Multi-Spindle Deep Hole Drilling — High-Production Systems

A manufacturer of shell-and-tube heat exchangers for nuclear power plants was producing 15,000 holes per heat exchanger through 200 mm thick SA-516 Gr70 tube sheets. With a single-spindle gun drilling machine running at 0.08 mm/rev feed, each hole took 2.5 minutes — the total drilling time per tube sheet was 625 hours (26 days of continuous operation). Installing a 5-spindle TARUS DHMS gundrilling machine with independent CNC-controlled spindle feed reduced the cycle time to 125 hours per tube sheet — 5.2 days — and eliminated the bottleneck that was constraining the entire heat exchanger production line. The $2.8 million machine investment had a payback period of 14 months based on labour, overhead, and throughput gains.

Why Multi-Spindle Deep Hole Drilling ​

Multi-spindle deep hole drilling machines increase production output by running two or more gun drills or BTA tools simultaneously on the same workpiece. The primary application is high-volume hole production where multiple identical holes are drilled in a single workpiece or where multiple identical workpieces are drilled simultaneously.

When multi-spindle drilling is justified:

• Tube sheets and baffle plates for heat exchangers: 500–15,000 holes per sheet
• Automotive connecting rods: 4–8 oil holes per rod
• Fuel injection components: multiple injector nozzle passages
• Hydraulic manifold blocks: multiple parallel valve bores
• Mould and die cooling channels: multiple parallel passages
• Structural steel plates: multiple fastener holes at high L/D

Productivity comparison:

The productivity gain is linear with spindle count only when all spindles can run simultaneously for the full drilling cycle. In practice, the gain is 80–90 % of the theoretical multiple due to tool change time, setup, and the need to avoid spindle collisions at the workpiece edge.

Machine Configurations ​

Multi-spindle deep hole drilling machines are available in several configurations:

Horizontal multi-spindle (most common for tube sheets):

The workpiece remains stationary while multiple horizontal spindles advance the drills simultaneously. The spindles are arranged in a row or grid pattern matching the hole pattern.

• Spindle count: 2–6 (typical for gundrilling), up to 60 for shallow drilling
• Spindle spacing: 50–300 mm (determined by minimum hole pitch)
• Max drilling depth: 500–3,000 mm
• Typical manufacturers: TARUS (DHMS series), DTI (HMDD series)

DTI's HMDD series (2024) offers 3 horizontal spindles capable of BTA, gun drilling, and conventional drilling/threading on a single machine. Max drilling diameter 81 mm, spindle power 25–41 kW, coolant 800 L/min at 110 bar.

TARUS DHMS series offers 2–5 spindles with independent CNC-controlled feed per spindle — each spindle's drill feed and bushing nose piece are independently driven, allowing any combination of spindles to run per cycle.

Vertical multi-spindle:

The workpiece is positioned on a rotary table or shuttle, and vertical spindles drill downward. Common for smaller components with multiple holes.

• Spindle count: 4–20+
• Workpiece indexing: rotary table for multi-position drilling
• Typical applications: automotive components, valve bodies
• Coolant delivery: through-spindle or external

Gun drill / BTA combination machines:

Some multi-spindle machines combine gun drilling and BTA capabilities:

• Kent CNC KMGD series: belt spindle up to 8,000 rpm, gun drill range 4–32 mm, with whip guides, drill grinder, and chip conveyor
• TARUS DHDM: dual-spindle — one dedicated gundrill/BTA, one for milling operations
• Dezhou Drillstar composite machine (2025): gun drill Φ10–30 mm / BTA Φ25–80 mm on one platform

Independent Spindle Control ​

The critical technology in multi-spindle deep hole drilling is independent spindle feed control. Without it, all spindles must start and stop simultaneously, limiting their use to regular hole patterns with equal drilling depths.

Benefits of independent CNC control:

1. Variable hole patterns: Each spindle can drill to a different depth or start at a different position — essential for tube sheets where the hole pattern does not align with equal spindle spacing
2. Edge avoidance: Spindles near the tube sheet edge can be programmed to start later or stop earlier, avoiding collision with the workpiece edge or clamps
3. Progressive feed control: Feed rate can be optimised per spindle based on chip evacuation conditions at each hole location
4. Individual tool monitoring: Torque and thrust force monitored per spindle — a problem on one spindle stops only that spindle, not the entire machine
5. Staggered entry: Spindles can start drilling 0.5–2 seconds apart to reduce peak coolant demand and power consumption

TARUS DHMS was the first system to implement fully independent CNC spindle control for multi-spindle deep hole drilling, and this capability remains a differentiating feature for complex tube sheet patterns.

Tooling and Setup ​

Tool holding:

• Each spindle uses a standard gun drill holder or BTA tool holder
• Quick-change tooling systems reduce tool change time — particularly important when 5 spindles need tool changes
• Torque limiters on each spindle protect against tool breakage

Guide bushing arrangement:

• Each spindle has an individual guide bushing mounted on a bushing plate
• The bushing plate is CNC-positioned to align with the hole pattern
• For tube sheets, the bushing plate is often a sacrificial template that matches the hole pattern
• Bushing clearance: 0.005–0.015 mm (depending on diameter and tolerance)

Work-holding:

• Tube sheets are clamped on a CNC-controlled X/Y table
• For large sheets (> 2 m × 2 m), hydraulic clamps with vacuum assist prevent distortion
• Multiple sheets and baffles can be stacked and clamped together for simultaneous drilling (stack height up to 500 mm)
• Alignment pins through precision-drilled dowel holes ensure consistent positioning

Coolant management:

• Total coolant flow scales with spindle count — a 5-spindle machine may require 400–800 L/min at 80–120 bar
• Central coolant filtration system with 10–20 µm absolute filtration
• Individual coolant flow control per spindle — typically manual valves or individually controlled pumps
• Return coolant temperature monitoring — multi-spindle operation generates more heat in the coolant system

Cycle Time Optimisation ​

Maximising the productivity of a multi-spindle deep hole drilling machine requires attention to several factors:

1. Feed rate optimisation per spindle: Although all spindles may run at the same feed, independent control allows feed variation based on:

• Hole position in the tube sheet (edge holes may require reduced feed)
• Material variation across the workpiece
• Tool wear state per spindle

2. Tool change synchronisation:

• All spindles should be changed at the same interval to avoid interrupted cycles
• If one spindle wears faster (e.g., drilling at the edge of a clad tube sheet), change all spindles at the same time
• Pre-set tool holders with offline tool presetting reduce change time to under 2 minutes per spindle

3. Chip evacuation:

• Multi-spindle operation generates more chips per minute — the chip conveyor system must handle peak chip load
• Chip form monitoring: if one spindle produces unfavourable chip form, correct the parameters for that spindle before it affects others
• Coolant filtration must handle the increased chip load — self-cleaning filters with 100+ micron pre-filtration followed by 20 µm absolute filtration

4. Workpiece handling:

• For tube sheet drilling, the workpiece is typically loaded and unloaded by overhead crane or forklift
• Shuttle systems allow one tube sheet to be drilled while the next is being set up
• Automated pallet changers reduce non-cutting time to under 5 minutes

5. Cycle time calculation:

For a tube sheet with N holes, S spindles, drilling depth D, and feed rate f, the theoretical cycle time is:

T_cycle = (N × D) / (S × f × RPM)

In practice, add 10–20 % for tool changes, positioning time, and chip clearing.

Applications by Industry ​

Heat exchanger manufacturing (primary application):

• Tube sheets for shell-and-tube heat exchangers

  • Material: SA-516 Gr70, 316L, duplex, titanium Grade 2
  • Hole count: 500–15,000 per sheet
  • Thickness: 25–500 mm
  • Drilling method: Gun drilling or BTA depending on diameter
  • Stack drilling: multiple tube sheets + baffles clamped together

• A 5-spindle TARUS DHMS machine can drill a 5,000-hole tube sheet in 5 days versus 25 days on a single-spindle machine.

Automotive manufacturing:

• Connecting rod oil holes

  • 4–8 holes per rod (oil passage, wrist pin lubrication)
  • Diameter: 3–8 mm
  • Depth: 30–120 mm
  • Production rate: 30–60 rods per hour per spindle

• Fuel injector bodies

  • Multiple fuel passages per body
  • Diameter: 2–6 mm
  • Depth: 20–80 mm
  • Multi-spindle machines with rotary table indexing

• Transmission valve bodies

  • Hydraulic control passages
  • Diameter: 4–15 mm
  • Depth: 50–300 mm

Oil and gas:

• BOP choke and kill line bores

  • Parallel bores in BOP ram blocks
  • Multi-spindle BTA drilling for 50–76 mm diameter × 3,000 mm depth
  • 2 or 3 parallel bores per block

• Manifold blocks

  • Multiple parallel flow passages
  • Drilled simultaneously for reduced cycle time

Mould and die:

• Cooling channels in injection moulds
  • Multiple parallel passages
  • Diameter: 6–20 mm
  • Depth: 200–1,500 mm
  • Multi-spindle gun drilling reduces mould manufacturing time by 3–5×

Comparison of Commercial Systems ​

Coolant and Filtration Systems ​

Multi-spindle deep hole drilling places extreme demands on coolant systems:

Coolant volume requirements:

Filtration requirements:

• Multiple spindles generate 3–5× the chip volume of a single spindle
• Gravity bed filtration with paper or fabric media for coarse filtration
• Magnetic drum separators for ferrous chip removal
• Cartridge or bag filters for 10–20 µm final filtration
• Self-cleaning filtration systems are essential for production operation

Coolant temperature control:

• Multi-spindle operation heats coolant faster — a heat exchanger or chiller in the coolant circuit maintains stable temperature
• Coolant temperature should be maintained at 25–35 °C for consistent drilling accuracy
• Thermal expansion of the workpiece from hot coolant must be considered for close-tolerance drilling

Troubleshooting Multi-Spindle Systems ​

Frequently Asked Questions ​

1. How many spindles can a multi-spindle deep hole drilling machine have? For true deep hole drilling (L/D > 10:1), 2–6 spindles is typical. For shallow drilling (L/D < 5:1), machines with up to 60 spindles are available for tube sheet and baffle plate drilling.

2. What is the main advantage of independent spindle CNC control? Each spindle can start, stop, and feed independently, allowing irregular hole patterns, staggered entry to reduce peak coolant demand, and individual tool monitoring — if one drill breaks, only its spindle stops.

3. Which industries use multi-spindle deep hole drilling most? Heat exchanger manufacturing (tube sheets) is the dominant application. Automotive component manufacturing (connecting rods, fuel injectors, transmission valve bodies) is the second-largest application.

4. How much floor space does a multi-spindle deep hole drilling machine require? A 5-spindle horizontal machine with coolant system and chip handling typically requires 50–100 m² of floor space. Vertical multi-spindle machines are more compact at 20–40 m² for the same spindle count.

5. Can multi-spindle machines do BTA drilling as well as gun drilling? Yes. The DTI HMDD series supports BTA, gun drilling, and conventional drilling/threading on the same machine. TARUS DHDM offers one gundrill/BTA spindle and one milling spindle.

6. What is the typical payback period for a multi-spindle deep hole drilling machine? 12–18 months for high-volume applications (heat exchanger tube sheets, automotive components). The payback comes from reduced cycle time — a 5-spindle machine producing 5× the output of a single-spindle machine with only 50–80 % higher operating cost.

7. How are multi-spindle machines programmed? The hole pattern is typically imported as a DXF or CSV file with X/Y coordinates. The CNC control assigns holes to spindles based on position within the spindle grid. Independent feed profiles can be programmed per spindle for specific hole ranges.

8. What is the typical accuracy of multi-spindle deep hole drilling? Positional accuracy is determined by the machine's X/Y table positioning (±0.02 mm typical). Hole diameter tolerance is the same as single-spindle gun drilling (±0.025 mm). Straightness is slightly reduced (0.1–0.2 mm per 100 mm versus 0.05–0.1 mm for single-spindle) due to the larger machine structure and potential spindle alignment variation.

9. How is tool breakage detected in multi-spindle operation? Each spindle monitors torque and thrust force independently. A sudden torque increase or decrease (indicating drill breakage) triggers an immediate feed stop for that spindle only. Acoustic emission monitoring is also used for real-time breakage detection.

10. What maintenance is specific to multi-spindle systems? Spindle alignment verification (every 6–12 months) is the most critical maintenance item. Misaligned spindles cause oversized holes and rapid tool wear. Coolant system capacity verification (flow, pressure, temperature) should be checked monthly. Guide bushing plate condition should be inspected before each production run.

Summary ​

Multi-spindle deep hole drilling is the enabling technology for high-volume production in heat exchanger, automotive, and mould manufacturing. The key decision factors are hole count per workpiece (500+ justifies multi-spindle), required production volume (yearly output targets), and hole pattern complexity (independent spindle control is essential for irregular patterns). With the global deep hole drilling machine market projected to grow at 5.4 % CAGR to $6.8 billion by 2032, multi-spindle systems represent the highest-growth segment as manufacturers seek productivity improvements through simultaneous drilling rather than faster single-spindle feeds.