How to Choose CNC Machining Materials for Your CNC Milling Applications
Introduction
One of the first and most critical choices when CNC machining parts is selecting the optimal material. The workpiece material impacts key factors from machinability and tool wear to dimensional stability, strength, and cosmetic appearance.
With the wide range of materials available for CNC milling, deciding on the right one for your application can be challenging. Properties like hardness, rigidity, chemical resistance, friction, thermal behavior, damping, and cost all come into play depending on functional requirements.
This article provides a detailed overview of common CNC machining materials, their properties, and how to select the ideal option whether your goal is wear resistance, strength-to-weight ratio, precision dimensions, or another critical performance metric.
Machining Metals
Metals make up a significant portion of CNC machined components across industries like aerospace, automotive, medical, and more. Here are popular machining metals and their notable qualities:
Aluminum
Known for its lightweight, aluminum is highly machinable and can be categorized into alloys like 6061, 7075, 2024, 356, etc. Pure aluminum grades offer softness and ductility but lower strengths. Alloying improves hardness, strength, and temperature resistance, though some series like 7075 are more difficult to machine.
Steel
From mild low-carbon steels to hardened tool and alloy steels, various steel grades provide higher strength than aluminum but with added weight. Low-carbon steels offer easy machining. Tool steels like D2 and A2 provide excellent hardness after heat treatment but require rigidity and precision to avoid tool chatter. Stainless steels add corrosion resistance.
Titanium
With a high strength-to-weight ratio and corrosion resistance, titanium alloys like Ti-6Al-4V are valued in aerospace applications but are challenging to cut and expensive. Titanium’s low thermal conductivity also demands careful tooling selection and operating parameters to avoid overheating.
Inconel/Superalloys
Superalloys like Inconel, Waspaloy, and Hastelloy are nickel, cobalt, or iron-based alloys with impressive high-temperature properties. However, their strength and abrasive nature lead to extreme tool wear during machining.
Brass and Bronze
Copper alloys like brass and bronze offer appealing aesthetics, some antimicrobial properties, and easier machining than steel. Leaded brass varieties enhance machineability. Copper alloys allow intricate detail with excellent surface finishes.
Exotics
Exotic alloys are complex metal blends with exceptional properties like the high-temperature strength of Inconels or cobalt chrome alloys. Components like turbine blades push the limits of exotic alloys. However, their performance comes at the cost of extensive tool wear during machining.
Plastics
Engineering thermoplastics are the most commonly CNC machined plastics thanks to their melt processability. Here are popular choices:
ABS – An amorphous plastic with good strength, ABS is easily machined and fitted but has lower chemical and heat resistance.
Acetal – Also known as POM or Delrin, acetal homopolymer provides toughness, low friction, and some dimensional stability, but can be prone to chipping.
Nylon – With high wear resistance, low coefficient of friction, and some flexibility, nylon is stronger than acetal but can be more abrasive to cut.
Polycarbonate – Optically clear polycarbonate has very high impact resistance but requires sharp tools to avoid gumming when machining.
Peek – An expensive but high-performance choice, Peek withstands temperatures over 300°C, and resists chemicals, and machines smoothly.
Machining Wood and Wood Composites
Wood naturally offers an attractive finish but can present issues like tool clogging and tear-out. Still, CNC milling expands possibilities for shaping solid woods and wood composites:
Hardwoods – Oak, maple, and ash have density for furniture and fixtures. Teak’s natural oils lubricate cutting but can shorten tool life. Machining along wood grain minimizes splintering.
Softwoods – Pine, cedar, spruce, and others require more delicate machining to avoid tear-out but offer cost savings.
Plywood – Built of laminated wood layers, plywood provides stability and fast production but edges may need finishing.
MDF – Made from wood fiber and glue composite, medium-density fiberboard has consistent machining with smooth edges but emits fine dust.
Considering Material Properties
Analyzing design requirements and operating conditions helps identify suitable materials for the task at hand. Here are key factors to consider:
Hardness and Wear Resistance
Harder materials withstand force and abrasion but quickly wear cutting tools. Ensure adequate rigidity and use new sharp inserts. Softer metals like aluminum cut easily and prolong insert life but lack durability.
Strength-to-Weight Ratio
Lower-density materials like aluminum, magnesium, and titanium alloys reduce component weight. Strength-to-weight ratio impacts aerospace, automotive, and consumer product designs.
Thermal Properties
Metals conduct and dissipate heat during machining while plastics insulate. Materials also expand at different rates when heated. This influences dimensional accuracy and tool pressures.
Corrosion Resistance
Some alloys, including stainless steel and titanium, resist oxidation and chemical attack. Others require protective platings or coatings if exposed to corrosive environments in use.
Part Accuracy
Metals hold tighter tolerances than plastics that creep under load. Precision aerospace parts demand metals while plastics suit consumer products. The stability of the raw material stock and rigidity during machining also factor into achievable accuracy.
Surface Finish Requirements
Most plastics can achieve glossy, molded-like finishes. Many metals polish to fine finishes but some alloys like magnesium are prone to galling and work hardening.
Cost Considerations
Exotic alloys, engineering-grade plastics, and some hardwoods have higher material costs. Soft alloys like 6061 aluminum, brass, and acetal plastics provide cost savings for high production volumes. Evaluating total manufactured part cost and production volumes influences decisions.
Recyclability
Metals hold the advantage over plastics for sustainability. Copper and aluminum retain excellent recyclability. Engineers increasingly consider end-of-life factors.
Best Practices for Material Selection
Here are some best practices when considering materials for a CNC machining application:
- Thoroughly analyze operating loads, life cycle forces acting on the component, temperatures, exposure to chemicals or grease, precision needs, assembly interfaces, potential for corrosion/galling, and other service conditions the part will encounter. This reveals ideal material property targets.
- Research the range of material options capable of meeting functional requirements and property goals using resources like CES EduPack. Compare relative merits and downsides. Get samples to evaluate firsthand if possible.
- Discuss options with your machining partner early in the design phase to assess manufacturability, including machinability, tooling considerations, scrap rates, and capabilities around secondary operations often needed like plating, heat treating, surface finishing, etc. An experienced machinist will provide practical guidance.
- Evaluate tradeoffs like higher performing materials against cost targets and production volumes. Sometimes an adequate lower-cost substitute meets the need.
- Don’t overspecify properties beyond what conditions actually require. This avoids unnecessary material expense.
- Consider supply chain factors like material availability, especially for exotic or specialty grades. Check procurement lead times.
- Run machining simulations if possible to predict and resolve potential work-holding or tooling issues before committing to a material. Digital twins help assess feasibility.
Case Study: Aerospace Bracket Material Selection
Let’s examine material selection for an aircraft engine mounting bracket. Heat resistance, high strength, lightweight, and long fatigue life are critical. 6061 aluminum could provide adequate strength if generously sized but has lower heat resistance. Titanium offers strength and temperature resistance but at a high cost. Inconel displays good elevated temperature strength and fatigue life. However, Inconel’s extreme hardness necessitates tooling upgrades to avoid premature failure during machining. The added machining costs outweigh material savings relative to titanium. In this case, titanium’s ideal balance of properties aligns with functional requirements despite higher raw material pricing. Thorough analysis pointed to the optimal machining material.
Conclusion
Choosing the right material sets the stage for CNC milling success. Metals, plastics, woods, and composites each carry distinct properties suitable for different applications. Hardness, strength, thermal behavior, precision requirements, corrosion and wear resistance, aesthetics, sustainability, supply availability and cost all come into play. Collaborating early with experienced machinists ensures manufacturability. Materials will continue advancing, providing engineers with more options to balance performance and manufacturability through careful selection.