Copper and its alloys are among the most “gummy” metals a deep hole drill can meet. Pure copper refuses to break its chip, welds onto the cutting edge as built-up edge (BUE), and expands under heat to squeeze the tool — yet it is drilled every day at L/D ratios up to 50:1 and beyond for electrical connectors, heat exchanger tubes, and water fittings. The whole game is chip control and coolant pressure.
| Challenge | Why It Matters | Consequence |
|---|---|---|
| Gummy chip flow | Ductile copper flows over the rake face instead of shearing | Long ribbon / snarled chips that jam the flute |
| Built-up edge (BUE) | Workpiece atoms bond to the tool tip under pressure and heat | Poor finish, unpredictable hole size, tool failure |
| Bird nesting | Stringy chips wrap around the drill and pack the hole | Tool breakage mid-bore, re-cut chips on peck retract |
| Work hardening | OFHC / oxygen-free copper work-hardens while cutting | Increasing cutting forces, rapid tool wear |
| Thermal expansion | Heat swells the workpiece and squeezes the drill in the bore | Drills seized in deep holes, scrap parts |
| High thermal conductivity | Heat conducts away fast — but friction heat still concentrates at the edge | BUE forms even though the bulk part stays cool |
The effective countermeasures are consistent across every source (Tormach, Harvey Performance, Asian Star CNC, Practical Machinist): run slower speeds with higher feeds, never let the tool dwell, keep the edge sharp, and flood high-pressure coolant straight at the tool tip. A heavier chip is far easier to break than a thin, flimsy one.
| Alloy | Machinability | Character | Deep Hole Notes |
|---|---|---|---|
| C36000 free-cutting brass | 100% (benchmark) | Leaded, free-breaking chips | Gundrilled to Ø0.5 mm at Ra 0.4 μm; 500+ fittings per tool at 3,000 RPM |
| C83600 leaded bronze (85-5-5-5) | ≈80–90% | 20% Pb gives chip control | Ø3 mm × 150 mm (L/D 50:1) gundrilled without chip welding |
| Cartridge / naval brass | ≈30% | More zinc, less lead | Needs chip-breaker geometry; watch chip packing |
| Bearing bronze (C93200) | ≈70% | Tin bronze, abrasive oxide film | Moderate speeds, good coolant filtration |
| Phosphor bronze (C51000) | ≈20% | Harder, more abrasive | Run 80–200 m/min milling reference; lower for deep bores |
| Free-cut phosphor bronze (C54400) | ≈80% | Leaded, improved chip break | Preferred where bronze conductivity is needed |
| Oxygen-free copper (TU1 / OFHC) | Poor | Pure Cu, extreme ductility | L/D 50:1 proven; feed 0.019–0.023 mm/r is the key lever |
| Aluminum bronze (C95400) | Low | ≈400 HB, very abrasive | Vibration-assisted drilling cut tool wear ~60% |
| Beryllium copper (BeCu) | Fair–poor | Ductile, work-hardens, glazes | Drill grabs; needs through-tool coolant and pecking |
| Operation | Material | Vc (m/min) | Feed (mm/rev) | Notes |
|---|---|---|---|---|
| Gundrill, Ø10 mm solid carbide | Oxygen-free copper TU1 | ~47 | 0.019–0.023 | Optimized; 2.1–2.4 MPa coolant, C-type chips |
| Gundrill (carbide-tipped) | Copper / bronze / brass | 80–150 | 0.003–0.07 by Ø | Botek range; oil 2.5–6.0 MPa, flow 0.2–0.65 L/s |
| Gundrill | General copper alloys | 66–99 | 0.015–0.03 | Machine-builder guidance (deepholemachines.com) |
| DeepTri-Drill, Ø22 mm | Copper alloy | 110 | 0.10 | Tungaloy case; doubled speed vs competitor 55 m/min |
| Gundrill, Ø3 mm × 150 mm, TiAlN carbide | Leaded brass C83600 | ~14 | 0.08–0.10 | 1,500 RPM, 7% water-soluble emulsion, L/D 50:1 |
| Twist / gun drill (HSS) | Brass | 46–76 | 0.10–0.45 by Ø | Somta; leaded brass 61–92 m/min; 118° point |
| Twist / gun drill (HSS) | Bronze | 30–61 | 0.10–0.45 by Ø | Somta; high-tensile bronze 22–30 m/min |
| BTA drilling, Ø52 mm | Oxygen-free copper | ~58 | 0.07 | 355 r/min spindle, published optimum |
| Gundrill, Ag-Cu alloy | Silver-copper | 55–73 | 0.01–0.02 | Literature; successful at L/D > 100 |
| Parameter | Brass / Bronze | Copper (pure) | Source |
|---|---|---|---|
| Point angle | 118° | 130–140° (gundrill), wider for ductile Cu | Somta, BNEC |
| Lip clearance | 15° (12° for aluminum) | 12–15° typical | Somta, BNEC |
| Chisel edge angle | 125–135° | Keep web thin, positive rake | BNEC |
| Edge | Sharp, polished flutes | Polished / mirror rake face to shed BUE | Tormach, Harvey |
| Coating | TiN or uncoated for brass | TiAlN, DLC, or low-friction coating (>40% less adhesion) | Asian Star, Neway |
| Drill type | Parabolic flute, non-ferrous specific | Non-ferrous gun drills, screw-machine-length HSS | Guhring GT80, Mikron |
| Alloy | Recommended Coolant | Avoid | Notes |
|---|---|---|---|
| Brass | Dry, soluble oil, or low-viscosity oil | High-chlorine / staining fluids | Somta: dry or soluble; BNEC lists kerosene for hand drilling |
| Bronze | Soluble oil | Alkaline fluids that attack tin | Penrite dilution 20:1 for drilling brass/bronze |
| Pure / OFHC copper | High-pressure emulsion or EP gun drill oil | Coolants with corrosive amine packages | 2.1–2.4 MPa internal coolant for Ø10 mm TU1 |
| Gun drilling (all) | Dedicated gun drill oil or chlorinated soluble (Phillips 66 Soluble Oil HD, Cut-Max IN 2010) | — | 5–15 μm filtration for small-diameter drills |
In copper and brass deep hole drilling, chip form is the process. The goal is a C-type (C-shaped) chip — short, curled, and small enough to ride the coolant stream out the flute without packing. Researchers quantify this with a chip evacuation coefficient (gullet-to-chip area ratio) and a chip deformation coefficient; the optimized TU1 conditions delivered values of 4.477 and 3.316 respectively.
Short C-hooks are good. Long spirals, pagodas, or snarled nests mean the chip won’t evacuate.
Feed is the most influential factor (Han et al., 2025). A heavier chip snaps against the chip breaker. Typical deep-hole feeds: 0.019–0.023 mm/r for Ø10 mm TU1.
Lower cutting speed keeps copper less plastic, so chips fragment more easily. Drop speed before touching feed on ductile alloys.
The speed–pressure interaction is significant. Internal coolant at 2.1–2.4 MPa flushes the flute and flash-cools long chips so they snap.
For twist drills in OFHC copper, peck ~half a diameter and retract 0.3 mm (documented on brass) to break the chip and avoid re-cutting on retract.
Ultrasonic axial vibration (~20 kHz) on electrolytic copper ECu 57 improved drilling moment, surface quality, and chip form at low feed; on C95400 aluminum bronze it cut tool wear ~60%.
The reference study (Han Xiaolan et al., 2025) gundrilled 9 × Ø10 mm holes, 500 mm deep, through Ø80 mm TU1 oxygen-free copper bars — an aspect ratio of 50 — on a KB1300 machine with a Botek 113 solid carbide gun drill. Chip packing is the killer: with only a thin annular gap for evacuation, one long chip means re-cutting, straightness deviation, and scrap.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Long spiral / snarled chips | Feed too low, speed too high | Raise feed (0.02–0.024 mm/r), drop speed, boost coolant pressure |
| Built-up edge, rough bore | Edge heat, adhesive welding | Sharper edge, DLC/low-friction coating, higher feed, keep tool fed |
| Drill breaks mid-bore | Chip packing; thermal expansion squeezing the tool | Internal coolant at 2+ MPa, peck, reduce depth-to-ream, verify filtration |
| Bird nesting around tool | Stringy chips, no chip breaker | Chip-breaker geometry, peck cycle, heavier feed |
| Work hardening / glaze (BeCu) | Heat glazing the surface | Flood coolant, cut at least 0.007 in depth under the glaze |
| Poor straightness at depth | Entry error, coolant pressure dip, drift | Spot-face a square start, hold pressure, monitor torque/pressure |
| Discolored / stained bore | Coolant incompatible with copper alloy | Switch to copper-safe fluid; check amine and chlorine content |
| Tool grabs on entry (BeCu) | Excessively sharp grab on ductile edge | Flat on the lip, through-tool coolant, controlled feed-in |