Appearance
A manufacturer of shell-and-tube heat exchangers for the power generation industry was drilling 8 mm diameter × 500 mm deep tube sheet bores in C11000 electrolytic tough pitch copper (99.9% Cu) and C23000 red brass (85% Cu, 15% Zn). In pure copper, gun drilling with standard C2 grade carbide tools at 118° point angle, Vc = 75 m/min, f = 0.035 mm/rev, 60 bar coolant pressure produced severe built-up edge at the outer corner of the gun drill point — visible as a welded layer of copper on the cutting edge that formed within 50 mm of drilling. The BUE caused the effective cutting diameter to increase progressively, and hole diameter grew by 0.08 mm over the first 200 mm of drilling — exceeding the H8 tolerance band (+0.022 mm for Ø8 mm). Tool life was 9 meters per edge, limited by BUE-induced edge chipping. Switching to a gun drill with 100° point angle, polished flute surface, PVD coating (TiB₂ for its low affinity to copper), and increasing coolant pressure to 100 bar eliminated the built-up edge entirely. Hole diameter stabilized within 0.015 mm of nominal, surface finish improved from Ra 2.2 µm to Ra 0.6 µm, and tool life increased to 38 meters per edge — a 4.2× improvement. In C23000 red brass, the same tool with 118° point angle at Vc = 120 m/min, f = 0.050 mm/rev, 60 bar coolant produced Ra 0.4 µm surface finish and tool life exceeding 60 meters per edge. The difference in behavior between copper and brass illustrates the dramatic effect of zinc content on chip formation and built-up edge tendency in copper alloys.
Metallurgical Characteristics of Copper Alloy Families
Copper alloys span a wide range of machinability, from free-machining leaded brass (machinability rating of 100% — the reference standard for copper alloys) to pure copper (machinability rating of approximately 20%). The machinability difference is driven by three factors: the alloying element type and content, the presence of lead or other chip-breaking additives, and the work-hardening rate of the alloy.
Copper Alloy Classification and Machinability
| Alloy Family | Typical Grades | Cu Content | Key Alloying Elements | Machinability Rating | Chip Form | Primary Drilling Challenge |
|---|---|---|---|---|---|---|
| Pure copper | C11000 (ETP), C10100 (OFHC), C10200 | 99.9%+ | None (<0.05% O for ETP) | 20% (fair) | Long, stringy, continuous | Severe BUE, poor chip breaking, edge chipping |
| Leaded brass | C36000 (free-cutting), C35300 | 60–63% | Zn 35–37%, Pb 2–3% | 100% (excellent) | Short, broken chips | Minimal — best machinability of any copper alloy |
| Red brass | C23000, C24000 | 85–90% | Zn 10–15% | 40–50% (good) | Continuous but manageable | Moderate BUE tendency at high speeds |
| Naval brass | C46400, C48200 | 60–62% | Zn 38–39%, Sn 1%, Pb 0–1% | 60–70% (very good) | Short to medium | Chip breaking in large diameters |
| Tin bronze | C90300, C90500, C90700 | 86–89% | Sn 8–10%, Zn 2–4% | 30–40% (fair to good) | Medium, slightly abrasive | Abrasive wear from tin-rich phases, chip packing |
| Aluminum bronze | C95400, C95500, C95900 | 80–88% | Al 9–11%, Fe 3–5% (some with Ni) | 20–30% (fair) | Medium, abrasive | High abrasive wear, work hardening, edge breakdown |
| Beryllium copper | C17200, C17510 | 97–98% | Be 1.8–2.0%, Co 0.2% | 20–25% (fair) | Stringy, work-hardening | Work hardening, rapid flank wear, surface finish control |
| Silicon bronze | C65100, C65500 | 95–97% | Si 1.5–3%, Mn <1% | 30% (fair) | Stringy, gummy | BUE, poor chip breaking, tearing |
| Copper-nickel | C70600 (90/10), C71500 (70/30) | 70–90% | Ni 10–30%, Fe 1–2% | 25–35% (fair) | Long, continuous | Work hardening, BUE, high cutting forces |
Cutting Mechanism Differences
Pure copper and high-copper alloys (>95% Cu) exhibit the highest built-up edge tendency of any copper alloy group. Copper has high ductility (elongation 30–55%), low work-hardening exponent (n = 0.3–0.4), and high thermal conductivity (390 W/m·K — approximately 10× that of steel). The high thermal conductivity rapidly conducts heat away from the cutting zone, keeping the chip temperature low and preventing thermal softening that would otherwise promote chip breaking. The result is a long, continuous chip that adheres to the tool cutting edge, forming a BUE that alters the effective tool geometry and degrades hole quality. BUE formation in copper drilling increases the effective rake angle locally, which can increase cutting forces and cause edge chipping when the BUE breaks away.
Brass, in contrast, contains zinc as the primary alloying element. Zinc reduces the thermal conductivity of the alloy (to approximately 120 W/m·K for 60/40 brass) and embrittles the chip through the formation of micro-cracks at the chip root — the beta phase in duplex brass (such as C36000) acts as a natural chip breaker. Leaded brass contains fine lead particles (2–3%) that act as a solid lubricant at the tool-chip interface, further reducing cutting forces and preventing BUE formation. The combination of chip embrittlement and lead lubrication makes leaded brass the most machinable copper alloy.
Bronze alloys contain tin, aluminum, or silicon as primary alloying elements. Tin bronze forms hard Cu-Sn intermetallic phases that abrade the cutting edge. Aluminum bronze forms a hard, adherent aluminum oxide layer on the surface (Al₂O₃) that is abrasive and causes rapid flank wear — similar to the behavior of aluminum-silicon alloys. Aluminum bronze also work-hardens significantly (hardness increases from 150–200 HB to 300–400 HB in the deformation zone), requiring tools with sharp edges and low runout to minimize work hardening.
Tool Geometry and Parameter Selection
Point Geometry by Alloy
| Alloy | Point Angle | Rake Angle | Relief Angle | Edge Preparation | Recommended Tool Material | Coating |
|---|---|---|---|---|---|---|
| Pure copper (C11000, C10100) | 100° | +6 to +12° positive | 8–12° | Sharp, polished (hone <5 µm) | Fine-grain WC, 10–12% Co | TiB₂, diamond-like carbon (DLC), or uncoated polished |
| Leaded brass (C36000) | 118° | +5 to +10° positive | 8–10° | Sharp (hone <10 µm) | Standard WC, 6–8% Co | Uncoated (polished) or TiAlN |
| Red brass (C23000) | 118° | +5 to +10° positive | 8–10° | Sharp (hone <10 µm) | Standard WC, 6–8% Co | Uncoated or TiAlN |
| Naval brass (C46400) | 118° | +5 to +8° positive | 8–10° | Sharp to slight hone | Standard WC, 6–8% Co | TiAlN or AlTiN |
| Tin bronze (C90300, C90500) | 118° | +3 to +8° positive | 7–10° | Light hone (10–15 µm) | Submicron WC, 8–10% Co | AlTiN or AlCrN |
| Aluminum bronze (C95400) | 130–140° | 0 to +5° positive | 6–8° | Light hone (10–20 µm) | Submicron WC, 8–10% Co | AlCrN or TiAlSiN |
| Beryllium copper (C17200) | 118–130° | +3 to +8° positive | 7–10° | Sharp to light hone | Fine-grain WC, 8–10% Co | AlTiN or TiAlN |
| Silicon bronze (C65100) | 100–110° | +5 to +10° positive | 8–12° | Sharp, polished | Fine-grain WC, 10–12% Co | DLC, TiB₂, or uncoated polished |
| Copper-nickel (C70600) | 118–130° | +3 to +8° positive | 7–10° | Light hone (10–15 µm) | Submicron WC, 8–10% Co | AlCrN or TiAlSiN |
Cutting Parameters for Deep Hole Drilling
| Alloy | Cutting Speed Vc (m/min) | Feed Rate f (mm/rev) | Coolant Pressure (bar) | Expected Ra (µm) | Expected Tool Life (m/edge) |
|---|---|---|---|---|---|
| Pure copper | 20–40 (gun drill) | 0.015–0.040 | 80–150 | 0.8–2.0 | 15–40 |
| Leaded brass | 60–180 (gun drill/BTA) | 0.030–0.080 | 40–80 | 0.2–0.8 | 50–100+ |
| Red brass | 50–120 (gun drill) | 0.025–0.060 | 40–80 | 0.3–1.0 | 40–80 |
| Naval brass | 50–130 (gun drill/BTA) | 0.025–0.070 | 40–100 | 0.3–1.2 | 30–70 |
| Tin bronze | 30–70 (gun drill/BTA) | 0.020–0.050 | 60–120 | 0.5–1.5 | 15–40 |
| Aluminum bronze (150–200 HB) | 25–50 (gun drill/BTA) | 0.020–0.050 | 80–150 | 0.6–1.8 | 10–25 |
| Aluminum bronze (200–250 HB) | 18–35 (gun drill/BTA) | 0.015–0.040 | 100–180 | 0.8–2.0 | 6–18 |
| Beryllium copper (solution treated) | 20–50 (gun drill) | 0.015–0.040 | 80–150 | 0.6–1.6 | 12–30 |
| Silicon bronze | 15–35 (gun drill) | 0.015–0.035 | 80–150 | 0.8–2.2 | 8–20 |
| Copper-nickel (90/10) | 20–45 (gun drill/BTA) | 0.015–0.045 | 80–150 | 0.6–1.8 | 12–28 |
| Copper-nickel (70/30) | 15–35 (gun drill/BTA) | 0.012–0.035 | 100–180 | 0.8–2.0 | 8–20 |
Coolant Strategy by Alloy
Coolant selection and pressure requirements vary significantly by copper alloy:
Pure copper and high-copper alloys require high coolant pressure (80–150 bar) and soluble oil coolant at 5–8% concentration. The primary purpose is BUE prevention — the high coolant flow rate flushes chips away from the cutting edge and prevents adhesion. Coolant filtration to 10 µm or better is important because copper chips are ductile and can clog filters or recirculate as fines that promote BUE formation.
Brass can be drilled with soluble oil or dry (leaded brass self-lubricates). When running dry, coolant is used only for chip flushing, not lubrication. Minimum coolant pressure of 40–60 bar is recommended for chip evacuation in deep hole drilling (L/D > 10:1). A chip conveyor is essential — brass chips are typically short and granular, and they settle rapidly in the coolant tank.
Aluminum bronze requires high coolant pressure (80–180 bar) and high-concentration soluble oil (8–12%) to manage the abrasive oxide layer and reduce cutting zone temperatures. The coolant should have extreme pressure (EP) additives to prevent adhesive wear at the guide pads.
Beryllium copper requires special attention to coolant filtration — beryllium particles in the coolant are toxic and require a dedicated coolant system with HEPA filtration and proper handling protocols. Coolant pressure of 80–150 bar with soluble oil at 5–8% is standard.
Application-Specific Strategies
Heat Exchanger Tube Sheets (Copper, Red Brass, Copper-Nickel)
Heat exchanger tube sheets require closely spaced, parallel bores that must maintain dimensional stability as adjacent holes are drilled — the inter-hole ligament stress can cause bore distortion. Recommended strategy: drill from the center outward (to manage stress distribution), use gun drilling for diameters 6–25 mm, coolant pressure 80–150 bar, and maintain coolant temperature within ±2 °C to prevent thermal expansion from affecting hole spacing. For copper-nickel tube sheets (common in marine heat exchangers), reduce cutting speeds by 30–40% from pure copper speeds and use AlCrN-coated carbide tools to manage the work-hardening tendency.
Electrical Connectors and Bus Bars (Pure Copper, Beryllium Copper)
Electrical components require burr-free holes with precise diameter control for press-fit connector pins. Recommended strategy: gun drilling with polished carbide tools and 100° point angle for pure copper, coolant pressure 100–150 bar, and a peck cycle (peck depth 3–5× diameter) to ensure chip breaking in stringy copper. For beryllium copper, solution-treated condition is preferred for drilling (the material is softer), with precipitation hardening performed after drilling.
Valve Bodies and Plumbing Fittings (Brass, Silicon Bronze)
Valve bodies often involve intersecting bores at various angles — cross-port drilling and angled drilling. For brass valve bodies, gun drilling or BTA drilling with standard carbide tools at high speeds (Vc = 80–180 m/min) is productive and economical. For silicon bronze valve bodies (common in marine valves), gun drilling with DLC-coated or TiB₂-coated tools at Vc = 15–35 m/min is recommended. Intersecting bores require deburring — the burr height at brass intersections is typically less than 0.05 mm when drilling from the larger diameter toward the smaller diameter.
Marine Hardware and Propeller Shafts (Aluminum Bronze, Nickel-Aluminum Bronze)
Large marine components cast in nickel-aluminum bronze (C95500) require deep hole drilling for propeller shaft bores, rudder stock bores, and hydraulic passages. BTA drilling with AlCrN-coated carbide inserts at Vc = 20–35 m/min, f = 0.03–0.06 mm/rev, coolant pressure 100–180 bar is recommended. The as-cast surface scale on castings is highly abrasive — the tool should enter through a pre-machined spot face, and the first 5–10 mm of drilling should be at 50% feed to negotiate the surface scale.
FAQ
What is the best point angle for gun drilling pure copper?
The recommended point angle for gun drilling pure copper is 100°, significantly flatter than the standard 118° used for most steels and brass. The flatter point angle reduces the effective rake angle at the cutting edge, which reduces the tendency for the chip to adhere to the tool face and form a built-up edge. The 100° point also produces a thinner chip at a given feed rate, reducing the cutting force per unit width of cut. Some high-copper alloys (99.9%+ Cu) benefit from an even flatter point angle of 90–95°. Crucially, the cutting edge must be sharp and polished — any edge honing larger than 5–10 µm will increase cutting forces and promote BUE formation. A polished flute surface (mirror finish, Ra < 0.1 µm) further reduces chip adhesion and improves chip evacuation.
Can brass be deep hole drilled without coolant?
Leaded brass (C36000) can be deep hole drilled without coolant for short holes (L/D < 20:1) because the lead content provides internal lubrication and the chips are short and broken. However, for deep hole drilling with L/D exceeding 20:1, coolant is still required for chip evacuation even if not needed for lubrication — the coolant flushes chips through the flute or chip passage and prevents chip packing. The coolant pressure requirement for chip evacuation in brass is lower than for most materials (40–60 bar is typically sufficient). For unleaded brass and red brass, coolant with lubrication properties is recommended to prevent BUE formation, particularly at cutting speeds above 60 m/min.
Why does aluminum bronze cause rapid tool wear in deep hole drilling?
Aluminum bronze causes rapid tool wear through two mechanisms: abrasive oxide formation and work hardening. The aluminum content (9–11%) forms a hard, adherent aluminum oxide layer (Al₂O₃, hardness 2,000–2,500 HV) on the workpiece surface at the cutting zone temperatures. This oxide layer abrades the tool cutting edge and guide pads. Additionally, aluminum bronze work-hardens significantly — the surface hardness in the deformation zone can reach 300–400 HB from a bulk hardness of 150–200 HB. The combined effect is that tool life in aluminum bronze is typically 25–40% of tool life in tin bronze at equivalent cutting parameters. AlCrN-coated or TiAlSiN-coated carbide tools with submicron grain size and 8–10% cobalt content provide the best wear resistance in this material.
What coolant concentration is recommended for deep hole drilling copper alloys?
For pure copper and high-copper alloys, soluble oil at 6–8% concentration with extreme pressure (EP) additives is recommended. The EP additives form a boundary lubrication layer that prevents metal-to-metal contact at the tool-chip interface, reducing BUE formation. For brass, soluble oil at 4–6% or even dry machining is acceptable — the lubricity requirement is lower because brass does not form BUE as readily. For aluminum bronze, higher concentration (8–12%) with EP additives is recommended to manage the abrasive oxide and reduce guide pad wear. Semi-synthetic coolants are generally not recommended for copper alloys because they provide insufficient lubricity for BUE prevention. Coolant temperature should be maintained below 35 °C to prevent thermal expansion effects on hole diameter and to maintain lubricant stability.
How does surface finish compare between gun drilling and BTA drilling in copper alloys?
In copper alloys, gun drilling consistently produces better surface finish than BTA drilling, with the gap widening in pure copper and high-copper alloys. Gun drilling of leaded brass can achieve Ra 0.2–0.4 µm, while BTA drilling of the same material typically produces Ra 0.8–2.0 µm. In pure copper, gun drilling with optimized geometry (100° point, polished flute) can achieve Ra 0.5–1.0 µm, while BTA drilling of copper is rarely attempted due to chip evacuation difficulties with the long, stringy copper chip through the internal tube passage. For aluminum bronze, gun drilling produces Ra 0.6–1.5 µm versus Ra 1.5–3.0 µm for BTA. The superior finish from gun drilling is attributable to the single-point cutting action and the burnishing effect of the guide pads on the bore surface.
Disclaimer: The process parameters, tool selection recommendations, and performance data presented in this article are based on published technical literature, tooling manufacturer specifications, and industry-reported experience with deep hole drilling of copper alloys. Actual results depend on specific alloy composition and temper, casting or wrought condition, machine tool rigidity, coolant system capability, and tooling quality. The cutting parameters provided should be used as starting recommendations and verified through process development trials for each specific application. Beryllium copper machining requires appropriate health and safety measures due to the toxicity of beryllium particles in chips and coolant. No guarantee of specific tool life, bore quality, or process stability is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.