Aluminium, Steel, Brass or Plastic? How to Choose a Material for CNC Parts

Material choice decides how strong, light, durable and expensive your CNC part will be, often more than any other decision on the drawing. This guide compares the materials most often machined for industrial parts and includes a grade-equivalence table for buyers sourcing from India, the USA and Europe.

Start with what the part has to do

Before picking a material, write down what the part actually needs. Most decisions come down to a few questions:

• Strength and stiffness: what loads, impacts or pressures will it see?
• Weight: does every gram matter, as in aerospace, robotics or handheld products?
• Environment: will it face moisture, salt water, chemicals or high temperatures?
• Wear and friction: does it slide, rotate or mate with other parts?
• Electrical or thermal behaviour: does it need to conduct, or to insulate?
• Regulations: are there food-contact, medical or industry requirements?
• Budget and quantity: material and machining cost both matter at volume.

Aluminium: the all-rounder

Aluminium is the most widely machined metal for good reason. It is light, machines quickly, resists corrosion and takes anodizing well.

• 6061-T6 is the general-purpose choice: good strength, excellent machinability and weldability. It suits brackets, housings, fixtures and enclosures.
• 6082-T6 is the European workhorse. It is similar to 6061 with slightly higher strength, and it is often the default grade for European drawings.
• 7075-T6 is a high-strength alloy used in aerospace and high-load parts. It is stronger than 6061 but less corrosion resistant and generally not welded.

Stainless steel: corrosion resistance and hygiene

Stainless steel is chosen when parts must resist corrosion, stay clean or keep their strength at higher temperatures. It is harder to machine than aluminium, which raises cost.

• 303 is a free-machining stainless steel, quick to machine for fittings, shafts and fasteners, but with lower corrosion resistance and poor weldability.
• 304 is the most common stainless steel, with good general corrosion resistance for food equipment, enclosures and industrial parts.
• 316 / 316L contains molybdenum for better resistance to chlorides and chemicals. It is the choice for marine, chemical-processing and many medical applications.
• 17-4 PH is a precipitation-hardening grade that can be heat treated to high strength while keeping reasonable corrosion resistance.

Carbon and alloy steel: strength at low cost

Where corrosion is not a concern, or the part will be coated, carbon and alloy steels offer high strength at a lower material price than stainless.

• Mild and medium carbon steels (such as EN8 / C45 / 1045) are economical for shafts, pins and general mechanical parts.
• Alloy steel 4140 (42CrMo4, often specified as EN19 in India and the UK) can be heat treated for high strength and toughness. It suits gears, axles and heavily loaded components.

Carbon steels rust, so they usually need a protective finish such as zinc plating, black oxide or painting.

Brass and copper: machinability and conductivity

Free-machining brass is one of the easiest metals to machine. It produces excellent surface finishes and suits fittings, valve parts, electrical connectors and decorative components. Copper is chosen for its outstanding electrical and thermal conductivity, as in busbars and heat-sink parts, but it is soft and gummy, which makes it slower to machine well.

Titanium: strength without the weight

Titanium alloys such as Ti-6Al-4V combine high strength, low weight and excellent corrosion resistance, and they are biocompatible. They are used in aerospace and medical implants. Titanium is expensive and slow to machine, so it is normally reserved for parts where its properties are essential.

Engineering plastics: light, quiet and insulating

• POM (acetal, often known by the brand name Delrin) is stiff, low-friction and dimensionally stable. It is ideal for precision plastic parts such as gears, bushings and guides.
• Nylon (PA6) is tough and wear resistant, but it absorbs moisture and can change size slightly, so it is less suited to very tight tolerances.
• PEEK is a high-performance plastic for high temperatures and harsh chemicals, used in aerospace, oil and gas and medical devices. The raw material is costly.
• PTFE has excellent chemical resistance and very low friction, but it is soft and holding tight tolerances is difficult.
• Acrylic (PMMA) and polycarbonate are used for clear covers, light guides and display parts.

Quick comparison

Ratings are a general guide for comparison; exact properties depend on grade, temper and heat treatment.

Grade equivalents: India, USA and Europe

International projects often mix naming systems. One drawing says 304, the supplier's stock list says 1.4301, and a local standard calls it something else. The table below shows common close equivalents. Always confirm the exact standard with your supplier for critical parts.

Five tips for specifying material on your drawing

1. Give the full grade and temper, such as "6061-T6", not just "aluminium".
2. Name the standard (ASTM, EN or IS) when it matters, especially for export parts.
3. Say whether equivalents are acceptable. This speeds up sourcing and can lower cost.
4. Request material test certificates (for example to EN 10204 3.1) when traceability is required.
5. Match the finish to the material: anodizing for aluminium, passivation for stainless steel, plating or black oxide for carbon steel.

Conclusion

For most industrial parts, aluminium 6061 or 6082 is the sensible starting point. Move to stainless steel for corrosion and hygiene, alloy steel for high loads, brass for easy machining and conductivity, titanium only where its properties are essential, and engineering plastics for light, low-friction or insulating parts. Material also has a big effect on price, as explained in what drives CNC machining cost.

Not sure which grade fits your application? Share your drawing and requirements and our engineers will recommend a suitable material through our CNC machining services. Request a quote.