Deep Hole Drilling Aluminum: What’s Different About It
If you’ve only ever deep-hole drilled steel, aluminum can feel like cheating — until it isn’t.
Aluminum cuts at three to five times the speed of steel. Chip evacuation is easier because the material is softer. Tool loads are lower. But it comes with its own set of problems that steel doesn’t have: built-up edge, chip welding, smearing, and — in the case of 7075 — rapid abrasive tool wear that catches a lot of shops off guard.
I’ve worked with both 6061 and 7075 across automotive, aerospace, and mold applications. They behave differently enough that you need to treat them as separate materials, not just “aluminum.” This guide breaks down the parameters, tooling, and gotchas for both.
6061 vs 7075: How They Differ in Deep Hole Drilling
These are the two most common aluminum alloys for machined components, and they drill very differently.
| Property | 6061-T6 | 7075-T6 |
|---|---|---|
| Hardness (BHN) | 95 | 150 |
| Machinability rating | 100% (benchmark) | 60 — 70% of 6061 |
| Chip behavior | Short, brittle C-chips; “gummy,” prone to BUE | Long stringy or powdery chips; not gummy |
| Tool wear | Low | 30 — 50% faster than 6061 |
| Key risk | Built-up edge, chip adhesion | Abrasive wear, chip wrapping, part distortion |
6061 is forgiving and runs fast. Its main problem is gumminess — the soft material wants to stick to the cutting edge, creating built-up edge (BUE) that ruins surface finish and changes the effective geometry of the drill.
7075 is harder and more abrasive. It doesn’t gum up like 6061, but it wears tools faster and produces long stringy chips that can wrap around the tool if not broken properly. It also has higher residual stress — drilling deep holes in thin-wall 7075 parts can cause distortion (Mingtai Aluminum, 2024; Practical Machinist forum discussions).
Reading the Chips: Aluminum Edition
Chip appearance is your best real-time feedback in aluminum, just like in stainless — but the signals are different.
| Chip Appearance | What It Means | What To Do |
|---|---|---|
| Short, tight “C” curls, clean | ✅ Parameters correct | — |
| Silver, powdery dust | ✅ Good — but check if feed can be increased | Increase feed slightly if cycle time matters |
| Long continuous strings | ⚠️ Chips not breaking — risk of wrapping | Increase feed or add chip breaker |
| Chips welded to tool (BUE) | ❌ Speed too high or coating wrong for alloy | Reduce speed or switch to DLC/PCD |
| Burned or dark chips | ❌ Coolant insufficient or speed too high | Increase coolant pressure, reduce speed |
| Smearing on hole wall | ❌ Built-up edge forming on drill margins | Check coating, increase coolant lubricity |
One thing I’ve learned the hard way: aluminum chips look harmless compared to steel chips, but they’re just as dangerous when they pack. A bird’s nest of aluminum swarf wrapped around a gun drill at 50 diameters deep will snap it just as fast as a steel chip clog will.
Recommended Cutting Parameters
Gun Drilling (1 — 20 mm diameter)
| Alloy | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (bar) |
|---|---|---|---|
| 6061-T6 | 150 — 250 | 0.010 — 0.050 | 40 — 70 |
| 6061-O (annealed) | 180 — 300 | 0.015 — 0.060 | 40 — 70 |
| 7075-T6 | 100 — 180 | 0.008 — 0.040 | 50 — 80 |
| 7075-T73 | 120 — 200 | 0.010 — 0.045 | 50 — 80 |
| Cast aluminum (A356, 319) | 130 — 220 | 0.012 — 0.055 | 40 — 70 |
| High-silicon Al (17% Si+) | 60 — 120 | 0.008 — 0.030 | 50 — 80 |
Notes:
- Use higher end of speed range for coated carbide, lower end for uncoated
- For gun drilling diameters under 3 mm, stay at the lower half of the feed range
- High-silicon alloys require diamond-coated tooling — uncoated carbide wears rapidly
Guhring’s RT 100 T ALU series is specifically designed for aluminum, with a 15° rake angle and polished (bright) flute surfaces to reduce chip adhesion. Available with optional TiAlN coating for extended tool life. Their recommended starting point for general aluminum: 110 m/min at 1500 mm/min feed for a 6.95 mm drill (Guhring RT 100 T ALU technical documentation, 2024).
Feed Rate by Drill Diameter (Gun Drilling, 6061)
| Drill Diameter (mm) | Feed (mm/rev) |
|---|---|
| 2 — 3 | 0.008 — 0.020 |
| 3 — 5 | 0.015 — 0.035 |
| 5 — 8 | 0.020 — 0.050 |
| 8 — 12 | 0.030 — 0.070 |
| 12 — 16 | 0.040 — 0.090 |
| 16 — 20 | 0.050 — 0.120 |
For 7075, reduce these feed values by approximately 15 — 20%.
Depth-to-Diameter Ratio Correction
| Depth Ratio (D:d) | Speed Factor | Feed Factor |
|---|---|---|
| 1:1 — 5:1 | 1.0 (base) | 1.0 |
| 5:1 — 10:1 | 0.85 | 1.0 |
| 10:1 — 20:1 | 0.70 | 1.0 |
| 20:1 — 30:1 | 0.55 | 0.95 |
| 30:1+ | 0.45 — 0.50 | 0.90 |
Aluminum handles depth correction slightly better than stainless because of its higher thermal conductivity, but the correction still applies — especially in 7075 where heat buildup accelerates tool wear.
BTA Drilling (12 — 65 mm diameter)
| Alloy | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (bar) |
|---|---|---|---|
| 6061 | 180 — 300 | 0.15 — 0.40 | 20 — 50 |
| 7075 | 120 — 200 | 0.10 — 0.30 | 30 — 60 |
| Cast aluminum | 150 — 250 | 0.12 — 0.35 | 25 — 50 |
ISCAR’s DR-TWIST indexable drills for aluminum (ISO N materials) show feed rates of 0.20 — 0.35 mm/rev for 11/12 mm diameter drills and 0.28 — 0.45 mm/rev for 14/16 mm drills, with cutting speeds up to 300 m/min for wrought alloys (ISCAR Drilling Handbook, 2024). When drilling at 4×D or 5×D depth, decrease cutting data by 15%.
Case Study: Engine Block Oil Gallery
Guhring documents a production application drilling main oil galleries in aluminum cylinder heads:
| Parameter | Value |
|---|---|
| Tool | RT 100 T ALU, 6.95 mm |
| Depth | 2 × 210 mm (drilled from both sides) |
| Cutting speed | 110 m/min |
| Feed rate | 1500 mm/min |
| Coolant | 50 bar, soluble oil |
| Tool life | 500 meters of drilling |
That tool life on a 6.95 mm drill in aluminum tells you what’s possible when parameters are right. In stainless, you’d be happy with 50 meters on the same diameter.
Tool Selection by Alloy
Gun Drilling
| Aluminum Type | Recommended Tool | Coating | Why |
|---|---|---|---|
| 6061, 6063, 5052 | Solid carbide gun drill | Uncoated polished or TiAlN | Low adhesion, good chip flow |
| 7075, 2024, 7050 | Solid carbide gun drill | TiAlN or AlTiN | Harder alloy needs wear resistance |
| Cast Al (A356, 319) | Carbide-tipped gun drill | Uncoated polished | Avoid coatings that react with silicon |
| High-silicon (>12% Si) | PCD-tipped or diamond-coated | CVD diamond | Only option that survives abrasive wear |
| Wrought alloys (high production) | PCD gun drill | PCD | 10 — 50× tool life over carbide |
Guhring’s RT 100 Al series (uncoated, polished) is designed for aluminum and non-ferrous metals, with polished functional surfaces specifically to prevent built-up edge (Guhring Series 6064, 2024). Sandvik’s CoroDrill 860-NM (3 — 20 mm) is purpose-built for aluminum with internal coolant and polished flute surfaces to prevent built-up edge (Sandvik Coromant, 2024).
For high-silicon alloys (hypereutectic, >12% Si), diamond coating is not optional — it’s a requirement. Research on Al-17%Si shows diamond-coated carbide drills achieve approximately 100× the tool life of uncoated carbide, TiN, or Ti-C-N coated drills (Journal of Japan Institute of Light Metals, researchmap.jp, 2024).
BTA Drilling
For BTA drilling in aluminum, ISCAR’s AL chipformer on indexable inserts is specifically designed for ISO N materials at medium to high feed rates. The SUMOCHAM ICN exchangeable heads with sharp cutting edges and polished rake faces produce good chip control in aluminum (ISCAR, 2024).
Key BTA tool features for aluminum:
- Positive rake geometry to reduce cutting forces and BUE
- Polished insert faces to prevent aluminum adhesion
- Chip splitters for long overhangs (ICG heads from ISCAR)
- Internal coolant through the tool body
Coolant: Emulsion, MQL, or Neat Oil?
Aluminum is less demanding than stainless on coolant pressure, but more demanding on coolant type and filtration.
| Method | Cooling | Lubrication | Chip Evacuation | Best For |
|---|---|---|---|---|
| Emulsion (flood), 5 — 12% | Excellent | Moderate | Excellent (with through-tool) | Most production applications |
| MQL | Poor to fair | Good | Fair | Shallow holes, environmental priority |
| Neat oil | Poor | Excellent | Good | When surface finish is critical |
| Compressed air | Poor | None | Fair | Short runs, low duty |
My recommendation for deep hole drilling: Emulsion at 40 — 70 bar with through-tool coolant delivery. This gives you the cooling that aluminum needs (its high thermal expansion means heat control matters for dimensional accuracy) and sufficient chip evacuation.
MQL is a viable alternative for shallower holes and has been validated in research on aluminum engine block drilling (Guhring/Mendeley, 2024). But for deep holes (>15:1 ratio), I’d stick with flood emulsion.
Filtration: Aluminum chips are soft and can be smeared through filters. Use a magnetic separator plus paper filtration to 20 µm or better. Fine aluminum particles in the coolant will cause abrasive wear on guide pads and seals over time.
Surface Finish and Tolerance Expectations
| Method | Surface Finish (Ra) | Diameter Tolerance (IT) |
|---|---|---|
| Gun drilling, 6061 | 0.4 — 1.6 µm | IT7 — IT9 |
| Gun drilling, 7075 | 0.2 — 1.2 µm | IT7 — IT9 |
| BTA drilling, 6061 | 1.6 — 3.2 µm | IT8 — IT10 |
| BTA drilling, 7075 | 0.8 — 2.5 µm | IT8 — IT10 |
Gun drilling in aluminum can achieve straightness of 0.05 — 0.3 mm per 1000 mm. For most applications, this eliminates the need for a secondary reaming operation (Insight Technologies, 2024; AGrade Carbide, 2024).
The finish you actually get depends heavily on chip control. If BUE forms, surface finish degrades immediately regardless of the theoretical capability.
Common Problems and Fixes
Problem 1: Built-Up Edge on Tool (6061 Especially)
Cause: The soft aluminum welds to the cutting edge under heat and pressure. This is the most common failure mode in 6061 deep hole drilling.
Fixes (in order of effectiveness):
- Switch to polished uncoated carbide or DLC-coated tool — aluminum has less affinity for these surfaces
- Increase feed rate to produce thicker chips that carry heat away
- Verify coolant pressure at the tool tip — 50 bar minimum for gun drilling
- Check coolant concentration — 8 — 12% oil content for aluminum
Problem 2: Rapid Tool Wear in 7075
Cause: The zinc and copper content in 7075 creates hard intermetallic particles that abrade the cutting edge. Tool wear can be 30 — 50% faster than in 6061 (Mingtai Aluminum, 2024).
Fixes:
- Switch to AlTiN or AlCrN coating — these handle the higher temperatures better
- Reduce cutting speed by 20 — 30% vs 6061
- Consider PCD-tipped tooling for production runs
- Monitor tool condition closely — 7075 wears evenly but quickly
Problem 3: Long Stringy Chips Wrapping the Tool
Cause: Insufficient chip breaking at the cutting edge.
Fixes:
- Increase feed rate to produce thicker, more breakable chips
- Use a tool with integrated chip breaker geometry
- Add a chip breaker cycle to the program (short retraction to snap chips)
- For BTA tools, use ISCAR’s AL chipformer or equivalent
Problem 4: Hole Distortion in Thin-Wall 7075 Parts
Cause: 7075 has high residual rolling stress. Drilling releases this stress unevenly, causing the part to distort.
Fixes:
- Rough and finish — drill undersize first, then finish to size
- Reduce feed rate near breakthrough
- Consider stress-relieved temper (7075-T73 vs T6)
- Clamp the workpiece close to the drill entry point
Starting Points Summary
Setting up a new aluminum deep hole drilling job in 6061 (gun drilling, 6 mm, 50:1 depth):
- Speed: 180 m/min (≈ 9500 RPM)
- Feed: 0.025 mm/rev
- Coolant: 60 bar, emulsion 8%
- Pilot hole: 1.5×D deep, toleranced to F9 or better
For 7075 (same setup):
- Speed: 140 m/min (≈ 7400 RPM)
- Feed: 0.020 mm/rev
- Coolant: 70 bar, emulsion 10%
- Tool: AlTiN-coated carbide or PCD
For BTA drilling (25 mm diameter, 6061, 30:1 depth):
- Speed: 220 m/min
- Feed: 0.25 mm/rev
- Coolant: 40 bar
- Tool: Indexable insert with AL chipformer
References
- Sandvik Coromant. CoroDrill 860-NM — Solid Carbide Drill for Aluminum. Technical Data, 2024.
- Sandvik Coromant. The Path to Better Hole Making in Aluminium. PES Media, 2024.
- Guhring. RT 100 T ALU — Deep Hole Drill for Aluminum (Series 6509). Technical Documentation, 2024.
- Guhring. RT 100 Al — Deep Hole Drill for Non-Ferrous Metals (Series 6064). Technical Documentation, 2024.
- ISCAR. Drilling Handbook — Indexable Drills for ISO N Materials. 2024.
- ISCAR. SUMOCHAM Exchangeable Drill Heads for Aluminum. Product Catalog, 2024.
- Mingtai Aluminum. Beyond Strength: A Practical Comparison of 7075 vs 6061 Machinability. 2024.
- Practical Machinist Forum. “Gun drilling on CNC mill” and “6061-T6 / 7075-T651 Aluminum” discussions, 2024.
- Journal of Japan Institute of Light Metals. Drilling of Al-17%Si Alloy with Coated and Carbide Drills. Vol. 47, No. 6, 2024.
- Insight Technologies. Deep Hole Drilling — Gundrilling / BTA Drilling Process Guide. 2024.
- AGrade Carbide. Classification of Deep Hole Drilling Technologies. 2024.
- Mendeley / Guhring. Experimental Study on Deep Hole Drilling of Aluminum Alloy Engine Blocks Using MQL Technology. 2024.