Deep hole drilling parameters for the materials that don’t behave — gummy copper, pyrophoric magnesium and zirconium, abrasive graphite, fiber-tearing composites, work-hardening superalloys. Start at the lower bound, verify chip form, then climb.
One table, four rules, and most of the trouble is over.
| Rule | Why It Matters |
|---|---|
| Start from the lower bound of Vc and feed | Every material in this table punishes overconfidence. The low end is safe; the high end is where you optimize after proof. |
| Never use water-based coolant on magnesium or zirconium | Magnesium reacts with water to produce flammable hydrogen gas; fine zirconium chips are pyrophoric. Dry machining or oil-based coolant only. |
| Raise feed on copper to force thick, breakable chips | Low feed on gummy copper produces long stringy tangles that pack the flute and seize the drill. |
| Inspect chip form on every trial cut | Chip shape is the feedback loop. Short, broken, consistent chips mean a stable process; ribbons, powders, or balled chips mean stop and adjust. |
1. Find the closest material group. 2. Take the lower bound of Vc and feed. 3. Trial-cut 10–20 mm, inspect chip shape and surface. 4. Optimize incrementally from chip condition. Watch the first 5–10 holes for tool-wear patterns before raising throughput.
Machinability and hazard are two different axes. Magnesium cuts beautifully but ignites; graphite machines fast but coats every surface in conductive dust. Grade each material on difficulty and risk before you load the machine.
| Material | Difficulty | Typical Grade | Vc (m/min) | Feed* (mm/rev) | Coolant Pressure | Notes |
|---|---|---|---|---|---|---|
| Copper (pure) | Moderate | C110, C101 | 60–120 | 0.02–0.10 | 30–50 bar | Gummy, long stringy chips. Raise feed to make thick breakable chips. Coolant at 20–22°C, filtration <20 μm. IT8–IT9 achievable. EP oil, sharp polished edges, positive rake. |
| Brass | Easy | C360 (free-cutting) | 80–150 | 0.02–0.08 | 20–40 bar | Easiest of the copper family — C360 is the 100% machinability benchmark. Watch for burrs at exit. Lead improves chip breaking; reduce Vc ~20% on dezincification-resistant grades. |
| Bronze | Difficult | C932 (SAE 660) | 60–100 | 0.02–0.06 | 30–50 bar | Abrasive high-tin alloy, faster tool wear. Use K-grade carbide. Aluminum bronzes are more abrasive than tin bronzes; phosphor bronze (C510) is the worst. |
| Magnesium alloy | Easy cut · fire risk | AZ31, AZ91 | 100–200 | 0.05–0.15 | 20–30 bar (oil or dry) | ⚠️ No water-based coolant — produces hydrogen gas. Dry or light mineral oil only. Sharp tooling, heavy feed for thick chips. Class D extinguisher + dry sand on hand. |
| Nickel-based alloy | Difficult | Monel 400 | 20–35 | 0.01–0.03 | ≥80 bar | Severe adhesion and work hardening. Polished rake face reduces material pickup. Continuous feed is mandatory — no dwell, no rubbing. |
| Cobalt-based alloy | Extreme | Stellite 6B | 15–25 | 0.01–0.02 | ≥100 bar | Extremely wear-resistant. PCBN tooling recommended for production; carbide feasible for short runs with short life. |
| Tool steel (annealed) | Moderate | D2, O1, A2 | 50–70 | 0.02–0.05 | 60–100 bar | High carbide content — D2 carries ~12% Cr and machines at ~35% of W1. Monitor guide-pad wear. AlTiCrN coating recommended. |
| Tool steel (hardened) | Extreme | D2 (HRC 58–62) | 15–25 | 0.01–0.03 | ≥80 bar | PCBN or CBN-coated tooling only. See the hardened-steel guide for detail. |
| Maraging steel | Difficult | C300, C350 | 30–50 | 0.015–0.04 | ≥80 bar | High toughness. Excellent finish possible — Ra <0.4 μm with stable cutting. Maintain continuous feed. |
| Carburized steel (post-case) | Difficult | 20CrMnTi, 8620 | 20–35 | 0.01–0.03 | ≥80 bar | Hard surface (>HRC 58) over a tough core. Reduce feed ~20% crossing the case-to-core transition zone. |
| Tungsten carbide (green) | Extreme | WC-Co (pre-sintered) | 10–20 | 0.005–0.015 | 30–50 bar | Extremely abrasive. Diamond / PCD tooling mandatory. High-volume coolant flushes the slurry. Never drill sintered (finished) carbide. |
| Powder metallurgy | Moderate | Various | 30–60 | 0.02–0.05 | 30–60 bar | Porosity changes thermal conductivity. Avoid excessive coolant pressure that could impregnate pores. Exit burr formation is unpredictable. |
| Plastic / Composite | Difficult | PEEK, CFRP, GFRP | 50–150 | 0.01–0.05 | 10–20 bar (air or mist) | Delamination risk at entry and exit. PCD tooling for production. Support board at exit; reduce feed at exit to prevent fiber tear-out. |
| Tantalum | Extreme | Ta, Ta-10W | 15–30 | 0.01–0.03 | ≥80 bar | High density, high melting point, gummy. Sharp tooling and continuous feed. High coolant pressure essential to lift chips out of deep holes. |
| Zirconium | Difficult · fire risk | Zr 702, Zr 705 | 25–45 | 0.01–0.04 | ≥60 bar | Fine chips are pyrophoric like titanium. No water-based coolant — oil-based only. Operation falls under NFPA 484 (combustible metals). |
* Feed reference values for D = 10 mm. For smaller diameters reduce feed proportionally; for larger diameters increase. Difficulty grades the machining window, not the safety risk — see the Safety section for fire and dust hazards.
The gummy family: great conductivity, terrible chip behavior. Chip control — not cutting force — is the real problem.
| Alloy | Machinability | Point Angle | Coolant | Key Behavior |
|---|---|---|---|---|
| Pure copper (C110, C101) | ~20% | 100° | EP oil, 30–50 bar | Built-up edge and stringy chips; copper does not like slow speeds. Sharp, polished, positive-rake edges; uncoated carbide often outlasts coated on copper. |
| Free-cutting brass (C360) | 100% (benchmark) | 118° | Dry or soluble oil | Drills dry or with soluble oil. Lead gives clean chip breaking; non-leaded brasses are markedly harder. |
| Bronze (C932 SAE 660) | 20–80% | 118° | Soluble oil, 30–50 bar | Harder and more abrasive than brass — high tin and phosphor content wear carbide fast. K-grade carbide preferred. |
Research on large-aspect-ratio deep drilling of oxygen-free copper (RSM + genetic-algorithm optimization) found feed rate is the single most influential parameter, ahead of cutting speed and coolant pressure. Optimal windows landed near Vc 47 m/min, feed 0.019–0.023 mm/rev, and fluid pressure 2.1–2.4 MPa (21–24 bar), with the speed × pressure interaction dominating chip evacuation. Practical takeaway: on copper, tune feed first and keep coolant temperature steady at 20–22°C.
Magnesium cuts like a dream and burns like a flare. The machining window is wide; the safety envelope is narrow.
High-carbide and high-toughness steels that trade cutting ease for wear resistance. Drill them in the softest condition you can schedule.
| Condition | Tooling | Vc (m/min) | Feed (mm/rev) | Practical Note |
|---|---|---|---|---|
| D2 annealed (~210–250 HB) | Carbide-tipped or solid carbide, AlTiCrN | 35–50 (carbide); ~10 HSS | 0.08–0.25 (conventional) | D2 machines at ~35% of W1 due to chromium carbides. Gundrill before hardening whenever possible; split-point geometry stops walking. |
| D2 hardened (58–62 HRC) | PCBN / CBN-coated | 15–25 | 0.01–0.03 | PCBN hard machining runs 70–120 m/min on turning but drilling keeps speeds low. Expect Ra 0.12–0.28 μm on finish passes. |
| Maraging C300/C350 | Carbide, continuous feed | 30–50 | 0.015–0.04 | High toughness, gummy if feed stalls. Continuous feed prevents local hardening; Ra <0.4 μm is achievable. |
| Carburized 8620 / 20CrMnTi | Carbide, coated | 20–35 | 0.01–0.03 | Hard case >HRC 58 over a tough core. Reduce feed ~20% as the tool crosses from case to core. |
Work-hardening superalloys where chip evacuation and heat management decide everything. Coolant pressure is the first knob to turn.
| Material | Vc (m/min) | Feed (mm/rev) | Coolant | Tooling / Notes |
|---|---|---|---|---|
| Monel 400 | 20–31 | 0.01–0.03 | ≥80 bar | Severe adhesion; polished rake face, high-pressure oil. Consistent feed prevents work-hardening the cut zone. |
| Nickel-base (Inconel, Hastelloy, Waspaloy, Rene) | 22–35 (<40 HRC); 18–31 (>40 HRC) | 0.010–0.076 by diameter | Up to 100 bar | Carbide, then PCBN gun drills for throughput. Full-heat-treated precipitation-strengthened grades are harder than solution-treated. |
| Stellite 6B (cobalt-base) | 15–25 | 0.01–0.02 | ≥100 bar | PCBN for production; carbide for short runs only. |
Without proper chip evacuation, gun drills fail under intense thermal and mechanical loading in Inconel-class alloys; chips pack, recut, score the bore and cause dimensional taper. High-pressure through-tool coolant is the mechanism — providers report 80–100 bar on Hastelloy and Inconel, and small-diameter Inconel 718 gun drilling at extreme L/D cites at least 500–1000 psi (35–70 bar). Coolant demand scales with depth: extended-length toolholders multiply required pressure and flow roughly in proportion to extension (a 3XL holder ~3×).
Abrasive fibers, soft matrix, and two failure modes — delamination at entry/exit and melting at the cutting edge. Heat and thrust are the enemies.
| Material | Vc (m/min) | Feed (mm/rev) | Coolant | Tooling |
|---|---|---|---|---|
| PEEK (pure) | 30–60 | 0.02–0.10 | Strong air or MQL | Carbide, 118–130° point, polished flutes; frequent retraction for deep holes |
| PEEK GF30 / CF30 | Lower than pure | 0.01–0.05 | Air / mist | PCD or diamond-coated — fibers destroy carbide quickly |
| CFRP / GFRP | 50–150 | 0.01–0.05 | 10–20 bar air or mist; vacuum extraction | PCD / diamond-coated mandatory; carbide wears fast on carbon and glass fibers |
Graphite makes powder, not chips — fine, abrasive, electrically conductive dust that ruins machines and lungs if not captured.
| Hole Size | Tooling | Speed | Technique |
|---|---|---|---|
| ≤1.5 mm | HSS (carbide is brittle at this size) | ~4,000 rpm | Slow feed; check taper frequently — HSS wears quickly on graphite |
| >1.5 mm | Solid carbide, preferably diamond-coated | 900–2,000 rpm | Diamond/PVD coatings give 8–30× the life of uncoated; PCD tips last longest |
| Large core holes | Carbide | 125–600 rpm | Hand-feed entry at ~125 rpm, then raise; vacuum collection |
Gummy refractory metals with high melting points — and fire rules that are not optional. Zirconium and tantalum machining falls under NFPA 484 (combustible metals).
| Material | Vc (m/min) | Feed (mm/rev) | Coolant | Behavior |
|---|---|---|---|---|
| Tantalum, Ta-10W | 15–30 | 0.01–0.03 | ≥80 bar | High density, high melting point, gummy. Sharp tooling, continuous feed, high-pressure oil to lift chips out of the bore. |
| Zirconium, Zr 702/705 | 25–45 | 0.01–0.04 | ≥60 bar, oil-based only | Fine chips and fines are pyrophoric. Never water-based coolant. Store chips sealed and dry; Class D fire plan in force. |
| Tungsten carbide (green) | 10–20 | 0.005–0.015 | 30–50 bar, high volume | Extremely abrasive pre-sintered block. PCD tooling mandatory; flood the slurry out. Never on sintered carbide. |
| Powder metallurgy | 30–60 | 0.02–0.05 | 30–60 bar | Porosity changes thermal behavior; avoid pressure that impregnates pores. Exit burrs vary. |
Virtually every failure on non-standard materials traces back to a chip that could not form or could not leave. Chip geometry and coolant pressure are where you intervene.
Ductile materials (copper, aluminum, tough steels) need chip-splitting or groove-type breakers to segment long ribbons. A groove-type breaker on tough steel shortened chips from 25–80+ mm to under 10 mm and roughly halved tool wear; ISCAR’s splitter and Tungaloy’s NDL geometries are designed for low-feed gun-drill machines (≤0.1 mm/rev).
Extended-length holders multiply the required pressure and flow roughly in proportion to extension — a 3XL holder needs ~3× the pressure and flow of a standard holder. Plan coolant capacity for the deepest hole, not the typical one. Filtration matters too: copper builds up on bearing pads at >20 μm contamination.
| Fault | Material Symptom | Fix |
|---|---|---|
| Long stringy chips | Pure copper, aluminum, tough steels | Raise feed, add chip-splitter / groove breaker geometry |
| Built-up edge | Copper, Monel, low-ductility alloys | EP coolant, sharp polished positive-rake edges, adequate chip load |
| Powder / fine chips | Magnesium, zirconium, graphite | Heavier feed for thicker chips; sharp tools; watch ignition risk |
| Sudden coolant pressure drop | All deep-hole materials | Blocked chip train — retract, clear, inspect bore |
| Exit delamination / tear-out | CFRP, GFRP, PEEK | Reduce feed at exit, support board, larger point angle |
| Guide-pad wear | D2, bronze, graphite | Cleaner filtration, coated pads, higher-pressure flush |
Published numbers vary by tool maker, machine, coolant and geometry — treat every cell in this page as a launch point, not a specification.