Appearance
At 20 bar, coolant boils on contact with the cutting edge and creates a steam barrier that insulates the tool from further cooling. At 70 bar, the fluid jet penetrates that vapour barrier, reaches the cutting interface, and changes the physics of chip formation entirely.
High-pressure coolant is not simply more coolant. It is a fundamentally different cooling regime in which the fluid momentum overcomes the vapour pressure generated at the tool-chip interface, delivering lubrication and cooling directly to the cutting zone while simultaneously ejecting chips through hydraulic force.
For deep hole drilling — where chip evacuation and cutting-edge cooling are the two factors that limit productivity — high-pressure coolant systems operating at 70 bar (1,000 psi) and above have become essential technology rather than optional upgrades.
The Vapour Barrier Problem
Why Conventional Cooling Fails
At the tool-chip interface in deep hole drilling, temperatures reach 600–1,000°C depending on material and cutting parameters. Conventional flood coolant at low pressure (5–20 bar) cannot reach this interface because:
- Coolant flashes to steam on contact with hot surfaces above 100°C
- The steam forms a stable vapour layer (Leidenfrost effect) that insulates the cutting zone
- Liquid coolant is deflected away from the cutting edge by the vapour pressure
- Cooling occurs only after the cutting edge has passed, limiting its effectiveness
This phenomenon is particularly severe in deep hole drilling because the cutting edge is at the bottom of a deep cavity where coolant has already lost pressure and velocity.
The 70 Bar Threshold
| Pressure | Regime | Cooling Effectiveness | Typical Application |
|---|---|---|---|
| 5–20 bar | Conventional | Steam barrier forms | General machining |
| 20–40 bar | Medium pressure | Partial vapour penetration | Standard drilling |
| 40–70 bar | High pressure | Vapour barrier disrupted | Deep hole drilling |
| 70–100 bar | High pressure | Vapour barrier defeated | Titanium, Inconel |
| 100–200 bar | Ultra-high pressure | Jet-assisted cutting | Exotic alloys |
| 200+ bar | Hyper-pressure | Chip breaking dominates | Superalloys, CFRP |
The threshold of approximately 70 bar (1,000 psi) is the inflection point where coolant jet momentum is sufficient to penetrate the vapour barrier and deliver liquid coolant to the cutting interface. Below this pressure, the cooling effect is significantly reduced regardless of flow volume.
The Hydraulic Wedge Effect
When high-pressure coolant successfully penetrates the tool-chip interface, it creates a hydraulic wedge between the chip and the tool rake face. This has four measurable effects:
1. Reduced Contact Length
High-velocity coolant flowing between the chip and the insert face shortens the chip-tool contact length. This reduces:
- Frictional heat generation by 30–50%
- Cutting forces by 10–20%
- Built-up edge formation
2. Chip Breaking
The hydraulic wedge exerts a bending moment on the chip as it forms, promoting fracture at shorter chip lengths. This is critical in deep hole drilling where long, stringy chips cause jamming and tool breakage.
| Material | Chip Length at 20 bar | Chip Length at 70 bar | Reduction |
|---|---|---|---|
| Low-carbon steel | 50–100 mm | 10–25 mm | 75% |
| Stainless steel 304 | 30–60 mm | 8–15 mm | 75% |
| Aluminium 6061 | 100–200 mm | 20–40 mm | 80% |
| Titanium Ti-6Al-4V | 20–40 mm | 5–12 mm | 70% |
3. Heat Removal
High-pressure coolant removes a significantly larger fraction of cutting-zone heat:
| Parameter | Flood at 20 bar | HPC at 70 bar | HPC at 140 bar |
|---|---|---|---|
| Heat carried away by coolant | 40–50% | 65–75% | 75–85% |
| Heat into tool | 25–30% | 10–15% | 5–10% |
| Heat into chip | 20–25% | 15–20% | 10–15% |
| Heat into workpiece | 5–10% | 3–5% | 2–3% |
4. Lubrication
At the cutting interface, the hydraulic wedge ensures that coolant additives (extreme pressure agents, lubricity modifiers) reach the zone where they are needed. This is particularly important for oil-based coolants used in gun drilling.
Tip: The hydraulic wedge effect is maximised when coolant is delivered through the tool's internal passages at the correct angle (30–60° relative to the tool face) and at the closest possible point to the cutting edge. A 1 mm change in coolant outlet position can reduce or eliminate the wedge effect.
Case Study: ChipBlaster — 20 bar vs. 70 bar
A documented production comparison in 1018 steel using a 3.3 mm carbide drill at 10 diameters depth demonstrates the magnitude of improvement:
| Parameter | Without HPC (20 bar) | With HPC (70 bar) | Improvement |
|---|---|---|---|
| Spindle speed | 3,300 rpm | 10,000 rpm | 3× |
| Feed rate | 10 ipm (254 mm/min) | 60 ipm (1,524 mm/min) | 6× |
| Feed per revolution | 0.003 ipr | 0.006 ipr | 2× |
| Peck cycle | Full retract every 1×D | No pecking required | Eliminated |
| Tools used for 3,000 holes | 53 drills | 1 drill | 53× reduction |
| Tool changes per shift | 5+ | 0 | Eliminated |
The combination of higher RPM, doubled feed per revolution, and elimination of peck cycles resulted in a cycle time reduction of approximately 80% per hole.
This case study illustrates a critical point: high-pressure coolant allows the operator to trade improved cooling for higher productivity rather than longer tool life. The same technology that can extend tool life by 50× can also triple production rate.
Coolant Pump Selection
Pump Types
| Pump Type | Max Pressure | Max Flow | Best For | Limitations |
|---|---|---|---|---|
| Piston/plunger (positive displacement) | 200+ bar | 20–80 L/min | Steel, general machining | Pulsation without dampener, particulate sensitive |
| Diaphragm (seal-less) | 100 bar | 40–200 L/min | Abrasive coolants, cast iron | Lower max pressure |
| Centrifugal (multi-stage) | 40 bar | 100–1,000+ L/min | High-flow BTA systems | Lower pressure, pressure varies with flow |
| Hydraulic intensifier | 1,000+ bar | 5–20 L/min | Ultra-high pressure | Low flow, high cost |
Pump Selection Criteria
For deep hole drilling coolant systems, the primary selection factor is the relationship between pressure and flow:
Gun drilling requires high pressure (70–200 bar) at moderate flow (20–80 L/min) to deliver coolant through small-diameter internal passages in the drill.
BTA drilling requires moderate pressure (10–50 bar) at very high flow (200–800+ L/min) to fill the annulus between the bore wall and the drill tube while maintaining chip transport velocity.
| Application | Recommended Pump | Pressure | Flow |
|---|---|---|---|
| Gun drilling, 3–10 mm | Piston, 7.5–15 HP | 70–140 bar | 20–40 L/min |
| Gun drilling, 10–25 mm | Piston, 15–30 HP | 70–140 bar | 40–80 L/min |
| BTA drilling, 20–80 mm | Piston or multi-stage centrifugal | 20–50 bar | 200–500 L/min |
| BTA drilling, 80–250 mm | Multi-stage centrifugal | 10–30 bar | 500–800+ L/min |
Integrated Coolant System Packages
Several manufacturers offer integrated coolant systems combining pump, filtration, and controls in a single unit:
| Manufacturer | Model | Pressure | Flow | Filtration | Special Features |
|---|---|---|---|---|---|
| WULI Pump | WBP-2240M | 140 bar | 41 L/min | Paper drum, 15/30/50 µm | VFD control, ceramic plunger, M-code integration |
| WULI Pump | WBP-2040M | 70 bar | 41 L/min | Paper drum, 15/30/50 µm | VFD control, 7.5 HP |
| All World Machinery | SpindleShot | 70 bar | Varies | Cyclonic (no bag filters) | Self-cleaning, chiller option |
| NOP | YTH-EP | 70 bar | 28.8 L/min | Turbulence self-cleaning | TAZUNA fluid control software |
Filtration Requirements
High-pressure coolant systems demand finer filtration than conventional systems because:
- Nozzle diameters in coolant-through tools are small (0.5–1.2 mm) and clog easily
- Abrasive particles recirculated at high velocity accelerate pump wear
- Chip fines compact into aggregates that block coolant passages
Filtration Level Recommendations
| Coolant Pressure | Minimum Filtration | Recommended Filtration | Application |
|---|---|---|---|
| <40 bar | 50 µm | 30 µm | Standard drilling |
| 40–70 bar | 30 µm | 15–20 µm | Deep hole drilling |
| 70–140 bar | 15 µm | 5–10 µm | Gun drilling, titanium |
| 140+ bar | 10 µm | 5 µm | Ultra-high pressure, superalloys |
Filtration Technologies
| Technology | Filtration Level | Flow Capacity | Maintenance | Best For |
|---|---|---|---|---|
| Paper drum filter | 15–50 µm | High | Automatic indexing | Deep hole drilling, consistent quality |
| Vacuum media filter | 10–25 µm | Very high (100+ gpm) | Automatic, modular | High-flow BTA systems |
| Cyclonic separator | 10–20 µm | Moderate | Self-cleaning, no media | Roughing, pre-filtration |
| Cartridge/bag filter | 5–50 µm | Moderate | Replaceable media | Fine polishing, low-flow |
| Magnetic separator | Ferrous only | High | Automatic discharge | Steel/iron chip removal |
Multi-Stage Filtration
For deep hole drilling coolant systems operating above 70 bar, a multi-stage filtration strategy is recommended:
- Coarse stage: Drag conveyor or chip trapper removes large swarf (1–10 mm)
- Medium stage: Paper drum or vacuum filter removes medium particles (15–50 µm)
- Fine stage: Cartridge or bag filter removes fine particles (5–10 µm)
- Optional polish: Magnetic separator removes ferrous fines
Warning: Filtration failures are the single most common cause of unscheduled downtime in high-pressure coolant systems. A clogged nozzle causes immediate flow stoppage at the cutting edge, leading to catastrophic tool failure within seconds. Install pressure sensors at the tool holder inlet with an alarm threshold set at 20% below nominal operating pressure.
Coolant Temperature Control
High-pressure coolant systems generate heat through pump work and fluid friction. Without temperature control:
- Coolant temperature rises by 5–15°C above ambient during operation
- Thermal expansion of the machine tool and workpiece introduces dimensional errors
- Coolant viscosity changes affect chip evacuation and cutting zone lubrication
- Emulsion stability degrades above 50°C
Temperature Control Recommendations
| Coolant Type | Target Temperature | Maximum | Cooling Method |
|---|---|---|---|
| Water-miscible emulsion | 20–25°C | 40°C | Chiller with plate heat exchanger |
| Oil-based (gun drilling) | 25–35°C | 50°C | Chiller or cooling tower |
Reservoir Sizing
Sandvik Coromant recommends coolant tank volume of 5–10 times the pump's per-minute flow rate. This provides sufficient residence time for:
- Fines settlement
- Air bubble release
- Temperature stabilisation
- Emulsion equilibration
For a 40 L/min pump, the reservoir should be 200–400 litres. For a 500 L/min BTA system, the reservoir should be 2,500–5,000 litres.
Coolant Delivery Through the Tool
Internal Coolant Channel Design
Coolant-through drills must balance coolant flow area against tool rigidity:
| Drill Diameter | Typical Coolant Hole Diameter | Number of Holes | Total Flow Area |
|---|---|---|---|
| 5–10 mm | 0.5–1.0 mm | 1–2 | 0.2–1.6 mm² |
| 10–20 mm | 1.0–1.5 mm | 2 | 1.6–3.5 mm² |
| 20–40 mm (gun drill) | 1.5–2.5 mm | 2 | 3.5–9.8 mm² |
| BTA drill 40–80 mm | Annular gap | 1 (annular) | 50–200 mm² |
The coolant outlet angle relative to the cutting edge significantly affects performance:
| Outlet Angle | Effect | Best For |
|---|---|---|
| 30–45° | Directs coolant at tool-chip interface | General deep hole drilling |
| 45–60° | Improves chip breaking through hydraulic force | Ductile materials, long-chip materials |
| 60–90° | Maximises cooling at expense of chip breaking | Hard/brittle materials |
Through-Spindle Coolant Delivery
Through-spindle coolant (TSC) systems deliver coolant from the machine's coolant pump through the spindle, tool holder, and into the drill's internal passages. Key design requirements:
- Rotary union: Seals must handle both pressure and rotation speed. Carbide-face seals are standard for pressures above 40 bar.
- Tool holder sealing: The interface between tool holder and drill must seal at operating pressure without leaking. HSK or Capto interfaces provide superior sealing to steep taper (BT/CAT) holders.
- Pressure drop: Each junction in the coolant path (pump → hose → rotary union → holder → drill) introduces pressure drop. System design should allow 15–25% pressure margin above the required tool-tip pressure.
BTA Drilling Coolant Systems
BTA drilling presents different coolant challenges than gun drilling because coolant is delivered through the annulus between the bore wall and the drill tube, while chips evacuate through the drill's centre.
BTA Coolant Parameters by Diameter
| Bore Diameter | Coolant Pressure | Flow Rate | Chip Transport Velocity | Filtration |
|---|---|---|---|---|
| 20–40 mm | 30–50 bar | 100–300 L/min | 3–6 m/s | 30–50 µm |
| 40–80 mm | 20–40 bar | 200–500 L/min | 3–5 m/s | 30–50 µm |
| 80–150 mm | 15–30 bar | 300–600 L/min | 2–4 m/s | 50–100 µm |
| 150–250 mm | 10–20 bar | 500–800+ L/min | 2–3 m/s | 50–100 µm |
Pressure Head Design
The BTA pressure head seals against the bore entrance and directs coolant into the annulus. Critical design features:
- Seal type: Lip seals for lower pressures (<30 bar), mechanical face seals for higher pressures
- Seal wear monitoring: Pressure drop across the seal indicates wear rate
- Coolant return: Pressure head must also accommodate chip-laden coolant returning through the drill centre
- Airlift prevention: The chip return tube must be submerged in the coolant tank to prevent air ingress
FAQ
What pressure is needed for effective deep hole drilling cooling?
The minimum effective pressure for deep hole drilling is approximately 70 bar (1,000 psi), which is the threshold at which coolant penetrates the vapour barrier at the cutting interface. Below this pressure, a significant portion of the coolant flashes to steam before reaching the cutting edge, reducing cooling effectiveness by 40–60%.
How does high-pressure coolant improve tool life?
High-pressure coolant improves tool life through three mechanisms: (1) reducing cutting-zone temperature by 200–400°C, which slows diffusion and crater wear; (2) providing a hydraulic wedge that reduces chip-tool contact length and frictional forces; and (3) ensuring consistent chip evacuation, preventing chip jamming that causes edge chipping. Documented improvements range from 50% to 3,000% depending on material and baseline conditions.
What filtration is required for a 70 bar coolant system?
A 70 bar coolant system requires minimum 15 µm filtration, with 5–10 µm recommended. The small coolant nozzles (0.5–1.2 mm diameter) in coolant-through tools clog rapidly if particles exceed the nozzle diameter by even a small margin. Multi-stage filtration — coarse chip removal, medium paper/vacuum filtration, and fine cartridge polishing — is recommended for reliable operation.
What is the difference between gun drilling and BTA coolant requirements?
Gun drilling uses high pressure (70–200 bar) at moderate flow (20–80 L/min) because coolant must pass through small internal drill passages and exit at the cutting tip. BTA drilling uses lower pressure (10–50 bar) but much higher flow (200–800 L/min) because coolant must fill the annulus between the bore wall and the drill tube while maintaining sufficient velocity to transport chips through the centre of the drill.
Can high-pressure coolant eliminate peck cycles?
Yes, this is one of the primary benefits. In the ChipBlaster case study, increasing coolant pressure from 20 bar to 70 bar eliminated the need for peck cycles entirely when drilling 10×D deep holes in 1018 steel. The hydraulic force of the coolant jet evacuates chips continuously, removing the chip packing problem that makes peck cycles necessary.
Does high-pressure coolant reduce surface roughness?
Yes, typically by approximately 50%. In the ChipBlaster case study, surface roughness improved from Ra 3.2–6.3 µm to Ra 1.6–3.2 µm. The improvement comes from reduced built-up edge formation, more consistent chip formation, and better cooling of the workpiece surface after the cutting edge passes.
What pump type is best for high-pressure coolant in deep hole drilling?
Piston/plunger (positive displacement) pumps are the most common choice for gun drilling systems requiring 70–200 bar at moderate flow. They provide consistent pressure regardless of flow demand. For high-flow BTA systems, multi-stage centrifugal pumps are often more economical, though they have a pressure-flow curve (pressure drops as flow increases). Diaphragm pumps are preferred when coolant contains abrasive particulates.
How is coolant temperature controlled in high-pressure systems?
Chillers with plate heat exchangers are the standard solution for high-pressure coolant temperature control. Piston pumps add significant heat to the coolant through compression and mechanical friction. The target temperature is typically 20–25°C for water-miscible coolants and 25–35°C for oil-based coolants. Sandvik recommends reservoir sizing at 5–10 times the pump flow rate to allow sufficient residence time for temperature equilibration and fines settlement.
Summary
| Parameter | Low Pressure (5–20 bar) | Medium (20–40 bar) | High (70–100 bar) | Ultra-High (100–200 bar) |
|---|---|---|---|---|
| Vapour barrier | Intact | Partial penetration | Defeated | Defeated with margin |
| Hydraulic wedge | None | Partial | Effective | Maximum |
| Tool life vs. baseline | 1× | 1.5–2× | 3–10× | 5–50× |
| Chip breaking | Poor | Moderate | Good | Excellent |
| Peck cycle required | Yes (≥3×D) | Sometimes | Often eliminated | Eliminated |
| Feed rate increase vs. baseline | — | 20–40% | 60–200% | 100–300% |
| Best pump type | Centrifugal | Piston/diaphragm | Piston (positive displacement) | Piston or intensifier |
| Minimum filtration | 50 µm | 30 µm | 15 µm (5–10 µm recommended) | 10 µm (5 µm recommended) |
| Coolant temp control | Optional | Recommended | Required | Required |
| Reservoir sizing (× pump flow/min) | 3–5× | 5–8× | 5–10× | 8–10× |
| Gun drilling application | Shallow only | Moderate depths | Standard | Deep, exotic alloys |
| BTA application | Light duty | Moderate | Standard | Large diameters |