Computer Numerical Control (CNC) machining is a versatile, high-precision traditional manufacturing process that’s ideal for medium to high-volume production runs of complex parts. CNC machining is a manufacturing technique for removing material. It uses sharp cutting tools, available in a variety of shapes and sizes, which are precisely controlled by a computer. With CNC machine rapid prototyping, manufacturers can rapidly produce prototypes and durable parts for specific applications. 

Material selection is an incredibly important part of any manufacturing process, but engineers and product teams must be especially diligent when it comes to selecting materials for CNC machining. Since this process is compatible with a wide variety of materials — from metal to fiberglass to wood — it’s easy to mistakenly choose a material that’s sufficient for the project but not the best-suited. In this article, we’ll touch on key considerations for CNC machining material selection and give an overview of some common options.

Key considerations for choosing CNC machining materials 

Before getting into the particulars of material selection, product teams must first take stock of their project requirements. Start by considering these four questions: 

1. How will the part be used? 

Engineers must consider their part’s end-use and where it will be used, as these factors will have a significant impact on material selection. For example, stainless steel and carbon steel are both suitable for CNC machining, but only stainless steel is naturally corrosion-resistant. Choosing the wrong kind of steel here would compromise a part’s longevity. FDA, FAA, as well as any industry-specific regulations should also be top of mind. 

2. Does weight matter? 

If a part must be light — a common requirement when manufacturing for automotive and aerospace applications — there are plenty of low-density metal options available. Manufacturers might also consider using a plastic like ABS to keep weight down if they’re willing to sacrifice strength. 

3. Does the part require high strength or heat resistance? 

Strength can be measured in many different ways (tensile strength, wear-resistance, etc.), so engineers should determine what kind of strength — and to what degree it matters — is required for their part. These parameters will impact the size of the pool of viable materials. Similarly, temperature requirements will exclude certain materials from the outset. Engineers should also think about whether their part will be exposed to fluctuating temperatures, as nearly all materials expand or contract in response to temperature changes.

4. What is the project budget? 

Material costs almost always influence material selection. The goal of most product teams is to select a material that maximizes desired material properties and performance while minimizing expenditure.  Material costs are especially important when it comes to CNC machining because this process produces a relatively large amount of scrap material behind. Some materials — such as specialty metals — are extremely expensive, which could have a major impact on the cost-efficiency of the entire project, considering scrap material output. Engineers should carefully evaluate their budget before making any final decisions. 

What Materials Can Be Used in a CNC Machine? 

A better question might actually be what materials can’t be used. CNC machines cut through nearly any material you can think of. While you can use a wide range of materials in a CNC machine, the most commonly employed materials used are: 

Metals: Such as aluminum, brass or steel 

Plastics: Such as Acetal (POM), Acrylics (PMMA), Polycarbonate (PC) and Polypropylene (PP) 

Wood: Such as hardwood, plywood or softwood  

Foam: Such as carving foam and rigid foam, which tend to be more lightweight yet durable. 

Since any material — metal or plastic — that’s hard enough for machining can be used for CNC manufacturing, product teams are spoiled for choice. This short list provides a quick gloss of some of the most common metals  materials used in CNC manufacturing today. 

• Stainless Steel 303 – Perhaps the most important thing to know about stainless steel 303, is that it looks very similar to other stainless steels, so it’s vital to test the material to ensure it actually is stainless steel 303. Other stainless-steel materials can rest very easily, which is not ideal for products. Due to the sulfur used to make the material, stainless steel 303 is not ideal for cold forming or being heat treated. This material is most often used for nuts, bolts, shafts and gears.
• Stainless Steel 304 – Stainless Steel 304, sometimes referred to as 18/8, is the most common form of stainless steel. 304 is very tough, non-magnetic, readily machinable and typically corrosion resistant making it ideal for kitchen accessors, tanks and pipes used in industry, architecture and automotive. It is found in a wide variety of consumer and industrial parts and products. 303 and 304 stainless are almost the identical metallurgy, 303 usually has a small sulfur content to aid with machining and is typically only for round bar stock; whereas 304 is mostly for sheet, plate, and flat stock. These two materials are effectively interchangeable.
• Stainless Steel 316 – Often considered a marine-grade stainless steel, 316 is tough and easy to weld. The material is very corrosion resistant making it ideal for architecture, marine fitting, industrial pipes and automotive purposes.
• Aluminum 6061 – This is the most common general-purpose aluminum grade and its typically used for auto parts, bicycle frames, sporting goods, frames for RC vehicles and more. Similar to other aluminum alloys, it has a good strength-to-weight ratio and is naturally resistant to atmospheric corrosion. While this material has great workability and machinability, it can be pricier than other metals. It also doesn’t perform as well when exposed to salt water or other chemicals, which is something to keep in mind depending on the use of the product.
• Aluminum 7075 – On the higher end of aluminum grades, 7075 is one of the strongest aluminum alloys. Unlike 6061, aluminum 7075 has average workability due to its strength, making it hard to bend into difficult and complex shapes. The material is ideal for high-strength recreational equipment, and automobile and aerospace frames – applications that require a metal that will undergo regular stress. However, it should be avoided in cases where welding is required.
• Magnesium AZ31 – An alloy with aluminum and zinc, magnesium AZ31 is up to 35 percent lighter than aluminum with the equivalent strength. However, this material is typically more expensive and is often used for aircraft components. While the material is easy to machine, it has flammable properties. Due to its light weight and high strength capabilities, it’s also often used for laptop cases, power tools, camera bodies and other uses around the house.
• Carbon Steel 1045 – This strong and tough material is a grade of mild steel, which is often less expensive than stainless steels. The material can be hardened, and heat treated, making it easy to machine and weld. It’s most often used in industrial applications and mechanical parts that require a high degree of toughness and strength such as for nuts and bolts, gears, shafts, and connecting rods.
• Titanium – Perhaps most well-known for its high strength, light weight and toughness, titanium can be easily welded and anodized for protection and appearance. However, it doesn’t polish well and is a poor conductor of electricity, although it is a good conductor of heat. This material is typically more expensive than other metals. It is abundant in the Earth’s crust, but hard to refine. It’s most commonly used for military, aerospace, industrial and bio-medical applications.
• Brass – Brass is commonly found in plumbing fitting, home décor and musical instruments largely due to its soft material and easy workability. Due to Brass’s high polish, it is often found in cosmetic applications and makes for a nice material to use for home décor. It also doesn’t spark when struck with another metal, making it ideal for use in environments that are potentially explosive. Free-cutting brass historically had added lead to help with machining but can now be found without lead. Naval brass is typically much harder to machine than ‘free-cutting’ brass, but it can be polished very highly. It is also naturally resistant to corrosion at sea.

Tap into expert advice on material selection 

This isn’t an exhaustive list, but it’s clear to see that CNC machining is an extremely versatile manufacturing process that requires thorough evaluation of project requirements in order to select a material best-suited for the part in question. Still, achieving the ideal balance of cost, quality, and time can be challenging.

With a trusted on-demand manufacturing partner like GT, material selection is made easy. Our manufacturing experts have years of experience guiding product teams through the entire product development and production processes, starting with design and material selection. Let’s make something incredible together — contact us to get started.