Machining Aluminium: Cutting Data, Built-Up Edge, Thin Walls
Knowledge · CNC machining
Why is aluminium so fast to machine, and where is the catch?
Cutting forces are only a fraction of steel, so aluminium jobs run at several hundred to over 1,000 m/min where structural steel sits at 150 to 250 m/min. The catch is adhesion: soft aluminium welds itself to the cutting edge under wrong conditions and forms a built-up edge that ruins surface finish and dimensional accuracy before breaking out with the corner of the insert. The remedies: very sharp, polished cutting edges with large rake angles, high cutting speed and reliable coolant supply. Dull “steel tools” are the most common beginner mistake on aluminium.
Which alloy for which part?
| Alloy | Properties | Typical use |
|---|---|---|
| EN AW-6082 T6 | machines well, welds well, balanced strength | standard for milled machine parts |
| EN AW-7075 T651 | very strong, machines excellently, barely weldable | highly loaded precision parts, fixtures |
| EN AW-5083 | seawater-resistant, welds well, smears slightly | welded structures, plates |
| Cast plate (e.g. G.AL) | low residual stress, very flat | base plates, fixture building |
A practical tip: for plate and fixture parts choose stress-relieved cast or stretched qualities (T651). Standard rolled plate can release its residual stress when large pockets are milled and warp; that costs straightening or scrap.
Why are thin-walled aluminium parts the real art?
Because the material is soft and everything yields: excessive clamping forces dent walls that spring back after unclamping and lose their dimension; machining heat and released residual stress amplify the effect. Practice works with adapted clamping (vacuum tables, soft jaws, low forces), material removal distributed over several passes and a finishing allowance in the last cut. For designers: wall thicknesses below about 2 mm and deep thin-walled pockets drive effort sharply; where function allows, one extra millimetre of wall pays off directly in price.
What does thermal expansion mean for tight tolerances?
Aluminium expands at about 23 µm/(m·K), roughly twice steel. A 500 mm part grows a good 0.01 mm per degree; between a warm machine and a climatised measuring room lie several hundredths quickly. Tight tolerances on aluminium are therefore measured at the 20 °C reference temperature after the part has acclimatised. Agree the measuring concept together with hundredth-range fits, otherwise two measuring rooms argue about a part that simply had two temperatures.
Frequently asked questions
Is aluminium cheaper to machine than steel?
For the same geometry usually yes: lower cutting forces, higher speeds, shorter runtimes. Material per kilogram costs more, and thin-walled designs can eat up the time advantage.
What is a built-up edge?
Workpiece material welded onto the cutting edge, typical for soft aluminium at low cutting speed with dull tools. It ruins surface and accuracy; sharp polished edges, high speed and good cooling prevent it.
Which tolerances are achievable on aluminium parts?
Fundamentally the same as on steel, including fits in the hundredths range. Because of doubled thermal expansion, tight tolerances belong on an acclimatised part measured at 20 °C.
Why do large aluminium plates warp during milling?
Rolled plates contain residual stress that releases when large pockets are cut. Stress-relieved cast plates or stretched T651 qualities and material removal balanced on both sides prevent it.