process capable of shaping a vast range of materials, certain substances are considered effectively unmachinable using conventional methods. This is less about absolute impossibility and more about practical limitations where a material’s properties—such as extreme hardness, excessive brittleness, a “gummy” softness, or hazardous nature—make it unsuitable for subtractive manufacturing. Key examples include technical ceramics, fully hardened tool steels, pure elastomers like rubber, and materials that produce toxic dust, which often require alternative processing methods like grinding, molding, or laser cutting for successful fabrication.

What Factors Define a Material’s Machinability?

Understanding  why certain materials resist CNC machining is crucial for successful product development. The challenge isn’t arbitrary; it’s rooted in fundamental physics and material science. At Hirung, our engineering expertise begins with analyzing these core properties to guide our clients toward the best manufacturing outcomes. Several key factors determine if a material can be efficiently and safely machined.

Extreme Hardness and Abrasiveness

Perhaps the most intuitive barrier to machining is hardness. When a workpiece material is harder than, or nearly as hard as, the cutting tool (typically made from high-speed steel, carbide, or diamond), the tool cannot effectively penetrate and shear the material. Instead of creating a clean chip, the tool will rub, generate immense heat, and wear down at an accelerated rate. Abrasive materials, like some composites filled with glass or carbon fibers, act like sandpaper on the cutting tool, dulling its edge almost instantly. This leads to poor surface finishes, dimensional inaccuracies, and a constant need for expensive tool replacement, making the process economically unviable.

Excessive Brittleness

Unlike ductile materials that deform and shear cleanly, brittle materials do not bend before they break. When a cutting tool applies force to a highly brittle substance like glass or ceramic, the stress causes micro-fractures that propagate unpredictably. Instead of a controlled chip, the material shatters, cracks, or chips away, making it impossible to achieve precise dimensions or a smooth surface finish. The final part is often structurally compromised and fails to meet any functional or aesthetic requirements. This is why materials like technical ceramics are typically shaped through grinding or casting processes rather than traditional milling or turning.

Extreme Softness and Lack of Rigidity

On the opposite end of the spectrum, materials that are too soft or flexible also pose significant machining challenges. Substances like pure silicone, soft rubber, or low-density foams lack the rigidity to resist cutting forces. Instead of being sheared, they deform, compress, or tear. The material may wrap around the cutting tool, melt due to friction, or result in a fuzzy, irregular surface. Holding these materials securely in a fixture is also difficult, as clamping pressure can distort their shape, leading to inaccurate results. Specialized techniques like cryogenic (freeze) machining can sometimes work, but often, processes like injection molding or die-cutting are far more suitable.

Hazardous or Reactive Properties

The safety of our machinists and the integrity of our equipment are paramount. Some materials produce byproducts that are dangerous. For example, machining certain plastics or composites can release toxic fumes or dust that require specialized ventilation and personal protective equipment (PPE). More dramatically, fine dust from materials like magnesium can be highly flammable or even explosive under the right conditions. Others, like beryllium, produce highly toxic particulate matter that poses severe health risks if inhaled. Machining these materials is not undertaken without extensive safety protocols, dedicated equipment, and often, regulatory compliance, placing them in the “practically unmachinable” category for most shops.

Economic Impracticality and Cost-Effectiveness

Finally, some materials might be *technically* machinable but are economically impractical. If a material requires extremely slow cutting speeds, causes excessive tool wear, and has a high rate of part failure, the cost per part can skyrocket. In these cases, even if a part can be produced, the time and expense involved make it an unfeasible option for prototyping or production. A key part of engineering is finding the sweet spot between material performance and manufacturing cost. At Hirung, we help our clients evaluate these trade-offs to select a material that meets their design needs without breaking their budget.

A Deep Dive: Which Materials Present the Greatest CNC Machining Challenges?

With the “why” established, let’s explore the specific material categories that challenge the limits of CNC machining. These are the substances our engineers carefully evaluate before proposing a manufacturing plan.

Category 1: The Extremely Hard and Brittle

• Technical Ceramics (Alumina, Zirconia, Silicon Carbide): These materials are valued for their extreme hardness, heat resistance, and electrical insulation. However, their brittleness makes them prone to catastrophic fracture under the stress of a cutting tool. They are almost exclusively shaped through diamond grinding, lapping, or formed in a green (unfired) state before being sintered.
• Fully Hardened Tool Steels: While steels are a staple of CNC machining, once they are heat-treated to their maximum hardness (typically above 65 HRC), they become incredibly difficult to cut with conventional tools. Machining is typically done *before* the final hardening process for this reason.
• Diamond and Tungsten Carbide: As the hardest known materials, these cannot be machined by conventional means. Diamond, being the ultimate abrasive, can only be cut or shaped by other diamonds, lasers, or specialized grinding. Tungsten carbide, used to make cutting tools themselves, is shaped via grinding or Electrical Discharge Machining (EDM).

Category 2: The Overly Soft and Flexible

• Pure Elastomers (Rubber, Silicone): These materials stretch and deflect under the cutting tool, resulting in tearing rather than a clean cut. Their high coefficient of friction also generates significant heat. Molding is the standard manufacturing method for these parts. While some harder durometer urethanes can be machined, it requires special expertise.
• Soft, Low-Density Foams: Similar to elastomers, these materials compress and tear. Securing them without distortion is a major challenge, and the resulting finish is often poor. Hot-wire cutting, waterjet cutting, or molding are better alternatives.
• Gummy or Sticky Materials: Certain plastics or pure metals like copper can be very “gummy,” leading to built-up edge (BUE) on the cutting tool. This phenomenon, where workpiece material welds itself to the tool tip, degrades the cutting action and ruins the surface finish. It requires careful selection of tool geometry, coatings, and coolants.

Category 3: The Hazardous and Unstable

• Untreated Magnesium: While magnesium is a light and machinable metal, its fine chips and dust are highly flammable. Dry machining of magnesium is a significant fire hazard, requiring specialized flood coolant systems, Class D fire extinguishers, and rigorous cleanup protocols. Many shops refuse to machine it due to these risks.
• Beryllium and Specific Alloys: Beryllium dust is a known carcinogen and highly toxic, causing a chronic, life-threatening lung disease. Machining it requires isolated environments with sophisticated air filtration and handling procedures, placing it beyond the capabilities of all but a few specialized facilities.
• Materials with Toxic Dust: This includes certain composites, especially those containing asbestos (now largely banned), and some phenolic resins that can release harmful compounds when heated during cutting.

Machinability on a Spectrum: “Difficult” vs. “Impossible”

It is critical to view machinability not as a binary state but as a spectrum. Many materials listed are not truly “impossible” to machine but fall into the “extremely difficult” category, requiring specialized equipment, tooling, and deep process knowledge. The decision to machine them is a trade-off between the desired outcome and the associated cost, time, and risk.

The table below summarizes the challenges and alternative approaches for these demanding materials:

How Hirung Navigates Complex Material Challenges

At Hirung, we pride ourselves on being more than just a service provider; we are your manufacturing partner. While we acknowledge the limitations discussed, our deep expertise and advanced technology allow us to successfully machine materials that others find challenging. We have extensive experience with difficult-but-machinable materials like superalloys (Inconel, Monel), titanium, and a wide range of engineering plastics (PEEK, Ultem).

Our process begins with a thorough consultation. When a client presents a project with a challenging material, our engineers don’t just say no. We ask the right questions: What is the application? What specific properties are essential? Is there an alternative material that offers similar performance but superior machinability? This collaborative approach ensures that your final part is not only manufactured to the highest standards of precision and quality but is also produced in the most efficient and cost-effective way possible.

If your project involves a complex material, don’t let a “maybe” stop you. Contact the engineering team at Hirung today to leverage our expertise and find a viable manufacturing solution.

Conclusion: The Right Approach to Material Selection for CNC Machining

In conclusion, the question “What materials cannot be CNC machined?” has a nuanced answer. While a short list of materials like technical ceramics and pure elastomers are practically unmachinable by conventional methods, many others simply present a higher degree of difficulty. The true barrier is often a combination of a material’s physical properties, safety risks, and the economic feasibility of the process.

The key to success lies in knowledge and collaboration. By understanding the fundamental challenges of hardness, brittleness, softness, and safety, you can make more informed material choices early in the design phase. Partnering with an expert machining service like Hirung provides access to the experience and technology needed to push the boundaries of what is possible, turning potential manufacturing roadblocks into successful, high-quality components.