Gun Drill & BTA Composite Machine: Single-Platform Solutions

A shop that buys separate machines for gun drilling and BTA drilling commits to twice the floor space, twice the maintenance, and twice the work-in-process handling. A composite machine collapses both processes into one footprint — but the changeover penalty between the two methods can erase the benefit if the design is not carefully matched to the production mix.

Deep hole drilling shops have traditionally treated gun drilling and BTA drilling as separate processes requiring separate machines. Gun drilling (external chip removal) handles small diameters with precision, while BTA drilling (internal chip removal) tackles larger diameters with higher material removal rates. The two methods use different tooling, different coolant delivery systems, and different machine configurations — so the conventional wisdom has been to dedicate a machine to each.

Composite machines challenge this assumption. By integrating both drilling methods on a single platform, they offer the flexibility to handle a wider range of work without the capital cost of a second machine. This article surveys the current state of composite deep hole drilling machine technology, compares the design approaches used by different manufacturers, and provides practical guidance for shops considering a single-platform solution.

Why Composite Machines? ​

The Problem with Dedicated Machines ​

A typical contract deep hole drilling shop needs to handle parts ranging from 5 mm diameter gun-drilled holes in hydraulic components to 80 mm BTA-drilled bores in aerospace actuators. With dedicated machines, this means:

• Capital cost: Two complete machines with duplicate coolant systems, control cabinets, and workholding
• Floor space: Typically 30–50 m² per machine, plus access areas
• Work-in-process: Parts must move between machines, with queue time and handling damage risk
• Operator specialisation: Different skills for gun drilling setup vs. BTA setup
• Maintenance: Two sets of coolant filters, seals, guide bushings, and wear parts

The Composite Machine Solution ​

A composite machine addresses these pain points by providing both processes on a single bed with shared coolant, control, and workholding systems. The key trade-off is changeover time: switching between gun drilling and BTA modes takes anywhere from 10 minutes to several hours depending on the design approach.

Tip: Composite machines are most economical when the shop has a mixed workload of gun drilling and BTA jobs that does not require simultaneous operation of both processes. For high-volume production of a single part type, dedicated machines remain the better choice.

Design Approaches ​

Composite deep hole drilling machines fall into three architectural categories.

Dual-Spillde Composite Machines ​

Two independent spindles are mounted on the same machine bed — one configured for gun drilling and one for BTA. The workpiece moves between spindles via a mobile table or gantry system.

Advantages:

• No changeover time — switch between processes by moving the workpiece
• Each spindle is optimised for its process
• Both processes can be used on the same part without re-fixturing

Disadvantages:

• Higher cost than shared-spindle designs (two complete spindles and drive systems)
• Larger footprint than single-spindle composite designs
• More complex coolant routing with two independent systems

Shared-Spillde Composite Machines ​

A single spindle serves both gun drilling and BTA processes. The operator manually changes the tool holder, coolant delivery system (pressure head vs. guide bushing), and chip evacuation setup when switching between methods.

Advantages:

• Lowest capital cost of any composite approach
• Smallest footprint (single spindle on single bed)
• Simple, proven mechanical design

Disadvantages:

• Changeover takes 30 minutes to several hours (tooling, coolant circuit, and chip evacuation must be reconfigured)
• Spindle design is a compromise — cannot be fully optimised for either process
• Limited to sequential operations

Fast-Change Pressure Head Design ​

The most advanced approach, exemplified by UNISIG's UNI-50BTA, uses a specially designed pressure head that accepts both gun drill and BTA tooling without dismantling. The changeover between methods takes approximately 10 minutes.

The UNI-50BTA pressure head incorporates a seal design that accommodates both the gun drill guide bushing and the BTA pressure head interface. The machine operator can switch from a BTA drilling operation to a gun drilling operation — or use both methods on the same part — without breaking down the setup.

Advantages:

• Fastest changeover time of any composite design
• Can drill with both methods on the same part in the same setup
• Maintains process capability for each method (no compromise on spindle design)
• Counter-rotation capability to minimise centreline drift

Disadvantages:

• Higher cost than shared-spindle designs
• Pressure head design is proprietary and may have more limited service options
• Diameter range may be narrower than some dual-spindle designs

Manufacturer Comparison ​

UNISIG (USA) ​

UNISIG offers the most comprehensive range of composite and multi-process deep hole drilling machines:

The USC-M series is notable for combining 5-axis CNC milling with both deep hole drilling processes on a single machine platform. These machines use dedicated parallel spindles for milling and drilling, eliminating the need for any changeover when switching between operations.

UNISIG machines use Heidenhain CNC controls with absolute scale feedback and are available with single or multiple spindles.

Dezhou Drillstar (China) ​

Drillstar launched its gun drill and BTA composite three-coordinate machine in late 2024:

The machine achieves IT7–IT10 accuracy with surface roughness Ra 0.4–1.6 µm. It uses a dual-spindle structure with manual changeover between tooling and coolant circuits.

Guanlu (China) ​

Guanlu's ZSK2108F uses a shared-spindle approach with a 22 kW servo spindle (max 2,500 rpm):

The single spindle design keeps costs lower but requires manual changeover between methods, including swapping the oil supply and chip evacuation setup.

DTI (Poland) ​

DTI's HMDD (Horizontal Multi-Spindle Drilling Machine Centre), delivered in August 2024, supports three drilling methods:

The HMDD represents the multi-process extreme of composite machine design, integrating three distinct drilling and machining methods on one platform.

Shin-IL Machinery (South Korea) ​

Shin-IL offers multi-axis (4-axis and 5-axis) CNC deep hole drilling machines that support both gun drilling and BTA:

Shin-IL's approach emphasises geometric flexibility — the multi-axis capability allows drilling angled and offset holes that would require additional setups on conventional composite machines.

TBT Tiefbohrtechnik (Germany) ​

TBT's ML series (ML200, ML250, ML300, ML500, ML700) offers modular multi-spindle configurations:

TBT machines are typically configured for either gun drilling or BTA per machine variant, though the ML platform can support both processes across different machine configurations within the same series.

Applications and Use Cases ​

Aerospace Actuators ​

Aerospace hydraulic actuators often require both small-diameter gun-drilled passages and larger BTA-drilled bores in the same component. A composite machine, particularly the fast-change pressure head design, allows both operations in a single setup — maintaining bore-to-passage positional accuracy without re-fixturing.

UNISIG specifically cites aerospace landing gear actuators and helicopter main shafts as target applications for the UNI-50BTA.

Hydraulic Cylinders ​

Hydraulic cylinder manufacturing involves drilling the main bore (typically BTA for larger diameters) and smaller oil passage holes (gun drilling). A composite machine with adequate depth capability can complete both operations without moving the workpiece between machines.

Mould and Die Cooling Lines ​

Mould making requires deep, small-diameter cooling channels (gun drilling) and occasionally larger-diameter bores (BTA) for insert fitting or heating elements. UNISIG's USC-M series was designed specifically for this market, combining 5-axis milling with both deep hole drilling processes.

Diesel Engine and Transmission Components ​

Engine blocks and transmission housings contain a mix of small-diameter oil galleries (gun drilling) and larger coolant passages (BTA). DTI's HMDD machine was developed for this application, adding standard drilling and threading to the process mix.

Warning: Composite machines are not a universal replacement for dedicated equipment. If a shop's workload is 90% gun drilling below 15 mm diameter and 10% BTA above 50 mm, maintaining a dedicated gun drilling machine and outsourcing the BTA work may be more economical than investing in a composite platform.

Selection Criteria ​

When evaluating a composite deep hole drilling machine, consider these factors in order of priority:

1. Changeover Frequency ​

2. Diameter Range Overlap ​

If the gun drilling diameter range (e.g., 5–30 mm) and BTA range (e.g., 25–80 mm) overlap significantly in the 25–30 mm zone, evaluate which process is preferred for that overlap. Some shops run both methods on the same diameter depending on length-to-diameter ratio and surface finish requirements.

3. Depth Capability ​

Composite machines often have a single maximum depth that applies to both processes. If gun drilling depths (typically shallower) and BTA depths (potentially much deeper) differ significantly, the machine must be specified for the deeper requirement — which adds cost.

4. Coolant System Design ​

Gun drilling typically requires higher pressure (5–15 MPa) and lower flow, while BTA requires lower pressure (2–6 MPa) and very high flow. The coolant system must accommodate both regimes:

A composite machine needs either two independent coolant systems or a single variable-pressure system with changeover valving.

5. Control System Integration ​

The CNC control must support both drilling methods with appropriate cycle parameters. Key considerations:

• Peck cycle programming (G73 vs. G83) for each process
• Coolant pressure monitoring and alarm thresholds per method
• Tool life management for both gun drill and BTA tooling
• Torque and thrust force monitoring with process-specific limits

6. Floor Space and Foundation ​

Composite machines are longer than their single-process counterparts due to the additional spindle or tooling systems. Verify that the available floor space and foundation (vibration and weight) can accommodate the composite machine before specifying.

Market Trends ​

Growth Projection ​

The global deep hole drilling machine market is projected to grow at a compound annual growth rate of approximately 5.4% from 2025 to 2030, reaching an estimated value of $6.8 billion by 2032. Composite machines represent a growing segment of this market as manufacturers seek to reduce capital equipment costs and improve production flexibility.

Multi-Process Centres ​

The line between deep hole drilling machines and machining centres is blurring. The UNISIG USC-M series and DTI HMDD exemplify the trend toward machines that combine deep hole drilling (both gun drilling and BTA) with milling, boring, and threading. These multi-process centres reduce the number of setups and handling operations for complex parts that require both deep holes and conventional machining.

Chinese Market Development ​

Chinese manufacturers are aggressively developing composite deep hole drilling machines. Sanjia, Dezhou Drillstar, and Guanlu have all introduced composite machines in the past two years, and the Chinese domestic market for these machines is growing rapidly. The availability of lower-cost composite machines from Chinese manufacturers is putting downward price pressure on the global market while also expanding access to composite technology for smaller shops.

FAQ ​

What is a composite deep hole drilling machine? ​

A composite deep hole drilling machine integrates both gun drilling (external chip removal) and BTA drilling (internal chip removal) on a single machine platform. It allows a shop to perform both processes without purchasing and maintaining two separate machines. Composite machines use either dual-spindle, shared-spindle, or fast-change pressure head designs to accommodate both tooling systems.

Which composite machine design is best for frequent changeover between processes? ​

The UNISIG UNI-50BTA fast-change pressure head design is the market leader for frequent changeovers, with a changeover time of approximately 10 minutes between gun drilling and BTA modes. This is particularly valuable for shops that run mixed batches and need to switch processes multiple times per shift.

What are the disadvantages of composite deep hole drilling machines? ​

Composite machines are a compromise. Shared-spindle designs cannot optimise the spindle for either process individually. The coolant system must accommodate two very different pressure and flow regimes. Changeover time — even with fast-change designs — adds non-productive time to each job change. For shops with a stable, single-process workload, a dedicated machine remains more economical.

Can a composite machine drill the same part with both gun drilling and BTA? ​

Yes, and this is one of the key advantages. Fast-change pressure head designs (UNISIG UNI-50BTA) and dual-spindle designs (Sanjia ZSK2309A) allow both processes on the same part without re-fixturing. This maintains bore-to-bore positional accuracy and eliminates handling time between operations.

How much does a composite deep hole drilling machine cost? ​

Pricing varies significantly by design approach and manufacturer. Shared-spindle composite machines from Chinese manufacturers start at approximately $80,000–$150,000. Dual-spindle composites range from $200,000–$400,000. Fast-change pressure head machines like the UNISIG UNI-50BTA and multi-process centres like the USC-M series are priced at $300,000–$800,000 depending on configuration and automation level.

What diameter ranges are available in composite machines? ​

Gun drilling on composite machines typically covers 4–35 mm diameter, with some machines extending to 40 mm. BTA drilling on composite machines typically covers 20–100 mm diameter. The practical overlap zone where both processes can handle the same diameter is approximately 20–35 mm, though the specific range varies by manufacturer and machine model.

Which industries benefit most from composite deep hole drilling machines? ​

Aerospace (actuators, landing gear components), hydraulics (cylinders, valve blocks), mould and die (cooling channels), automotive (engine and transmission components), and defence (gun barrel features) all benefit from composite machine flexibility. The common factor is parts that require both small precision deep holes and larger bores.

What is the trend in composite machine development? ​

The market is moving toward multi-process centres that combine gun drilling, BTA drilling, milling, boring, and threading on a single platform. UNISIG's USC-M series and DTI's HMDD machine exemplify this trend. Chinese manufacturers are also expanding their composite machine offerings, bringing lower-cost options to the market and broadening access to composite technology.

Summary ​