Built-up edge is the whole problem
A built-up edge is a lump of workpiece material welded to the cutting edge. It grows, becomes unstable and breaks off, taking a piece of the coating with it. Because it changes the effective rake angle as it forms, the hole or surface it produces varies continuously.
The remedy has three parts: a sharp polished edge, a smooth low-friction surface, and a cutting speed high enough that the chip leaves before it can weld. A hard coating applied to a blunt edge makes aluminium worse rather than better.
| Alloy | Behaviour | Tooling direction |
|---|---|---|
| Pure and low-silicon | Very ductile, gummy | Polished 2-flute, high helix, DLC or uncoated |
| 6082 and 6061 | General structural | 3-flute for finish, 2-flute for chip room |
| 7075 | Harder, better chip breaking | Standard high-helix geometry works well |
| Hypereutectic (high silicon) | Abrasive silicon particles | PCD-tipped tooling; carbide wears rapidly |
| Aluminium castings | May contain hard inclusions or porosity | PCD for abrasive grades, sharp edge otherwise |
Flute count and chip room
Aluminium produces a large volume of chip for the volume of material removed, so chip room matters more than edge strength. Two flutes provide the largest gullet and are the standard choice for drilling and slotting.
Three flutes improve surface finish and productivity in pocketing where chip evacuation is not the limiting factor. Beyond three flutes, the gullets become too small for aluminium and the tool packs with chip.
DLC, uncoated and PCD
DLC provides a low-friction, non-sticking surface that resists built-up edge while adding some wear resistance. Uncoated polished carbide works on the same principle and is frequently the better choice where the polished flute finish is the main requirement.
For hypereutectic aluminium with hard silicon particles, neither is enough because the wear mechanism is abrasion by the silicon rather than adhesion. PCD is the economic answer there: it costs considerably more per tool and lasts many times longer, so the cost per part falls.
Coolant, mist and dry cutting
Aluminium is frequently machined with minimum-quantity lubrication or dry, because the material conducts heat well and the chip carries most of it away. Where coolant is used, it is primarily for chip evacuation and lubrication rather than for cooling.
The exception is deep-hole drilling, where chip evacuation is the limiting factor and through-tool coolant or high-pressure air is required to clear the hole.
Frequently asked questions
Why is my aluminium part coming out oversize?
Almost always a built-up edge. Material welds to the cutting edge, changes the effective geometry, and produces a hole or surface that varies in size. Increase the speed, use a sharper polished edge, and make sure the coating is smooth rather than hard and rough.
Should I use coolant when machining aluminium?
Usually not for milling, because the chip carries the heat away and coolant can cause thermal shock on the edge. Use minimum-quantity lubrication or dry cutting, except in deep holes where chip evacuation requires through-tool coolant or high-pressure air.
How many flutes should an aluminium end mill have?
Two or three. Two gives the largest chip room for slotting and drilling, and three improves finish and productivity in pocketing. More than three flutes leaves too little gullet volume for aluminium's large chip.
When is PCD worth the cost for aluminium?
For hypereutectic alloys with high silicon content, metal matrix composites, and any aluminium with hard abrasive inclusions. Silicon abrades carbide rapidly, and PCD resists it many times longer, so the cost per part falls despite the much higher tool price.
What is the best coating for aluminium?
DLC for a low-friction non-sticking surface, or uncoated polished carbide where the flute finish is the main requirement. Hard coatings such as TiAlN are the wrong choice, because they blunt the edge and encourage built-up edge.