There is a word you will keep hearing in precision manufacturing — deep hole drilling — and it gets used loosely. Some people mean any long hole. Some mean a specific machine. Some mean a process they have heard about but never quite understood. It is worth being precise, because the difference between deep hole drilling and ordinary drilling is not size. It is a different way of making a hole entirely.
So, what is deep hole drilling?
What Is Deep Hole Drilling?
Deep hole drilling is a machining process for producing holes whose depth is a large multiple of their diameter — far beyond what a conventional drill can manage — while holding a straightness, surface finish, and accuracy that ordinary drilling cannot maintain at those depths.
The key word is ratio, not depth. A hole’s depth-to-diameter ratio (written L/D) is what decides whether you are doing deep hole drilling or just drilling a long-ish hole.
How Deep Is “Deep”?
There is no single agreed threshold, but the conventions cluster around the same idea:
- Anything beyond roughly 4:1 is generally called deep.
- 10:1 and up is where specialized methods start to be necessary rather than optional.
- Purpose-built machines routinely run 100:1, and specialized setups reach 300:1 — a 10mm hole drilled a full three meters, straight, in a single pass.
The ratio matters because it decides which physics problem you are solving. Drill a 10mm hole 40mm deep and chip evacuation is an afterthought. Drill that same 10mm hole a meter deep, and chips, heat, and tool rigidity stop being negotiable. The mechanics of how deep hole drilling solves all three at once is covered in our guide to how deep hole drilling works.
The Three Classic Methods
The term “deep hole drilling” covers three related methods, distinguished mainly by how the tool manages chips and coolant:
Gun drilling. The oldest method, developed in the 19th century for rifling gun barrels. A single-lip tool drills the hole while high-pressure coolant is pumped through it and flushes the chips back along an external V-groove. It owns the small-diameter range — roughly 1mm to 50mm — and is the standard for extreme depth-to-diameter ratios. That is why the gun drilling machine is the workhorse of precision deep hole work.
BTA drilling. Standardized by the Boring and Trepanning Association in the 1930s, BTA inverts the flow: coolant travels down between the tube and the hole wall, and chips evacuate through the hollow center of the drill tube. Because the chip path is far larger, BTA removes metal several times faster. It takes over where gun drilling thins out — roughly 20mm upward.
Ejector drilling. A double-tube variant that uses a venturi to create suction and pull the chips out. It avoids the sealed pressure head that BTA requires, so it can run on conventional machines.
For the full mechanics of each — the cutting edge, the guide pads, the coolant pressures — see how deep hole drilling works, or the practical crossover story of a part that outgrew gun drilling and switched to BTA.
Why Manufacturers Choose It
Deep hole drilling is not the cheap way to make a hole. It is the only way to make certain holes. The advantages that keep industries using it:
- One pass, no pecking. A finished deep hole in a single cut, instead of repeated retract-and-advance cycles.
- Straightness you cannot get any other way. Self-guiding tools hold holes straight to a few hundredths of a millimeter per 100mm of depth.
- Clean surfaces. Many deep drilled holes need no reaming or honing afterward.
- Consistent quality at volume. The process is repeatable, which matters when every part has to match.
- Material versatility. Steel, stainless, aluminum, titanium, hardened alloys — the methods adapt.
The limits are equally real: specialized machines and tooling cost more than a drill press, and the process rewards proper engineering. Our article on why we drill a customer’s part before building the machine shows how we de-risk exactly that.
Where Deep Hole Drilling Is Used
Wherever a part’s function depends on a long, straight, precisely finished bore:
- Firearms and ordnance — where the process began
- Automotive — crankshafts, camshafts, fuel injection components
- Medical devices — cannulated screws and surgical instruments drilled end to end
- Oil & gas and hydraulics — drill collars, cylinders, valve bodies that have to hold pressure
- Mold & die — cooling channels routed deep inside injection molds
- Aerospace and power generation — landing gear, engine and generator shafts
Many of these parts run on machines we build, from medical gun drilling to multi-spindle production.
Deep Hole Drilling vs. Conventional Drilling
How do you know when “a long drill” stops being the answer? A useful rule of thumb:
- L/D below about 5 — conventional twist drilling, often with pecking, is faster and cheaper.
- L/D from about 5 to 10 — the crossover zone. High-pressure internal coolant and careful technique can stretch a twist drill, but deep hole methods start to win on straightness and cycle time.
- L/D above about 10 — specialized deep hole drilling, on a purpose-built machine, is the reliable answer.
If your parts are crossing into that territory, it is worth a conversation before you commit to tooling. The story of the part that outgrew gun drilling is a good example of how the crossover actually plays out.
FAQ
Is deep hole drilling the same as gun drilling? No. Gun drilling is one method within deep hole drilling — the oldest and most common, for small diameters. BTA and ejector drilling are the other two classic methods.
How deep can deep hole drilling go? Depth-to-diameter ratios of 100:1 are routine, and specialized setups reach 300:1.
What is the L/D ratio? Depth divided by diameter. A 50mm-deep hole that is 5mm in diameter has an L/D of 10:1.
Does deep hole drilling need special machines? For ratios beyond roughly 10:1, almost always. A purpose-built machine provides the high-pressure coolant, guide support, and counter-rotation that a standard CNC cannot.
Is peck drilling the same as deep hole drilling? No. Pecking is a workaround for chips accumulating in a deep hole; deep hole drilling removes chips continuously so pecking is not needed.
The Takeaway
What is deep hole drilling? In one sentence: it is the family of processes that makes holes whose depth far exceeds their diameter — straight, smooth, and in a single pass. The three classic methods, gun drilling, BTA, and ejector, share the same physics and differ in how they manage chips and coolant.
If you have parts crossing the deep hole line, send us your specifications — diameter, depth, material, and volume — and we will tell you honestly which method and machine belongs on the job. The conversation costs nothing, and it is how the right process gets chosen. Talk to our engineers.