Appearance
Copper does not cut — it tears. A pure copper chip under a microscope shows a surface of ductile rupture, not clean shear. The chip is continuous, gummy, and long enough to wrap around the drill shank three times before it breaks under its own weight. In deep hole drilling, where the chip must travel the full length of the bore to be evacuated, a continuous copper chip is a guaranteed tool failure. The solution is not to cut the material more efficiently — it is to force the chip to break at the cutting edge by controlling the feed, the coolant pressure, and the tool geometry with precision that exceeds what is needed for steel. Copper deep hole drilling is a test of process discipline. Get the parameters right and the holes are clean, straight, and smooth. Get them wrong and the tool breaks within seconds of entering the material.
Copper Alloy Families and Machinability
Machinability Classification
| Classification | Rating (% of free-cutting brass) | Examples | Deep Hole Drilling Difficulty |
|---|---|---|---|
| Free-machining | 80–100% | C36000 (leaded brass), C14500 (tellurium copper), C14700 (sulphur copper) | Low |
| Moderate | 40–60% | C18200 (chromium copper), C15000 (zirconium copper), CuNi30 | Moderate |
| Difficult | 20–30% | C11000 (ETP copper), C10200 (OFHC), C10100 (oxygen-free) | High |
| Very difficult | 10–20% | C17200 (beryllium copper), CuCr1Zr, tough-pitch copper | Very high |
Key Copper Alloys for Deep Hole Drilling Applications
| Alloy | Designation | Tensile Strength | Conductivity (% IACS) | Application | Drilling Challenge |
|---|---|---|---|---|---|
| ETP copper | C11000 | 220–260 MPa | 101% | Cooling plates, electrical busbars | Severe BUE, long chips |
| OFHC copper | C10200 | 220–270 MPa | 102% | Vacuum components, electronics | Severe BUE, chip control |
| Deoxidised copper | C12000/C12200 (DHP) | 220–280 MPa | 85–95% | Heat exchangers, plumbing | Moderate BUE |
| Chromium copper | C18200 | 350–450 MPa | 80% | Resistance welding electrodes | Moderate, lower BUE |
| Zirconium copper | C15000 | 350–420 MPa | 93% | High-temp electrical | Moderate |
| CuCr1Zr | — | 450–550 MPa | 80–92% | Rocket engines, moulds | Moderate — alloy hardens |
| Beryllium copper | C17200 | 1,200–1,400 MPa | 22–25% | Downhole tools, bearings | Low BUE, high tool wear |
| CuNi30 | — | 350–450 MPa | 5–10% | Marine heat exchangers | Moderate, work-hardening |
| Tellurium copper | C14500 | 250–300 MPa | 93% | Electrical connectors | Easy — free machining |
| Leaded brass | C36000 | 340–470 MPa | 26% | Fittings, valves | Easy — chips break naturally |
WARNING
The machinability rating of copper is not about tool wear — it is about chip control. Free-machining copper alloys contain lead, tellurium, or sulphur that act as chip breakers by creating stress raisers in the chip. Pure copper has none of these. A gun drill that lasts for 500 holes in C14500 tellurium copper may last only 5 holes in C11000 ETP copper before a chip jam breaks the tool. Do not assume that parameters from one copper alloy transfer to another.
Built-Up Edge and Chip Control
The BUE Problem
Built-up edge (BUE) is the dominant failure mode in copper deep hole drilling:
| BUE Parameter | Characteristic |
|---|---|
| Formation temperature | 150–300°C (lower than steel) |
| Adhesion strength | High — copper welds to carbide |
| BUE stability | Unstable — builds, breaks off, re-forms |
| Effect on bore surface | Tearing, poor finish, oversize holes |
| Effect on tool life | Edge chipping when BUE breaks away |
Chip Morphology Control
| Chip Type | Characteristic | Desirability | How to Achieve |
|---|---|---|---|
| Long ribbon chip | Continuous, snarled, metres long | Never | Low feed, dull tool, insufficient coolant |
| Spiral chip | Coiled, diameter > hole diameter | Acceptable | Moderate feed, sharp tool |
| C-shaped chip | Short, curved, compact | Ideal | Optimised feed + coolant pressure |
| Segmented chip | Broken into short pieces | Good | High feed + vibration assistance |
Feed rate is the dominant parameter controlling chip morphology in copper drilling. The 2025 optimisation study on oxygen-free copper (TU1) found that:
- Feed rate has the most significant effect on chip shape (p < 0.0001)
- Cutting speed has a secondary effect (p = 0.0254)
- Coolant pressure has a tertiary effect (p = 0.3968)
Chip Breaking Strategies
| Strategy | Mechanism | Effectiveness |
|---|---|---|
| Increase feed rate | Thicker chip bends and breaks more easily | Most effective single parameter |
| High coolant pressure (> 2.0 MPa) | Hydraulic force breaks chip, flushes segments | Essential — prevents packing |
| Ultrasonic vibration assistance | Intermittent cutting segments chip at source | Very effective for pure copper |
| Step feed (peck drilling) | Retract tool to break chip | Reduces productivity |
| Chip breaker geometry | Groove on rake face curls chip to breaking | Tool must be custom-ground |
Tool Selection and Geometry
Carbide Grade Selection
| Copper Type | Recommended Grade | Coating | Why |
|---|---|---|---|
| Pure copper (C11000, C10200) | K10–K15 | Uncoated or DLC | Sharp edge essential — coatings can increase BUE |
| Chromium/zirconium copper | K15–K20 | TiAlN or uncoated | Moderate wear resistance needed |
| Beryllium copper (C17200) | K20–K30 | TiAlN or AlCrN | High hardness requires tougher grade |
| CuNi30 | K15–K20 | TiAlN | Work-hardening requires wear resistance |
| Brass / free-machining | K10–K15 | Uncoated | Low wear — coating unnecessary |
Edge Geometry for Copper
| Geometry Feature | Recommended | Why |
|---|---|---|
| Rake angle | 8–15° positive | Positive rake reduces cutting forces and BUE formation |
| Point angle | 110–120° | Lower angle reduces thrust, helps chip curl |
| Edge preparation | Sharp (0.005–0.015 mm hone) | Sharp edge cuts cleanly; honed edge increases BUE |
| Flute finish | Polished (< Ra 0.2 µm) | Reduces friction for chip evacuation |
| Margins | Narrow (0.2–0.4 mm) | Reduces friction against bore wall |
| Clearance angles | 12–18° | Generous clearance prevents rubbing |
TIP
A polished flute is worth more than any coating in copper deep hole drilling. Copper chips adhere to rough surfaces, building up layer by layer until the flute is blocked. A gun drill flute polished to Ra 0.1 µm or better will evacuate chips reliably; a standard-ground flute at Ra 0.4–0.8 µm will accumulate copper build-up within the first 50 mm of drilling. Diamond paste polishing of the flute after regrinding is standard practice in copper gun drilling shops.
Gun Drilling Parameters
Recommended Starting Parameters
| Diameter | Copper Type | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure | Expected Chip Form |
|---|---|---|---|---|---|
| 3–6 mm | Pure copper (C11000, C10200) | 30–50 | 0.008–0.025 | 2.0–2.5 MPa | C-shaped to spiral |
| 3–6 mm | CuCr1Zr, CrCu | 40–60 | 0.010–0.030 | 1.8–2.4 MPa | C-shaped |
| 3–6 mm | BeCu (C17200) | 20–35 | 0.005–0.015 | 1.5–2.0 MPa | Segmented |
| 3–6 mm | Brass (C36000) | 60–100 | 0.020–0.060 | 1.0–1.5 MPa | Natural chip breaking |
| 6–12 mm | Pure copper | 35–55 | 0.015–0.035 | 2.0–2.5 MPa | C-shaped |
| 6–12 mm | CuNi30 | 30–50 | 0.012–0.030 | 1.5–2.0 MPa | Segmented |
| 6–12 mm | Free-machining copper | 80–120 | 0.030–0.080 | 1.0–1.5 MPa | Short, broken |
| 12–20 mm | Pure copper | 30–50 | 0.020–0.045 | 2.0–2.5 MPa | C-shaped to spiral |
Optimised Parameters from Research
The 2025 multi-objective optimisation study on TU1 oxygen-free copper (10 mm diameter, L/D ≈ 50) using RSM and genetic algorithm found:
| Parameter | Optimal Value | Effect on Chip Evacuation |
|---|---|---|
| Cutting speed | 47.1 m/min | Secondary influence (p = 0.0254) |
| Feed rate | 0.019 mm/rev | Dominant influence (p < 0.0001) |
| Coolant pressure | 2.4 MPa | Tertiary influence (p = 0.3968) |
| Result | Chip evacuation coefficient 3.30 | Predominantly C-shaped chips |
BTA Drilling Parameters
For larger diameters in copper alloys, BTA drilling is used:
| Diameter | Copper Type | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Flow | Coolant Pressure |
|---|---|---|---|---|---|
| 20–40 mm | Pure copper | 25–45 | 0.04–0.10 | 200–350 L/min | 1.5–2.0 MPa |
| 20–40 mm | CuCr1Zr | 30–50 | 0.05–0.12 | 200–350 L/min | 1.2–1.8 MPa |
| 20–40 mm | Brass | 60–100 | 0.08–0.20 | 150–250 L/min | 0.8–1.2 MPa |
| 40–80 mm | Pure copper | 20–35 | 0.06–0.15 | 300–500 L/min | 1.2–1.8 MPa |
BTA drilling of copper requires careful attention to the guide pads. The high ductility of copper can cause material to smear onto the carbide pads, reducing the bore burnishing effect and creating a poor surface finish. PCD-tipped guide pads are recommended for production quantities in pure copper.
Coolant Strategy
Copper-Specific Coolant Requirements
| Parameter | Recommendation | Why |
|---|---|---|
| Coolant type | Neat oil (high EP) or synthetic | Water-based coolant increases BUE in copper |
| Pressure | 2.0–2.5 MPa (300–360 PSI) for pure copper | Hydraulic chip breaking |
| Flow rate | Adequate for chip transport | Copper chips are heavy — high velocity needed |
| Filtration | ≤ 20 µm | Copper fines are adhesive — prevent recirculation |
| Temperature control | 20–30°C | Thermal stability for precision holes |
| Chlorine-free | Required for electrical applications | Chlorine causes corrosion in electrical contacts |
Coolant Pressure vs. Chip Form
| Coolant Pressure | Chip Form | Tool Life |
|---|---|---|
| < 1.5 MPa | Long ribbons, frequent jamming | Very poor — tool breakage likely |
| 1.5–2.0 MPa | Mixed spiral and C-shaped | Poor-moderate |
| 2.0–2.4 MPa | Predominantly C-shaped | Good |
| > 2.4 MPa | Short C-shaped and segmented | Best |
Applications
Copper Cooling Plates
| Application | Material | Hole Pattern | Key Requirement |
|---|---|---|---|
| Furnace cooling staves | OFHC or DHP copper | Parallel channels, 10–25 mm diameter | Leak-tight, good thermal contact |
| Heat shields (furnace) | Pure copper (> 99.95% Cu) | Deep-drilled cooling channels | No welding joints — minimises leakage |
| Power electronics cooling | C11000 or CuCr1Zr | Small diameter, tight pitch | Positional accuracy, surface finish |
| Fusion reactor components | CuCrZr | Deep, small-diameter channels | Precision, defect-free bores |
Electrical Components
| Component | Material | Drilling Requirement |
|---|---|---|
| Resistance welding electrodes | C18200 (CrCu) | Cooling channel through centre |
| Electrical busbars | C11000 | Connection holes, deep and accurate |
| Switchgear components | C14500 (TeCu) | Precision small holes |
| Rotor short circuit rings | OFHC copper | Deep holes for connector pins |
Heat Exchangers
| Application | Material | Condition |
|---|---|---|
| Marine condensers | CuNi30 | Seawater corrosion resistance |
| Desalination plants | CuNi30 or DHP copper | Tube sheet and support plate holes |
| Hydraulic coolers | Brass or CuNi | Small diameter deep holes |
Surface Finish and Quality
Expected Results
| Copper Type | As-Drilled Ra (Gun Drilling) | After Reaming Ra | Application |
|---|---|---|---|
| Pure copper (C11000) | 0.8–1.6 µm | 0.4–0.8 µm | Cooling plates |
| CuCr1Zr | 0.6–1.2 µm | 0.3–0.6 µm | High-performance cooling |
| Beryllium copper | 0.3–0.6 µm | 0.15–0.3 µm | Downhole tools |
| Brass | 0.4–0.8 µm | 0.2–0.4 µm | Fittings, connectors |
Common Defects
| Defect | Cause | Fix |
|---|---|---|
| Built-up edge on tool | Low cutting speed, insufficient coolant | Increase speed, increase coolant pressure |
| Oversize bore | Tool deflection from BUE | Reduce feed, sharpen tool, check BUE |
| Rough bore surface | BUE fragments embedded in bore | Increase coolant pressure, polish flutes |
| Chip packing (tool breakage) | Continuous chip, insufficient pressure | Increase feed, increase coolant pressure, use peck cycle |
| Bell-mouth at entry | Abrasive entry burr from ductile chip | Use entry bush, chamfer entry |
| Spiral chip in bore | Feed too low | Increase feed rate |
FAQ
Q: Why is copper difficult to deep hole drill? Copper is highly ductile and forms long, continuous chips that are difficult to evacuate from deep holes. It also readily forms built-up edge (BUE) because copper welds to the cutting edge at cutting temperatures. These two problems — chip evacuation and BUE — are the primary failure modes in copper deep hole drilling.
Q: What cutting speed is recommended for gun drilling pure copper? 30–50 m/min for diameters 3–20 mm. The 2025 optimisation study on oxygen-free copper found 47.1 m/min to be optimal for 10 mm diameter holes with L/D ≈ 50. Lower speeds increase BUE risk; higher speeds increase heat without improving chip breaking.
Q: What coolant pressure is needed for deep hole drilling copper? Minimum 2.0 MPa (300 PSI), recommended 2.0–2.5 MPa for pure copper. For free-machining copper alloys and brass, lower pressure (1.0–1.5 MPa) is acceptable. The high pressure provides hydraulic chip breaking — the coolant jet physically breaks the ductile chip into manageable segments.
Q: What is the best carbide grade for gun drilling copper? K10–K15 uncoated carbide for pure copper provides the sharpest edge, which is essential for clean cutting. DLC (diamond-like carbon) coating can reduce BUE by providing a non-stick surface. Avoid TiAlN for pure copper — it increases BUE formation.
Q: How does feed rate affect chip formation in copper drilling? Feed rate is the dominant parameter controlling chip morphology. Higher feed rates produce thicker, more rigid chips that break into C-shaped segments. Lower feeds produce thin, ribbon-like chips that wrap around the tool. The optimal feed for pure copper gun drilling is 0.019–0.035 mm/rev.
Q: Can gun drilling be used for beryllium copper? Yes, but with caution. C17200 beryllium copper at full hardness (35–42 HRC) requires K20–K30 carbide, reduced speeds (20–35 m/min), and lower feeds (0.005–0.015 mm/rev). BUE is less of a problem than with pure copper, but tool wear is higher due to the material's hardness.
Q: What chip form should be targeted in copper deep hole drilling? C-shaped chips are ideal. They are short, curved segments that flush easily through the bore and coolant chip pan. Spiral chips are acceptable if they break into short coils. Long ribbon chips indicate a problem — reduce speed or increase feed.
Q: Can ultrasonic vibration assistance help with copper deep hole drilling? Yes. Research on ECu 57 electrolytic copper (Heisel et al., CIRP Annals 2008) showed that 20 kHz ultrasonic axial vibration significantly improves chip form and reduces drilling torque. Vibration assistance is particularly effective for pure copper where chip breaking is most difficult.
Q: What are the main applications for deep-drilled copper components? Cooling plates for furnaces and power electronics (gun-drilled channels for water cooling), resistance welding electrodes (central cooling bore), electrical busbars (connection holes), heat exchanger tube sheets, and marine condenser components.
Q: Is brass easier to deep hole drill than pure copper? Yes, significantly. Free-machining brass (C36000) contains lead that acts as a chip breaker, producing short, broken chips naturally. Brass can be drilled at 60–100 m/min with standard gun drills, standard coolant pressure, and without special chip breaking measures.