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Aluminum CNC Machine: Benefits and Possible Alternatives

For CNC machining projects, aluminum is one of the most popular material choices due to its desirable physical properties. It is strong, which makes it ideal for mechanical parts, and its oxidized outer layer is resistant to corrosion from the elements. These benefits have made aluminum parts common across all industries, though they are particularly favored in the automotive, aerospace, healthcare and consumer electronics spheres.

Aluminum also offers specific advantages that simplify and improve the process of CNC machining. Unlike many other metals with similar material properties, aluminum offers excellent machinability: many of its grades can be effectively penetrated by cutting tools, chipping easily while being relatively easy to shape. Because of this, aluminum can be machined more than three times faster than iron or steel.

This article explains some of the key advantages of aluminum CNC turning — reasons why it is one of our most widely requested prototyping and production processes — but also suggests machining alternatives to aluminum.

Other metals and plastics can provide similar benefits to aluminum, in addition to the unique benefits of their own.

Machinability

One of the main reasons why engineers choose aluminum for their machined parts is because, quite simply, the material is easy to machine. While this would appear to be more of a benefit for the machinist manufacturing the part, it also has significant benefits for the business ordering the part, as well as the end-user that will eventually use it.

Because aluminum chips easily, and because it is easy to shape, it can be cut quickly and accurately with aluminum CNC milling. This has some important consequences: firstly, the short timeframe of the machining job makes the process cheaper (because less labor is required from the machinist and less operating time is required from the machine itself); secondly, good machinability means less deformation of the part as the cutting tool goes through the workpiece. This can allow the machine to meet tighter tolerances (as low as ±0.025 mm) and leads to higher accuracy and repeatability.

Corrosion resistance

Different aluminum grades differ greatly in their resistance to corrosion — the degree to which they can withstand oxidization and chemical damage. Fortunately, some of the most popular grades for brass CNC turning are the most resistant. 6061, for example, offers excellent corrosion resistance, as do other alloys on the lower end of the strength spectrum. (Strong aluminum alloys may be less resistant to corrosion due to the presence of alloyed copper.)

Strength-to-weight ratio

Aluminum has desirable physical properties that make it ideal for both mechanical and aspect parts. Two of the most important are the metal’s high strength and its lightweight, both of which make the material favorable for critical parts such as those required in the aerospace and automotive industries. Aircraft fittings and automotive shafts are two examples of parts that can be successfully machined with aluminum.

However, different grades of aluminum serve different purposes. Because of their favorable strength-to-weight ratio, general-use grades like 6061 can be used for a wide variety of parts, while notably high-strength grades like 7075 may be preferred in aerospace and marine applications.

Electrical conductivity

CNC machined aluminum parts can be useful for electrical components due to their electrical conductivity. Though not as conductive as copper, pure aluminum has an electrical conductivity of about 37.7 million siemens per meter at room temperature. Alloys may have lower conductivities, but aluminum materials are significantly more conductive than, for example, stainless steel.

Machined aluminum parts are especially popular in the consumer electronics industry, not just for strength and weight demands, but because of important aesthetic considerations. As well as being receptive to paints and tints, aluminum can be treated with anodization, a surface finishing procedure that thickens the protective and oxidized outer layer of the part.

The anodization process, which generally takes place after machining is completed, involves passing an electric current through the part in an electrolytic acid bath and results in a piece of aluminum that is more resistant to physical impact and corrosion.

Importantly, anodizing makes it easier to add color to a machined aluminum part, since the anodized outer layer is highly porous. Dyes can find their way through the porous sections of the outer layer and are less likely to chip or flake since they are embedded within the tough exterior of the metal part.

Another benefit of aluminum is its high recyclability, which makes it preferable for businesses seeking to minimize their environmental impact or for those who simply want to reduce material wastage and recoup some of their expenditure. Recyclable materials are particularly important in CNC machining, where there is a relatively large amount of waste material in the form of chips from the cutting tool.

Alternatives to aluminum in CNC machining

Businesses may seek alternatives to aluminum for brass CNC milling for any number of reasons. After all, the metal has a few weaknesses: its oxide coating can damage tooling, and it is generally more expensive than alternatives like steel, partly due to the high energy costs of aluminum production.

Here are some potential machining alternatives to aluminum, with an emphasis on their differences and similarities to the popular silver-gray metal.

Steels and stainless steels are widely used materials in CNC machining. Because of their high strength, steels tend to be favored for high-stress applications and those that require strong welds. Steels are resistant to very high temperatures, and stainless steels can be heat treated to enhance their corrosion resistance.

However, while machining steels are engineered for improved machinability, aluminum remains the more machinable of the two materials. Steels are also heavier and have a higher hardness than aluminum, which may or may not be desirable depending on the application.

If temperature resistance is a key consideration and weight is not, steel may be an ideal alternative to aluminum.

Titanium

Better than aluminum for:

Strength-to-weight ratio

Worse than aluminum for:

Cost

Titanium may be used as a like-for-like replacement for aluminum since its primary advantage is an exceptional strength-to-weight ratio — also one of the main benefits of aluminum. Titanium has a similar weight to aluminum but is almost twice as strong. Like aluminum, it is also highly resistant to corrosion.

These advantages are reflected in the higher price point of titanium. Though the material is an excellent choice for parts like aircraft components and medical devices, its cost can be prohibitive.

Titanium is a suitable alternative to aluminum when lightweight is a primary concern and, importantly, when the manufacturing budget has some flexibility.

Magnesium

Better than aluminum for:

Machinability

Weight

Worse than aluminum for:

Machining safety

Corrosion resistance

Although not the most common machining material, the lightweight metal magnesium offers many of the benefits of common aluminum alloys. In fact, magnesium is one of the most machinable metals out there, making the machining process fast and efficient.

One potential downside for machine shops? Magnesium chips are extremely flammable and are aggravated further by water, which means machinists must take caution while clearing debris.

Brass

Better than aluminum for:

Some aesthetic applications

Worse than aluminum for:

Cost

A metal with a golden appearance, brass is a highly machinable metal available at a slightly higher price point than aluminum. It is commonly seen on parts such as valves and nozzles, as well as structural components, while its high machinability makes it suitable for high-volume orders.

Copper

Better than aluminum for:

Electrical conductivity

Worse than aluminum for:

Machinability

Copper shares several material properties with aluminum. However, the superior electrical conductivity of copper can make it preferable for various electrical applications. While pure copper is difficult to machine, many copper alloys offer similar machinability to popular aluminum grades.

CNC machining projects need not be limited to metals. In fact, several engineering thermoplastics can match or exceed some of the benefits of aluminum, depending on the application.

Since aluminum is often favored for its excellent machinability, one viable plastic alternative is POM (Delrin), which is, like aluminum, highly suited to the machining process. POM has a low melting point but impressively high strength for a plastic.

POM is an electrical insulator, making it suitable for parts like electronic enclosures. It is also suitable for mechanical parts. However, given its radically different insulating behavior compared to aluminum, it should only be used as a like-for-like replacement in situations where thermal and electrical conductivity is of negligible importance.

If aluminum remains the preferred material choice for a project, there are ways to combine CNC machining with other manufacturing processes in order to create more complex, higher-performing aluminum parts. Doing so can maximize the functionality of aluminum while reaping the benefits of multiple production processes.

In addition to being an all-in-one manufacturing process, stainless steel CNC turning can be used to refine or modify parts made using other machinery. Extrusion, casting and forging processes can each be complemented with the machining process to make better aluminum components.

Aluminum extrusion + CNC machining

Extrusion is the process of forcing molten material through an aperture in in a die, producing an elongated component with a continuous profile. While aluminum extrusion is an effective way of producing functional components with quality surface finishes and complex cross-sections, it is limited in scope, since those cross-sections must be consistent across the part.

Unless, of course, the part is modified after extrusion. Because aluminum extrusion tends to involve malleable, ductile and machinable aluminum grades like 6061 & 6063, the extruded parts can then be post-machined — cut in various ways using a CNC machining center.

Combining aluminum extrusion and CNC machining is a great way to produce resilient parts with complex cross-sections and irregular geometries.

Die casting + CNC machining

Pressure die casting is a manufacturing process in which molten metal is forced into a mold cavity with high pressure. It is generally used when making parts in larger quantities since the required tool steel dies are expensive to make.

Along with steel, magnesium, and zinc, aluminum is one of the more popular metals for pressure die casting, and die-cast aluminum parts generally have an excellent surface finish and dimensional consistency.

These advantages can be combined with the advantages of CNC machining. By die casting aluminum components then adding further cuts using a machining center, it is possible to create parts with an exceptional finish and more complex geometries that would be possible using either process on its own.

Gravity dies casting can be used instead of pressure die casting if reducing cost is more important than ensuring high precision or creating thin walls.

Investment casting + CNC machining

Investment casting is a metal casting process that uses wax patterns to create metal parts. Like other casting processes, it produces parts with an excellent surface finish and high dimensional accuracy.

The process also produces unique advantages: it can be used to create more intricate parts than would be possible with die casting, and parts emerge with no parting lines.

Aluminum alloys are a common material used for investment casting, and the cast aluminum parts can be post-machined for refinement.

Forging + CNC machining

Many machinable aluminum alloys are also suited to the age-old process of forging, which involves shaping metal through compressive force. (This often involves hitting the metal with a hammer.)

Aluminum 6061, for example, is suited to hot forging with a closed die — a process commonly used to produce automotive and industrial components.

The forged pieces of aluminum can be post-machined with a CNC machining center. This can be beneficial compared to machining alone since forged parts are generally stronger than fully cast or fully machined equivalents. However, post-machining allows for the creation of more complex geometries without wholly compromising the integrity of the part.
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