A plastic injection mold needs a Ø6 mm × 350 mm water channel drilled into P20 steel for cooling — that is a depth-to-diameter ratio of 58:1. A standard twist drill will deflect, wander, and snap before reaching the bottom. A landing gear trunnion needs a Ø20 mm × 600 mm bore through 7075-T6 aluminum for a hydraulic line — that is 30:1. A surgical cannula needs a Ø1.5 mm × 80 mm bore through 304 stainless for biopsy — that is 53:1. All three are deep hole drilling CNC jobs — the most unforgiving operation in the precision machining shop. When the depth-to-diameter ratio exceeds about 10:1, conventional drilling fails and dedicated deep hole drilling methods take over: gun drilling for small diameters (1–50 mm) and short-to-medium depths, BTA (Boring and Trepanning Association) for medium-to-large diameters (20–250 mm) and high production rates, ejector drilling for cost-sensitive medium depths, and EDM for the hardest materials or non-conventional geometries. This guide covers what we have learned running deep hole drills on OEM mold, aerospace, automotive, medical and hydraulic parts across 23 years in Dongguan: how the four methods differ, the depth-to-diameter sweet spots, the coolant pressure and chip evacuation rules that decide whether the drill cuts or breaks, the four tolerances that separate a usable deep hole from a scrap one, and the five application scenarios where deep hole drilling is non-negotiable. For the broader CNC drilling framework, our CNC machining guide covers the general process.
4 deep hole drilling methods and when to use each
The four methods differ on diameter range, depth-to-diameter capability, surface finish, and cost. Picking the right one is the first decision.
1. Gun drilling — for small diameters (1–50 mm) and depths up to 100:1
Gun drilling is the classic deep hole method. A single-flute drill with an internal coolant hole delivers high-pressure coolant (7–20 MPa) through the drill body to evacuate chips from the cutting zone. Gun drills are the right answer for Ø1–50 mm holes at depths of 30:1 to 100:1 — the sweet spot for mold cooling channels, hydraulic lines, medical cannulas, and aerospace pins. Surface finish Ra 0.4–1.6 μm straight off the drill; roundness and straightness typically 0.02–0.05 mm per 100 mm depth.
2. BTA drilling — for medium-to-large diameters (20–250 mm) and depths 20:1–100:1
BTA (Boring and Trepanning Association) drilling uses a multi-tooth head with coolant supplied externally around the outside of the tool; chips evacuate up through the inside of the tool. BTA is 3–5× faster than gun drilling on diameters above 20 mm and produces better straightness on long bores. BTA is the right answer for Ø20–250 mm hydraulic cylinders, landing gear bores, and large mold cooling circuits. Cycle time per 100 mm depth: BTA 1–2 minutes vs gun drill 5–10 minutes at Ø30 mm.
3. Ejector drilling — for medium depths at lower cost
Ejector drilling is the legacy single-tube method: a two-piece drill with an inner tube that carries chips back up against the coolant flow. Ejector drills are cheaper than BTA but slower and limited to about 20:1 depth-to-diameter. Ejector is the right answer for hydraulic cylinder bores up to about Ø100 mm and depths of 20:1–40:1 where cost matters more than cycle time.
4. EDM deep hole drilling — for hard materials and non-standard geometries
EDM (electrical discharge machining) deep hole drilling uses a brass or copper tube electrode that erodes the workpiece with electrical sparks while dielectric fluid flushes debris. EDM is the right answer for materials too hard to cut (>HRC 55), for hole geometries that cannot be machined (small diameters in carbide, blind holes with curved paths, cross-holes in assemblies), and for exotic alloys (titanium, Inconel, Waspaloy). EDM is slower than mechanical drilling (typical 1–3 mm/min material removal) and produces a recast layer (~0.02 mm) that may need post-EDM finishing for fatigue-critical parts.
| Method | Diameter range | Depth ratio | Surface roughness | Cycle time (per 100 mm) | Best for |
|---|---|---|---|---|---|
| Gun drilling | 1–50 mm | 30:1–100:1 | Ra 0.4–1.6 μm | 5–10 min at Ø30 | Mold cooling, hydraulic, medical cannula |
| BTA drilling | 20–250 mm | 20:1–100:1 | Ra 0.8–3.2 μm | 1–2 min at Ø30 | Landing gear, large mold, hydraulic cylinder |
| Ejector drilling | 12–100 mm | 10:1–40:1 | Ra 1.6–3.2 μm | 3–8 min at Ø30 | Cost-sensitive hydraulic, short bores |
| EDM drilling | 0.3–10 mm | 20:1–200:1 | Ra 0.8–3.2 μm (recast) | 1–3 mm/min | Carbide, hard materials, exotic alloys |
Coolant pressure, chip evacuation, and tool wear
Three failure modes dominate deep hole drilling: drill breakage, hole wander, and poor chip evacuation. The coolant system and tool wear management prevent all three.
Coolant pressure: 7–20 MPa for gun drilling
Standard CNC flood coolant delivers 0.5–1 MPa. Deep hole drilling needs 7–20 MPa to evacuate chips through the flute — without it, chips pack in the cutting zone, the drill binds, and the tool snaps. A dedicated high-pressure coolant system (separate pump, filter, and tank) is mandatory for deep hole work. We use 10 MPa for Ø6–20 mm gun drills in steel, 8 MPa for Ø20–50 mm, and 15 MPa for Ø1–3 mm micro-drills.
Chip evacuation and peck drilling cycle
For depths above 30:1, the drill must retract periodically (peck) to clear chips from the flute and re-establish coolant flow. Standard peck depth: every 5–10× drill diameter (so a Ø6 mm drill pecks every 30–60 mm). Failing to peck on a deep hole guarantees drill breakage. On a BTA drill, the chip flow is reversed (up through the tool) and continuous retract is not needed, but coolant flow must be monitored.
Tool wear and tool life
Deep hole drills wear on the outer corner first (the cutting edge that determines hole size) and on the margin (the bearing surface that determines straightness). Typical tool life: 50–200 holes per gun drill in steel at Ø6–10 mm before hole size drifts out of tolerance. We measure drill wear on a tool microscope after every 20 holes in steel and replace drills before the wear land exceeds 0.1 mm.
4 critical tolerances for deep hole drilling
Deep hole tolerances are different from conventional drilling tolerances. Depth, straightness, and surface finish matter more than position accuracy.
1. Diameter: ±0.05 mm (typical) — tighter for precision
Standard gun drill hole diameter tolerance is ±0.05 mm at first article; tighter ±0.02 mm is achievable with controlled feed and dress cycles. BTA drilling holds ±0.05 mm consistently. Diameter is checked with a plug gauge or air gauge at the entrance and exit of the bore.
2. Straightness: 0.02–0.05 mm per 100 mm depth
Straightness is the single tolerance that separates a usable deep hole from a scrap one. A Ø6 × 350 mm gun-drilled hole should hold 0.07–0.18 mm total indicator reading (TIR) at the exit — equivalent to 0.02–0.05 mm per 100 mm depth. Out-of-straightness holes cause problems downstream: hydraulic line binding, cooling channel blockage, assembly interference. We measure with a long-reach dial indicator or a laser scanner at the bore exit.
3. Surface roughness: Ra 0.4–3.2 μm
Gun drilling delivers Ra 0.4–1.6 μm directly off the drill (better than conventional drilling's Ra 1.6–3.2 μm). BTA drilling is Ra 0.8–3.2 μm. EDM drilling is Ra 0.8–3.2 μm but with a 0.01–0.03 mm recast layer. For hydraulic lines, Ra 0.8 μm is the standard; for medical cannula, Ra 0.4 μm is preferred to reduce tissue trauma.
4. Entrance / exit perpendicularity: 0.02 mm
The bore entrance must be square to the starting surface within 0.02 mm. If the drill starts on a curved or angled surface, it will wander and the hole will exit off-target. Pilot drilling (a short Ø3–5 mm starter hole) or a flat spotting face is recommended for critical bores. We check exit position with a coaxial indicator on the workpiece.
5 applications where deep hole drilling is non-negotiable
Plastic injection mold cooling channels
Mold cooling channels are typically Ø6–12 mm at depths 100–500 mm in P20, H13, or S7 tool steel. The channels must be placed close to the cavity surface (typically 1.5–3× channel diameter) to extract heat efficiently, and they must be straight to allow EDM wire cutting and to avoid hot spots. Gun drilling is the standard method. Reference: our DFM analysis guide covers the cooling channel placement rules.
Aerospace hydraulic and pneumatic lines
Landing gear trunnions, actuator bodies, and hydraulic manifolds need Ø10–30 mm bores at depths up to 600 mm in 7075-T6 aluminum, 15-5 PH stainless, or titanium. BTA drilling is the standard for diameters above 20 mm; gun drilling for below 20 mm. Reference: our aerospace and drone CNC guide covers the broader aerospace part portfolio.
Medical cannulas and biopsy needles
Surgical cannulas (Ø0.5–3 mm) and biopsy needles (Ø1–5 mm) at depths 50–300 mm in 304 / 316 stainless or titanium. Gun drilling is the standard; EDM is used for the smallest diameters below 1 mm where mechanical drill strength is insufficient. Surface finish Ra 0.4 μm is required for tissue-smooth insertion. Reference: our medical device CNC guide covers the regulatory framework.
Automotive transmission shafts and cylinder bores
Transmission shafts need Ø15–40 mm oil channels at depths 100–400 mm; cylinder heads need Ø8–14 mm coolant passages; fuel injector bodies need Ø4–8 mm at depths 50–150 mm. Gun drilling and ejector drilling dominate. Reference: our IATF 16949 CNC guide covers the automotive certification framework.
Hydraulic cylinder barrels
Hydraulic cylinders need Ø50–250 mm bores at depths 200–1500 mm in 4140 alloy steel or 304 stainless. BTA drilling is the standard production method. Cycle time per cylinder: 20–60 minutes at Ø100 mm depending on depth and material. Reference: our stainless steel CNC guide covers the 4140 and stainless machining behavior.
Conclusion
Deep hole drilling is not conventional drilling with a longer tool — it is a separate discipline with dedicated machines, high-pressure coolant, and disciplined chip evacuation. Pick the method for the diameter and depth (gun drill for 1–50 mm at high depth ratios, BTA for 20–250 mm at high production rates, ejector for cost-sensitive medium depths, EDM for hard materials and exotic geometries), hold coolant pressure above 7 MPa, peck the drill every 5–10× diameter, monitor tool wear on a tool microscope, and verify straightness at the bore exit with a long-reach indicator. If you have a deep hole drilling job — mold cooling channels, hydraulic bores, medical cannulas, transmission oil passages — send your drawing and we will quote with the method, the cycle time, and the tolerance guarantee. Request a quote today and let our 23 years of precision drilling experience work for your part.
Need a deep hole drilling CNC quote? Send your drawing with depth-to-diameter ratio and material spec — DFM review included, firm quote within 24 hours.