Deep Hole Drilling

Straight holes.
100× diameter deep.
Gun drilled.

Specialty deep hole drilling for hydraulic components, oil and gas hardware, aerospace fuel systems, and precision tooling. Gun drilling, BTA drilling, and ejector drilling achieve straightness and accuracy impossible with twist drills.

Deep Hole Drilling Quote All processes

Up to 2 m depth 100:1 L/D ±0.05 mm/100 mm 3–150 mm dia

01 · What it is

How Deep Hole Drilling works.

Deep hole drilling refers to specialty drilling processes for holes where the depth-to-diameter ratio exceeds ~10:1 — the practical limit of conventional twist drilling. Three primary specialty processes handle different size ranges:

Gun drilling: Single-flute carbide drill with high-pressure through-coolant. Used for 3–50 mm diameter holes up to 2 m deep. Precision straightness. Ideal for hydraulic cylinder bores, injector bodies, aerospace fuel system deep holes, small-diameter precision work.

BTA drilling (Boring and Trepanning Association): Ejector-style drilling for larger diameters (50–150 mm+). Coolant flows through annulus between drill and hole wall; chips ejected back through the drill. For large hydraulic cylinders, gun barrels, aerospace cannons, large-bore applications.

Ejector drilling: Medium depth/diameter range (10–60 mm diameter, up to 500 mm depth). Compromise between gun drill speed and BTA drill robustness. Used when gun drilling is too slow but BTA is overkill.

02 · Specifications

Capability specs.

3–150 mm

Diameter range

Gun drill 3–50 mm, ejector 10–60 mm, BTA 50–150+ mm

100:1

Max L/D ratio

Length-to-diameter ratio achievable. Conventional drilling limit: ~10:1

2,000 mm

Max depth

Gun drilling practical limit. BTA can reach similar depths on larger bores

±0.05 mm

Straightness per 100 mm

Typical straightness accuracy — better than any alternative process

±0.025 mm

Diameter tolerance

Hole diameter achievable. Reaming to ±0.01 mm for precision applications

Ra 1.6 µm

Surface finish

Interior surface finish — typical for gun-drilled holes

20–40 bar

Coolant pressure

High-pressure through-coolant essential for chip evacuation and heat management

Any metal

Material

Steel, stainless, Inconel, titanium, tool steel — any machinable metal

03 · Applications

Where Deep Hole Drilling excels.

Hydraulic cylinders

Hydraulic cylinder bores — precision straightness critical for seal performance

Injection mold cooling

Cooling channel passages in injection molds — conformal cooling layouts

Oil & gas mandrels

Downhole tool internal bores — sour service materials (Inconel, duplex)

Gun barrels

Firearm barrels — rifled or smooth bore precision deep-hole drilling

Aerospace fuel system

Deep cross-drilled fuel passages in aerospace manifolds and valve bodies

Heat exchanger tubes

Tube sheet holes for shell-and-tube heat exchangers

Print roller cores

Printing industry roller cores with heated/cooled passages

Medical tubing

Deep small-diameter bores for medical catheter tooling

Research hardware

Pressure vessel deep bores, particle accelerator components

04 · When not to use it

Not suitable for:

Every process has its limits. Being honest about where Deep Hole Drilling isn\'t the right answer saves time and money.

Shallow holes under 10:1 L/D — conventional drilling faster and cheaper
Holes smaller than 3 mm diameter — micro-machining territory
Short production runs of 1–5 pieces — setup cost per part high
Material harder than 55 HRC — deep hole drilling limited on hardened material
Non-round cross-sections — deep hole drilling is round bore only

FAQ

Deep Hole Drilling questions.

Twist drill deflection and chip evacuation problems limit practical depth to ~5× diameter. Beyond that, drills wander off-center, chips pack and break tools, heat builds up catastrophically. Gun drilling solves all three: single-flute design improves rigidity, through-coolant at high pressure flushes chips efficiently, steady-rest design maintains straightness. Result: 100:1 L/D achievable vs 5:1 with twist drill.

Conventional drill: can wander 0.5–2 mm off-center at depths over 10× diameter. Gun drill: 0.05 mm drift per 100 mm depth — at 1 meter depth, total drift under 0.5 mm. For precision applications (hydraulic cylinders, aerospace fuel systems, gun barrels), this straightness is mandatory. Post-drilling honing further improves straightness and surface finish.

Yes. Gun drilling Inconel requires: slower speeds (15–25 m/min vs 60 for steel), careful coolant pressure management, premium carbide tooling. Typical inconel gun drilling cost: 3–4× equivalent steel cost. Our experience with difficult materials includes gun drilling NACE-qualified Inconel 718 for oil and gas downhole tool work.

Smooth bore gun drilling: standard process, produces straight round hole. Rifled bores (for firearms): gun drill produces smooth bore, then button rifling or broaching adds the rifling grooves. We handle smooth bore through partner for rifling operations. For hydraulic cylinders and general deep hole work: smooth bore is what's needed.

Gun drilling uses high-pressure through-coolant (20–40 bar) that flows through the center of the drill and exits at the cutting edge. This flushes chips back along the outside of the drill. For very long holes, intermediate chip-breaking cycles may be needed. Specialized cutting fluid (not standard machine coolant) matched to material is required.

Typical deep hole drilling: 7–14 business days. Depends on: specific material availability, drill stocking (specialty diameters may need special tooling), combined operations (usually machined externally too). For one-off custom-diameter work, additional 5–10 days for custom tool procurement. For recurring production of standard bores, 7 days typical.