About CNC machining

 

You can machine aluminium by a number of the CNC machining
processes available today. Some of these processes are as follows.
CNC Turning
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
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

Face turning
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.
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.

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

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.
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.
Despite its machinability, never cut aluminium dry as this promotes the
formation of built-up edges. The appropriate cutting fluids for aluminium
CNC machining are soluble-oil emulsions and mineral oils. Avoid cutting
fluids that contain chlorine or active sulphur as these elements stain
aluminium.
After machining an aluminium part, there are certain processes that you
can carry out to enhance the physical, mechanical, and aesthetic features of
the part. The most widespread processes are as follows.
Bead blasting is a finishing process for aesthetic purposes. In this
process, the machined part is blasted with tiny glass beads using a highly
pressurised air gun, effective removing material and ensuring a smooth.

surface. It gives aluminium a satin or matte finish. The main process
parameters for bead blasting are the size of the glass beads and the
amount of air pressure used. Only use this process when the dimensional
tolerances of a part are not critical.
Other finishing processes include polishing and painting.
Besides bead blasting, there is also sandblasting, which uses a high￾pressure stream of sand to remove material.
This involves coating an aluminium part with another material such as
zinc, nickel, and chrome. This is done to improve the parts processes and
may be achieved through electrochemical processes.

Anodising is an electrochemical process in which an aluminium part is
dipped in a solution of diluted sulphuric acid, and an electric voltage is
applied across the cathode and anode. This process effectively converts the
exposed surfaces of the part into a hard, electrically non-reactive aluminium
oxide coating. The density and thickness of the coating created is
dependent on the consistency of the solution, the anodising time, and the
electric current. You may also carry out anodisation to colour a part.
The powder coating process involves coating a part with colours
polymer powder, using an electrostatic spray gun. The part is then left to
cure at a temperature of 200°C. Powder coating improves strength and
resistance to wear, corrosion, and impact.

Parts made from heat-treatable aluminium alloys may undergo heat
treatment to improve their mechanical properties.
As stated earlier, aluminium alloys have a number of desirable
properties. Hence, CNC machined aluminium parts are indispensable in
several industries.

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