CNC Materials: How To Choose The Right Materials for CNC Machining

One of the great advantages to using CNC machining is its versatility. That’s because precision CNC milling and turning works successfully with a very wide variety of raw materials to produce finished parts. This gives design engineers many options when it comes to creating prototypes and commercial products.

Most CNC turned and milled parts are made from metal. This is because metal is strong and rigid and can withstand the rapid material removal caused by modern tools.Let’s first take a look at the most common metals used for CNC machining.

Common Metal Materials for CNC Machining

In this section, you will learn the various common metal materials that are valuable for CNC machining. We’ve listed these materials below.

Aluminum 6061

This is the most common general purpose aluminum used for CNC machining. The main alloying elements are magnesium, silicon, and iron. Like all aluminum alloys it has a good strength-to-weight ratio and is naturally resistant to atmospheric corrosion. Other advantages of this material are that it has good workability and CNC machinability, can be welded and anodized, and its wide availability means that it’s economical.

When heat-treated to a T6 temper, 6061 has a considerably higher yield strength than annealed 6061, although the price is slightly higher. One of the drawbacks to 6061 is poor corrosion resistance when exposed to salt water or other chemicals. It’s also not as strong as other aluminum alloys for more demanding applications.

6061 is a material that’s typically used for auto parts, bicycle frames, sporting goods, some aircraft components, and frames for RC vehicles.

Aluminum 7075

7075 is a higher grade of aluminum, alloyed mainly with zinc. It’s one of the strongest aluminum alloys used in machining, with excellent strength-to-weight characteristics.

Because of the strength of this material it has average workability which means it tends to spring back to its original shape when being cold-formed. 7075 is also machinable and can be anodized.

Hi-end tent stakes from MSR are made with 7075-T6 aluminum.

7075 is often hardened to T6. However, it’s a poor choice for welding and this should be avoided in most cases. We routinely use 7075 T6 for making plastic injection mold tools. It’s also used for high-strength recreational equipment for mountain climbing, as well as for automotive and aerospace frames, and other stressed parts.

Brass

Brass is an alloy of copper and zinc. It’s a very soft metal, and can often be machined without lubrication. It’s a material that’s also highly workable at room temperature, so it often finds applications that doesn’t require great strength. There are many kinds of brass, largely depending on the percentage of zinc. As this percentage increases, corrosion resistance decreases.

Brass mallets are dense, non-sparking and soft.

Brass takes a high polish that looks much like gold. This is the reason it’s often found in cosmetic applications. Brass is electrically conductive but non-magnetic, and can be easily recycled.

Brass can be welded but is most often joined with low-temperature processes like brazing or soldering. Another feature of brass is that it doesn’t spark when struck with another metal, so it finds use for tools in potentially explosive environments. Interestingly, brass has natural anti-bacterial and anti-microbial properties, and its use in this regard is still being studied.

Brass is common in plumbing fittings, home decorative hardware, zippers, naval hardware, and musical instruments.

Magnesium AZ31

Magnesium AZ31 is an alloy with aluminum and zinc. It is up to 35% lighter than aluminum, with equivalent strength, but it’s also a bit more expensive.

The body of this camera was pressure die cast with magnesium.

Magnesium is a material that’s easy to machine but it’s very flammable especially in powder form, so it must be machined with a liquid lubricant. Magnesium can be anodized to improve its corrosion resistance. It’s also highly stable as a structural material and is an excellent choice for pressure die casting.

Magnesium AZ31 is often used for aircraft components in which light weight and high strength are most desirable, and can also be found in the housings for power tools, laptop cases and camera bodies.

Stainless Steel 303

There are many varieties of stainless steel, so called because of the addition of chromium that helps to deter oxidation (rust). Because all stainless steels look alike, great care must be taken to test incoming raw material with modern metrology equipment like OES detectors to confirm the characteristics of the steel you’re using for machining.

In the case of 303, sulfur is also added. This sulfur helps to make 303 the most readily machined stainless steel, but it also tends to reduce its corrosion protection somewhat.

303 isn’t a good choice for cold forming (bending), nor can it be heat treated. The presence of sulfur also means it’s not a good candidate for welding. It does have excellent machining properties but care must be taken with speeds/feeds and the sharpness of cutting tools.

303 is often used for stainless nuts and bolts, fitting, shafts, and gears. It should not, however, be used for marine grade fittings.

Stainless Steel 304

This is the most common form of stainless steel found in a wide variety of consumer and industrial products. Often called 18/8, this refers to the addition of 18% chromium and 8% nickel to the alloy. These two elements also make this machining material especially tough and non-magnetic.

304 is a material that is readily machineable, yet unlike 303 it can be welded. It’s also more corrosion resistant in most normal (non-chemical) environments. For machinists, it should be processed with very sharp cutting tools, and not contaminated with other metals.

Screws, nuts and other attachment hardware is often made from 304 stainless.

Stainless Steel 304 is an excellent material choice for kitchen accessories and cutlery, tanks, and pipes used in industry, architecture, and automotive trim.

Stainless Steel 316

The addition of molybdenum makes 316 even more corrosion resistant, so it’s often considered a marine-grade stainless steel. It’s also tough, and easy to weld.

316 stainless was used to make this shackle for a boat.

316 is used in architectural and marine fittings, for industrial pipes and tanks, automotive trim, and kitchen cutlery.

Carbon Steel 1045

This is a common grade of mild steel, i.e., not stainless. It’s typically less expensive than stainless steels, but considerably stronger and tougher. It’s easy to machine and to weld, and it can be work hardened and heat treated for various hardnesses.

Carbon steel can stand up to repeated hammer blows

1045 steel (in the European standard, C45) is used in many industrial applications for nuts and bolts, gears, shafts, connecting rods, and other mechanical parts requiring a higher degree of toughness and strength than stainless. It’s also used in architecture, but if exposed to the environment it will usually be surface treated to prevent rust.

Titanium

Titanium is well-known for possessing high strength, light weight, toughness, and corrosion resistance. It can be welded, passivated and anodized for increased protection and to improve its appearance. Titanium doesn’t polish especially well, is a poor conductor of electricity but a good conductor of heat. It’s a tough material to machine and only specialty cutters should be used.

This replacement hip joint and socket were 3D printed from titanium

Titanium is generally bio-compatible, and has a very high melting point. Although more expensive than other metals in commercial form, it’s a material used in machining that’s actually very abundant in the Earth’s crust but is more difficult to refine.

Titanium works well for powder bed 3D metal printing. It finds applications in the most demanding aerospace, military, bio-medical, and industrial fields, where it stands up well against heat and corrosive acids.

Common Plastic Materials for CNC Machining

Plastic resins used for CNC milling and turning must be rigid enough to hold their shape while they’re clamped in a vise or fixture. That’s one consideration that narrows the field of available materials. The following types of plastic resin have proven themselves over the years because they are stable, strong, easily machined, and produce great finished parts and prototypes.

ABS

ABS is an excellent choice for CNC machining. ABS is a tough, impact-resistant plastic that’s also resistant to chemicals and electrical current.

ABS is easy to color so it produces good cosmetic results. Because of its versatility and strength, it’s the most common plastic that we use for rapid prototyping. You’ll find it in automotive components, power tools, toys and sporting goods, among many other applications. ABS is less expensive than other engineering plastics like PEEK or Ultem but it doesn’t withstand high temperatures for long periods of time.

Nylon

Nylon has many of the same desirable characteristics as ABS. It has greater tensile strength which is why we use it for fabric and rope. Nylon and ABS resins are often mixed together, along with glass fibers, to enhance their desirable properties. Nylon can replace many mechanical parts, and because it has good surface lubrication it’s used for moving gears and sliding components. One drawback to nylon is that it does absorb moisture over time so it’s not suitable for marine applications. And it can be tough on cutting tools during machining.

PMMA Acrylic

PMMA is a rigid, transparent resin used as a substitute for glass or when making other clear optical parts. It resists scratching but is less impact resistant than polycarbonate. One advantage of PMMA is that it doesn’t contain Bisphenol-A, so it can be used for food storage. After machining, acrylic shows a hazy, matte surface. The surface can be treated with vapor polishing, which we do at Star Rapid, to make it optically clear. One thing to be aware of about acrylic is that it’s susceptible to heat deformation, so it should be stress-relieved before machining. PMMA is used for display screens, light pipes, lenses, clear enclosures, food storage, and to replace glass if strength is not an issue.

PEEK

PEEK is a true high-strength and stable engineering plastic. It can be used as a substitute for metal in many applications and it can withstand prolonged exposure to high temperatures. PEEK is used for advanced medical, aerospace, and electronic components. It’s also a great choice for lightweight fixtures because it doesn’t tend to creep, or deform, over time like other resins. PEEK is much more expensive than many other plastics so it tends to be used only when nothing else will do. In many cases, it’s necessary to anneal it during the machining process, or else it will form stress fractures.

UHMWPE

This long name means “ultra high molecular weight polyethylene”. There are in fact several different kinds of PE, with different mechanical and chemical properties. UHMWPE is especially hard and strong, very resistant to chemicals, and has a naturally slippery surface. All of these traits make UHMWPE the standard of care for joint replacements. This material is also used in marine environments, food and chemical processing, and for gear trains and conveyor belts.

Other CNC Machining Materials

In this chart, you will find additional CNC machining materials that are found in the industry.

How To Choose The Right CNC Machining Materials? Our Step-by-Step Guidelines

The above information can help inform your decision about what material will best suit your application, bearing in mind that in many cases more than one choice will work just fine.

We always advise our partner clients to consider the environment that the part will be used in, and what kinds of forces it will be subjected to throughout its service life. Although there are many variables, in our experience these are the areas that have the largest effect on raw material suitability.

Moisture

Does the product need to withstand salt or freshwater? Some metals and plastics are naturally resistant to corrosion, while other materials may need additional surface treatments such as painting, plating, or anodizing. And yes, even many types of plastic, such as nylon, can absorb water over time which will lead to premature part failure.

Strength

There are several different ways to understand the concept of strength as it applies to material science, and the subject is a very complex and technical one. In general, product engineers are usually concerned about:

• Tensile strength: How well does the material resist a pulling force?
• Compression or load bearing: How well does the material resist a constant load?
• Toughness: How well does the material resist tearing?
• Elasticity: How well does the material snap back to its original shape after a load is removed?

All materials differ in the various types of strength that they exhibit, so it’s critical to know what your tolerable limits are and then choose a material that has an adequate safety factor well above those limits.

The good news is that there are many online material data websites that provide comprehensive technical information about all available commercial metal and plastic, so these should be consulted in advance.

Heat

All materials expand and contract in the presence of heat. This could potentially affect your part if it’s going to be subjected to many heating and cooling cycles. As parts get hotter they also get softer and more flexible before they reach their melting point. Heat can also cause some plastic resins to outgas or to undergo thermal degradation that destroys its chemical bonds. Therefore, to prevent critical part failure always use a material that will be thermally stable at a temperature far above your expected working conditions.

Corrosion Resistance

Corrosion involves much more than just exposure to water. Any adverse chemical reaction with another foreign substance could potentially cause part failure. These substances include oils, reagents, acids, salts, alcohols, cleaners, etc. Consult the relevant material data sheets to verify that your metal or plastic can withstand any expected chemical exposure.

Machinability

Not so much an issue with relatively soft plastic, machinability can be a big deal with certain types of metal or carbon fiber. Extremely tough materials, and that does include carbon fiber, can quickly destroy expensive cutting tools. Others will require very careful control of cutting speed and feed rates. In addition, some materials can be processed faster than others. For longer production runs, using a metal that machines quickly can save significant time and money over the long term.

Cost

Obviously there are cost considerations with all raw materials. However, we strongly encourage all product developers to consider that saving cost by choosing a lower grade of material is never a good idea in the long term. Rather, choose the best material that you can afford which still offers all the necessary functionality. This helps to guarantee that the finished part will be durable.

Request Your Free CNC Milling Service Quote Today

Although these are the most common metal alloys we use for most CNC milling and turning jobs, there are many more available to suit almost any application. We’ll be happy to work with you to recommend the best solution when you upload your CAD files for a quote.