Why Is My Aluminum Welding to the Router Bit?

What built-up edge actually is

The lump on your cutting edge has a name: built-up edge, or BUE. It is not swarf that failed to clear and it is not burnt material. It is aluminum that has pressure-welded itself to the carbide.

Aluminum has a low melting point, it is ductile rather than brittle, and a freshly cut aluminum surface is chemically raw — the protective oxide layer is sliced away at the moment of cutting. That clean, hot, soft metal is pressed against the rake face of the flute at high pressure. It sticks.

Once a few microns have welded on, the geometry is ruined. The bit no longer has a sharp edge presented at the right angle; it has a rounded blob. A blob cannot shear metal, so it ploughs and rubs instead, which generates far more heat than cutting did, which welds on more aluminum. The failure accelerates — which is why a cut that started fine can destroy itself within a single pass.

Lubrication is not optional in aluminum

In wood and most plastics, air blast is enough — you are clearing chips and cooling them. Aluminum is different, because the problem is adhesion, not just temperature. You need something physically between the metal and the carbide.

That does not mean flood coolant. Most CNC routers have no coolant containment, and soaking an MDF spoilboard is its own problem. The standard answer on a router is MQL — minimum quantity lubrication: a very small, metered amount of lubricant delivered to the cutting zone, usually with air. Parts come off dry to the touch, and there is no tank, no sump and no swarf slurry to manage.

This is what Coolube is for. It is a pure lubricant rather than a water-based coolant emulsion — it works by film strength, not by evaporation, so it stops the weld forming in the first place instead of trying to carry heat away after the fact.

The second half: chipload

Lubricant alone will not save a bit that is rubbing. Chipload is the thickness of material each flute removes per revolution:

Chipload = Feed rate (IPM) ÷ (RPM × number of flutes)

A worked example. 18,000 RPM, 60 IPM, 2-flute bit:

60 ÷ (18,000 × 2) = 0.0017″ per flute. On a 1/4″ bit that is roughly half of what Onsrud's aluminum series calls for, and well under half of what their single-flute series calls for. At that chip thickness the edge is burnishing hot aluminum against carbide — ideal conditions for welding.

Spindle speed matters more here than in wood. Router spindles idle high, and 24,000 RPM in aluminum is usually too fast: you cannot feed quickly enough to keep the chip thick, so the tool rubs. Dropping to 12,000–16,000 RPM often fixes a welding problem on its own, because the same feed rate suddenly produces a real chip.

Published chiploads for aluminum

These two series disagree, and that is not an error. Onsrud publishes different application data for different geometries, and we show both rather than averaging them into a number no manufacturer stands behind.

Source: LMT Onsrud 2025 Production Cutting Tools catalog technical data, as published per series. Use the figures for the bit you actually own — every bit we sell lists its own on its product page. If you are unsure which applies, the Cut Doctor reads the series off your bit and uses that chart rather than a generic one.

Fixing it, in order

1. Add lubrication. This is the one change that addresses the actual mechanism. Nothing else on this list substitutes for it.
2. Calculate your real chipload with the formula above or the feed rate calculator.
3. Drop the RPM before you raise the feed. Most router spindles run aluminum far too fast, and lowering speed is usually easier on the machine than doubling feed.
4. Use fewer flutes. One or two. Aluminum chips are bulky and need somewhere to go; a 4-flute end mill packs the gullets and re-cuts its own swarf.
5. Never let it re-cut chips. Recutting is the fastest route to BUE. Air blast to clear the slot even when using MQL, and avoid full-width slotting at depth — step over instead.
6. Limit depth of cut and take more passes. Aluminum rewards light, fast, well-lubricated passes over heavy grinding ones.
7. Inspect the bit. If BUE has already formed, clean it off before running again — a welded lump will not clear itself, and the bit will keep smearing until it is removed.

Bits made for aluminum

Aluminum-specific geometry matters: high rake, polished flutes so chips slide instead of sticking, and wide gullets. A general-purpose wood bit has none of these and will weld far sooner.

Browse the full range in aluminum cutting router bits, or use the Bit Finder to match a bit to your alloy and thickness.

Common questions

Can I cut aluminum dry on a CNC router?

You can cut it, but you should not expect a good edge or reasonable tool life. Aluminum is the one common router material where lubrication changes the outcome rather than merely improving it, because the failure is adhesion rather than heat alone. Light shallow passes in thin sheet may survive dry; anything sustained will build up edge.

Is WD-40 good enough for cutting aluminum?

It is a common shop workaround and it is better than nothing, but it is a penetrant and solvent rather than a cutting lubricant — it evaporates quickly, has to be reapplied constantly, and leaves residue. A dedicated MQL lubricant is formulated for film strength at the cutting edge and is applied in far smaller quantities.

What RPM should I use for aluminum on a router?

Usually lower than your spindle's comfortable range. Many router spindles will not go below 8,000–10,000 RPM, and aluminum would often prefer less. Work out the feed your chipload target needs at your minimum RPM; if the required feed is beyond your machine, that is the real constraint, and a smaller-diameter bit or a lighter pass is the answer.

Why does the first pass look fine and later passes tear?

Because built-up edge accumulates. The bit is progressively less sharp with each pass, and the surface degrades as the blob grows. It is also a sign the tool is recutting chips — clear the slot and check your stepover.

Does anodised or cast aluminum behave differently?

Cast alloys are generally more abrasive and less gummy; wrought alloys such as 6061 are the classic welders. Anodising is only a surface layer and does not change the bulk behaviour beneath it. The chipload targets above apply across common alloys; start at the conservative end and verify on scrap.

Chipload ranges are published manufacturer application data for the series named, not a generic table. Manufacturers differ — sometimes substantially — for the same material and diameter, so use the figures for the bit you own. Every bit we sell lists its own recommended feeds and speeds on its product page.