Appearance
A manufacturer installing a new BTA drilling system for 60 mm × 2,000 mm bores in 4140 steel selects a centrifugal pump rated at 40 bar and 400 L/min based on initial cost. During production, the pump cannot maintain pressure beyond 1,200 mm depth due to increasing flow resistance from chip loading — causing chip packing and surface finish degradation. Replacing the centrifugal pump with a three-screw positive displacement pump (60 bar, 450 L/min, VFD-controlled) eliminates chip packing, reduces cycle time by 12%, and extends tool life by 30%.
Coolant Pump Requirements
The coolant pump in deep hole drilling serves two critical functions: delivering coolant to the cutting zone for lubrication and heat removal, and providing hydraulic power for chip evacuation through the chip tube or annular gap.
| Process | Pressure Range | Flow Range | Pump Type | Primary Coolant Function |
|---|---|---|---|---|
| Gun drilling (STS) | 35–170 bar (500–2,500 PSI) | 20–110 L/min | Three-screw or piston | Chip evacuation through drill flute |
| BTA drilling (DTS) | 10–70 bar (150–1,000 PSI) | 200–1,500 L/min | Three-screw or multistage centrifugal | Chip evacuation through chip tube |
| BTA trepanning | 10–50 bar (150–750 PSI) | 300–2,000 L/min | Three-screw | Chip evacuation for large diameters |
| Gun reaming | 20–100 bar (300–1,500 PSI) | 30–150 L/min | Three-screw or piston | Lubrication and chip flushing |
| BTA reaming | 10–40 bar (150–600 PSI) | 200–800 L/min | Centrifugal or screw | Lubrication and chip flushing |
Pump Types
Three-Screw Pump
The three-screw positive displacement pump is the most common type for deep hole drilling coolant systems. It uses three intermeshing screws — one power rotor and two idler rotors — to move fluid axially through the pump housing.
| Parameter | Specification |
|---|---|
| Pressure range | 10–200 bar (150–2,900 PSI) |
| Flow range | 20–900 L/min |
| Efficiency | 75–85% (high) |
| Particle tolerance | Up to 100 µm (hardened screws) |
| VFD compatibility | Excellent (linear flow vs speed) |
| Maintenance interval | 8,000–12,000 hours |
| Initial cost | Moderate |
| Noise level | 65–80 dB(A) |
Applications: BTA drilling (all diameters), gun drilling (medium to large diameters), central coolant systems.
Multistage Centrifugal Pump
Centrifugal pumps use an impeller to accelerate fluid and convert velocity to pressure through a volute. Multistage designs stack multiple impellers for higher pressure.
| Parameter | Specification |
|---|---|
| Pressure range | 5–60 bar (70–870 PSI) |
| Flow range | 50–2,000 L/min |
| Efficiency | 60–75% (moderate) |
| Particle tolerance | < 50 µm (fine clearance at wear rings) |
| VFD compatibility | Limited (flow drops significantly below 50% speed) |
| Maintenance interval | 4,000–8,000 hours |
| Initial cost | Low to moderate |
| Noise level | 70–90 dB(A) |
Applications: BTA reaming, lighter-duty BTA drilling, central coolant systems where pressure requirement is below 50 bar.
Limitation for deep hole drilling: Centrifugal pump flow drops significantly as system back-pressure increases. In deep hole drilling, chip loading increases back-pressure along the bore — the centrifugal pump cannot maintain constant flow under these conditions, leading to chip packing.
Piston Pump
Piston pumps (axial or radial) use reciprocating pistons to displace fluid. Seal-less diaphragm versions (e.g., Hydra-Cell) handle abrasive particles without rapid wear.
| Parameter | Specification |
|---|---|
| Pressure range | 20–350 bar (300–5,000 PSI) |
| Flow range | 5–200 L/min |
| Efficiency | 80–92% (highest) |
| Particle tolerance | Up to 250 µm (seal-less diaphragm type) |
| VFD compatibility | Good |
| Maintenance interval | 6,000–10,000 hours |
| Initial cost | High |
| Pulsation | Requires dampener |
| Noise level | 75–95 dB(A) |
Applications: High-pressure gun drilling (small diameters), applications requiring > 100 bar.
Pressure and Flow Requirements by Diameter
Gun Drilling (STS)
| Bore Diameter | Coolant Pressure | Coolant Flow | Annular Velocity |
|---|---|---|---|
| 5 mm | 100–170 bar | 12–20 L/min | 15–25 m/s |
| 10 mm | 70–100 bar | 20–30 L/min | 12–18 m/s |
| 20 mm | 50–80 bar | 30–50 L/min | 10–15 m/s |
| 30 mm | 40–70 bar | 40–65 L/min | 8–12 m/s |
| 40 mm | 35–60 bar | 55–80 L/min | 6–10 m/s |
BTA Drilling (DTS)
| Bore Diameter | Coolant Pressure | Coolant Flow | Annular Velocity |
|---|---|---|---|
| 20 mm | 15–30 bar | 100–200 L/min | 8–14 m/s |
| 30 mm | 12–25 bar | 150–300 L/min | 6–12 m/s |
| 40 mm | 10–20 bar | 200–400 L/min | 5–10 m/s |
| 60 mm | 8–15 bar | 300–600 L/min | 4–8 m/s |
| 80 mm | 6–12 bar | 400–800 L/min | 3–6 m/s |
| 100 mm | 5–10 bar | 500–1,000 L/min | 2–5 m/s |
Effect of Bore Depth
Pressure requirement increases with bore depth due to friction losses along the coolant flow path. As a rule of thumb, add 2–5 bar per metre of bore length for BTA drilling, depending on diameter.
| Bore Depth | Pressure Increase (30 mm BTA) | Pressure Increase (60 mm BTA) |
|---|---|---|
| 500 mm | +3 bar | +2 bar |
| 1,000 mm | +6 bar | +4 bar |
| 2,000 mm | +12 bar | +8 bar |
| 3,000 mm | +18 bar | +12 bar |
Pump Sizing Calculation
Step-by-Step Sizing Procedure
- Determine required flow rate from tool manufacturer's recommendation based on diameter and process
- Calculate annular velocity V = Q / A where A is the annular flow area
- Verify velocity meets minimum chip transport velocity (typically 4–8 m/s for BTA, 8–15 m/s for gun drilling)
- Calculate pressure drop ΔP = ΔP_tool + ΔP_tube + ΔP_depth
- Select pump with rated pressure ≥ 1.2 × calculated ΔP and rated flow ≥ 1.1 × required Q
Example Calculation (40 mm BTA, 1,500 mm depth)
| Parameter | Value |
|---|---|
| Bore diameter | 40 mm |
| Drill tube OD | 32 mm |
| Annular area | π(40² − 32²)/4 = 452 mm² |
| Required flow | 250 L/min = 0.00417 m³/s |
| Annular velocity | 0.00417 / 0.000452 = 9.2 m/s ✅ (within 5–10 m/s target) |
| Tool pressure drop | 8 bar (from tool manufacturer) |
| Tube friction loss | 3 bar (at 250 L/min, 32 mm ID, 1.5 m) |
| Depth pressure increase | 6 bar (4 bar/m × 1.5 m) |
| Total required pressure | 17 bar |
| Selected pump rating | 22 bar × 280 L/min |
Filtration System Design
Coolant filtration is essential for deep hole drilling — particles larger than 20 µm embedded between the guide pad and bore surface create scoring marks and accelerate pad wear.
Filtration Stages
| Stage | Filtration Method | Particle Size Removed | Application |
|---|---|---|---|
| 1 — Primary | Chip conveyor or drag flight | > 1 mm | Remove large chips from coolant return |
| 2 — Secondary | Magnetic separator | > 50 µm (ferrous) | Remove ferrous fines from grinding and drilling |
| 3 — Tertiary | Paper band filter | > 10–20 µm | Achieve required cleanliness for high-pressure pump |
| 4 — Polishing (optional) | Cartridge or bag filter | > 5 µm | Precision applications; guide pad protection |
Filter Sizing
| Pump Flow | Paper Band Filter Area | Magnetic Separator Capacity | Tank Volume |
|---|---|---|---|
| 100 L/min | 1.0–1.5 m² | 50 L/min rating | 1,000–2,000 L |
| 250 L/min | 2.0–3.0 m² | 150 L/min rating | 2,500–5,000 L |
| 500 L/min | 3.5–5.0 m² | 300 L/min rating | 5,000–10,000 L |
| 1,000 L/min | 6.0–8.0 m² | 600 L/min rating | 10,000–20,000 L |
Coolant Tank Design
| Feature | Recommendation |
|---|---|
| Tank capacity | 10–20× pump flow per minute (for settling and heat dissipation) |
| Baffle plates | Minimum 3 baffles to prevent short-circuit flow |
| Return inlet | Below liquid level to minimise aeration |
| Pump suction | 100 mm above tank bottom; anti-vortex plate |
| Access cover | Removable for cleaning |
| Drain | Sloped bottom with drain valve at lowest point |
| Level indicator | Sight glass or float switch (low-level alarm) |
| Temperature monitoring | Thermocouple or RTD with alarm at 50°C |
Variable Frequency Drive Control
VFD control of the coolant pump provides significant advantages for deep hole drilling:
| Benefit | Mechanism | Typical Saving |
|---|---|---|
| Pressure control | VFD adjusts pump speed to maintain set pressure | Eliminates pressure relief bypass |
| Flow matching | Reduce flow for reaming vs drilling operations | 30–50% energy reduction |
| Soft start | Ramp up speed over 5–10 seconds | Eliminates pressure surge at cycle start |
| Process integration | PLC adjusts pressure based on depth or diameter | Optimised chip evacuation |
| Energy efficiency | Pump runs at required speed, not full speed | 20–40% energy saving |
VFD Sizing
| Pump Motor Power | VFD Rating | Recommended Features |
|---|---|---|
| 15 kW | 20 kVA | Sensorless vector control |
| 30 kW | 40 kVA | PID pressure control loop |
| 55 kW | 75 kVA | PID + fieldbus (Profibus or EtherNet/IP) |
| 90 kW | 120 kVA | Line reactor + output sine filter |
Pump Maintenance
| Component | Inspection Interval | Typical Service Life | Wear Indicator |
|---|---|---|---|
| Screw set (three-screw pump) | 6,000 hours | 12,000–20,000 hours | Pressure drop at rated speed; increased noise |
| Mechanical seal | 3,000 hours | 6,000–10,000 hours | Visible leakage; seal flush line flow |
| Bearings | 8,000 hours | 15,000–25,000 hours | Vibration increase; temperature rise |
| Relief valve | 1,000 hours | 5,000–8,000 hours | Pressure chatter; setpoint drift |
| VFD cooling fan | 6,000 hours | 15,000–20,000 hours | Over-temperature alarm on VFD |
| Paper band filter media | As needed | 50–200 hours (depending on chip load) | Pressure differential across filter |
| Coolant (water mix) | Weekly | 4–12 weeks (depending on contamination) | Concentration drift; bacterial growth |
Coolant Temperature Control
Coolant temperature affects both pump performance and drilling quality. High coolant temperature reduces viscosity (reducing lubrication at the guide pad interface) and causes thermal expansion of the drill tube.
| Coolant Temperature | Effect on Drilling | Corrective Action |
|---|---|---|
| < 20°C | High viscosity; reduced chip evacuation | May need lower-viscosity coolant |
| 20–35°C | Optimal range for most operations | None required |
| 35–45°C | Marginal — viscosity reduction affects pad lubrication | Increase coolant concentration; monitor surface finish |
| 45–55°C | Poor — thermal expansion of drill tube increases bore diameter | Install heat exchanger or chiller |
| > 55°C | Unacceptable — seal damage; coolant degradation risk | Stop production; install chiller |
For production systems, a plate heat exchanger with cooling tower or chiller maintains coolant temperature within ±2°C of the setpoint to eliminate thermal drift in bore diameter.
Troubleshooting Coolant Delivery Problems
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| Pressure drops as bore depth increases | Centrifugal pump cannot maintain pressure with chip loading | Replace with positive displacement screw pump |
| Pressure adequate but chip evacuation poor | Coolant velocity too low | Increase flow rate; verify annular gap; check for blockage |
| Fluctuating pressure (sawtooth pattern) | Intermittent chip packing in annular gap | Reduce feed; increase coolant flow; check chip breaker |
| Pump noisy or vibrating | Cavitation from restricted suction | Check suction strainer; increase suction pipe diameter; reduce coolant temperature |
| Mechanical seal leaking | Abrasive particles in coolant | Upgrade filtration; check seal flush line |
| Pump motor overloading | Pump running at excessive pressure | Check relief valve setting; verify back-pressure not excessive |
| Coolant temperature rising over shift | Heat load exceeds system cooling capacity | Add heat exchanger; increase tank volume; check cooler performance |
| Foaming at coolant return | Air entrainment from return flow | Lower return pipe below liquid level; add defoamant |
| Paper band filter clogging rapidly | Chip load exceeding filter capacity | Add magnetic pre-separator; increase filter area |
| Flow decreases gradually over weeks | Progressive filter clogging or pump wear | Replace filter media; inspect screw set for wear |
FAQ
What type of coolant pump is best for deep hole drilling?
Three-screw positive displacement pumps are the best choice for deep hole drilling, offering 10–200 bar pressure range, 20–900 L/min flow, high efficiency (75–85%), and good tolerance of abrasive particles in recycled coolant. They maintain constant flow regardless of back-pressure, which is critical for consistent chip evacuation along the full bore depth. Centrifugal pumps can be used for lower-pressure BTA applications below 50 bar but will not maintain flow under chip loading conditions.
What coolant pressure is required for BTA drilling?
BTA drilling typically requires 10–70 bar (150–1,000 PSI) depending on bore diameter. Small diameters (20 mm) require higher pressure (15–30 bar) to achieve adequate chip transport velocity in the annular gap. Large diameters (100 mm) require lower pressure (5–10 bar) but much higher flow (500–1,000 L/min). Pressure also increases with bore depth — add 2–5 bar per metre of bore length depending on diameter.
What coolant pressure is required for gun drilling?
Gun drilling requires 35–170 bar (500–2,500 PSI) — significantly higher than BTA drilling because coolant must pass through the small coolant hole in the gun drill and return chips through the V-flute. Small diameters (5 mm) require the highest pressure (100–170 bar). Large diameters (40 mm) require lower pressure (35–60 bar) but higher flow.
How do you size a coolant pump for deep hole drilling?
Calculate required flow from the annular velocity needed for chip transport (4–8 m/s for BTA, 8–15 m/s for gun drilling). Calculate total pressure requirement as the sum of tool pressure drop, tube friction loss, and depth-related pressure increase. Select a pump with rated pressure ≥ 1.2 × calculated total and rated flow ≥ 1.1 × required flow. For example, a 40 mm BTA bore at 1,500 mm depth requires approximately 280 L/min at 22 bar, corresponding to a 30 kW three-screw pump.
What filtration is needed for deep hole drilling coolant?
A three-stage filtration system is recommended: primary chip conveyor (removes > 1 mm chips), magnetic separator (removes > 50 µm ferrous particles), and paper band filter (removes > 10–20 µm particles). This achieves the coolant cleanliness required to prevent guide pad scoring and pump seal wear. For precision grinding combined with drilling, a final cartridge or bag filter (5 µm) may be added.
Can a centrifugal pump be used for BTA drilling?
Centrifugal pumps can be used for BTA reaming or light-duty BTA drilling where pressure requirements are below 50 bar and bore depths are under 1,000 mm. However, centrifugal pumps cannot maintain constant flow under increasing back-pressure — as chips accumulate in the annular gap, flow decreases, worsening the chip packing problem. Three-screw positive displacement pumps are strongly recommended for production BTA drilling.
What coolant flow rate is needed for a given bore diameter?
For BTA drilling: 100–200 L/min for 20 mm diameter, increasing to 500–1,000 L/min for 100 mm diameter. For gun drilling: 12–20 L/min for 5 mm diameter, increasing to 55–80 L/min for 40 mm diameter. These values ensure annular coolant velocity of 4–15 m/s depending on process, which is the critical parameter for chip evacuation.
How does coolant temperature affect drilling quality?
Coolant temperature affects viscosity (affecting pad lubrication), drill tube thermal expansion (affecting bore diameter), and seal life. Optimal range is 20–35°C. Above 45°C, thermal expansion of the drill tube increases bore diameter by approximately 0.005–0.010 mm per 10°C for a 40 mm steel tube. For production systems, a plate heat exchanger maintaining ±2°C temperature stability is recommended. An 80 kW BTA drilling machine typically requires 20–50 kW of cooling capacity depending on the material removal rate.
What is the difference between STS and DTS coolant requirements?
STS (Single Tube System / gun drilling) requires high pressure (35–170 bar) and low flow (20–110 L/min). DTS (Double Tube System / BTA) requires low to moderate pressure (10–70 bar) and high flow (200–1,500 L/min). The difference arises from coolant path geometry — gun drilling forces coolant through a small tube (3–8 mm diameter) inside the drill, while BTA pumps coolant through the annular gap between the drill tube and bore wall.
How often should coolant pump maintenance be performed?
Three-screw pumps require inspection every 6,000 hours, with screw set replacement at 12,000–20,000 hours depending on coolant cleanliness. Mechanical seals should be inspected at 3,000 hours. Paper band filter media is replaced as needed (50–200 hours depending on chip load). Coolant should be tested weekly for concentration and bacterial growth. Weekly inspection of suction strainers and monthly lubrication of pump bearings is recommended.
Summary
The high-pressure coolant pump is a critical component of any deep hole drilling system. Three-screw positive displacement pumps are the recommended choice for production deep hole drilling, offering constant flow regardless of back-pressure, pressure capability up to 200 bar, and tolerance of abrasive particles in recycled coolant. Gun drilling requires 35–170 bar at 20–110 L/min; BTA drilling requires 10–70 bar at 200–1,500 L/min. Three-stage filtration (chip conveyor, magnetic separator, paper band filter) provides the < 20 µm cleanliness required for guide pad protection and pump seal life. VFD control enables demand-based pressure regulation, energy savings of 20–40%, and integration with the machine PLC. Coolant temperature should be maintained at 20–35°C with a plate heat exchanger for stable drilling conditions. Proper pump sizing, filtration, and maintenance prevent chip packing, extend tool life, and maintain consistent bore quality.