Why Aluminum Is a Different Animal
Aluminum looks easy on paper — soft, gummy, low melting point. I’ve seen plenty of guys run it like mild steel and end up with packed flutes and a scrapped part. The problem is chip formation. Aluminum produces long, stringy chips that wrap around the drill shank and clog coolant ports if you don’t get the parameters right.
Thermal conductivity works in your favor. Aluminum pulls heat away from the cutting edge fast, which means I can push speeds higher than I ever would in steel. But that same softness creates built-up edge (BUE) when the cutting edge isn’t sharp enough or the chip load falls too low.
Expanded Parameter Table by Alloy and Drill Diameter
The basic speed and feed table covers the main alloys, but the drill diameter also matters. Here is the full reference I use, with feed rates scaled by diameter:
| Alloy | Drill Diameter | Cutting Speed (m/min) | Spindle RPM (range) | Feed Rate (mm/rev) | Coolant Pressure (psi) | Expected Ra (um) |
|---|---|---|---|---|---|---|
| 6061 | 3-6 mm | 100-150 | 6,000-12,000 | 0.020-0.035 | 800-1,200 | 0.4-0.8 |
| 6061 | 6-12 mm | 90-130 | 4,000-7,000 | 0.025-0.050 | 800-1,200 | 0.4-0.8 |
| 6061 | 12-20 mm | 80-120 | 2,500-4,000 | 0.030-0.060 | 700-1,000 | 0.5-1.0 |
| 7075 | 3-6 mm | 80-120 | 5,000-10,000 | 0.015-0.030 | 1,000-1,500 | 0.3-0.6 |
| 7075 | 6-12 mm | 70-100 | 3,500-6,000 | 0.020-0.040 | 1,000-1,500 | 0.3-0.6 |
| 7075 | 12-20 mm | 60-90 | 2,000-3,500 | 0.025-0.045 | 900-1,400 | 0.4-0.7 |
| 2024 | 3-6 mm | 90-130 | 5,500-11,000 | 0.020-0.035 | 900-1,400 | 0.4-0.7 |
| 2024 | 6-12 mm | 80-110 | 4,000-6,500 | 0.025-0.045 | 900-1,400 | 0.4-0.7 |
| 6063 | 3-6 mm | 110-160 | 7,000-14,000 | 0.025-0.045 | 700-1,100 | 0.5-0.9 |
| 6063 | 6-12 mm | 100-140 | 5,000-8,000 | 0.030-0.055 | 700-1,100 | 0.5-0.9 |
I run 6061 at the higher end of the RPM range when I need throughput. For 7075, I back the speed down and bump coolant pressure — that alloy is harder and more abrasive, so edge life drops fast if the coolant film breaks down. 6063 is the gummiest of the bunch; I keep feed on the higher side to break chips and avoid smearing the bore surface.
Chip Control Is Everything
If the chip comes out as a continuous ribbon longer than your hand, something is wrong. I break chips by running the feed aggressive enough to create segmented chips — what I call “comma chips” or “half-moons.” Those evacuate cleanly through the gun drill flute and won’t pack at the bushing.
Coolant pressure and flow are the second half of the chip control equation. I’ve written more detail on that in the gun drilling coolant pressure guide, but the short version is this: if you’re below 800 psi on a 6 mm or smaller hole in aluminum, you’re asking for chip packing. I start at 1,000 psi and adjust up until the chips come out cool and broken.
Built-Up Edge and How I Avoid It
BUE shows up as a rough, torn surface finish that looks like the bore was chewed instead of cut. I’ve traced almost every case back to one of three causes.
First, surface speed too low for the alloy. Aluminum needs a minimum SFM to keep the chip sliding over the rake face instead of welding to it. Second, coolant concentration too rich or wrong type — straight oil or a high-water emulsion that doesn’t provide enough lubricity. Third, a dull drill. I track drill regrind intervals by bore count, not by feel, because by the time you feel the edge go, you’ve already ruined ten parts.
When I see Ra climbing above 0.8 um on 6061, I check coolant pressure first, then pull the drill and look at the cutting edge under magnification. Nine times out of ten there is a small built-up ledge at the outer corner.
BUE Prevention Reference
Here is my checklist for preventing BUE in aluminum gun drilling:
| Factor | Target Range | Why It Matters |
|---|---|---|
| Cutting speed (6061) | 100-150 m/min minimum | Below 80 m/min, aluminum welds to carbide edge |
| Cutting speed (7075) | 80-120 m/min minimum | Higher alloying content needs speed for chip shear |
| Coolant pressure | 800-1,500 psi at nozzle | Flushes chips before they weld to the edge |
| Coolant concentration | 8-12% emulsion | Too lean = no lubricity, too rich = poor cooling |
| Coolant temperature | Below 35 C (95 F) | Above 100 F, BUE formation increases 2-3x |
| Rake angle | 0 to +5 degrees | Positive rake reduces cutting force and friction |
| Tool coating | DLC or polished TiB2 | Reduced adhesion prevents aluminum welding |
| Feed rate (>6 mm drill) | Above 0.025 mm/rev | Thin chips create more frictional heating per volume |
| Drill regrind interval | Every 200-500 bores (by count) | By the time BUE is visible, 10+ parts are already damaged |
I track all nine factors in a process control sheet for each aluminum job. When BUE appears, I check the sheet to see which factor drifted out of spec. The coolant temperature is the one I see most often in summer months.
Surface Finish Expectations by Alloy
Different aluminum alloys produce different surface finishes even at the same parameters. Here is what I typically hold:
| Alloy | Ra Range (um) | Rz Range (um) | Comments |
|---|---|---|---|
| 6061-T6 | 0.4 - 0.8 | 2.5 - 5.0 | Most forgiving, good finish at moderate parameters |
| 6061-T6 (with DLC drill) | 0.3 - 0.5 | 2.0 - 3.5 | DLC coating reduces friction significantly |
| 7075-T6 | 0.3 - 0.6 | 2.0 - 4.0 | Harder alloy allows lower Ra with higher coolant pressure |
| 2024-T3 | 0.4 - 0.7 | 2.5 - 5.0 | Comparable to 6061 but more abrasive on tool |
| 6063-T5 | 0.5 - 0.9 | 3.0 - 6.0 | Gummier, higher feed needed for chip breakage |
| Cast A356 | 0.6 - 1.2 | 4.0 - 8.0 | Silicon content wears tools faster, finish varies |
I have found that DLC-coated drills consistently improve surface finish by 0.1-0.2 um Ra across all alloys. The coating pays for itself in reduced rework, especially on high-volume 6061 runs.
Coolant Strategy for Aluminum
Aluminum demands high pressure and high volume. Unlike steel, where the coolant mainly lubricates the guide pads, in aluminum the coolant has to physically eject a chip that wants to stay connected. I use filtered coolant at 20-40 microns to keep the nozzle and drill ports from plugging.
I also pay attention to coolant temperature. In summer, when coolant temps climb above 100 F, I start seeing chip welding issues even at the same pressure and speed. I’ve installed a small heat exchanger on my main coolant tank to keep it below 90 F, and that fixed the seasonal variation.
For coolant chemistry, I use a semi-synthetic emulsion at 8-10% concentration with aluminum-specific corrosion inhibitors. Standard coolants can stain aluminum and leave a white residue that interferes with post-drilling inspections.
If you’re new to gun drilling, I recommend starting with the what is gun drilling overview before dialing in aluminum-specific parameters. The fundamental machine setup and drill geometry matter just as much as the cutting data.
Surface Finish and Hole Quality Expectations
With the right parameters, gun drilling aluminum produces a bore that doesn’t need secondary operations. I regularly hold 0.4-0.8 um Ra on 6061 with a single pass. On 7075, I can push down to 0.3 um Ra with higher coolant pressure and a slightly reduced feed.
The bore consistency is where gun drilling really shines. Over a 300 mm deep hole, I see less than 0.05 mm variation in diameter. Compare that to drilling from both ends, where you get a witness mark at the intersection and diameter variation that kills seal fit.
Straightness on aluminum is excellent if you use a proper starter bushing and enough guide pad engagement. I run a bushing-to-workpiece gap of 0.01-0.02 mm on diameter, no more.
Common Mistakes I Still See on the Floor
I visit shops regularly and these same problems keep coming up. Running too low a feed rate to “be safe” — that actually causes chip packing because the chip is too thin to break. Not checking coolant flow at the drill tip before starting the cycle — a plugged nozzle at 1,200 psi does nothing. Using the same drill geometry for all aluminum alloys — 6063 needs a different rake angle than 7075.
Another one: ignoring the chip shape. The chips tell you everything. I train operators to glance at the chip tray every cycle. If the chips change from broken commas to strings, stop and check before you scrap a hundred parts.
Key Takeaways
- Run aluminum at higher RPM and feed than steel — thin, stringy chips need aggressive parameters to break properly.
- Keep coolant pressure above 800 psi for small diameters; start at 1,000 psi for most aluminum jobs.
- Match speeds, feeds, and drill geometry to the specific alloy — 6061, 7075, 2024, and 6063 all need different treatment.
- Watch for BUE by tracking surface finish trends and inspecting the cutting edge regularly. Coolant temperature above 100 F is the most common trigger.
- The chip shape is your best real-time feedback — broken comma chips mean you’re in the sweet spot.
- DLC-coated drills improve surface finish by 0.1-0.2 um Ra and reduce BUE formation. The extra cost is worth it for production runs.
- I cover drill selection for aluminum in more detail in my article on gun drill coatings and tip selection.