Skip to main content

Essentials of CNC Machining: Material Selection

Close-up photo of parallel-aligned stainless steel bars with a reflective metallic surface.The Importance of Material Selection in CNC Machining

Selecting the right material is as crucial as any design or engineering decision. Materials are not just the substance of the final product but a cornerstone upon which the success of the machining process rests. From predominant metals such as aluminum, brass, copper, and steel to exotic materials like Inconel and Monel, along with versatile plastics and composites, each material brings its unique properties and challenges to the table. In this introduction to materials, you’ll discover a vast world where entire disciplines, like Material Science, are dedicated to exploring the significance of material selection in CNC machining and how it affects the final product’s quality, functionality, and suitability for its intended application.

Material Properties

The first step in material selection is understanding the properties that each material possesses. These properties, which are crucial to the performance and longevity of the final product, include but are not limited to:

  • Strength: This is a fundamental property that measures the ability of a material to withstand mechanical forces without undergoing deformation. Strength can be further categorized into:
    • Tensile Strength: The maximum stress a material can withstand while being stretched or pulled before failing. It is typically measured in megapascals (MPa) or pounds per square inch (psi) using tensile tests.
    • Yield Strength: The stress at which a material begins to deform plastically. Beyond this point, the material will not return to its original shape when the load is removed. Yield strength is crucial for determining the load a material can handle before it begins to deform permanently.
    • Compressive Strength: The capacity of a material to withstand loads tending to reduce size. This is especially relevant for materials used in construction and manufacturing that must bear weight without collapsing.
  • Machinability: How easily a material can be cut into the desired shape with satisfactory finish quality. Machinability assesses the ease of cutting, the wear on the tool, and the quality of the finished surface. It is rated on a scale or index that compares the material’s performance to a standard, typically free-cutting steel.
  • Thermal Resistance: The ability of a material to maintain its properties and appearance under high temperatures. Important parameters include melting point, heat capacity, thermal expansion coefficient, and thermal conductivity. These metrics are essential for evaluating a material’s behavior in high-temperature applications.
  • Corrosion Resistance: The ability of a material to resist damage caused by oxidation or other chemical reactions. Corrosion resistance is determined through tests that expose the material to corrosive environments, measuring effects like weight loss, penetration rates, or thickness reduction.
  • Hardness: A measure of a material’s resistance to surface deformation, indentation, or scratching. Hardness is often assessed using tests such as Rockwell, Brinell, or Vickers, which involve indenting the material with a specific force and measuring the size or depth of the indentation. Hardness is a critical factor in wear resistance and durability.
  • Brittleness: Brittleness describes a material’s tendency to fracture or fail upon the application of stress without significant deformation. It is a crucial consideration for applications where impact resistance and flexibility are important. Brittleness is often inversely related to ductility, as materials that are more brittle are less likely to deform before breaking. This property is particularly relevant in selecting materials for structural components, safety equipment, and any application where sudden failure could lead to catastrophic outcomes.

Understanding these properties and how they are measured can guide the material selection process, ensuring that the chosen material meets the specific requirements of the CNC machining project and the final product’s intended application.

Types of Metals

Industrial material storage area with an array of metal bars, tubes, and rods in various sizes, organized on vertical storage racks, with a forklift parked on the left.

Metals are the most commonly used materials in CNC machining, prized for their strength, durability, and conductivity. Each type offers unique properties suited for various applications. There are far too many to provide a comprehensive list.  Entire university undergrad degrees are offered at prestigious schools like MIT or UC Berkley to study just this:

  • Aluminum: Lightweight and with excellent machinability, aluminum is ideal for aerospace, automotive, and consumer electronics. It offers good thermal and electrical conductivity. Common types include:
    • 6061: Combines strength, corrosion resistance, and machinability. Used in aerospace and marine.
    • 7075: Known for superior strength, used in aerospace and military.
    • 2024: High strength and fatigue resistance for aerospace structures.
    • 5052: Excellent corrosion resistance, suitable for marine applications.
    • 5083: High resistance to saltwater corrosion, used in marine and automotive.
    • 6063: Often used in architectural applications for its smooth surface.
    • 1100: Commercially pure, with excellent ductility and corrosion resistance.
  • Steel: Renowned for strength and durability, used in construction, automotive, and industrial applications. Types include:
    • Carbon Steel (1018, 1045, 1080): Ranges from low (1018) to high (1080) carbon content. 1018 is easily machined and welded, 1080 is very hard and used for cutting tools, and 1045 offers a balance of strength and ductility.
    • Alloy Steel (4140, 4340): Adds elements such as chromium, nickel, and molybdenum for improved properties like strength, hardness, and resistance to corrosion.
    • Stainless Steel (304, 316): Containing significant chromium offers corrosion resistance. 304 has excellent weldability. 316 has enhanced resistance to chloride corrosion.
    • Tool Steel (M2, D2): Used in cutting tools such as drills, taps, dies, and molds. M2 retains its hardness at high temperatures, and D2 is known for wear resistance.
  • Titanium: Offers the highest strength-to-weight ratio, making it perfect for aerospace and medical implants. Types include:
    • Grade 2 (CP Titanium): Commercially pure, used in aerospace and marine.
    • Grade 5 (Ti-6Al-4V): Widely used in aerospace for its strength and lightness.
    • Grade 23 (Ti-6Al-4V ELI): Extra low interstitials, for medical implants.
    • Grade 7: Added palladium for chemical processing applications.
    • Grade 9 (Ti-3Al-2.5V): High strength and corrosion resistance.
    • Grade 12 (Ti-0.3Mo-0.8Ni): For chemical and marine applications.
  • Brass: Known for its machinability and acoustic properties, used in decorative items and musical instruments. Types include:
    • C36000 (Free-Cutting Brass): Common for precision machined parts.
    • C26000 (Cartridge Brass): High ductility for deep drawing.
    • C28000 (Muntz Metal) and C46400 (Naval Brass): For marine and architectural.
    • C38500 (Architectural Bronze) and C37700 (Forging Brass): For decorative and complex shapes.
  • Copper: Outstanding electrical and thermal conductivity. Types include:
    • ETP Copper (C11000): Pure copper for electrical uses.
    • Oxygen-Free Copper (C10100): For high-tech applications.
    • Phosphorus-Deoxidized Copper (C12200): For plumbing and roofing.
    • Beryllium Copper (C17200): For electrical connectors and aerospace.
    • Cupro-Nickel (C70600, C71500): For marine applications.
  • Bronze: Valued for strength and corrosion resistance. Types include:
    • Tin Bronze (C90300, C90500, C90700): For bearings and marine applications.
    • Aluminum Bronze (C95400, C95500, C95800): High strength and resistance.
    • Silicon Bronze (C65500): For marine hardware and welding.
    • Manganese Bronze (C86300): For high load-bearing parts.
    • Phosphor Bronze (C51000): For electrical connectors and springs.
  • Inconel: Nickel-chromium-based superalloys for extreme environments. Types include:
    • 600, 625, 718: For chemical processing and aerospace.
    • X-750: High strength at temperatures up to 1500°F.
    • 725: For oil and gas extraction in sour gas environments.
  • Monel: Nickel alloys with strength and seawater corrosion resistance. Types include:
    • 400 and K-500: For marine engineering and chemical processing.
    • R-405: Free-machining version for valve parts and fasteners.
    • 404: For the electronics industry, offering low magnetic permeability.

Types of Plastics

Plastics represent a diverse and adaptable category of materials that bring together innovation, functionality, and design freedom. Unlike metals, plastics can be engineered and formulated to meet specific performance criteria, making them indispensable in industries ranging from healthcare to consumer electronics. With a wide range of physical properties, plastics allows for the creation of intricate and detailed components that would be challenging or impossible to manufacture from metal.

  • ABS (Acrylonitrile Butadiene Styrene): ABS is a popular thermoplastic due to its high toughness, impact resistance, and easy machinability. It’s often used for automotive components, consumer electronics casings, and prototypes because of its excellent finishing options and strength.
  • Polycarbonate (PC): Known for its outstanding toughness, clarity, and thermal resistance, polycarbonate is used in applications requiring transparency and high impact resistance, such as bulletproof windows, eyewear lenses, and medical devices.
  • PEEK (Polyether Ether Ketone): PEEK is a high-performance thermoplastic with exceptional mechanical, chemical resistance, and thermal properties, making it suitable for aerospace, medical implants, and automotive components. It’s particularly valued in applications that require longevity under harsh conditions.
  • Nylon (Polyamide): Nylon is renowned for its wear resistance, low friction coefficient, and good temperature resistance. It’s used for mechanical parts like gears, bearings, and fasteners, as well as in textile manufacturing.
  • PTFE (Polytetrafluoroethylene): Best known by the brand name Teflon, PTFE offers unparalleled chemical resistance and a very low coefficient of friction. It’s used in applications requiring non-stick surfaces, such as cookware, or in industrial settings for seals and gaskets.

Types of Composites

Composites open a new chapter in the evolution of materials used in CNC machining, offering a synergy of strength and lightness that metals alone cannot provide. These advanced materials harness the best attributes of their components, resulting in engineered solutions that are both robust and surprisingly lightweight.

  • Carbon Fiber-Reinforced Plastic (CFRP): Combining carbon fibers with a polymer matrix results in a material that’s incredibly strong and lightweight, with excellent fatigue resistance. CFRP is used in high-performance applications like racing cars, sports equipment, and aerospace components.
  • Fiberglass: Fiberglass is a composite material made from woven glass fibers bonded with a resin. It’s less expensive than carbon fiber and offers a good balance of strength and weight, making it suitable for boat hulls, automotive bodies, and building materials.
  • Kevlar-Reinforced Composites: Kevlar, known for its use in bulletproof vests, is also used in composites for its high tensile strength and resistance to impact. It’s used in protective gear, high-performance sports equipment, and aerospace for its durability and lightweight properties.
  • Graphene Composites: Although relatively new in the market, graphene composites offer unprecedented strength, conductivity, and thinness. They’re being explored for use in electronics, energy storage devices, and advanced coatings.

Both plastics and composites represent the forefront of material innovation in CNC machining, providing options that surpass traditional materials in certain applications. Their versatility, combined with advancing CNC technology, opens up new possibilities in design and functionality, from lightweight aerospace components to customized medical implants, showcasing the evolving landscape of manufacturing and engineering.

Machining Strategies for Optimal Quality

Each material requires specific machining strategies to optimize the quality of the final product. Factors such as cutting speed, feed rate, and tool choice must be tailored to the material’s properties to minimize defects, reduce wear on tools, and achieve the desired surface finish.

Metals:

A close-up action shot of a CNC machine's drill bit actively drilling into a metallic surface, with coolant fluid splashing and metal shavings scattered around the drill site.

  • Aluminum: Due to its relatively soft nature, aluminum allows for higher cutting speeds and more aggressive feed rates compared to harder metals. However, using sharp, polished tools can prevent material adhesion to the tool edges, enhancing surface finish and tool life. For example, machining aluminum 6061 might involve high-speed milling techniques with carbide tools to achieve excellent surface quality efficiently.
  • Steel: Steel, particularly alloy steels like 4140 or tool steels, requires careful consideration of cutting speeds and feed rates to avoid tool wear and achieve precise finishes. The use of coolant is often essential to dissipate heat and extend tool life. Tools coated with titanium nitride (TiN) or titanium carbonitride (TiCN) can offer the durability needed for cutting through tough steel grades.
  • Titanium: Known for its strength and heat-resistance, titanium challenges machinists with its tendency to adhere to cutting tools. Low cutting speeds, high feed rates, and ample coolant flow are key strategies to prevent tool wear and overheating. Tools made from cubic boron nitride (CBN) or coated with physical vapor deposition (PVD) coatings are often recommended for their hardness and thermal stability.
  • Brass: Brass is known for its excellent machinability, making it a favorite for detailed and intricate parts. The material’s softness allows for high cutting speeds and low tool wear, but sharp, uncoated carbide or high-speed steel (HSS) tools are recommended to achieve a clean finish. Coolants can be used but are not always necessary; instead, compressed air may be sufficient to remove chips and keep the cutting area clear.
  • Copper: Copper’s high thermal conductivity and ductility mean that it can quickly become gummy during machining, potentially leading to built-up edge on the cutting tool. Slow to moderate cutting speeds, sharp tools, and a continuous, non-interrupted cut are crucial to prevent work hardening and to achieve a smooth surface finish. Polycrystalline diamond (PCD) tipped tools are often used for their wear resistance and ability to maintain a sharp edge.
  • Bronze: Bronze varies widely in hardness and machinability depending on its alloy composition. Generally, it requires moderate cutting speeds and feed rates to balance between machining efficiency and minimizing tool wear. Tools with a positive rake angle are effective for reducing cutting forces and improving chip evacuation. Using coolants can help in extending tool life and preventing overheating, especially when machining harder bronze alloys.
  • Inconel: Inconel alloys are extremely hard and abrasive, leading to rapid tool wear and difficulty in machining. Low cutting speeds, heavy, consistent feed rates, and rigid tooling are necessary to manage the work hardening and high heat generated during machining. Tools coated with titanium aluminum nitride (TiAlN) or using ceramic inserts are preferred for their ability to withstand the high temperatures without losing hardness.
  • Monel: Monel, like Inconel, poses challenges due to its tendency to work harden and its high strength. Moderate to low cutting speeds with heavy feed rates help to cut below the work-hardened layer. Carbide tools are recommended for their hardness and resistance to the abrasive wear Monel can cause. The use of a water-soluble coolant is beneficial for both cooling and lubrication, helping to prevent tool build-up and ensuring a smoother finish.

Plastics:

  • ABS and Polycarbonate: These plastics can be machined at relatively high speeds, but they require sharp tools to prevent melting or chipping at the edges. Single-flute or two-flute end mills and drills can efficiently remove material while minimizing heat buildup. Implementing cool air blasts instead of traditional coolants can help maintain the integrity of the plastic during machining.
  • PEEK: This high-performance plastic requires careful temperature management to prevent melting during machining. Using tools with high rake angles and low cutting forces helps minimize heat generation. Coolant use should be controlled, as PEEK can absorb liquids, potentially affecting its material properties.

Composites:

  • Carbon Fiber-Reinforced Plastics (CFRP): Machining CFRP demands tools with diamond or polycrystalline diamond (PCD) coatings to withstand the abrasive nature of carbon fibers. Low-speed milling and drilling, combined with high feed rates, can prevent delamination and ensure the integrity of the composite material. Specialized tool geometries, such as compression routers, are effective in reducing fraying and splintering at the material surface.
  • Fiberglass: Similar to CFRP, fiberglass is abrasive to cutting tools. The use of carbide or diamond-coated tools, along with controlled cutting speeds, helps maintain surface quality and tool longevity. The direction of the cut relative to the fiber orientation can significantly impact the quality of the machined surface, with strategies developed to cut perpendicularly to fibers where possible to reduce delamination.

Optimizing machining strategies for each material not only involves adjusting cutting speeds, feed rates, and tool selection but also requires a holistic approach that considers the material’s behavior under machining stresses. Understanding these dynamics allows machinists to push the boundaries of what’s possible in CNC machining, leading to higher quality products, reduced manufacturing times, and lower costs. Tailoring these strategies to the unique challenges presented by each material ensures that the final products meet the rigorous standards of precision, durability, and aesthetic appeal required in today’s competitive markets.

The Role of Price in Material Selection

When selecting materials for CNC machining projects, price is a critical factor that cannot be overlooked. The cost of materials directly influences the overall budget of a project, affecting both the manufacturing process and the final product’s marketability. It’s essential to balance material properties with cost-effectiveness to ensure a project’s financial viability.

Budget Constraints and Material Choices: Budget limitations often dictate the choice of material. While exotic metals like titanium or Inconel offer superior properties, their high cost may not be justifiable for all projects. Materials such as aluminum or brass, on the other hand, provide a cost-effective alternative with a favorable balance between performance and price.

Volume and Economies of Scale: The quantity of parts required can significantly impact the material selection process. Bulk purchases of material can reduce costs, making higher-priced materials more accessible for large-scale projects. Conversely, for small, custom projects, the material cost becomes even more critical, as economies of scale are not in play.

Long-Term Considerations: The initial cost of a material is just one part of the equation. It’s important to consider the lifespan of the product, maintenance costs, and the material’s impact on operational efficiency. In some cases, investing in a more expensive material that offers durability and lower maintenance costs can be more economical in the long run.

Alternatives and Substitutes: Exploring alternative materials or substitutes can offer cost benefits without compromising significantly on quality or performance. Advances in material science frequently introduce new materials that mimic the properties of more expensive options at a fraction of the cost.

Negotiating with Suppliers: Establishing strong relationships with material suppliers can lead to better pricing, discounts, and access to premium materials within budget constraints. Knowledgeable supply chain managers can negotiate favorable terms that directly influence the cost-effectiveness of material selection.

Price plays a pivotal role in the material selection process for CNC machining projects. A thorough understanding of how material costs affect the project budget and final product pricing is essential for making informed decisions. By carefully considering cost alongside material properties, manufacturers can optimize their projects for both performance and economic efficiency, ensuring that the selected material meets the project’s technical requirements without exceeding budgetary limitations.

Conclusion

The choice of material in CNC machining is a complex decision that impacts every aspect of the production process, from design and machining strategies to the final product’s performance and application. Understanding the properties of various materials and how they respond to machining is critical for engineers and designers to make informed decisions. By carefully selecting the right material and adapting machining strategies accordingly, manufacturers can optimize product quality, meet application requirements, and push the boundaries of what is possible with CNC machining.

 

Contact Us

Request A Quote