Appearance
The coolant system is not a support accessory for a deep hole drilling machine — it is the enabling subsystem. Without the correct pressure and flow at the cutting zone, chip evacuation stops, heat builds up, and the drilling process fails within seconds. A properly sized coolant pump is as important as the spindle or the guide bushing.
Deep hole drilling coolant pumps operate under conditions that are extreme by general machining standards. Pressures range from 300 to over 3,000 psi (20–207 bar) depending on hole diameter. Flow rates range from a few gallons per minute for small-diameter gun drilling to over 400 GPM (1,500 L/min) for large BTA drilling. The pump must deliver this performance continuously, 24/7, in an environment contaminated with metal chips and cutting fluid degradation products.
This article covers coolant pump types, pressure and flow requirements for different deep hole drilling methods, hydraulic sizing calculations, filtration requirements, and complete system design considerations.
Why Coolant Pressure and Flow Matter
In deep hole drilling, the coolant performs three simultaneous functions:
| Function | Consequence of Failure | Effect |
|---|---|---|
| Chip evacuation | Chips pack in the bore | Tool jamming, workpiece scrapped |
| Heat removal | Overheating at cutting zone | Tool failure, surface burn |
| Lubrication of guide pads | Pad galling | Surface finish destroyed, tool damage |
Critical principle: Chip evacuation is the limiting factor in deep hole drilling penetration rates. A 10% increase in coolant flow rate can enable a 15–20% increase in feed rate, while a 10% decrease in flow rate risks chip packing and tool failure.
BTA vs. Gun Drilling Coolant Requirements
| Parameter | Gun Drilling | BTA/STS Drilling | Deep Hole Boring |
|---|---|---|---|
| Coolant path | Through tool OD, exits at tip | Through tool ID, exits at cutting head | Through tool or induction tube |
| Chip transport | Chips carried in tool V-flute, coolant flushes | Chips evacuated through tool ID | Chips pushed ahead or pulled through |
| Typical pressure | 300–1,500 psi (20–100 bar) | 600–750 psi (41–52 bar) | 200–500 psi (14–35 bar) |
| Typical flow rate | 2–40 GPM (8–150 L/min) | 90–400 GPM (340–1,500 L/min) | 20–100 GPM (75–380 L/min) |
| Pressure stability | Critical — affects chip formation | Important — affects chip transport | Moderate |
Pressure and Flow Requirements by Hole Diameter
Gun Drilling Pressure Requirements
The required coolant pressure for gun drilling is inversely proportional to hole diameter — smaller holes need higher pressure to overcome the restricted flow path:
| Hole Diameter | Recommended Pressure |
|---|---|
| 1.0–2.0 mm (0.040–0.078 in) | 2,000–3,000 psi (140–207 bar) |
| 2.0–4.0 mm (0.079–0.155 in) | 1,500 psi (100 bar) |
| 4.0–5.0 mm (0.156–0.186 in) | 1,300 psi (90 bar) |
| 5.0–5.5 mm (0.187–0.217 in) | 1,150 psi (80 bar) |
| 5.5–6.5 mm (0.218–0.249 in) | 1,050 psi (70 bar) |
| 6.5–8.0 mm (0.250–0.311 in) | 925 psi (60 bar) |
| 8.0–9.5 mm (0.312–0.374 in) | 775 psi (50 bar) |
| 9.5–11.0 mm (0.375–0.436 in) | 675 psi (45 bar) |
| 11.0–12.5 mm (0.437–0.499 in) | 600 psi (40 bar) |
| 12.5–14.0 mm (0.500–0.561 in) | 525 psi (35 bar) |
| 14.0–16.0 mm (0.562–0.624 in) | 500 psi (20 bar) |
| 16.0–19.0 mm (0.625–0.749 in) | 425–450 psi (28–30 bar) |
| 19.0–22.0 mm (0.750–0.874 in) | 400 psi (26 bar) |
| 22.0–25.4 mm (0.875–1.000 in) | 350 psi (24 bar) |
| > 25.4 mm (> 1.000 in) | 300 psi (20 bar) |
Note: These values are starting points. Actual required pressure depends on tool geometry, coolant hole configuration, coolant viscosity, and drilling depth. Deeper holes require higher pressure due to cumulative friction losses along the coolant path.
BTA Drilling Pressure and Flow Requirements
BTA drilling uses external chip evacuation through the tool ID, which requires high flow rates at moderate pressures:
| Bore Diameter | Typical Pressure | Typical Flow Rate |
|---|---|---|
| 20–30 mm (0.75–1.2 in) | 750–1,000 psi (52–70 bar) | 90 GPM (340 L/min) |
| 40–50 mm (1.5–2.0 in) | 600–750 psi (41–52 bar) | 125 GPM (475 L/min) |
| 60–80 mm (2.5–3.0 in) | 600–750 psi (41–52 bar) | 185 GPM (700 L/min) |
| 100–150 mm (4–6 in) | 600–750 psi (41–52 bar) | 300 GPM (1,135 L/min) |
| 150–250 mm (6–10 in) | 500–750 psi (35–52 bar) | 400 GPM (1,515 L/min) |
Flow Rate Estimation
For initial sizing, use the following rule-of-thumb calculations:
BTA drilling:
Flow (GPM) ≈ 0.4 × (bore diameter in mm)² × 0.001
Flow (L/min) ≈ 0.15 × (bore diameter in mm)² × 0.001Gun drilling:
Flow (GPM) ≈ 0.015 × (bore diameter in mm)²
Flow (L/min) ≈ 0.057 × (bore diameter in mm)²Important: These are approximate values for initial pump sizing. Final flow requirements must be verified against the specific tool manufacturer's recommendations and the planned cutting parameters.
Pump Types for Deep Hole Drilling
Pump Type Comparison
| Pump Type | Max Pressure | Max Flow | Best For | Limitations |
|---|---|---|---|---|
| Piston pump (axial or radial) | 5,000+ psi (345+ bar) | High | High-pressure gun drilling | Higher cost, pressure pulsation |
| Internal gear / gerotor pump | 2,000 psi (140 bar) | Moderate | Standard BTA drilling | Sensitive to contamination |
| External gear pump | 2,500 psi (172 bar) | Moderate | Through-spindle coolant | Higher noise, limited to clean fluids |
| Diaphragm pump | 1,500 psi (100 bar) | Low–Moderate | Through-spindle (can run dry) | Pulsating output, moderate cost |
| Multi-stage centrifugal | 500 psi (35 bar) | Very high | Flood coolant, chip transport | Low pressure, not for small holes |
| Screw pump | 1,500 psi (100 bar) | Moderate–High | Contaminated coolants | Higher cost, larger size |
Recommendation: For deep hole drilling, piston pumps (axial piston) are the standard for gun drilling applications requiring > 1,000 psi. Internal gear pumps are the most common choice for BTA drilling where pressures are moderate (600–750 psi) but flow requirements are high. For systems requiring both high pressure and high flow, a multiple-pump configuration (two or more pumps in parallel) may be needed.
Pump Drive Options
| Drive Type | Advantages | Disadvantages | Best For |
|---|---|---|---|
| Fixed speed AC motor + relief valve | Simple, low initial cost | Constant flow, energy waste, heat generation | Single-purpose machines |
| VFD-controlled AC motor | Variable flow, energy savings, soft start | Higher initial cost | Multi-diameter drilling, variable conditions |
| Servo-driven pump | Precise flow control, energy efficient | Highest cost | High-precision, automated systems |
| Hydraulic drive | Remote mounting, high power density | Complex, less efficient | Special applications |
VFD recommendation: Variable frequency drive control is strongly recommended for deep hole drilling coolant pumps. The ability to adjust flow and pressure for different hole diameters, tool types, and depths provides significant process flexibility. VFD control also reduces energy consumption by 20–40% compared to fixed-speed pumps with relief valves, because the pump only delivers what the process requires.
Hydraulic Sizing Calculations
The Core Formula
The relationship between pressure, flow, and required motor power is:
HP = (P × Q) / 1714Where:
- HP = Required hydraulic horsepower
- P = Pressure (psi)
- Q = Flow rate (GPM)
- 1714 = Conversion constant
Selection Worked Examples
Example 1: Gun drilling application
Requirements:
- Bore diameter: 6 mm (0.236 in)
- Required pressure: 1,050 psi
- Estimated flow: 6 GPM
HP = (1,050 × 6) / 1714 = 3.68 HPApply safety factors:
- Pump efficiency: 85% → 3.68 / 0.85 = 4.33 HP
- System safety margin: 20% → 4.33 × 1.2 = 5.19 HP
Selected motor: 5.5–7.5 HP (depending on available standard sizes)
Example 2: BTA drilling application
Requirements:
- Bore diameter: 50 mm (2.0 in)
- Required pressure: 750 psi
- Estimated flow: 125 GPM
HP = (750 × 125) / 1714 = 54.7 HPApply safety factors:
- Pump efficiency: 85% → 54.7 / 0.85 = 64.4 HP
- Safety margin: 15% → 64.4 × 1.15 = 74.0 HP
Selected motor: 75 HP (nearest standard size)
Example 3: Large BTA drilling application
Requirements:
- Bore diameter: 150 mm (6.0 in)
- Required pressure: 750 psi
- Estimated flow: 300 GPM
HP = (750 × 300) / 1714 = 131.3 HPApply safety factors:
- Pump efficiency: 85% → 131.3 / 0.85 = 154.5 HP
- Safety margin: 15% → 154.5 × 1.15 = 177.6 HP
Selected motor: 200 HP or dual 100 HP pumps
Note on large systems: For BTA machines requiring > 150 HP, consider using two pumps in parallel rather than a single large pump. This provides redundancy — if one pump requires service, the machine can continue operating at reduced capacity. Dual pumps also allow flexible operation (one pump for smaller diameters, both for larger diameters).
Metric Equivalent Formula
For systems specified in metric units:
Power (kW) = (P (bar) × Q (L/min)) / 600 × ηWhere η is pump efficiency (typically 0.80–0.92).
Pump Selection Checklist
| Parameter | Data to Gather | Source |
|---|---|---|
| Maximum hole diameter | Largest bore to be drilled | Production requirements |
| Minimum hole diameter | Smallest bore to be drilled | Production requirements |
| Maximum drilling depth | Deepest bore | Production requirements |
| Material being drilled | Workpiece material | Production requirements |
| Desired penetration rate | Feed rate target | Process engineering |
| Tool coolant hole size | Tool manufacturer data | Tool supplier |
| Coolant type | Oil or emulsion | Process specification |
| Ambient temperature | Shop floor conditions | Facility data |
Filtration System Design
Why Filtration Is Critical
Contamination in the coolant is the single most common cause of pump failure and process inconsistency in deep hole drilling:
| Particle Size | Effect on System |
|---|---|
| > 100 μm | Rapid pump wear, tool coolant hole blockage |
| 50–100 μm | Accelerated seal wear, reduced tool life |
| 20–50 μm | Gradual pump wear, surface finish degradation |
| < 20 μm | Normal operating range |
Rule of thumb: For coolant pressures above 1,000 psi (70 bar), filtration to 20–50 microns is required. For pressures above 2,000 psi (140 bar), 10–20 micron filtration is recommended. Every pump manufacturer specifies a maximum particle size — exceeding this voids the warranty and dramatically reduces pump life.
Filtration Stages
| Stage | Filtration Level | Purpose |
|---|---|---|
| 1 — Chip conveyor | Removes large chips | Pre-treatment before fine filtration |
| 2 — Magnetic separator | Removes ferrous fines (to ~50 μm) | Reduces load on final filter |
| 3 — Paper or drum filter | 15–50 micron media | Primary filtration for pump protection |
| 4 — Polishing filter (optional) | 5–10 micron | Ultra-fine filtration for critical applications |
Filter Sizing
| Parameter | Guideline |
|---|---|
| Filter media rating | ≤ 50 μm for standard, ≤ 20 μm for high-pressure |
| Flow rate through filter | Minimum 1.5× pump flow rate (to allow for media loading) |
| Filter area | Minimum 1 ft² per 5 GPM of pump flow |
| Differential pressure at clean filter | < 0.5 bar |
| Differential pressure at change-out | 1.5–2.0 bar |
| Automatic backwash | Recommended for production systems |
Filter type recommendation: For deep hole drilling, automatic drum filters or self-cleaning cyclonic filters are preferred over disposable bag filters. The high chip volume in deep hole drilling rapidly clogs disposable media, creating a maintenance burden that disrupts production.
System Layout and Component Selection
Tank Design
| Parameter | Recommendation |
|---|---|
| Tank capacity | Minimum 3× pump flow per minute (5–10× preferred) |
| Baffle configuration | Two baffles — first from bottom to 70% depth, second from top to 70% depth |
| Pump suction location | 200 mm above tank bottom, far from return lines |
| Return location | Opposite end from pump suction, near chip conveyor outlet |
| Tank material | Welded steel with corrosion-resistant coating |
| Drain slope | 2° minimum toward drain port |
| Access cover | Full-size for cleaning access |
| Level sensor | Required for low-level pump shutoff |
Piping and Valves
| Component | Specification |
|---|---|
| Suction line | Minimum 1.5× pump inlet diameter, as short as possible |
| Pressure line | Schedule 80 steel pipe or equivalent rated pressure |
| Flexible hoses | Required at pump outlet for vibration isolation |
| Relief valve | Set at 110–115% of maximum operating pressure |
| Pressure gauge | At pump outlet and at machine entry point |
| Flow meter | Recommended at machine entry for process monitoring |
| Accumulator | Recommended for pulsation dampening with piston pumps |
| Isolation valve | At pump outlet for maintenance access |
Piping critical rule: Use the largest practical pipe diameter and minimize elbows, couplings, and valves in the pressure line. Each 90° elbow adds the equivalent of 30–50 pipe diameters of friction loss. In a high-pressure deep hole drilling system, poorly designed piping can waste 10–20% of pump pressure before the coolant reaches the tool.
Coolant Induction Systems
The method of introducing coolant into the tool varies by drilling method:
| System | Coolant Introduction | Applications |
|---|---|---|
| Rotary coolant union | Through-spindle, rotating seal | Gun drilling and BTA with rotating spindle |
| Coolant induction tube | Stationary tube inserted into rotating workpiece | BTA with rotating workpiece |
| Static coolant connection | Threaded connection to stationary tool | Deep hole boring, trepanning |
| Pressure head | Sealed enclosure at workpiece entry | BTA with high-pressure requirements |
Temperature Control
Why Temperature Control Matters
| Issue | Cause | Effect |
|---|---|---|
| Thermal growth of machine | Heated coolant raises machine temperature | Alignment drift, bore diameter variation |
| Coolant degradation | Sustained temperature above 60°C | Reduced coolant life, bacterial growth (emulsions) |
| Pump cavitation | Coolant temperature too high at pump inlet | Reduced flow, pump damage |
| Inconsistent viscosity | Temperature variation | Variable pressure and chip evacuation |
Heat Load Calculation
The pump generates significant heat. Typically 25–35% of input horsepower is converted to heat in the coolant:
Heat load (BTU/hr) = HP × 2545 × (1 − η)Where η is pump efficiency (decimal).
Example: A 75 HP pump at 85% efficiency:
Heat load = 75 × 2545 × (1 − 0.85) = 75 × 2545 × 0.15 = 28,631 BTU/hrCooling Options
| Method | Capacity | Best For |
|---|---|---|
| Tank surface cooling | Low — limited to small systems | Low-power gun drilling |
| Heat exchanger (shell-and-tube) | Moderate–High | Most BTA systems |
| Coolant chiller | Precise temperature control | High-precision applications |
| Evaporative cooling | High | Large systems with high water availability |
Chiller sizing guideline: For systems above 50 HP pump power, or where bore diameter tolerance is tighter than H9, a coolant chiller is recommended. Target coolant temperature stability of ±2°C at the machine entry point.
Common Coolant System Design Mistakes
1. Undersized Tank Capacity
A tank that is too small cannot dissipate heat effectively and allows air to be entrained in the coolant.
Fix: Size the tank for 5–10 minutes of pump flow at maximum rate. For a 100 GPM pump, the tank should hold 500–1,000 gallons.
2. Inadequate Filtration
Fine chip particles from deep hole drilling quickly clog undersized or poorly designed filtration systems.
Fix: Install automatic self-cleaning filtration with a rating appropriate for the system pressure. Monitor differential pressure across the filter as a maintenance indicator.
3. Suction Line Restrictions
Restrictions on the pump inlet cause cavitation, which destroys pumps and reduces flow.
Fix: Keep suction lines short, use pipe diameter 1.5× the pump inlet, and never install a valve (even a fully open valve) on the pump suction line.
4. Ignoring Pressure Drop in Distribution
The pressure at the pump outlet is not the pressure at the tool. Long piping runs, undersized pipes, and excessive fittings all reduce available pressure.
Fix: Install pressure gauges at both the pump outlet and the machine entry point. Design the piping for a maximum pressure drop of 5% between pump and tool.
5. No Pressure or Flow Monitoring
Without instrumentation, the operator cannot detect gradual pump wear, filter clogging, or tool coolant hole blockage.
Fix: Install a pressure gauge and flow meter at the machine entry point. Monitor pressure trends — a gradual pressure rise indicates filter clogging; a sudden pressure drop may indicate a broken tool or seal failure.
Summary Table
| Aspect | Key Information |
|---|---|
| Gun drilling pressure range | 300–3,000 psi (20–207 bar), inversely proportional to hole diameter |
| BTA drilling pressure range | 500–750 psi (35–52 bar) for most applications |
| BTA flow rate range | 90–400 GPM (340–1,515 L/min) depending on bore diameter |
| Sizing formula | HP = (P × Q) / 1714, then apply efficiency and safety factors |
| Pump types | Piston pumps (high pressure), gear pumps (BTA), multi-pump for large systems |
| Drive preference | VFD control recommended for flexibility and energy savings |
| Filtration requirement | ≤ 50 μm for standard, ≤ 20 μm for > 1,000 psi systems |
| Tank capacity | 3–10× pump flow per minute |
| Heat management | 25–35% of pump HP becomes heat; chiller recommended for > 50 HP systems |
| Critical piping rule | Minimize elbows, maximize diameter, install gauges at both ends |
| Most common mistake | Undersized tank causing heat buildup and air entrainment |
FAQ
What coolant pressure do I need for a 10 mm gun drilling application?
For a 10 mm diameter gun drill hole, the recommended coolant pressure is approximately 675–775 psi (45–50 bar) at the tool entry point. This is based on the standard pressure-to-diameter relationship for gun drilling. The actual pressure required depends on the specific tool geometry (coolant hole configuration), drilling depth, and workpiece material. Deeper holes require higher starting pressure because friction losses in the coolant channel increase with length. Always verify with the tool manufacturer's recommendations for your specific application.
Can I use a single coolant pump for multiple deep hole drilling machines?
This is possible but requires careful system design. A central coolant system with a single large pump, manifold distribution, and individual flow control valves at each machine can be cost-effective for a facility with multiple similar machines. However, central systems have disadvantages: pressure fluctuations when one machine cycles, coolant contamination from one machine affecting all machines, and single-point failure risk. For machines drilling significantly different hole sizes or using different coolant types, separate dedicated pumps are strongly recommended.
How do I calculate the right pump size for a new deep hole drilling application?
Start with the largest hole diameter you plan to drill to determine flow requirements, and the smallest hole diameter to determine pressure requirements. Use the sizing formula HP = (P × Q) / 1714, where P is pressure in psi and Q is flow rate in GPM. Apply an efficiency factor of 0.80–0.92 (depending on pump type) and a safety margin of 15–25%. The result is the required motor horsepower. For BTA applications, flow rate is the dominant factor. For gun drilling applications, pressure is the dominant factor. Consider a VFD-controlled pump to allow adjustment for different hole sizes.
What type of filtration does a deep hole drilling coolant system need?
Multi-stage filtration is required: a chip conveyor or settling tank for bulk chip removal, a magnetic separator for ferrous fines, and a final filter rated at 20–50 microns for system pressures under 1,000 psi, or 10–20 microns for higher pressures. Self-cleaning automatic filters (drum-type or cyclonic) are strongly preferred over disposable media for production environments because the chip volume in deep hole drilling quickly clogs disposable filters. Monitor differential pressure across the final filter as a maintenance indicator.
Why is VFD control recommended for deep hole drilling coolant pumps?
Variable frequency drive control allows the pump speed — and therefore flow and pressure — to be adjusted for different hole diameters, depths, and tool geometries. A single VFD-controlled pump can serve a machine that drills a range of hole sizes, whereas a fixed-speed pump requires a relief valve to dump excess flow, which wastes energy and heats the coolant. VFD control also enables soft starting (reduces electrical and mechanical stress), pressure monitoring for process control, and energy savings of 20–40% compared to fixed-speed operation.