Why aluminum needs a different end mill
Aluminum alloys such as 6061-T6 and 7075-T6 are free-machining materials, but they produce a soft, gummy chip that sticks to cutting edges. A general-purpose steel end mill will gum up, weld aluminum to the flute, and fail within minutes. Selecting the correct tool comes down to four decisions: flute count, helix angle, coating and edge preparation.
1. Flute count
| Flutes | Best for | Chip evacuation | Finish |
|---|---|---|---|
| 1 (single flute) | High-speed profile cutting, very soft alloys, acrylic-aluminum hybrids | Excellent | Good |
| 2 | General purpose milling, slotting, pocketing | Very good | Good |
| 3 | Finishing passes, thin walls, higher feed rates on rigid machines | Moderate | Very good |
For most shops, a 2-flute 45-degree helix tool is the workhorse. Move to 3 flutes only when the machine, holder and workpiece clamping are rigid enough to support the higher metal removal rate.
2. Helix angle
- 30-38 degrees: heavier cuts, unstable setups, good chip control
- 40-45 degrees: general purpose, shears aluminum cleanly, reduces cutting force
- 45-60 degrees (high helix): excellent finish on thin walls and deep pockets; the upward shearing action pulls the part into the fixture
3. Coating choice
Uncoated polished flutes work, but modern coatings multiply tool life:
| Coating | Hardness (HV) | Max temp | Notes |
|---|---|---|---|
| ZrN | ~2600 | 500 C | Cheap, bright, low friction, ideal entry choice |
| DLC (diamond-like carbon) | ~3000 | 400 C | Lowest friction, best anti-galling on high-Si alloys |
| nACO (nano composite) | ~3200 | 700 C | All-rounder if you also cut steels |
| Uncoated, polished | n/a | n/a | Pure aluminum, resharpenable, lowest cost |
4. Geometry details that matter
- High rake, keen edge: a sharp positive edge reduces rubbing and heat
- Polished flutes: mirror-smooth flutes stop chips from welding
- Corner radius: a 0.5 mm radius corner adds 2-3x tool life versus a square end in pocketing
- Chipbreaker roughers: for heavy stock removal on plates, a corn-cob rougher at 8000 RPM removes aluminum fast without packing the flute
Starting parameters for 6061-T6
| Tool dia (mm) | RPM (carbide) | Feed (mm/min) | Depth of cut |
|---|---|---|---|
| 3 | 18000-24000 | 1800-2800 | 0.5 x D |
| 6 | 12000-16000 | 2200-3500 | 0.5 x D |
| 10 | 8000-11000 | 2500-4000 | 0.5 x D |
| 12 | 7000-9000 | 2500-3800 | 0.4 x D |
Start at the low end and increase feed until the finish begins to degrade, then back off 10 percent. Flood coolant or air blast is mandatory in deep pockets.
Summary
For 90 percent of aluminum work: 2-flute, 45-degree helix, ZrN or DLC coated, polished flute, corner radius for pocketing. Upgrade to 3-flute high-helix for finishing and thin-wall work, and single-flute for pure high-speed profiling on light machines.
