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CNC (Computer Numerical Control) machining of aluminium extrusions is a
subtractive manufacturing process used to create complex, high-precision parts
from standard linear profiles. Aluminium (usually 6061-T6 or 7075 alloys) is
favored for its excellent machinability, high strength-to-weight ratio, and
corrosion resistance. The process begins with selecting the correct extrusion
profile. Extrusions come in various shapes—square tubes, angle irons, and custom
hollow profiles. The machinist must account for the "stock allowance" because
the extrusion surface is not perfectly smooth or dimensionally exact; extra
material (typically 0.5mm to 2mm) must be left on the surfaces to be machined to
achieve a true finish.
The first step in achieving precision is workholding and fixturing.
Aluminium is soft and prone to vibration (chatter), which ruins surface finishes
and tolerance. Parts must be clamped securely using soft jaws (aluminium or
plastic) to avoid marring the soft metal. Vacuum tables are often used for
thin-walled extrusions to hold the part flat without distorting it. For complex
4-axis or 5-axis machining, custom fixtures are milled to match the part's
geometry, ensuring it doesn't move during the high-speed cutting process.
Tool selection is the second critical factor. Carbide end mills are
standard for aluminium, but the geometry of the flute matters. "O-flute" or
"chipbreaker" tools are designed to pull chips upward and out of the cut,
preventing them from being recut (which causes poor surface finish). High-speed
machining (HSM) strategies using high RPMs and shallow depths of cut are
employed to manage heat. Aluminium has a low melting point, and excessive heat
can cause the material to weld to the tool (built-up edge) or warp. Using air
blasts or mist coolant helps evacuate chips and keep the workpiece cool without
causing thermal shock.
The third consideration is programming and toolpaths. CNC machines rely on
G-code generated by CAM (Computer-Aided Manufacturing) software. To maintain
tight tolerances (often +/- 0.005 inches or less), the programmer must use
"climb milling" (where the cutter rotates in the same direction as the feed) to
reduce tool deflection. "Adaptive clearing" toolpaths maintain a constant chip
load on the tool, extending tool life and ensuring consistent accuracy. For
features like pockets or slots, "trochoidal milling" (circular movements) is
used to reduce tool wear in corners.
Finally, post-machining finishing and inspection are vital. As-machined
aluminium has sharp burrs that must be removed by hand deburring or tumbling in
a vibratory media bowl. For aesthetic parts, anodizing or powder coating is
applied to enhance durability and color. Inspection involves using digital
calipers, micrometers, and CMM (Coordinate Measuring Machines) to verify that
critical dimensions and geometric tolerances (flatness, parallelism) meet the
blueprint specifications. Because aluminium expands slightly when heated, parts
are often allowed to cool to room temperature before final measurement to ensure
dimensional stability in the end-use environment.
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