Bottle Boring & Contour Drilling: Non-Straight Deep Holes

Most deep hole drilling produces a straight cylindrical bore. But some of the most valuable components in aerospace, oil and gas, and hydraulic systems require something different — a bore that is narrow at the entrance and wide at the bottom, or one that changes diameter partway through, or one that follows a curved path. These are not standard operations. They require tools that expand, retract, and steer at depth.

Bottle boring, contour drilling, and chamber boring represent the frontier of deep hole drilling capability. While standard gun drilling and BTA drilling produce straight, cylindrical bores, these specialised techniques enable internal geometries that were previously impossible or required multiple operations with part transfers between machines.

This article covers the methods, tooling, and applications for non-straight deep hole geometries, including bottle boring, contour drilling, chamber boring, eccentric drilling, and active bore path control.

What Is Bottle Boring? ​

Definition ​

Bottle boring is a deep hole drilling technique that produces a bore where the internal diameter is larger than the entrance diameter — creating a shape resembling a bottle. The process typically involves two phases:

1. Pilot bore: A straight BTA or gun-drilled bore of the entrance diameter is created through the full component length.
2. Bottle boring pass: An expandable tool head is fed through the pilot bore to the target depth, where its cutting edges are expanded radially to create the larger internal cavity.

Key Capabilities ​

Why Bottle Boring Matters ​

Bottle boring eliminates the need to produce large-diameter bores from solid material. Instead, a small pilot bore is drilled first (fast, efficient), and only the section that requires the larger diameter is expanded. This saves material, reduces cycle time, and lowers machine power requirements.

The Bottle Boring Process ​

Step-by-Step ​

1. Drill the pilot bore: A standard BTA drill creates a straight, accurate pilot bore through the full component length. This bore serves as the guide path for the bottle boring tool.

2. Position the bottle boring tool: The expandable tool head is mounted on a BTA drill tube and fed through the pilot bore to the start position of the bottle section.

3. Expand the cutting edges: CNC-actuated axes push the cutting inserts radially outward. The expansion is precisely controlled — typically in increments of 0.01 mm.

4. Feed and expand simultaneously: The tool rotates and feeds axially while the cutting edges remain expanded, cutting the bottle-shaped cavity. The feed rate is coordinated with the radial expansion rate to produce the desired internal profile.

5. Retract the cutting edges: At the end of the bottle section, the cutting edges retract to the pilot bore diameter.

6. Withdraw the tool: The tool is withdrawn through the pilot bore, which is now connected to the expanded cavity.

Tip: Chip evacuation during bottle boring uses the BTA chip-forward method — chips travel through the centre of the drill tube and are carried away by coolant flow. This is critical because chips generated by the expandable cutting edges must be evacuated through the same path as the pilot bore chips.

Expandable Tooling Design ​

The bottle boring tool head is the critical enabling technology:

Patented design (EP1795287A1, Pieri): The cutting head incorporates retractable cutting elements controlled by a drive shaft within the drill tube. Low-friction guiding elements (bronze alloy) prevent tool shaft bending and vibration at extreme overhangs.

UNISIG B-Series Implementation ​

UNISIG has commercialised bottle boring through its B-Series deep hole drilling machines:

• Pilot bore: Created with standard BTA drilling to high accuracy
• Bottle boring tool: CNC-actuated axes expand and contract the cutting surfaces
• Chip-forward discharge: Eliminates tool breakage risk
• Material range: Stainless steel, Inconel, and other challenging alloys
• Industries: Aerospace landing gear, oil and gas, hydraulic actuators

Contour Hole Drilling and Chamber Boring ​

Continuous Profiling ​

While bottle boring creates a single enlarged cavity, contour drilling extends the concept to continuous profiling along the bore axis. The expandable tool head is actuated dynamically during the feed stroke to create:

• Stepped diameters
• Tapered transitions
• Sculpted internal contours
• Internal threads or grooves

Chamber Boring Systems ​

The Institute of Machining Technology (ISF) at TU Dortmund, in collaboration with BGTB GmbH, has developed chamber boring systems that extend deep hole drilling capabilities further:

Research finding (MM Science Journal, 2019): Chamber boring systems achieve reliable contour accuracy across varying cutting speeds, feeds, and workpiece materials. The process forces are predictable and correlate well with the radial expansion rate of the cutting elements.

Non-Circular Deep Bore Profiles ​

The same chamber boring technology can produce pipes with non-circular cross-sections in deep bores:

Expandable and Retractable Tooling ​

Tool Geometry ​

Expandable deep hole tools are available in two configurations:

Feed-Out Stroke Ratio ​

A critical tool design parameter is the feed-out stroke ratio — the relationship between the pilot bore diameter and the maximum expanded diameter. BGTB GmbH specifies:

• Large feed-out stroke ratio: When the bottle bore diameter significantly exceeds the pilot bore diameter, specialised tool body design is required to maintain stability.
• Maximum stability: Achieved by using the largest possible tool body and minimising guide pad recess.
• Simple change of feed-out slide: Modular designs allow diameter range changes without replacing the entire tool.

Actuation Methods ​

Eccentric and Offset Deep Hole Drilling ​

Eccentric Bores ​

Eccentric deep hole drilling produces a bore that is intentionally offset from the workpiece centreline. This is required for components with offset coolant passages, hydraulic galleries, or sensor ports.

Methods ​

Eccentric deep hole drilling is achieved through:

1. Offset tool alignment: The drill guide bush is positioned off-centre relative to the workpiece rotation axis.
2. Counter-rotation drilling: Both tool and workpiece rotate in opposite directions, enabling the tool axis to be offset from the workpiece centreline while maintaining concentric cutting.
3. Specialised machine configurations: Multi-functional machines that combine deep hole boring and gun drilling features can accommodate eccentric holes in very heavy workpieces.

Warning: Eccentric deep hole drilling requires careful consideration of wall thickness around the offset bore. If the eccentric bore breaks through the outer wall or intersects another internal feature, the component is scrap. Minimum wall thickness of 3 mm or 10% of the bore diameter (whichever is greater) is recommended for eccentric bores.

Active Gun Drill Steering ​

Real-Time Bore Path Control ​

Patent US20140050543A1 (Siemens/GE) describes a system for actively steering a gun drill during the boring process:

How It Works ​

1. The gun drill rotates and feeds normally.
2. Acoustic transmitters and receivers monitor the drill's orientation and position in real-time.
3. When a deviation from the desired bore path is detected, the system applies axial impulse forces at a specific azimuthal orientation using piezo-electric stacks.
4. The impulse forces push the drill head in the opposite direction, correcting the bore path.
5. The system accounts for drill twisting (torsion) by comparing the azimuthal orientation at the drill head vs. the external end.

Applications ​

Applications by Industry ​

Aerospace ​

Oil and Gas ​

Hydraulic Systems ​

Firearms and Defence ​

FAQ ​

What is bottle boring in deep hole drilling? ​

Bottle boring is a technique that creates a bore with a larger internal diameter than the entrance diameter. A pilot bore is drilled first, then an expandable tool head opens cutting edges radially at depth to create the enlarged cavity. The resulting shape resembles a bottle.

How does bottle boring differ from standard BTA drilling? ​

Standard BTA drilling produces a straight, constant-diameter bore. Bottle boring uses a CNC-actuated expandable tool head that opens cutting edges radially at depth, enabling variable internal diameters, stepped bores, and sculpted internal profiles.

What depth-to-diameter ratios are achievable in bottle boring? ​

Depth-to-diameter ratios of 14:1 to 20:1 are achievable in bottle boring. The limitation is the stiffness of the tool shaft and the guide system. UNISIG's B-Series machines achieve depths exceeding 30 feet in diameter ranges of 2–16 inches.

Can deep holes be non-circular? ​

Yes. Chamber boring systems from the ISF at TU Dortmund and BGTB GmbH can produce deep bores with wave-like cross-sections, non-circular profiles, and polygonal shapes. This is achieved by oscillating the cutting edges radially during rotation.

What is eccentric deep hole drilling? ​

Eccentric drilling produces a bore that is intentionally offset from the workpiece centreline. It is used for components requiring offset coolant passages or hydraulic galleries. Positional tolerances of ±0.020 inch at extreme depths are achievable with specialised BTA and gun drilling equipment.

Can a gun drill be steered to follow a curved path? ​

Yes. Active gun drill steering systems use acoustic monitoring and piezo-electric impulse actuators to correct bore path deviations or intentionally create curved trajectories. The technology was developed by Siemens/GE and is covered by patent US20140050543A1.

What industries use bottle boring? ​

Aerospace (landing gear struts, actuator cylinders), oil and gas (downhole tools, blowout preventer components), hydraulic systems (telescopic cylinders, accumulators), and defence (gun chambers, cannon barrels) are the primary industries that use bottle boring.

How does the expandable tool head work in bottle boring? ​

The tool head contains cutting inserts mounted on radially expandable slides. A CNC-controlled push rod within the drill tube actuates the slides outward. The expansion is coordinated with the axial feed to produce the desired internal profile. Guide pads maintain tool position during expansion.

Summary ​