Appearance
Aluminum is the most forgiving material for deep hole drilling in terms of cutting forces and power requirement, and the most unforgiving in terms of built-up edge and chip control. The difference between a good hole and a scrapped part is often measured in microns of edge build-up that forms and breaks away in a single cycle.
Overview
Aluminum alloys are classified as the easiest-to-machine group for deep hole drilling when the correct parameters and tooling are applied. The low cutting forces, high thermal conductivity, and excellent surface finish potential make aluminum ideal for high-productivity gun drilling. However, the material's ductility and chemical affinity for carbide create challenges that, if not managed, turn a productive process into a source of inconsistent hole quality and frequent tool changes.
| Aluminum Type | Machinability Rating | Primary Challenge | Typical Application |
|---|---|---|---|
| 6061 (structural) | Excellent | Chip control | Hydraulic components, structural |
| 2024 (aerospace) | Good | BUE formation | Aerospace structural, fittings |
| 7075 (aerospace) | Good | Surface finish consistency | Aerospace, high-strength parts |
| 2011 (free-cutting) | Excellent | None — most machinable | General purpose, high production |
| 380 (die cast) | Good | Porosity interruptions | Automotive, high volume |
| 7050 / 7475 | Moderate | Stress relief required before drilling | Aerospace plate |
Cutting Parameters
Recommended Starting Parameters
| Alloy | Vc (m/min) Carbide | Vc (m/min) HSS | Feed (mm/rev) — 6 mm Ø | Feed (mm/rev) — 12 mm Ø | Feed (mm/rev) — 20 mm Ø |
|---|---|---|---|---|---|
| 2011, 6061, 6262 | 60 – 100 | 30 – 50 | 0.04 – 0.10 | 0.10 – 0.20 | 0.15 – 0.30 |
| 2024, 7075 | 50 – 80 | 25 – 40 | 0.03 – 0.08 | 0.08 – 0.15 | 0.12 – 0.25 |
| 7050, 7475 | 40 – 60 | 20 – 30 | 0.02 – 0.06 | 0.06 – 0.12 | 0.10 – 0.20 |
| 380, A356 (cast) | 40 – 70 | 20 – 35 | 0.03 – 0.08 | 0.08 – 0.15 | 0.10 – 0.20 |
Speed and Feed Effects
| Parameter | Effect on Process | Optimization Strategy |
|---|---|---|
| Increasing cutting speed | Reduces BUE tendency, improves surface finish | Run at upper speed range for aluminum |
| Increasing feed rate | Increases MRR, coarsens surface finish | Balance against finish requirement |
| High speed + high feed | Maximum productivity, good finish | Preferred for production aluminum drilling |
| Low speed + low feed | BUE prone, poor surface finish | Avoid — promotes adhesion |
Aluminum is unique among deep hole drilling materials in that higher cutting speeds improve surface finish — the opposite of the relationship in steel. This is because higher speeds reduce the tendency for BUE formation by raising the temperature at the chip-tool interface above the adhesion activation range.
Chip Control
Chip Formation in Aluminum
Aluminum produces distinctive chip forms depending on the alloy and parameters:
| Chip Form | Cause | Acceptability |
|---|---|---|
| Tight conical spiral | Good chip breaker engagement, moderate feed | Ideal |
| Loose long spiral | Feed too low, chip breaker not engaging | Problematic — packing risk |
| Segmented / granular | High feed, aggressive breaker | Acceptable, excellent evacuation |
| Needle-like | Very high feed, brittle alloy | Acceptable but may pack |
| Built-up edge fragments | BUE forming and releasing intermittently | Unacceptable — surface damage |
Chip Breaker Requirements
Aluminum requires a more aggressive chip breaker than steel because:
- Higher ductility — aluminum chips resist bending fracture more than steel chips
- Higher volume — at 2–3× the metal removal rate of steel, more chip volume must be managed
- Lower chip curl stiffness — thin aluminum chips curl easily but do not fracture readily
| Parameter | Aluminum Chip Breaker | Steel Chip Breaker |
|---|---|---|
| Breaker width | 0.8 – 1.5 mm (wider) | 0.5 – 1.0 mm |
| Breaker height | 0.2 – 0.4 mm | 0.15 – 0.35 mm |
| Breaker distance from edge | 1.0 – 2.5 mm (farther) | 0.5 – 1.5 mm |
| Rake angle | +5° to +15° (more positive) | 0° to +6° |
Built-Up Edge Prevention
BUE is the most common quality problem in aluminum deep hole drilling. It forms when aluminum adheres to the carbide cutting edge, building up a layer of work-hardened material that periodically breaks away, taking fragments of the cutting edge with it.
BUE Prevention Strategies
| Strategy | Effectiveness | Implementation |
|---|---|---|
| Increase cutting speed | High | Run Vc ≥ 60 m/min for carbide |
| Use coated tools | High | DLC, diamond, or TiAlN coatings |
| Coolant lubricity | Medium | Use high-lubricity coolant (≥ 8% emulsion or oil) |
| Polish cutting edge | Medium | Edge hone 5 – 15 µm reduces adhesion sites |
| Positive rake angle | High | Use +8° to +15° rake |
| Coolant pressure | Low (BUE is chemical, not thermal) | Not a primary lever |
Tool Coatings for Aluminum
| Coating | BUE Resistance | Wear Resistance | Cost | Best For |
|---|---|---|---|---|
| DLC (diamond-like carbon) | Excellent | Good | High | Aerospace, high-finish requirements |
| CVD diamond | Excellent | Excellent | Very high | High-volume production |
| TiAlN | Good | Good | Moderate | General purpose aluminum |
| ZrN | Good | Moderate | Moderate | Reduced friction |
| Uncoated carbide | Poor | Good | Low | Low-volume, non-aerospace |
DLC coating is the gold standard for aluminum gun drilling
Diamond-like carbon (DLC) coating provides the best BUE resistance for aluminum deep hole drilling. The coating's low coefficient of friction (0.1–0.2 vs 0.4–0.6 for uncoated carbide) prevents aluminum from adhering to the cutting edge. While DLC-coated drills cost 30–50% more than uncoated, the improvement in hole quality consistency and the elimination of BUE-related scrap typically justify the premium in production applications.
Coolant and Filtration
Coolant Requirements
| Parameter | Recommendation | Why |
|---|---|---|
| Coolant type | Water-miscible emulsion (6–10%) or neat oil | Emulsion provides better cooling; oil provides better lubrication |
| Coolant pressure | 20 – 60 bar | Lower than steel — aluminum chips are less dense and easier to evacuate |
| Filtration | ≤ 20 µm | Aluminum chips are soft and can smear through coarser filters |
| Coolant temperature | 20 – 35°C | Temperature stability affects hole diameter by 0.01–0.02 mm per 10°C change |
Chip Evacuation
Aluminum produces 2–3× the chip volume of steel for the same hole volume (lower density, higher chip compression ratio). The coolant system must handle this increased volume:
| Hole Diameter | Chip Volume per 100 mm Depth (Aluminum) | Required Flow Rate |
|---|---|---|
| 6 mm | 3.5 – 5.0 cm³ | 15 – 30 L/min |
| 12 mm | 14 – 20 cm³ | 30 – 60 L/min |
| 20 mm | 40 – 55 cm³ | 50 – 100 L/min |
Tool Life and Wear
Expected Tool Life
| Tool Type | Aluminum Alloy | Holes per Regrind | Regrinds per Tool | Total Holes |
|---|---|---|---|---|
| Carbide gun drill (coated) | 6061 | 2,000 – 5,000 | 8 – 12 | 16,000 – 60,000 |
| Carbide gun drill (uncoated) | 6061 | 1,000 – 3,000 | 8 – 12 | 8,000 – 36,000 |
| Carbide gun drill (coated) | 2024 / 7075 | 1,500 – 3,500 | 8 – 10 | 12,000 – 35,000 |
| Carbide gun drill (uncoated) | 2024 / 7075 | 800 – 2,000 | 8 – 10 | 6,400 – 20,000 |
| Indexable insert | Various | 500 – 2,000 edges | N/A (indexable) | 500 – 2,000 per edge |
Wear Patterns
| Wear Type | Cause | Appearance | Action |
|---|---|---|---|
| Flank wear | Abrasion from oxide particles | Uniform wear on clearance face | Regrind at VB = 0.2 mm |
| BUE adhesion | Chemical affinity | Aluminum built up on cutting edge | Increase speed, check coating |
| Edge chipping | BUE release pulling carbide fragments | Irregular edge line | Switch to tougher grade |
| Crater wear | Diffusion at high speed | Depression on rake face | Reduce speed or use coated tool |
Surface Quality
Expected Surface Finish
| Alloy | Typical Ra (µm) | Typical Rz (µm) | Surface Characteristics |
|---|---|---|---|
| 6061-T6 | 0.3 – 0.6 | 2 – 4 | Bright, burnished |
| 2024-T3 | 0.4 – 0.8 | 3 – 6 | Matte, may show minor BUE marks |
| 7075-T6 | 0.4 – 0.7 | 3 – 5 | Good, consistent |
| 2011-T3 | 0.2 – 0.5 | 1.5 – 3 | Excellent — most machinable |
| 380 (cast) | 0.6 – 1.2 | 4 – 8 | May show porosity-related surface defects |
Dimensional Accuracy
| Parameter | Typical Tolerance | Best Possible |
|---|---|---|
| Diameter (gun drilling) | IT8 – IT9 | IT7 |
| Straightness | 0.05 – 0.15 mm per 100 mm | 0.02 mm per 100 mm |
| Roundness | 0.01 – 0.03 mm | 0.005 mm |
Summary
| Aspect | Recommendation |
|---|---|
| Cutting speed | 50 – 100 m/min (carbide), upper end reduces BUE |
| Feed rate | 0.02 – 0.30 mm/rev (diameter-dependent) |
| Tool coating | DLC or diamond for best BUE resistance |
| Coolant pressure | 20 – 60 bar (lower than steel) |
| Coolant type | Emulsion 6–10% or neat oil |
| Chip breaker | Wider and farther from edge than steel |
| Surface finish achievable | Ra 0.2 – 0.8 µm |
| Primary failure mode | BUE (preventable with correct speed and coating) |
FAQ
What is the best cutting speed for gun drilling 6061 aluminum?
60–100 m/min for carbide gun drills. Unlike steel, higher cutting speeds improve surface finish in aluminum by reducing BUE formation. Start at 60 m/min and increase until BUE is eliminated or surface finish targets are met. Speeds above 100 m/min are possible with DLC-coated tools and adequate coolant pressure.
Why does my gun drill produce rough surface finish in 7075 aluminum?
Rough surface finish in 7075 aluminum is most often caused by BUE forming on the cutting edge and intermittently releasing, leaving fragments embedded in the bore surface. Solutions: increase cutting speed (above 60 m/min), switch to a DLC or diamond-coated tool, increase rake angle (more positive), and verify coolant lubricity (bump emulsion concentration to 8–10%).
What coolant pressure is needed for gun drilling aluminum?
Lower than for steel — 20–60 bar is typically adequate. Aluminum chips are less dense than steel chips and are easier to evacuate. However, because aluminum drilling produces 2–3× the chip volume of steel, flow rate (L/min) is more important than pressure. Ensure the coolant system delivers adequate flow to the flute or drill tube.
Can I use the same gun drill for aluminum and steel?
Not ideally. Aluminum requires a different chip breaker geometry (wider, farther from the cutting edge), more positive rake angle, and a different coating (DLC vs TiAlN) than steel. A drill optimized for one material will perform poorly in the other. If one drill must serve both, compromise on a medium chip breaker and accept reduced performance.
What causes built-up edge in aluminum deep hole drilling?
BUE is caused by the chemical affinity between aluminum and carbide at elevated temperatures. Aluminum atoms bond to the carbide surface, building up a layer of work-hardened material. The primary levers for BUE prevention: cutting speed (increase to reduce adhesion time), tool coating (DLC or diamond prevents bonding), and rake angle (more positive reduces contact pressure).
How often should gun drills be reground for aluminum?
Less frequently than for steel. Carbide gun drills in aluminum typically achieve 1,000–5,000 holes per regrind depending on alloy and coating. Regrind at VB = 0.2 mm flank wear or when surface finish degrades below specification. The edge condition after regrind is critical — a sharp, burr-free edge with 5–15 µm hone is essential for BUE prevention.
Do I need coolant for deep hole drilling aluminum?
Yes. While aluminum can be dry-drilled at shallow depths, deep hole drilling requires coolant for chip evacuation and lubrication. MQL (minimum quantity lubrication) is feasible for aluminum in some applications at L/D < 20:1. For production deep hole drilling with L/D > 10:1, through-tool coolant (emulsion or oil) is recommended for reliable chip evacuation.
What is the typical hole straightness in gun-drilled aluminum?
Gun-drilled aluminum holes typically achieve straightness of 0.05–0.15 mm per 100 mm of depth. With counter-rotation (workpiece rotating opposite to the drill), straightness can improve to 0.02–0.08 mm per 100 mm. Aluminum's lower cutting forces result in less drill deflection than steel, making it one of the best materials for straight-hole drilling.
Cutting parameters for aluminum alloys depend on specific alloy composition, heat treatment, machine rigidity, and tool geometry. The values in this article are recommended starting points for production deep hole drilling. Always verify with tool supplier recommendations. This article reflects industry knowledge as of 2026.