Coolant delivery is the single most important process variable in deep hole drilling — more influential than cutting speed or feed rate on tool life and process reliability. Yet it is also the most commonly misunderstood.
In conventional machining, coolant is primarily for cooling and chip flushing. In deep hole drilling, it serves a third function that is equally critical: chip transport. Without sufficient coolant velocity, chips will not travel the length of the drill tube (which can exceed 10 m in drill collar drilling), and the process will stall within seconds.
This guide covers the formulas, reference data, and pump selection guidelines needed to specify a coolant system for deep hole drilling.
1. The Fundamental Difference: Pressure vs. Flow
The first concept to understand: volume moves chips, pressure overcomes resistance — but the two are linked.
A common mistake is specifying a high-pressure pump without checking whether it can deliver sufficient flow at that pressure. The relationship is:
P = k × Q²
Where P = pressure, Q = volumetric flow rate, and k = the hydraulic resistance of the entire system (tool, hoses, fittings, chip load). Doubling flow requires four times the pressure in a given system (ISCAR Drilling Handbook).
Practical implication: A pump rated for 100 bar at 10 l/min will not clear chips from a 30 mm BTA drill that needs 200 l/min. The pressure rating is meaningless without the flow rating to match.
For a general overview of how coolant systems differ between drilling methods, see The Four Deep Hole Drilling Methods Explained.
2. Gun Drilling: Coolant Calculation
Rule of Thumb: Volume-Per-Revolution Method
The most widely used starting point for gun drilling (Master Chemical/Rotem): for every revolution of the drill, deliver enough coolant to fill the volume of the hole being drilled.
Hole volume:
V = (π × D² / 4) × L
Flow rate:
Q = V × n
Where:
- V = volume per revolution (mm³ or in³)
- D = drill diameter
- L = axial advance per revolution (= feed rate f in mm/rev)
- n = spindle speed (RPM)
Example — 10 mm gun drill, 0.025 mm/rev feed, 3,000 RPM:
- Volume per rev = (π × 10² / 4) × 0.025 = 1.96 mm³/rev
- Flow rate = 1.96 × 3,000 = 5,880 mm³/min ≈ 5.9 l/min
Example — 20 mm gun drill, 0.050 mm/rev feed, 1,500 RPM:
- Volume per rev = (π × 20² / 4) × 0.050 = 15.7 mm³/rev
- Flow rate = 15.7 × 1,500 = 23,550 mm³/min ≈ 23.6 l/min
Nozzle Flow Formula
For estimating flow through a specific drill coolant hole:
Q = 30 × d² × √P
Where:
- Q = flow rate (GPM)
- d = coolant hole diameter (inches)
- P = pressure (psi)
Example — 0.060“ coolant hole at 1,000 psi:
- Q = 30 × (0.060)² × √1,000 = 30 × 0.0036 × 31.6 = 3.4 GPM (12.9 l/min)
Sources: Master Chemical/Rotem gun drilling guide.
Typical Gun Drilling Coolant Parameters
| Drill Diameter | Coolant Pressure | Coolant Flow (approx.) | Notes |
|---|---|---|---|
| 3–6 mm | 80–150 bar | 5–15 l/min | High pressure needed for small holes |
| 6–12 mm | 50–100 bar | 15–40 l/min | |
| 12–20 mm | 40–80 bar | 40–80 l/min | |
| 20–50 mm | 30–60 bar | 80–200 l/min | Flow becomes limiting factor |
Sources: ISCAR Drilling Handbook; Accurate Edge; JimmyTool.
For cutting parameters at these diameters, see the Speeds and Feeds Reference Tables.
3. BTA Drilling: Coolant Calculation
BTA drilling requires higher flow rates than gun drilling because the chip evacuation area is the full drill tube cross-section (>60% of hole area), and the chips must be transported through the entire tube length.
Coolant Pressure by Diameter (ISCAR BTA Data)
| Drill Diameter | Coolant Pressure (approximate) | Flow Rate (approximate) |
|---|---|---|
| 10 mm | 80–100 bar | 30–60 l/min |
| 20 mm | 50–70 bar | 80–150 l/min |
| 30 mm | 35–50 bar | 150–250 l/min |
| 40 mm | 25–40 bar | 200–350 l/min |
| 60 mm | 15–30 bar | 300–450 l/min |
Sources: ISCAR Drilling Handbook (pages 287–288, BTA coolant charts); JimmyTool.
The trend is clear: larger diameters need lower pressure but much higher flow. A 60 mm BTA drill needs roughly 30× the flow of a 10 mm drill but only one-third the pressure.
VDI 3209 Coolant Reference
VDI 3209 Blatt 1 (2024 edition) contains diagrams for cooling lubricant quantity and pressure as a function of drilling diameter specifically for BTA and ejector drilling. These are the most authoritative reference values available. The standard also covers machine drive power requirements corresponding to each diameter range. For more on VDI 3209 and related standards, see VDI Standards for Deep Hole Drilling.
Academic Model: Hydraulic Resistance (Jung & Ni, ASME 2003)
For precise pump sizing, the pressure drop in a gun drill or BTA drill tube can be modeled using the Blasius equation for turbulent flow:
Δp ≈ 0.241 × L × ρ^(3/4) × μ^(1/4) × d^(−4.75) × Q^(1.75)
Where:
- Δp = pressure drop (Pa)
- L = coolant channel length (m)
- ρ = fluid density (kg/m³)
- μ = dynamic viscosity (kg/(m·s))
- d = coolant channel hydraulic diameter (m)
- Q = volume flow rate (m³/s)
This model accounts for:
- Major losses — friction along the coolant channel (typically 70–80% of total pressure drop in long drills)
- Minor losses — at the drill head, sudden expansions, and directional changes (20–30%)
The exponent of −4.75 on diameter means that small changes in coolant hole diameter have an enormous effect on pressure drop. Reducing the coolant hole from 2.0 mm to 1.8 mm (10% reduction) increases pressure drop by approximately 60%.
Source: Jung & Ni, “Prediction of Coolant Pressure and Volume Flow Rate in Gundrilling,” ASME, 2003.
4. Chip Transport Velocity
For reliable chip evacuation, the coolant velocity in the chip evacuation channel must exceed the critical transport velocity — the speed at which chips settle out of the flow.
Minimum Velocity Estimates
| System | Chip Evacuation Channel | Minimum Velocity |
|---|---|---|
| Gun drilling (small dia) | V-groove | 5–8 m/s |
| Gun drilling (medium dia) | V-groove | 8–12 m/s |
| BTA (internal tube) | Hollow drill tube | 3–6 m/s |
| Ejector (inner tube) | Inner tube | 4–8 m/s |
Sources: ISCAR Handbook; TU Dortmund research (Rupasinghe et al., 2025).
BTA systems require lower minimum velocity than gun drilling because the chips travel through a smooth tube rather than an open groove, reducing the risk of chips catching on edges.
For difficult materials that produce long stringy chips (stainless steel, aluminum), increase target velocity by 30–50% above the minimum. If chip compaction is observed, increase pressure rather than flow — the higher pressure helps break chips into shorter segments.
5. Coolant Filtration
Filtration is not optional in deep hole drilling. Unfiltered particles cause abrasive wear on guide pads, leading to loss of straightness and surface finish degradation.
Filtration Requirements by Application
| Application | Recommended Filtration | Consequence of Insufficient Filtration |
|---|---|---|
| General gun drilling | < 20 μm | Guide pad wear, surface finish degradation |
| Precision small-diameter | < 10 μm (3–5 μm recommended) | Drill blockage, oversize holes |
| BTA drilling | < 20 μm | Pad wear, loss of pressure head seal life |
| Inconel / superalloys | < 10 μm | Abrasive carbides accelerate pad wear |
| Aluminum | < 30 μm (chip control) | Built-up edge, chip welding |
Sources: UNISIG Technical Reference; JimmyTool; Brinkmann Pumps; TechniDrill.
Recommended Multi-Stage Filtration System
For a single deep hole drilling machine or a small cell:
| Stage | Type | Target Removal | Notes |
|---|---|---|---|
| 1 | Magnetic separator | Ferrous particles > 50 μm | Removes bulk chips before fine filtration |
| 2 | Paper band filter | Particles > 20–30 μm | Standard for general machining |
| 3 | Cartridge filter | Particles > 5–10 μm | Required for precision small-diameter work |
Source: Lex Technoaid case study; Brinkmann Pumps filtration guide.
A 10% drop in coolant pressure during operation is a reliable early warning of filter clogging or a developing chip blockage. Most production deep hole machines have automatic pressure monitoring that triggers an alarm or tool retraction when this occurs (JimmyTool).
6. Pump Selection Guidelines
Pump Type Comparison
| Pump Type | Pressure Range | Flow Range | Particle Tolerance | Efficiency |
|---|---|---|---|---|
| Screw spindle (Brinkmann, others) | Up to 100 bar | Up to 500+ l/min | < 20–50 μm | 70–85% |
| Piston / plunger | Up to 200+ bar | Up to 200 l/min | < 10 μm | 80–90% |
| Sealless positive displacement (Hydra-Cell) | Up to 100 bar | Up to 200 l/min | Up to 250 μm+ | 78–90% |
| Centrifugal / multistage | Up to 40 bar | Up to 1,000+ l/min | < 50 μm | 60–75% |
Sources: Wanner Pumps; Brinkmann Pumps; various pump manufacturer data.
Selection Criteria
| Requirement | Recommended Pump Type |
|---|---|
| BTA drilling, 20–60 mm, high flow | Screw spindle or multistage centrifugal |
| Gun drilling, 3–20 mm, high pressure | Piston or sealless positive displacement |
| Mixed production (varying diameters) | Screw spindle with VFD |
| Ejector drilling retrofit | Existing machine pump + booster |
| Difficult materials (high pressure) | Piston pump, 100+ bar rated |
Sizing Example
Specification for a BTA machine drilling 30 mm × 1,000 mm in alloy steel:
| Parameter | Value | Source |
|---|---|---|
| Required pressure | 40–50 bar | ISCAR chart for 30 mm |
| Required flow | 180–250 l/min | ISCAR chart for 30 mm |
| Pump type | Screw spindle | Brinkmann FFS3 or equivalent |
| Motor power | approx. 25–35 kW | P = Q × p × efficiency factor |
| Filtration | 20 μm paper band + 10 μm cartridge | Multi-stage |
| Coolant tank capacity | 1,000–2,000 l | Minimum 5× pump flow/min |
For machine selection guidance, see the Machine Selection Checklist.
7. Coolant Temperature Control
Coolant temperature affects viscosity, which directly impacts hydraulic resistance and chip transport.
| Temperature | Effect on Coolant | Effect on Process |
|---|---|---|
| < 20°C | High viscosity | Higher pump pressure required; reduced chip transport |
| 25–40°C | Optimal viscosity range | Best balance of lubrication and chip transport |
| > 50°C | Reduced viscosity, additive breakdown | Lower lubricity; increased tool wear; risk of coolant degradation |
For high-production BTA drilling, a coolant chiller or heat exchanger is recommended to maintain the 25–40°C range. The coolant pump itself generates significant heat — a 30 kW pump running continuously adds approximately 25,000 kcal/h to the coolant.
8. Safety Considerations
Coolant at 100+ bar is a serious safety hazard. At pressures above 100 psi (6.9 bar), fluid can pierce human skin and cause injection injuries requiring immediate surgical intervention. Deep hole drilling coolant systems operate at 5–20× this threshold.
| Pressure Level | Risk | Required Control |
|---|---|---|
| < 40 bar | Low | Standard guarding |
| 40–100 bar | Moderate | Interlocked machine enclosures, hose whip restraints |
| > 100 bar | High | Full enclosure, pressure relief valves, automatic shutoff on leak detection |
See the Deep Hole Drilling Safety Guide for complete safety requirements for high-pressure coolant systems.
Summary
- Gun drilling coolant: 40–150 bar, flow rates of 5–200 l/min depending on diameter. Use the volume-per-revolution rule for initial estimates.
- BTA drilling coolant: 15–100 bar, flow rates of 30–450 l/min. Larger diameters need more flow but less pressure.
- The pressure-flow relationship is quadratic (P ∝ Q²) — doubling flow requires four times the pressure.
- Minimum coolant velocity for chip transport: 3–8 m/s in BTA tubes, 5–12 m/s in gun drill V-grooves.
- Filtration to < 20 μm is essential; precision small-diameter work requires 3–10 μm.
- Pump selection must consider both pressure and flow; a pump rated for high pressure at low flow will not clear chips from a large BTA drill.
Key Sources
- ISCAR Drilling Handbook, pages 287–288 — BTA coolant pressure and flow rate charts
- Jung & Ni, “Prediction of Coolant Pressure and Volume Flow Rate in the Gundrilling Process,” ASME, 2003 — analytical model
- Master Chemical / Rotem, “Gun Drilling Best Practices” — volume-per-revolution rule
- VDI 3209 Blatt 1:2024-01 — coolant quantity and pressure diagrams for BTA drilling
- Wanner Pumps, “Application and Pump Selection — Machine Tool” — pump comparison data
- Brinkmann Pumps, FFS3 screw spindle pump specifications — filtration requirements
- Lex Technoaid, single-process deep drilling filtration case study — multi-stage filtration
- TechniDrill high-pressure coolant system — small-diameter drilling filtration data
- Rupasinghe et al., “Numerical and Experimental Evaluation of Chip Evacuation and Lubricant Flow using Optimized Drill Heads for Ejector Deep Hole Drilling,” Production Engineering, 2025 — coolant velocity research
- UNISIG Technical Reference — coolant filtration guidelines
- JimmyTool — coolant pressure monitoring and filtration requirements
- Accurate Edge (UAE) — gun drilling coolant pressure data