Threading Aluminium: Adhesion, Built-Up Edge and the One Coating Rule You Cannot Break

Why Aluminium Rewards Speed and Punishes Patience

“Aluminium is soft — you can cut it with your eyes closed” is the most widespread and most expensive misconception on the shop floor. Apply steel-cutting tools and parameters to aluminium — low speed, slow feed, economy of effort — and the result is adhesion, built-up edge, burrs and poor finish, all at once. When steel chips or breaks you notice immediately; when aluminium gums up you may not, and by the time the finish looks wrong a whole batch has already gone through.

Aluminium’s difficulty is not hardness. It is three characteristics that sit opposite to steel’s.

Soft, but sticky. Aluminium is highly ductile, so chips neither break like cast iron nor curl and snap like steel — they run continuously. Worse, aluminium has a chemical affinity with the titanium in carbide and cold-welds to the cutting edge under heat and pressure. Steel’s problem is hardness, which can be overcome; aluminium’s problem is adhesion, which cannot be shaken off.

Conductive, but the edge still gets hot. Aluminium conducts heat several times better than steel, so heat has somewhere to go. But aluminium smeared on the edge rubs repeatedly against the cut, and local temperature is enough to weld it in place. Good conduction does not mean a cool edge.

Thermal expansion about twice that of steel. Dimensions move with temperature, so finish-machined parts should be measured once they have stabilised — otherwise thermal growth is misread as an out-of-tolerance cut.

Built-up edge: the common source of every defect

Poor finish, burrs, drifting dimensions and chipped edges in aluminium mostly trace back to one phenomenon: built-up edge. A layer of aluminium welds onto the edge under heat and pressure, turning the tool into an irregular blunt cutter and starting a closed loop — aluminium sticks, the edge effectively dulls, cutting turns from shearing into ploughing, ploughing generates heat, and more aluminium sticks. When the chip note goes dull, the surface turns from bright to hazy, or scratches of varying depth appear, stop the machine and check the tool rather than suspecting the machine.

Breaking the loop follows one counter-intuitive rule: aluminium punishes low speed, not high speed. At low cutting speeds the edge is not hot enough to soften and break the chip, which is exactly the condition cold-welding needs. Raise the speed and the chip is gone before it can stick. The verified industry magnitude: below roughly 100 m/min built-up edge forms most readily; above 300 m/min it largely stops. When adhesion appears, the first response is higher speed and more feed — the opposite of the instinctive move.

The coating rule: a good steel coating is the wrong aluminium coating

This is where aluminium machining wastes the most money. TiN, TiCN and TiAlN — titanium-containing coatings — are excellent in steel and actively harmful in aluminium. Aluminium and titanium are chemically compatible at temperature, so a titanium-bearing coating gives the chip a velcro pad: the more you cut, the more it sticks, and built-up edge grows at a multiple of the normal rate.

Three coating routes suit aluminium instead. Uncoated with a mirror-polished surface gives the sharpest edge and a flute surface with nothing to grip, making it the first choice for finishing pure aluminium and soft alloys such as 6061 and 6063. DLC-type diamond-like coating has an extremely low friction coefficient and suits high-volume continuous cutting and die-cast aluminium. ZrN (zirconium nitride) contains no titanium and is relatively slick, making it the practical general-purpose choice for aluminium and non-ferrous work.

Turned around, this also answers a common question: if a customer asks why their steel cutter gums up on aluminium, the answer is usually in that rule. Aluminium tools and steel tools are best kept as two separate product lines rather than mixed.

Threading: from tapping to thread milling

In aluminium, tapping typically runs at only tens of metres per minute and suffers the same titanium-coating trap and chip-evacuation problems. Thread milling behaves quite differently in aluminium: cutting speed can be raised into the hundreds of metres per minute, which sits well clear of the built-up-edge zone; and helical interpolation produces short chips that coolant washes away rather than wrapping inside the hole. For general-purpose alloys such as 6061 and 6063, a 55° single-tooth imperial thread mill for through holes and blind holes in aluminium alloy covers the most common imperial thread conditions.

Deep-hole threads are a special case in aluminium. Chip volume is high, and the deeper the hole the more likely chips are to pack and be re-cut. A high-precision 3-start thread milling cutter for deep holes in aluminium and non-ferrous metals addresses that risk through chip-room and evacuation geometry designed for deep-hole work.

The no-pilot-hole case also matters, and it overlaps with cast iron. A 2-row-tooth circular drill thread mill that requires no pre-drilled hole in aluminium alloy and cast iron merges drilling and threading into one operation, removing a set-up and a tool change.

Geometry for milling and turning

Aluminium end mills are selected on the reverse logic of steel cutters. Steel tooling favours more flutes to carry load; aluminium wants few — two, or three at most. The reason is chip room: aluminium chips are sticky and long, so a four-flute cutter packs them into a narrow groove where they are repeatedly rubbed, worsening both heat and adhesion.

High helix is the second keyword. Aluminium cutters run 40°–45° or above, which increases the effective working rake, makes the cut freer, curls the chip more tightly and prevents packing. Combine that with a strongly positive rake of 15°–25° so the edge shears rather than ploughs.

One professional detail on edge preparation: aluminium cutters take a very light hone in the 0.02–0.05 mm range. A perfectly sharp edge micro-chips, and each chip becomes an adhesion site; a light hone removes almost no sharpness while substantially improving resistance to micro-chipping.

Three operating rules follow. Raise the speed — small-diameter tools routinely run into the tens of thousands of rpm so that chips are thrown clear by centrifugal force. Do not be stingy with feed — 0.1 mm per tooth and above, because a thick chip carries heat away while a thin chip merely rubs. And climb mill throughout: conventional milling rubs before it cuts, and that rub generates the heat that guarantees adhesion.

For turning aluminium, the entry point is the ISO N group. Inserts designed with negative rake and thick coatings to survive impact are a liability in aluminium. Aluminium turning inserts are characterised by two features only: a strongly positive rake of around 20° and a polished rake face, with essentially no edge honing. The parameter rule is high rather than low — finish turning 6061-type alloys runs comfortably in the hundreds of metres per minute. On nose radius, R0.4 is common for finishing, but thin-walled and slender parts should go down to R0.2 to reduce radial force and prevent deflection. That trade-off protects part geometry, not tool life.

Grade by grade

• 1050 and 5xxx: softest and stickiest, low melting point, built-up edge territory. Needs adequate speed and generous lubrication, with uncoated mirror-polished tooling at the sharpest edge grade.
• 6061 / 6063: the most forgiving, wide parameter tolerance. Two or three flutes, high helix, uncoated or ZrN.
• 7075 / 2024: strength approaching steel, brittle chips that break easily, so finish is actually easier to achieve — but tool consumption is high. 2024 burrs noticeably, so use sharp aluminium tooling, reduce speed slightly and allow a deburring operation.
• High-silicon die-cast aluminium (Si > 8 %): silicon particles are highly abrasive and dull a carbide edge within tens of minutes, after which adhesion and finish collapse together. One signal is enough — above 8 % silicon, PCD moves from optional to mandatory.

Coolant: lubrication matters more than cooling

Because aluminium conducts well, coolant’s main job is lubrication and flushing — forming an oil film between edge and workpiece and driving chips out of the cut. Three practical points. Run the concentration at the upper end: 8–12 % for semi-synthetic coolant, richer than for steel, because lubrication depends on oil content. Never cut dry — without a coating buffer and oil film, dry aluminium cutting welds edge to workpiece directly. Where possible, use minimum-quantity lubrication so atomised oil reaches the edge, gives a uniform film and eliminates disposal of waste fluid. Aluminium is also chemically active, and cutting oils with heavy chlorine or sulphur EP additives will darken and corrode the workpiece, so a low-acid, chlorine-free aluminium-specific formulation is the right choice.

Starting parameters (confirm against manufacturer data)

• Rough milling: carbide 2–3 flute, from the low hundreds of metres per minute upward, 0.1–0.25 mm/tooth.
• Finish milling: same tooling or PCD, higher speed, narrower radial engagement, 0.05–0.1 mm/tooth.
• Finish turning: N-group AL geometry, 400–600 m/min.
• Drilling: aluminium twist drill, 100–300 m/min, peck drilling for deep holes.
• Tapping: spiral-flute or forming tap, tens of metres per minute.

Three rules summarise it: speed high rather than low, feed heavy rather than light, lubrication rich rather than thin.

Conclusion

Aluminium’s difficulty hides in its stickiness, and stickiness is decided jointly by speed, coating and geometry. Line all three up with the material and most adhesion and built-up-edge problems disappear. Set any one of them by steel-shop habit and the third day ends with a smeared, useless cutter.