Aluminium CNC Machining Processes
You can machine aluminium by a number of the CNC machining processes available today. Some of these processes are as follows.
In CNC turning operations, the workpiece rotates, while the single-point cutting tool stays stationary along its axis. Depending on the machine, either the workpiece or the cutting tool carries out feed motion against the other in order to achieve material removal.
CNC Milling operations are the most commonly used in machining aluminium parts. These operations involve the rotation of a multi-point cutting along its axis, while the workpiece stays stationary along its own axis. Cutting action and subsequently material removal is achieved by the feed motion of either the workpiece, the cutting tool, or both of them combined. This motion can be carried out along multiple axes.
Also known as pocket milling, pocketing is a form of CNC milling in which a hollow pocket is machined in a part.
Facing in machining involves creating a flat cross-sectional area on the surface of a workpiece through either face turning or face milling.
CNC Drilling is the process of making a hole in a workpiece. In this operation, a multi-point rotating cutting tool of a particular size move in a straight line perpendicular to the surface to be drilled, thereby effectively creating a hole.
Tools for machining aluminium
There are several factors that influence the selection of a tool for aluminium CNC machining.
There are different aspects of a tool geometry that contributes to its efficiency in machining aluminium. One of these is its flute count. In order to prevent difficulty in chip evacuation at high speeds, cutting tools for aluminium CNC machining should have 2-3 flutes. A higher number of flutes results in smaller chip valleys. This will cause the large chips produced by aluminium alloys to get stuck. When cutting forces are low and chip clearance is critical to the process, you should use 2 flutes. For a perfect balance of chip clearance and tool strength, use 3 flutes.
Tool flutes (harveyperformance.com)
The helix angle is the angle between the centre line of a tool and a straight line tangent along the cutting edge. It is an important feature of cutting tools. While a higher helix angle removes chips from a part more quickly, it increases the friction and heat during cutting. This may cause the chips to weld to the tool surface during high-speed aluminium CNC machining. A lower helix angle, on the other hand, produces less heat but may not remove chips effectively. For machining aluminium, a 35° or 40° helix angle is suitable for roughing applications, while a helix angle of 45° is best for finishing.
Helix angle (Wikipedia.com)
Clearance angle is another important factor for the proper functioning of a tool. An excessively large angle would cause the tool to dig into the work and chatter. On the other hand, a too small angle would cause friction between the tool and the work. Clearance angles between 6° and 10° are best for aluminium CNC machining.
Carbide is the preferred material for cutting tools used in aluminium CNC machining. Because aluminium is soft cutting, what is important in a cutting tool for aluminium is not hardness, but the ability to retain a razor sharp edge. This ability is present in carbide tools and it depends on two factors, carbide grain size and binder ratio. While a larger grain size results in harder material, a smaller grain size guarantees a tougher, more impact-resistant material which is actually the property we require. Smaller grains require cobalt to achieve the fine grain structure and the material’s strength.
However, cobalt reacts with aluminium at high temperatures, forming a built-up edge of aluminium on the tool surface. The key is to use a carbide tool with the right amount of cobalt (2-20%), in order to minimize this reaction, while still maintaining the required strength. Carbide tools are typically able to withstand better than Steel tools, the high speeds associated with aluminium CNC machining.
In addition to tool material, tool coating is an important factor in tool cutting efficiency. ZrN (Zirconium Nitride), TiB2 (Titanium di-Boride), and diamond-like coatings are some suitable coating for tools used in aluminium CNC machining.
Feeds and speeds
Cutting speed is the speed at which the cutting tool rotates. Aluminium can withstand very high cutting speed hence the cutting speed for aluminium alloys is dependent on the limits of the machine being used. The speed should be as high as is practical in aluminium CNC machining, as this reduces the possibility of the formation of built-up edges, saves time, minimises temperature rise in the part, improves chip breakage, and improves finishing. The exact speed used varies by the aluminium alloy and the tool diameter.
Feed rate is the distance the workpiece or tool moves per revolution of the tool. The feed used depends on the desired finish, the strength, and the rigidity of the workpiece. Rough cuts require a feed of 0.15 to 2.03 mm/rev while finishing cuts require a feed of 0.05 to 0.15mm/rev.