Milling Aluminium (ISO N): Alloy Differences, Built-Up Edge Control and High-Speed Machining

Aluminium falls into group N (non-ferrous) of the ISO 513 classification and, because of its low cutting force, is often regarded as an "easy" material. In practice, however, the factors that determine tool life and surface quality are entirely different: the limit is set not by cutting force but by adhesion, chip volume and the abrasiveness of silicon. This guide covers alloy differences in aluminium milling, geometry and coating choice, high-speed machining limits and thin-wall strategies.

1. The place of aluminium in the ISO N group

ISO 513 divides cutting materials and their fields of application into six main groups: P, M, K, N, S and H. Group N covers non-ferrous metals; aluminium alloys, magnesium, copper, brass and bronze belong to this group. The detail of the group classification is covered in the ISO material groups guide.

In Sandvik Coromant's material classification, the ISO N group typically covers soft metals below 130 HB and the specific cutting force (kc) lies in the 350–700 N/mm² range. This value is markedly below the figures given for steel and stainless steel. As a result, power and force are rarely limiting in aluminium; the limit is set by the chip's ability to leave the flute and by preventing adhesion on the edge.

In the sub-classification based on ISO 513, aluminium is separated according to its production route (wrought, cast, sintered) and its silicon content. In practice these two headings are also the main variables that steer tool selection.

2. Wrought and cast alloys

In wrought alloys the total alloying element content is low, whereas cast alloys contain a high proportion of silicon for castability. This difference directly changes the wear mechanism: in wrought alloys the dominant problem is adhesion, in cast alloys it is abrasive wear.

ASM International's compilation on aluminium machining states that alloys such as 2024-T4 and 7075-T6 produce continuous chips and need a chip breaker, that chip control of 6061-T6 is more difficult, and that soft alloys such as 1100 and 3003 give gummy chips.

The 12.6% threshold: The aluminium–silicon eutectic composition is around 12.6% Si. Above this level, primary silicon crystals form during solidification; because they are far harder than the aluminium matrix, they abrade the cutting edge directly.

3. Tool material according to silicon content

Silicon content is on its own the strongest indicator for the tool material decision.

Sandvik Coromant states that fine-grain uncoated carbide grades are suitable for aluminium with a silicon content below roughly 7–8%, while compositions exceeding 12% are regarded as highly abrasive and call for a move to PCD. Published comparisons report that PCD tools give tenfold and higher life compared with carbide in high-silicon cast alloys. On the carbide side, the ability of fine-grain grades to hold a sharp edge becomes decisive.

4. Built-up edge and its prevention

Built-up edge is the welding of the cut aluminium onto the rake face under the effect of pressure and temperature. The resulting layer distorts the geometry of the cutting edge, shifts the dimension and, when it breaks away, takes a fragment of carbide with it from the edge, creating sudden wear. Bright patches on the surface, dimensional deviation and a sudden change in sound are typical symptoms.

Three factors feed adhesion: a blunt or rough rake face, low cutting speed and chips remaining in the cutting zone. Against these, the following measures largely eliminate the problem when applied together:

• Polished flute and rake face. Grinding marks increase friction and the area available for the chip to grip; on a polished surface the chip keeps sliding.
• Sharp cutting edge and high rake angle. In aluminium the edge hone is kept to a minimum; a sharp edge lowers cutting force and deformation.
• High helix angle. It ensures the chip is carried rapidly up and out of the flute.
• Low flute count and wide chip flute. As flute volume increases, chip packing decreases.
• Sufficiently high cutting speed. At very low speeds the tendency to adhesion increases.
• Chip removal. Compressed air, MQL or flood coolant prevents re-cutting.

5. Tool geometry

The geometry of aluminium cutters is deliberately different from that of cutters for steel. The decisive design decisions are flute count, helix angle, core diameter and flute surface quality.

Harvey Performance publications state that a 35–40° helix is preferred in roughing and slotting, and a 45° helix in finishing and high-efficiency machining. Since a high helix increases the axial force, the tendency of the part to lift on thin, weakly clamped components has to be assessed separately.

6. Coating choice and the TiAlN problem

The coating decision in aluminium works the opposite way round to the logic used for steel. In steel, what is expected from a coating is temperature resistance; in aluminium, what is expected is low friction and low chemical affinity.

TiAlN (and AlTiN) coatings contain aluminium in their structure. The fact that these coatings form a protective aluminium oxide layer on the surface at high temperature provides an advantage in steel and heat-resistant alloys; in aluminium machining, however, cutting temperatures do not rise to a level that would trigger this mechanism. What remains is the chemical affinity between the coating and the workpiece, and this affinity accelerates adhesion. For this reason TiAlN is not preferred in aluminium.

The general differences between the coating families are covered in detail in the coatings guide.

7. High-speed machining (HSM) and the machine limit

In aluminium the cutting speed is largely unconstrained; the limit is set by the machine rather than the tool. Spindle speed and feed are calculated with the following relations:

n = (vc × 1,000) / (π × D) and vf = fz × Z × n

Harvey Performance's aluminium guide gives a range of roughly 245–455 m/min for wrought alloys and roughly 150–305 m/min for cast alloys. What these speeds mean at small diameters is seen from the calculation:

• For D = 10 mm and vc = 400 m/min, n = (400 × 1,000) / (3.1416 × 10) ≈ 12,732 rpm
• With Z = 3 and fz = 0.05 mm/tooth on the same tool, vf = 0.05 × 3 × 12,732 ≈ 1,910 mm/min
• For D = 6 mm and vc = 400 m/min, the required spindle speed rises above 21,000 rpm

The spindle of most workshop machines cannot deliver this speed. Once the spindle reaches its limit, the cutting speed necessarily drops and the productivity expected from aluminium is not achieved. In that case the solution is not to force the cutting speed but to reduce the radial engagement and increase the feed; the approach is explained in the trochoidal milling and high-efficiency machining guide. Because the chip thins at low radial engagement, the feed also has to be corrected.

At high spindle speed: The balance and runout of the tool holder become decisive. The balancing of rotating tools and tool systems is defined in the ISO 16084 standard. As runout increases, the load is distributed unevenly between the flutes; one flute takes a thicker chip than expected and tool life is shortened.

8. Thin-wall machining and workholding

On aluminium parts, as the wall thickness decreases the problem shifts from the tool to the part. The wall deflects under the cutting force, the tool takes an irregular chip, the resulting vibration amplifies the deflection and the process enters a self-feeding loop. In practice the following approaches are used together:

• Radial engagement is kept low and the number of passes is increased.
• The wall is thinned down step by step from the top; at each level the material remaining below acts as support.
• The final pass is left symmetrical; taking a balanced amount of material from both faces of the wall reduces distortion.
• The tool is clamped in the collet at the shortest possible length; deflection increases with the cube of the unsupported length.
• The axial force created by a high helix is taken into account; if the part tends to lift, a lower helix is preferred.

In workholding, mechanical clamps are often inadequate: the force needed to hold the part deforms the thin wall. For this reason a vacuum table is preferred on flat, wide parts, and symmetrical clamping on thick sections together with support blocks on prismatic parts. Clamping points have to be placed not on thin walls but on thick sections, ribs or sacrificial areas.

9. Chip evacuation, air and MQL

In aluminium the great majority of tool breakages arise not from wear but from re-cutting of the chip. A chip trapped in the flute heats up, welds to the edge and breaks the tool on the next revolution. The risk is highest in slotting, pocketing and plunging operations; in these operations keeping the flute count low and securing evacuation is mandatory.

In dry and MQL machining, chip clearance depends entirely on the air jet; the jet has to be aimed at the flute exit and directed continuously. The scope and limits of MQL application are described in detail in the MQL and dry machining guide. In workshops using emulsion, regular measurement of the concentration is important in order to prevent staining and surface degradation on aluminium.

10. Starting parameters

The table below is a starting framework and does not replace manufacturer data. The cutting speed ranges are based on the values in Harvey Performance's aluminium guide; the engagement ratios are strategy ranges in common use in practice. For exact values, the tool manufacturer's data sheet and the cutting parameter calculator can be used.

The product-side counterpart for tool selection in aluminium machining can be examined in the square end mill category.

Sources

• ISO 513:2012 — Classification and application of hard cutting materials for metal removal with defined cutting edges — Designation of the main groups and groups of application (iso.org)
• ISO 8688-2:1989 — Tool life testing in milling — Part 2: End milling (iso.org)
• ISO 16084:2017 — Balancing of rotating tools and tool systems (iso.org)
• ASM International — Machining of Aluminum and Aluminum Alloys, ASM Handbook, Volume 16: Machining
• Sandvik Coromant — Workpiece materials: ISO N — Non-ferrous metals (sandvik.coromant.com)
• Sandvik Coromant — Tool balancing and RPM (sandvik.coromant.com)
• Harvey Performance Company — Attacking Aluminum: A Machining Guide (harveyperformance.com)
• ISCAR — Machining Aluminum: Is It Really That Easy? (iscar.com)