Quick, practical answers to the questions engineers ask most about deep hole drilling — every answer links to the full guide that goes deeper.
A deep hole is usually defined by depth-to-diameter ratio (L/D). The industry threshold is roughly L/D 5:1 (VDI 3210 says >3×D); beyond about 10:1, standard twist drills can no longer evacuate chips, cool the edge, or hold the axis, and specialized deep hole drilling methods are mandatory.
They are the three core deep hole methods, chosen by diameter and depth. Gundrilling (0.5–50 mm, to 300:1 L/D) pushes coolant through the tool and evacuates chips along an external V-flute. BTA / single-tube (STS) (6–2000 mm, to 200:1) delivers coolant externally and pulls chips back through the tube center, feeding 5–10× faster. Ejector / double-tube (DTS) (18–250 mm, to 100:1) uses a Venturi ring and needs no workpiece face seal, so it retrofits onto ordinary lathes.
Single-lip gundrills routinely reach 300:1 length-to-diameter in production, and 400:1 is achievable with whip guides and careful setup. A 1 mm drill can reach 400 mm deep; a 10 mm drill, 4 meters or more.
Yes — a gundrill is not self-starting. On a machining center or lathe it needs a pilot hole about 1.5–3×D deep at gundrill diameter +0.01–0.02 mm; on a dedicated gun drilling machine a starting bushing does the job. The first few diameters decide the straightness of the whole hole.
It depends on method and diameter. Gundrilling needs 50–150+ bar (up to ~2,500 psi for micro drills) because the V-flute is small; BTA runs 15–100 bar with high flow; ejector needs only 10–50 bar but high flow, because its Venturi effect is driven by flow, not pressure.
Gundrilling holds straightness near 0.001 in/in (0.03 mm/m) with tool rotation, or ~0.001 in/ft (0.08 mm/m) with workpiece rotation. Counter-rotation of part and tool reaches ~0.0005 in/ft. BTA typically achieves 0.05 mm/m. An entry error of 0.1° becomes roughly 1.75 mm of drift over 1000 mm of depth.
As-drilled, gundrilling gives Ra 0.4–6.3 µm thanks to guide-pad burnishing. A follow-up skive & roller burnishing pass (one setup) reaches Ra 0.1–0.4 µm, and honing can go below that. BTA lands around Ra 0.2–1.6 µm as-drilled.
Yes, with limits. With through-spindle coolant (150–1,000 psi), a VMC or HMC can gundrill effectively to roughly 40:1 L/D. Beyond that, purpose-built machines win on straightness and productivity. Ejector drilling retrofits onto lathes even more easily because it needs no face seal.
A twist drill cannot evacuate chips or cool its edge beyond about 5×D — chips pack, heat builds, and the axis wanders. Deep hole drills use one cutting edge and a V-flute (gundrill) or internal chip tube (BTA) specifically so chips clear continuously at high L/D.
Practically any machinable metal: carbon and alloy steels, stainless, aluminum, titanium, superalloys (Inconel, Hastelloy), hardened steel (to HRC 65 with PCBN tooling), cast iron, copper alloys, and magnesium. For non-conductive or ultra-hard parts, EDM, laser, or waterjet take over.
Cost per hole depends on diameter, depth, material, and method. Gundrilling is cheapest per hole at low volume; BTA wins at high volume (break-even around 500–1,000 parts/month). Machine investment runs $30k–$100k for an ejector retrofit, $50k–$200k for a gundrill machine, $200k–$800k+ for dedicated BTA.
Typically every 50–200 m of drilled length, depending on material and diameter. Gundrills are regrindable (8–10+ times), which keeps tool cost per hole low. Watch guide-pad wear and coolant pressure — regrind before wear causes straightness loss.
Chips are the only real-time window into the cutting zone, which is out of sight. Tight C-shaped chips mean healthy evacuation; ribbons, dust, or tangles mean feed, coolant, or chip-breaker problems. Chip shape change is the first warning before tool breakage — stop and investigate.
Gundrilling holds IT6–IT11 (optimally IT5–IT7, ±0.02 mm typical in aerospace). BTA holds IT8–IT10, ejector IT9–IT11. Diameter, straightness, and surface finish are the three independent tolerances to specify for a deep hole.
BTA feeds roughly 5–10× faster than gundrilling on the same diameter because its internal chip tube evacuates chips without the thin V-flute limit. That is why small bores are gundrilled (feed is not the bottleneck) but large bores go BTA (metal removal is). Gundrilling wins on finish, straightness and the smallest diameters.
Skive & roller burnishing (SRB) finishes a drilled deep bore in one pass: a carbide blade skives a thin layer off the wall (0.1–0.4 mm), then tapered rollers cold-work the surface to Ra 0.1–0.4 µm. It is the fastest way to size and finish long open barrels — the standard for hydraulic cylinders — but it needs a round pre-bore and a through-hole to exit.
An unsupported gundrill tube flexes under cutting load and whips — deflecting off-axis, grabbing the bore wall, and ruining straightness. Supporting the tube every ~40× the drill diameter (and closer beyond ~200:1 L/D) is what lets a hole come out straight. Dedicated machines have steady rests and whip guides built along the axis.
Typical planning numbers: a Ø10 mm gundrilled hole in steel runs roughly $1–$4; a Ø50 mm BTA bore over a metre runs $15–$40; Inconel work costs more. The drivers are method choice, diameter, depth, material, and whether you already own a high-pressure coolant system. Machine investment runs $30k–$800k+ depending on method and class.
Trepanning cuts a large deep bore as a ring, leaving the center as a solid core (slug) that is recovered. It is used when the bore is big (> ~Ø100 mm), when the core has value, or when solid drilling would need impractical power. It needs a through-hole — a blind bore cannot be trepanned because there is no way to extract the core.
The classic causes: chip packing (feed or coolant flow wrong), coolant loss starving the edge, a dull edge that work-hardens and grabs, whip from an unsupported tube, a bad entry that starts a spiral, or dirty coolant above ~20–30 µm. Live monitoring of spindle torque and coolant pressure catches all of them before the tool snaps.
BTA holds ~200:1 L/D in production on medium-to-large diameters, with depth limited more by bar rigidity, chip evacuation and coolant pressure than by anything else. Above ~30–40:1, boring bars need intermediate supports and vibration damping; for very deep work, shops step-drill with intermediate bushings or switch to specialized long-bar machines.
Answers are starting points — always verify cutting parameters with tool manufacturers and test cuts. Browse all 98 guides from the homepage.