There is a question every machinist has asked at some point: how do you drill a hole a hundred times deeper than it is wide — and have it come out straight?
It is a question a standard drill press cannot answer. A conventional twist drill relies on its flutes to spiral chips out of the hole, and that system stops working the deeper the hole gets. Deep hole drilling exists precisely because of that limit. It is the family of methods that lets you drill deep, straight, single-pass holes that nothing else can make — and here is how it works.
What Counts as a Deep Hole
A “deep hole” is not defined by absolute depth. It is defined by the ratio of depth to diameter. Anything beyond roughly 4:1 is generally considered deep; purpose-built machines routinely reach 100:1, and specialized setups go beyond 300:1.
The ratio matters because it decides which problem you are actually solving. Drill a 10mm hole 40mm deep and chip evacuation is a minor concern. Drill that same 10mm hole 1,000mm deep, and the chips — if they are allowed to accumulate — will do more damage to the hole than the cutting edge ever could.
Why a Normal Drill Fails at Depth
The conventional twist drill works beautifully at shallow depth. As the hole deepens, three things go wrong at once:
- Chip evacuation collapses. The spiral flutes can only move so much material, and the return path gets longer with every millimeter. The standard fix is “pecking” — repeatedly retracting the drill to clear the hole — which destroys cycle time and, on a long slender drill, invites the tool to wander every time it re-enters.
- Heat has nowhere to go. Deep holes only flush part of the cutting heat away through the chips. The rest stays in the tool and the workpiece, and at depth it concentrates where it can do the most damage.
- The tool goes floppy. A drill a hundred times longer than its diameter flexes like a needle. Without support it bends, vibrates, and drills a hole that isn’t straight — long before it breaks.
Deep hole drilling methods solve all three at the same time. That is what makes them a discipline rather than a drill size.
The Three Principles Behind Every Method
Every deep hole drilling method — gun drilling, BTA, ejector — rests on the same three ideas:
Single-pass cutting with continuous chip removal. No pecking. The tool enters once and the chips never get the chance to pile up. The difference between peck drilling and deep hole drilling is the difference between shoveling snow by hand and turning on a snow blower.
High-pressure coolant through the tool. Coolant is pumped through the drill itself at pressures far beyond what a standard CNC delivers — in most gun drilling applications, well over 1,000 psi. The coolant does three jobs at once: it flushes every chip out of the hole, it cools the cutting edge, and it lubricates the cut. It is not an accessory. It is the mechanism.
A tool that guides itself. The cutting edge is deliberately asymmetric, balanced by guide pads that ride against the wall of the freshly cut hole. Instead of following the path of least resistance, the tool steers itself. That is how a 100:1 hole comes out straight instead of banana-shaped.
Gun Drilling: The Method Behind the Name
Gun drilling is the oldest deep hole method, and it got its name honestly — it was developed over a century ago to rifle gun barrels. A gun drill is a single-lip tool with a V-shaped groove running along its length.
The key detail is how the chips leave the hole. Chip evacuation is external: high-pressure coolant is pumped through a channel inside the drill, exits at the cutting tip, and carries the chips back out along the V-groove on the outside of the tool. Because the whole assembly is self-piloting, the tool supports itself as it cuts.
Gun drilling owns the small-diameter end of the spectrum — typically from well under 1mm up to around 50mm — and it is the method of choice when extreme depth-to-diameter ratio matters more than raw metal removal. It produces holes so straight and so clean that secondary operations like reaming and honing are often eliminated entirely. It is also the method our customers lean on for materials that punish tooling, as in our article on taming the heat when gun drilling titanium. The machines built to run it — from single-spindle to three-axis CNC — are collected on our gun drilling machines range page.
BTA Drilling: When the Hole Gets Bigger
BTA (named after the Boring and Trepanning Association, which standardized the system) takes the opposite approach to chip evacuation. The chips exit internally — through the center of the drill tube — while coolant is delivered under pressure between the tube and the hole wall.
That single change has two consequences. Because the chips travel through the tool’s interior instead of scraping along the freshly cut bore, the hole wall stays cleaner. And because the cutting edges are indexable carbide inserts rather than a solid-carbide drill, BTA removes metal several times faster than a gun drill.
BTA takes over where gun drilling thins out, typically from around 20mm diameter upward — up to the largest deep holes made in industry. We walk through a real-world crossover on our blog, in the story of a part that outgrew gun drilling and switched to BTA, and the machine that runs those parts is the BTA deep hole drilling machine.
Ejector Drilling: A Third Option
Less common but worth knowing: ejector drilling uses a double-tube system. Part of the coolant is diverted through a venturi inside the tool, creating a partial vacuum that sucks the chips back through the inner tube. It avoids the sealed pressure head that BTA requires, which can be an advantage on certain machines. For most practical decisions, gun drilling and BTA are the two methods that matter.
Where Deep Hole Drilling Is Used
Deep hole drilling shows up wherever a part’s function depends on a long, straight, precisely finished bore:
- Automotive — crankshafts, camshafts, transmission shafts, fuel system components
- Medical devices — cannulated screws and surgical instruments drilled end to end, the kind of work done on our medical gun drilling machines
- Oil & gas — valve bodies, manifolds, and downhole tools that have to hold pressure
- Mold & die — cooling channels routed deep inside injection molds
- Aerospace — structural and hydraulic components where a failed hole means a failed part
Common Mistakes to Avoid
- Reaching for a standard CNC and a long twist drill. It will peck, wander, and eventually break. The tooling and the coolant system are not optional extras.
- Skipping the starter hole or guide bushing. The first few millimeters decide the hole’s direction. Start it crooked and no amount of self-guiding will fix it.
- Cutting corners on coolant. If the pressure or filtration is wrong, the chips win. Coolant is the process.
- Changing one variable without the others. Feed, speed, and tool geometry are locked together. Change one and you will end up diagnosing a tool-life problem that is really a process problem.
FAQ
How deep can deep hole drilling actually go? Depth-to-diameter ratios of 100:1 are routine, and specialized setups reach 300:1. Absolute depth depends on the method and the machine.
Is gun drilling the same as BTA drilling? No. They are the two main methods, differentiated mainly by chip evacuation — external (gun drilling) versus internal (BTA) — and by diameter range. See the sections above.
Does deep hole drilling need a starter hole? Almost always. A bushing or a short pilot start supports the tool for the first few millimeters, after which it becomes self-guiding.
Can I do deep hole drilling on a standard CNC mill? Gun drilling on a lathe or mill works for modest ratios. Beyond about 40:1 the tool needs the whip guides, counter-rotation, and high-pressure coolant that only a purpose-built machine provides.
Why are the holes so straight? Because the tool is self-guiding. The asymmetric cutting edge and guide pads keep it cutting on its own axis instead of wandering with the runout.
The Takeaway
Deep hole drilling works because it refuses to fight the three things that make deep holes hard. It removes chips continuously instead of pecking, it cools through the tool instead of hoping for the best, and it lets the tool steer itself instead of flexing. Get those three right, and a hole a hundred times deeper than it is wide is not a gamble — it is a process.
The right method still depends on your numbers. Small and deep is gun drilling territory; large diameters belong to BTA. Send us your real specifications — diameter, depth, material, and volume — and we will tell you honestly which process fits your part.