Aluminum is one of the most commonly used materials for machining today. In fact, the CNC machining process for aluminum is second only to steel in terms of frequency of use.
In its high-purity form, the chemical element aluminum is soft, ductile, non-magnetic, and has a silvery-white appearance. However, this element is not used only in its pure form. Aluminum is often alloyed with various elements, such as manganese, copper, and magnesium, to form a variety of aluminum alloys with significantly improved properties. The most common machined aluminum alloys and their grades in different standards can be found here.
Advantages of using aluminum for CNC machined parts
While there are countless aluminum alloys with varying degrees of characteristics, there are some basic characteristics that apply to basically all aluminum alloys.
Workability
Aluminum can be quickly shaped, fabricated, and machined by a variety of different processes. Aluminum can be cut quickly and easily by machine tools because it is soft, easy to cut, inexpensive, and requires less effort than steel. These characteristics are great advantages for both machinists and customers ordering parts. Furthermore, aluminum’s superior machinability means that it is less likely to deform during machining. Given that it allows CNC machines to achieve a higher degree of resistance, their accuracy will be higher.
Strength of Aluminum
Aluminum is about one-third the density of steel, making it relatively lighter in mass. Despite its light weight, aluminum is very strong. This combination of strength and weight is called the specific strength of the material. Aluminum’s high specific strength makes it suitable for parts needed in many industries, such as the automotive and aerospace industries.
Corrosion resistant
Aluminum resists scratching and corrosion in conventional marine and atmospheric environments. This property can also be enhanced by anodizing. It is important to note that the corrosion resistance of different grades of aluminum varies. The most regular and common CNC machined grades instead have the strongest resistance.
Low Temperature Performance
Most materials lose some of their quality properties below freezing. For example, carbon steel and rubber become brittle at low temperatures. Aluminum, on the contrary, retains its softness, ductility and strength at very low temperatures.
Conductivity
The electrical conductivity of pure aluminum is about 37.7 million Siemens/meter at room temperature . Although aluminum alloys are less conductive than pure aluminum, they are sufficient for applications in electronic components. On the other hand, if the desirable properties of a machined part are not electrical conductivity, aluminum would not be a suitable material for it.
Recyclability
Since the CNC machining process is a subtractive manufacturing process, it generates a large amount of chips, also known as scrap. Aluminum is highly recyclable and requires relatively little energy, effort and cost to recycle. This makes it a favorite among those who want to recoup expenses or reduce material waste. It also makes aluminum a more environmentally friendly material to process.
Anodizing possibilities
Anodizing, a surface machining process, improves the material’s resistance to wear and corrosion, which is extremely easy to achieve with aluminum. This process also makes it easier to color machined aluminum parts.
CNC machining process for aluminum
Aluminum is machined through some of the existing CNC machining processes. Some of these processes are listed below.
CNC Turning
In a CNC turning operation, the workpiece rotates while the single-point cutting tool remains stationary along the axis. Both the workpiece and the tool will rely on the machine tool to perform feed movements against each other to achieve material removal.
CNC Milling
CNC milling is the most common process used when machining aluminum parts. These processes involve the rotation of a multi-point cut along an axis, while the workpiece remains stationary along the axis. The cutting action and subsequent material removal is realized by a feed motion, either of the workpiece or of the cutting tool or a combination of both. This action can be carried out along multiple axes.
Notching
Recess machining, known as recess milling, is a form of CNC milling used to machine a hollow recess in a part.
End face machining
Face machining involves the use of face turning or face milling to create a flat cross-section on the surface of a workpiece.
CNC drilling
CNC drilling is the process of punching a hole in a workpiece. In this process, a multipoint rotary cutting tool of a specific size is perpendicular to the surface to be drilled and moves in a straight line to effectively punch the hole.
Tools for machining aluminum
In the choice of CNC machining tools for aluminum, there are so many influencing elements.
Tool Design
The different styles of a tool’s shape affect its efficiency in machining aluminum. One of them is the number of grooves in the tool. In order to avoid difficulties in discharging chips at high speeds, a cutting tool for CNC machining of aluminum should have 2-3 grooves. Too many notches means narrower chip discharge passages, which can lead to large chip jams from aluminum alloys. When cutting forces are low and chip removal is critical to the process, you should use a tool with 2 notches. If you want to achieve the perfect balance of chip removal and tool strength, you should use a 3-notch tool.
Helix angle
The helix angle is the angle formed between the center axis of the tool and the straight line cut along the cutting edge. It is an important feature of cutting tools. Although a larger helix angle clears part chips more quickly, it increases friction and heat during the cutting process. In high-speed CNC machining of aluminum, this can result in chips fusing to the surface of the tool. On the other hand, a smaller helix angle generates less heat and may not be as effective at removing chips. For machining aluminum, a 35- or 40-degree helix angle is suitable for roughing, while a 45-degree helix angle is best for finishing.
Clearance angle
The clearance angle is another important factor for a tool to function properly. Too much angle can cause the tool to drill into the workpiece and make a gurgling sound. On the other hand, too little of an angle can cause friction between the tool and the workpiece. The best clearance angle for CNC machining of aluminum is between 6 and 10 degrees.
Material of the tool
Tungsten carbide is the material of choice for use as a cutting tool for CNC machining of aluminum. Because aluminum is a soft cut, it is important that the tools used to cut aluminum are not hard, yet retain the ability to cut sharp edges. Carbide cutting tools have this ability, mainly by two points: alloy grain size and binder ratio. Although a larger grain size makes the material harder, a smaller grain size ensures that the material is more resilient and impact resistant, which are the properties we really need. Smaller grains allow cobalt to achieve a fine grain structure and material strength.
However, cobalt and aluminum react at high temperatures to form aluminum chip tumors on the tool surface. The key to minimizing this reaction is to use carbide tools with the right amount of cobalt (2 to 20%) while still maintaining the required strength. Carbide tools can usually withstand the high speeds of CNC machining of aluminum better than steel tools. In addition to the material of the tool, coatings are an important factor in cutting efficiency. Zirconium dinitride, titanium diboride and diamond-like coatings are all suitable tool coatings for CNC machining of aluminum.
Feed and speed
The cutting speed is the speed at which the cutting tool is rotating. Aluminum can withstand very high cutting speeds, so the cutting speed of aluminum alloys depends on the limits of the machine tool used. This speed should be as fast as the actual speed at which the aluminum is CNC machined, as this reduces the possibility of chip-rule formation, saves time, reduces part temperatures, improves chip breakage and improves finishing finishes. The exact speed used varies depending on the aluminum alloy and tool diameter.
The feed rate is the distance the workpiece or tool moves for each revolution of the tool. The choice of feed is dependent on the degree of finish, strength and stiffness of the workpiece desired. Rough cutting requires a feed rate of 0.15-2.03 mm/rev, while finish cutting requires a rate of 0.05-0.15 mm/rev.
Cutting fluid
Even if aluminum is a good machinable, never cut it in a dry state, as this will encourage the formation of chip tumors. Suitable cutting fluids for CNC machining of aluminum are oil-based emulsions and mineral oils. Avoid cutting fluids that contain chlorine or reactive sulfur, which can stain aluminum.
Post-Processing Flow
After finishing machining an aluminum part, there are specific processes you can perform to enhance its physical, mechanical, and aesthetic properties. The most widely used processes are listed below.
Pearlized and sandblasted finishes
Pearlizing is a finishing process with a decorative effect. The process consists of a high-pressure air gun spraying small glass beads onto the machined part, effectively cleaning the material and ensuring that the surface of the part is smooth and flat and that the aluminum can have a smooth or matte finish. The main operating parameters of the bead finishing process are the size of the glass beads and the amount of high or low air pressure used. This process is only suitable if the dimensional tolerances of the part are not critical. Other processes, including polishing and lacquering, are also available in addition to bead-blasting, i.e. cleaning the material with a high-pressure stream of sand.
Protective layer
This process covers the use of other materials such as zinc, nickel, and chrome plating to overcoat aluminum parts. This is done to perfect the machining of the part and may be possible through an electrochemical reaction process.
Anodic oxidation
Anodizing is an electrochemical process in which an aluminum part is immersed in a dilute sulfuric acid solution and a voltage is applied to both the positive and negative terminals. This process effectively transforms the exposed surface of the part into a hard, non-reactive aluminum oxide coating. Both the density and thickness of the resulting coating depend on the consistency of the solution, the length of the oxidation, and the current. You can also use anodizing to color parts.
Powder Coating
The powder coating process consists of coating the parts with colored polymer powder using an electrostatic spray gun. The parts are then stored at a temperature of 200 degrees Celsius. Powder coating improves strength and resistance to abrasion, corrosion and impact.
Hot treatment
Parts made of heat-treatable aluminum alloys receive heat treatment to improve their mechanical properties.
CNC Machining of Aluminum Parts in Industry
As mentioned above, aluminum alloys possess many desirable properties. Therefore, CNC machining of aluminum parts is indispensable in many industries, as follows:
Aerospace: Given the high specific strength of aluminum alloys, many aircraft components are made from machined aluminum.
Automotive: Similar to the aerospace industry, many components in the automotive industry, such as driveshafts and other components, are made of aluminum.
Electrical: CNC machined aluminum parts are often used as electronic components in household appliances due to their high electrical conductivity.
Food/Pharmaceuticals: Aluminum parts do not react with most organic substances, so they play an important role in the food and pharmaceutical industries.
Sports: Aluminum is often used to make sports equipment such as baseball bats and sports whistles.
Cryogenic: Aluminum retains its mechanical properties at temperatures below freezing, making it popular for low-temperature applications.
