Appearance
A machinist who has only ever drilled steel will be shocked the first time they gun drill 6061 aluminum. The cutting edge loads up with welded aluminum within seconds. The chips come out as long, continuous ribbons that wrap around the tool holder and pack solid in the flute. The hole entrance bell-mouths, the exit burr is huge, and the surface finish looks like it was chewed rather than cut. Aluminum is not harder to machine than steel — it is harder to machine well. The difference is that steel failures are gradual and give warning. Aluminum failures happen instantly, and they happen because the fundamental rules are different: sharper edges, higher speeds, more coolant, and a cutting edge that must be kept clean by preventing the work material from welding to it.
Getting deep hole drilling right in aluminum means understanding that every alloy is different. What works for 6061 will fail on 7075. What works on 7075 will fail on 390 cast aluminum. This article covers the full range of aluminum alloys and how to drill them.
Aluminum Alloy Families
Classification by Machinability
| Machinability | Alloy Examples | Key Characteristics | Deep Hole Drilling Suitability |
|---|---|---|---|
| Excellent (free-machining) | 2011, 6013, 6262 | Contains Pb, Bi, or Sn for chip breaking | Best — predictable chip form, long tool life |
| Good | 6061, 6082, 3003 | Balanced ductility, moderate chip control | Good with proper parameters and coolant |
| Fair | 2024, 7075, 7050 | High strength, more abrasive, tougher chips | Requires sharp tools, high coolant pressure |
| Poor | 5052, 5083, 5086 | Very ductile, stringy chips, galling tendency | Difficult — special chip breaker geometry needed |
| Abrasive | 390, A390, 4032 | High silicon content (11–23%), extremely hard Si particles | PCD tooling mandatory |
Common Alloys in Detail
| Alloy | Tensile Strength | Hardness | Elongation | Thermal Conductivity | Primary Applications |
|---|---|---|---|---|---|
| 2011 | 310 MPa | 100 HB | 15% | 151 W/m·K | Screw machine parts, fittings |
| 2024 | 470 MPa | 120 HB | 18% | 121 W/m·K | Aerospace structures, wing skins |
| 5052 | 230 MPa | 75 HB | 25% | 138 W/m·K | Marine, fuel tanks |
| 5083 | 290 MPa | 85 HB | 22% | 117 W/m·K | Marine, pressure vessels |
| 6061 | 290 MPa | 95 HB | 20% | 167 W/m·K | Structural, automotive, general |
| 6063 | 210 MPa | 75 HB | 25% | 201 W/m·K | Architectural extrusions |
| 6082 | 310 MPa | 100 HB | 16% | 180 W/m·K | Structural, bridges, cranes |
| 6262 | 310 MPa | 110 HB | 12% | 145 W/m·K | Free-machining, threaded components |
| 7050 | 510 MPa | 135 HB | 11% | 157 W/m·K | Aerospace, high-strength plates |
| 7075 | 540 MPa | 150 HB | 11% | 130 W/m·K | Aerospace, tooling, high-stress parts |
| A390 (17% Si) | 275 MPa | 120 HB | < 1% | 134 W/m·K | Engine blocks, cylinder liners, wear surfaces |
Why Aluminum Is Different from Steel
The Comparison That Matters
| Property | Aluminum (6061) | AISI 4140 Steel | Effect on Deep Hole Drilling |
|---|---|---|---|
| Thermal conductivity | 167 W/m·K | 42.6 W/m·K | Heat dissipates well — advantage |
| Coefficient of thermal expansion | 23.6 µm/m·K | 12.3 µm/m·K | Holes shrink on cooling — 2× steel |
| Melting point | 650°C | 1,420°C | Lower but BUE forms before melting |
| Modulus of elasticity | 68.9 GPa | 205 GPa | Workpiece deflection 3× steel |
| Elongation | 20% | 15% | More ductile, stringier chips |
| Chemical affinity to carbide | High | Low | BUE forms readily on cutting edge |
The Temperature-Diameter Problem
The thermal expansion coefficient of aluminum is double that of steel. A hole gun-drilled to 10.000 mm at 40°C (typical drilling temperature at the bore surface) will contract to 9.996 mm when measured at 20°C — a 4 µm difference. This is smaller than an H7 tolerance band (±15 µm for Ø10 mm), but the effect compounds at larger diameters and tighter tolerances.
For precision work, measure holes at a controlled temperature after allowing the part to stabilise. A gun drill that cuts on-size in a warm steel part may cut undersize in an aluminum part that cools before measurement.
Built-Up Edge (BUE) — The Primary Challenge
Aluminum has a strong chemical affinity for cobalt, the binder phase in tungsten carbide. At cutting temperatures above 150°C, aluminum atoms diffuse into the cobalt binder and form a micro-weld at the cutting edge. This adhesion grows into a built-up edge that changes the effective tool geometry:
| BUE Stage | Appearance | Effect on Drilling |
|---|---|---|
| Initial adhesion (0–5 sec) | Thin aluminium film on rake face | No measurable effect |
| BUE growth (5–30 sec) | Visible lump on cutting edge | Cutting forces increase 20–40% |
| BUE break-off (30–60 sec) | Fragment detaches, taking carbide grains | Edge chipping begins |
| Full BUE cycle established | Cyclic adhesion + break-off | Rough surface, oversize hole, tool failure |
The solution is preventing adhesion at the cutting edge through a combination of:
- Sharp, polished cutting edges (reduces contact pressure)
- High cutting speed (reduces contact time, raises temperature above BUE range)
- High lubricity coolant (prevents metal-to-metal contact)
- Non-aluminium-affinity coatings (TiB₂, DLC, diamond)
Tool Material and Coating Selection
Carbide Grades
| Application | Recommended Grade | Cobalt % | Grain Size |
|---|---|---|---|
| General aluminium (6061, 2024) | K10–K20 | 6–10% | Fine (0.5–1.0 µm) |
| Abrasive aluminium (7075, 7050) | K15–K25 | 8–12% | Fine to medium |
| High-silicon cast (390, A390) | PCD (not carbide) | N/A | N/A |
| Free-machining (2011) | K10–K15 | 6–8% | Fine |
For deep hole drilling of aluminum, the carbide grade should be tougher (higher cobalt) than for cast iron but harder than for titanium. The primary wear mode is not abrasion but edge chipping from BUE cyclic loading.
Coatings
| Coating | Why It Works | Limitation | Best For |
|---|---|---|---|
| TiB₂ (titanium diboride) | Chemically inert to aluminum — no adhesion | Expensive, limited availability | Production aluminum |
| DLC (diamond-like carbon) | Very low friction (µ < 0.1), releases chips | Limited temperature (~400°C) | General aluminum |
| CVD diamond | Extremely hard, no chemical affinity | High cost, edge sharpness | High-silicon aluminum |
| Uncoated (polished) | No coating issues, sharpest edge possible | Requires more frequent regrind | Small diameters, short runs |
Critical note: Do NOT use TiAlN or AlTiN coatings for aluminum. The aluminium in the coating chemically bonds with the workpiece aluminium, accelerating BUE formation.
Tool Geometry
| Feature | Aluminum-Optimised | Steel-Optimised | Why for Aluminum |
|---|---|---|---|
| Point angle | 130–140° | 118–130° | Lower thrust, reduces BUE |
| Rake angle | 10–15° positive | 0–5° positive | Shears rather than pushes |
| Flute finish | Polished (Ra < 0.2 µm) | Ground (Ra 0.4–0.8 µm) | Prevents chip adhesion |
| Helix angle (gun drill margin) | 30–45° | 15–25° | Faster chip evacuation |
| Cutting edge preparation | Sharp, no hone | 0.02–0.05 mm hone | Sharp edge shears cleanly |
| Margin width | Narrowed (50–60% of standard) | Standard | Reduces friction |
Gun Drilling Parameters
Recommended Starting Parameters
| Diameter | Alloy Group | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure | Coolant Flow |
|---|---|---|---|---|---|
| 3–6 mm | Free-machining | 150–250 | 0.010–0.035 | 100–150 bar | 15–30 L/min |
| 3–6 mm | High-strength (7075) | 120–200 | 0.008–0.025 | 120–180 bar | 15–30 L/min |
| 3–6 mm | Ductile (5052) | 100–180 | 0.005–0.020 | 100–150 bar | 15–30 L/min |
| 6–12 mm | Free-machining | 200–300 | 0.020–0.060 | 80–120 bar | 30–60 L/min |
| 6–12 mm | High-strength (7075) | 150–250 | 0.015–0.050 | 100–150 bar | 30–60 L/min |
| 6–12 mm | Ductile (5052) | 130–220 | 0.010–0.040 | 80–120 bar | 30–60 L/min |
| 12–25 mm | Free-machining | 200–300 | 0.040–0.100 | 60–100 bar | 60–120 L/min |
| 12–25 mm | High-strength (7075) | 150–250 | 0.030–0.080 | 80–120 bar | 60–120 L/min |
Critical Rules for Aluminum Gun Drilling
| Rule | Reason |
|---|---|
| Coolant on before tool enters workpiece | Prevents immediate BUE formation on hot start |
| Never interrupt feed while cutting | Feed mark creates a ridge that initiates BUE |
| Polished flute margins | Reduces aluminium adhesion to guide pads |
| Sharp cutting edges | Dull edge increases pressure, accelerates BUE |
| Higher speed, not lower feed, controls BUE | BUE decreases above ~150 m/min for most alloys |
| Check coolant concentration daily | Water evaporation changes lubricity rapidly |
| Retract with coolant flowing | Clears chips from bore before they can score the surface |
Feed Rate Adjustment by L/D Ratio
| L/D Ratio | Feed Adjustment (relative to starting feed) |
|---|---|
| < 20:1 | 100% (nominal) |
| 20:1 – 50:1 | 80–90% |
| 50:1 – 100:1 | 65–80% |
| > 100:1 | 50–65% |
Aluminium's low modulus of elasticity means the tool deflects more at depth. Reducing feed at high L/D ratios prevents the drill from walking and maintains straightness.
BTA Drilling Parameters
Recommended Starting Parameters
| Diameter | Alloy Group | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Flow | Coolant Pressure |
|---|---|---|---|---|---|
| 20–40 mm | Free-machining | 150–250 | 0.10–0.30 | 200–400 L/min | 30–60 bar |
| 20–40 mm | High-strength (7075) | 120–200 | 0.08–0.25 | 200–400 L/min | 40–70 bar |
| 20–40 mm | Ductile (5052) | 100–180 | 0.06–0.20 | 200–400 L/min | 30–60 bar |
| 40–80 mm | Free-machining | 150–250 | 0.15–0.40 | 300–600 L/min | 20–50 bar |
| 40–80 mm | High-strength (7075) | 120–200 | 0.10–0.35 | 300–600 L/min | 30–60 bar |
BTA Tool Design for Aluminum
| Feature | Recommendation |
|---|---|
| Insert grade | Uncoated fine-grain carbide or PCD |
| Wiper edge | Required for surface finish |
| Guide pad material | Carbide (not bronze — aluminium welds to bronze) |
| Guide pad geometry | Increased relief angle, polished surface |
| Chip breaker | Step-type recommended (groove type packs with aluminium) |
Coolant Selection
Types Compared
| Coolant Type | Lubricity | Cooling | Chip Evacuation | Surface Finish | Environmental |
|---|---|---|---|---|---|
| Neat oil (chlorinated EP) | Excellent | Good | Excellent | Best | Disposal issues |
| Neat oil (chlorine-free) | Very good | Good | Excellent | Very good | Preferred |
| Semi-synthetic (8–12%) | Good | Very good | Good | Good | Lower cost |
| Emulsion (5–8%) | Moderate | Excellent | Good | Moderate | Most economical |
| MQL | Good (low volume) | Poor | Fair | Good | Lowest impact |
For deep hole drilling of aluminium, neat cutting oil is strongly preferred for the following reasons:
- Maximum lubricity prevents BUE formation
- High film strength protects guide pads
- Consistent viscosity maintains seal at the rotary union
- Superior surface finish — Ra 0.2–0.4 µm routinely achievable
Semi-synthetic and emulsion coolants can be used for less demanding applications but require higher concentration (8–12%) and frequent monitoring.
Additives
| Additive | Function | Recommendation for Aluminum |
|---|---|---|
| Sulphurised EP | Extreme pressure lubricant | Not recommended — stains aluminum |
| Chlorinated EP | Extreme pressure lubricant | Effective but regulated (waste disposal) |
| Phosphorus EP | Extreme pressure for non-ferrous | Recommended — does not stain |
| Ester lubricity improver | Reduces friction | Recommended |
| Antioxidant | Extends oil life | Recommended for circulating systems |
Chip Control
Chip Formation in Aluminum
The chip form is the most important indicator of process health in aluminum deep hole drilling:
| Chip Form | What It Means | Action |
|---|---|---|
| Short, segmented, C-shaped | Ideal — good chip breaking | None |
| Short, helical | Acceptable — some ductility | Monitor |
| Long, continuous string | Chip breaker not engaging | Reduce feed, check chip breaker geometry |
| Long, snarled ribbons | Severe ductility — material welding | Increase speed, improve lubricity |
| Powder / dust | Tool rubbing, not cutting | Increase feed, check tool sharpness |
| Discoloured (blue/brown) | Overheating, friction galling | Increase coolant pressure, check flow |
Chip Breaking Strategies
| Method | How It Works | Effectiveness in Aluminum |
|---|---|---|
| Chip breaker step on rake face | Deflects chip to break | Good — most common approach |
| Modulated feed / vibration | Periodic feed variation breaks chip | Very good — for long L/D and ductile alloys |
| High coolant pressure | Hydraulic force breaks chip | Good — 100+ bar effective |
| Alloy selection | Free-machining grades contain chip breakers | Best — but limited to specific alloys |
| Feed increase | Thicker chip breaks more readily | Limited by surface finish and tool load |
TIP
For ductile alloys like 5052 and 5083 that produce continuous stringy chips, a vibration-assisted feed (low-frequency modulation at 0.5–2 Hz, amplitude 0.05–0.15 mm) is the most effective chip breaking method. The modulated feed creates chip thickness variation that initiates fracture without increasing average chip load.
Chip Evacuation
| Hole Depth | Chip Evacuation Method | Risk |
|---|---|---|
| < 20× diameter | Standard coolant pressure | Low |
| 20–50× diameter | Increased pressure + chip breaker | Moderate — chip packing in long flutes |
| 50–100× diameter | High pressure + vibration assistance | High — flute packing is primary failure mode |
| > 100× diameter | Maximum pressure + optimised chip form | Very high — requires careful parameter control |
Surface Finish and Hole Quality
Achievable Quality
| Parameter | Typical (Gun Drilling) | Best (Optimised) |
|---|---|---|
| Surface finish Ra | 0.4–0.8 µm | 0.2–0.4 µm |
| Diameter tolerance | H8–H9 | H7 |
| Roundness | 0.010–0.025 mm | 0.005–0.015 mm |
| Straightness (per 100 mm) | 0.03–0.10 mm | 0.02–0.05 mm |
Common Defects and Fixes
| Defect | Cause | Fix |
|---|---|---|
| Bell-mouth entry | BUE at entry, bushing clearance | Increase speed, check bushing fit |
| Oversize hole (uniform) | High coolant pressure deflecting tool | Reduce pressure, or accept if within tolerance |
| Oversize exit | Tool wander at depth, low feed | Increase feed at depth, check chip evacuation |
| Rough surface finish | BUE on cutting edge | Increase speed, improve lubricity, polish tool |
| Spiral marks / rifling | Chatter from low stiffness | Adjust speed, increase damping |
| Exit burr (large) | Ductile alloy, high feed at break-through | Reduce feed in last 2–3 mm, use chamfer |
| Hole undersize (thermal) | Part cooled below 20°C before measurement | Stabilise temperature, or cut +0.005 mm oversize |
| Galling on bore wall | Chip scoring during retraction | Retract with coolant flowing |
| Taper (entry larger) | Tool deflection, bushing wear | Check bushing, reduce overhang |
Troubleshooting Guide
| Symptom | Likely Cause | First Action |
|---|---|---|
| BUE forms immediately | Speed too low (< 100 m/min) | Increase speed to > 150 m/min |
| BUE forms after steady operation | Coolant lubricity lost | Check concentration, change coolant |
| Chips pack in flute | Feed too low, chip breaker not engaging | Increase feed 10–20%, check chip form |
| Coolant pressure builds up | Chip packing restricting flow | Retract, clear chips, check chip breaker |
| Tool squeals during cut | BUE altering effective geometry | Retract immediately, inspect edge |
| Surface finish degrades mid-hole | BUE forming as tool heats | Increase coolant flow, reduce speed |
| Hole diameter at exit oversize | Tool walking at depth | Reduce feed, increase pressure |
| Guide pads show galling | Insufficient lubrication | Increase oil lubricity, check filtration |
| Tool breaks at entry | BUE shock loading | Ensure coolant on before cut, check sharpness |
| Tool breaks at depth | Chip packing | Increase pressure, improve chip breaking |
FAQ
Q: What is the best cutting speed for gun drilling 6061 aluminum? 150–250 m/min for small diameters (3–12 mm), 200–300 m/min for larger diameters. Speed should be high enough to prevent BUE formation but low enough to avoid built-up edge from thermal softening.
Q: Why does aluminum cause built-up edge on carbide tools? Aluminum has a high chemical affinity for cobalt, the binder in tungsten carbide. At cutting temperatures above 150°C, aluminium atoms diffuse into the cobalt, forming a micro-weld. TiB₂ or DLC coatings prevent this by providing a chemically inert barrier.
Q: What coolant works best for deep hole drilling aluminum? Neat cutting oil with EP additives (chlorine-free preferred) provides the best results. Minimum 80 bar pressure for gun drilling. Semi-synthetic coolants at 8–12% concentration are acceptable for less demanding applications.
Q: How does 7075 aluminum compare to 6061 for deep hole drilling? 7075 is stronger (540 MPa vs 290 MPa), more abrasive, and produces tougher chips. It requires sharper tools, higher coolant pressure, and slightly lower cutting speeds. Tool life in 7075 is typically 60–70% of that in 6061.
Q: What tool coating prevents aluminum adhesion in deep hole drilling? TiB₂ (titanium diboride) is the best coating for aluminum — it is chemically inert to aluminum and prevents adhesion. DLC (diamond-like carbon) is also effective due to its low friction coefficient. TiAlN and AlTiN must be avoided — the aluminum in the coating reacts with the workpiece.
Q: How is chip breaking achieved in ductile aluminum alloys? A chip breaker step on the rake face is the primary method. For ductile alloys (5052, 5083), vibration-assisted feed modulation is the most effective approach. Free-machining alloys (2011, 6262) contain lead or bismuth additives that inherently produce short chips.
Q: What surface finish can be achieved when gun drilling aluminum? Ra 0.2–0.4 µm is routinely achievable with sharp tools, proper parameters, and high-lubricity coolant. The roller-burnishing action of the guide pads contributes to the polished surface.
Q: What causes oversize holes at the exit when drilling aluminum? Tool walking at depth due to aluminum's low modulus of elasticity (68.9 GPa — one-third of steel). The drill deflects more at depth, producing a larger exit diameter. Reducing feed at high L/D ratios and maintaining high coolant pressure minimises this effect.
Q: Can MQL be used for deep hole drilling aluminum? MQL is effective for L/D ratios up to about 20:1 in free-machining and high-strength alloys. Beyond this depth, the lubricant film does not reach the cutting edge consistently, and BUE forms rapidly. For deep holes (> 50:1 L/D), high-pressure oil is required.
Q: How many holes can be drilled between regrinds in aluminum? With proper parameters and coolant, carbide gun drills in 6061 can achieve 1,000–3,000 holes between regrinds. In 7075, 500–1,500 holes is typical. In high-silicon cast aluminum (A390), carbide may last only 10–50 holes — PCD tooling is required for production quantities.