There is a point where a drilling method stops being the right tool — not because it fails, but because it becomes the wrong tool. A customer found that point recently, and the way we handled it is worth explaining, because it is a decision every deep hole drilling buyer eventually faces.
The part was a steel component for the energy sector, requiring a Ø60mm hole, 1500mm deep, with straightness and surface finish requirements that mattered. The customer was already running gun drilling machines and assumed the answer was another one — bigger and more powerful.
It wasn’t. The right answer was to switch methods, from gun drilling to BTA single-tube drilling — and to make that call before they spent money on a machine that would have been the wrong tool.
Why Gun Drilling Has a Diameter Ceiling
Gun drilling is brilliant in its home territory: holes from roughly Ø3mm up to about Ø30–40mm, at extreme depth-to-diameter ratios, with excellent straightness. Within that range it is hard to beat. A well-tuned gun drilling machine can push further — our own machines reach Ø50mm — but the crossover is not about whether it can. It is about whether it should.
As the diameter grows, gun drilling stops scaling gracefully:
- Tool cost climbs steeply. A gun drill is a solid-carbide tool, and at Ø60mm the tool is expensive. Large gun drills exist, but the cost per hole climbs with every millimeter.
- Chip evacuation gets harder. A gun drill relies on chips being flushed back along the tool’s external V-groove. At larger diameters, the cutting edge removes far more material per revolution — more chips, faster — and the single groove cannot keep up. Chip packing slows the cut and endangers the tool.
- Metal removal is slow. At big diameters, a gun drill cuts a much bigger annulus of material per revolution. The feed has to be conservative, and the cycle time stretches.
There is overlap between the methods — BTA systems start around Ø20mm — but somewhere in the Ø30–50mm band the economics flip, and where exactly depends on depth, material, and volume.
What BTA Single-Tube Drilling Changes
BTA (Boring and Trepanning Association) single-tube drilling works on a different principle, and it is the principle that matters at big diameters:
- Chips come out through the center of the drill tube. Coolant is pumped down under high pressure through the annulus between the tube and the hole wall, then returns through the tube’s hollow center, carrying the chips with it.
- The chip path is a straight, open channel. There is no narrow groove to clog. At large diameters, where chip volume is high, that channel is exactly what is needed.
- Higher metal removal rates. BTA tooling for large diameters delivers more power to the cut, so the cycle time for a big hole is dramatically shorter than gun drilling the same hole.
The trade-off: BTA is a specialist process, and its machines are configured differently — different spindles, higher coolant volume, higher torque. That is why the decision matters: you do not want a BTA machine if you only drill small holes, and you do not want a gun drilling machine if your work is heading into large-diameter territory.
The Diagnosis: Reading the Customer’s Real Numbers
We did not tell the customer “you need BTA” on the first call. We asked for the numbers that decide the question:
- Required diameter: Ø60mm — well into BTA territory.
- Depth: 1500mm — comfortably within a BTA machine’s 2000mm range.
- Annual volume: high enough that cycle time and tool cost per hole dominated the economics.
- Hole quality requirements: straightness and surface finish (around Ra 1.6µm) that the BTA process meets in a single pass.
When those numbers are laid out, the choice becomes obvious. A Ø60mm hole drilled by gun drilling would have been slow, expensive per hole, and fragile at depth. BTA single-tube drilling was built for exactly this.
We proposed a machine configured for BTA single-tube drilling within the customer’s diameter range and, as we do with every order, cut trial parts before anything shipped. The trial verified the numbers we had promised.
The Result
The switch produced the results that matter on a production floor:
- Cycle time for the Ø60mm × 1500mm hole dropped by more than half compared with the gun drilling attempt — the metal removal rate of the BTA tool is that much higher.
- Tool cost per hole fell, because BTA tooling for this diameter is more economical than pushing a gun drill to its limit.
- Straightness and surface finish met spec in a single pass — no secondary operations to correct the hole.
- The machine was right-sized. The customer got the process their parts actually need, not a gun drilling machine that would have fought the job for years.
The Takeaway: Match the Process to the Diameter, Not the Other Way Around
The most expensive mistake in deep hole drilling is buying a machine that is the wrong method for your holes — either a gun drill stretched past its diameter range, or a BTA machine for work that never needed it.
A simple heuristic: small and deep is gun drilling territory; large diameter is BTA territory. If your holes are crossing into the Ø30–40mm range and beyond, it is worth a conversation before you commit to a machine.
That conversation should start with your real numbers — diameter, depth, material, and volume. Send us those, and we will tell you honestly which process fits: a gun drilling machine, a BTA single-tube system, or something in between. Matching the machine to the part is how you avoid buying the wrong one.