Appearance
A manufacturer of gun-drilled automotive fuel injection components (Ø3 mm × 200 mm bores in 4140 steel, L/D 67:1, 60,000 parts per year) was experiencing erratic tool life — some tools lasting 800 mm and others fracturing after only 50 mm, with no correlation to tool batch or operator. The machine was equipped with a plunger pump (45 kW, 150 bar rated, 80 L/min) that had been in service for 8 years. Investigation revealed that the pump's discharge check valve was worn (0.08 mm seat wear), causing pressure ripple of ±25 bar at 50 Hz (the plunger stroke frequency) versus the ±3 bar normal for a plunger pump in good condition. The pressure ripple caused intermittent chip packing — when pressure dropped below 100 bar, chip evacuation became unreliable, causing instantaneous torque overload and tool fracture. Rebuilding the pump with new check valves, plunger seals, and discharge valve seats (€6,800 total) restored pressure ripple to ±3 bar, stabilized tool life at 400–550 mm (consistent with the 500 mm expected life), and reduced tool fracture rates from 8% to 0.3%. A preventive maintenance program was implemented: check valve inspection every 12 months (4,000 operating hours), plunger seal replacement every 18 months (6,000 hours), and full pump rebuild every 5 years (15,000 hours).
Pump Type Comparison
Primary Pump Technologies for Deep Hole Drilling
| Pump Type | Pressure Range | Flow Range | Efficiency | Typical Power | Initial Cost | Maintenance Cost | Best Application |
|---|---|---|---|---|---|---|---|
| Plunger pump (positive displacement, reciprocating) | 50–300 bar | 10–200 L/min | 85–92% | 15–150 kW | €15,000–60,000 | Medium: seal and valve replacement every 1–3 years | Gun drilling (50–200 bar, 10–200 L/min); single or dual machines |
| Piston pump (axial piston, swash plate) | 50–350 bar | 20–500 L/min | 82–90% | 30–300 kW | €20,000–80,000 | Medium-high: piston shoes, valve plate, and seal replacement | BTA drilling (10–100 bar, high flow); multi-machine central systems |
| Multistage centrifugal pump (radial, 2–12 stages) | 10–100 bar | 200–3,000 L/min | 65–78% | 50–400 kW | €25,000–100,000 | Low: bearing and seal replacement every 3–5 years | BTA drilling (10–80 bar, high flow); central coolant systems |
| Progressive cavity pump (screw type) | 5–40 bar | 50–500 L/min | 70–80% | 10–60 kW | €10,000–40,000 | Medium: stator and rotor replacement every 2–4 years | Low-pressure BTA or roughing; slurry handling |
| Gear pump (external or internal) | 10–50 bar | 20–200 L/min | 75–85% | 5–40 kW | €5,000–20,000 | Low: bearing and seal replacement every 2–3 years | Low-pressure coolant boost; transfer or circulation pumps |
| Hydraulic intensifier | Up to 1,000 bar | 2–20 L/min | 60–75% | 5–30 kW | €30,000–80,000 | High: seal replacement every 6–12 months | Micro-drilling (< 1 mm diameter); ultra-high-pressure applications |
Selection Criteria by Drilling Method
| Drilling Method | Recommended Pump Type | Pressure (bar) | Flow (L/min) | Pump Features Required |
|---|---|---|---|---|
| Single-lip gun drilling, Ø1–5 mm | Plunger pump | 80–200 | 10–40 | Low flow ripple (< ±5 bar), pulsation damper, fine filtration (5–10 µm) |
| Single-lip gun drilling, Ø5–20 mm | Plunger pump | 80–200 | 40–150 | Pressure control valve, flow meter, dual filtration (20 µm primary + 5 µm polishing) |
| Multi-spindle gun drilling, 2–12 spindles | Plunger pump (per spindle) or piston pump (central) | 50–200 | 40–80 per spindle | Individual pressure control per spindle, flow monitoring per spindle |
| BTA drilling, Ø18–80 mm | Multistage centrifugal or piston pump | 15–80 | 200–1,000 | High flow at moderate pressure; flow control valve, bypass for tool retraction |
| BTA drilling, Ø80–300 mm | Multistage centrifugal pump | 10–40 | 500–3,000 | Very high flow; VFD for flow control; large filter system |
| Ejector drilling, Ø18–180 mm | Multistage centrifugal pump | 10–50 | 200–1,500 | Moderate pressure; VFD for pressure control |
| Counter-rotation drilling | Piston pump (high pressure) or centrifugal (high flow) | 20–80 | 500–2,000 | Two independent pressure circuits (inner and outer tube) |
Pump System Design
Pressure and Flow Requirements
The required coolant pressure at the tool is determined by the pressure drop through the system — the pump must overcome the pressure losses through the filter, piping, rotary union, and drill tube to deliver the required pressure at the cutting zone.
| System Component | Typical Pressure Drop | Notes |
|---|---|---|
| Filter (clean element) | 0.5–2.0 bar | Increases as element loads; replace at 3–5 bar drop |
| Piping (supply line) | 1–10 bar (depends on length, diameter, bends) | Use pipe velocity < 3 m/s for supply, < 2 m/s for return |
| Rotary union | 1–5 bar | Increases with seal wear; replace at > 10 bar drop |
| Drill tube (gun drilling, L/D 50:1) | 20–80 bar | Major component; increases with L/D ratio and decreases with tube ID |
| BTA drill tube (L/D 20:1) | 5–20 bar | Lower pressure drop due to larger annulus |
| Cutting zone (pressure margin) | 10–30 bar | Pressure margin above the minimum required for chip evacuation |
The total required pump pressure is the sum of all pressure drops plus the required cutting zone pressure. For gun drilling at 150 bar cutting zone pressure with a clean filter (1 bar drop), piping (5 bar), rotary union (3 bar), and drill tube (50 bar at L/D 50:1), the pump discharge pressure must be approximately 209 bar. As the filter loads and the rotary union wears, the pump pressure increases — a properly sized pump should have at least 15% pressure margin above the calculated requirement.
Pulsation Dampening
Positive displacement pumps (plunger and piston pumps) produce pressure pulsations at the pumping frequency — the plunger stroke frequency creates pressure waves that travel through the coolant system to the cutting zone. These pressure pulsations can disrupt chip evacuation and cause bore surface finish variation if the amplitude exceeds ±5 bar at the tool.
| Pulsation Control Method | Attenuation | Cost | Application |
|---|---|---|---|
| Pulsation damper (bladder type) | 70–90% of pulsation amplitude | €800–3,000 | Gun drilling; required for all plunger pump installations |
| Pulsation damper (piston type) | 80–95% | €1,500–5,000 | BTA drilling with tight surface finish requirements |
| Multiple plungers (3, 5, or 7) | 3-plunger: 85% ripple; 5-plunger: 92%; 7-plunger: 96% | Built into pump design | Select pump with sufficient plungers; 5-plunger recommended for gun drilling |
| Inline accumulator (10–20% of system volume) | 50–70% | €500–2,000 | Supplementary damping for long piping runs |
| Flexible hose section near pump | 20–40% | €200–500 | Minimum damping; not sufficient alone for gun drilling |
Seal Technology and Maintenance
Pump Seal Types
| Seal Type | Pressure Rating | Temperature Range | Life Expectancy | Leakage Rate | Application |
|---|---|---|---|---|---|
| Packed gland (braided PTFE/graphite) | Up to 300 bar | −50 to 260 °C | 2,000–6,000 hours | 5–20 drops per minute | Plunger pumps; traditional, adjustable, low cost |
| Mechanical seal (stationary/rotary) | Up to 200 bar | −20 to 150 °C | 4,000–12,000 hours | < 1 drop per minute | Centrifugal pumps; low leakage, moderate cost |
| Lip seal (PTFE or polyurethane) | Up to 100 bar | −30 to 100 °C | 1,000–3,000 hours | < 1 drop per minute | Piston pumps; compact, low cost, limited pressure |
| Labyrinth seal (non-contact) | Up to 50 bar | −50 to 200 °C | 10,000+ hours | 0 (no contact) | Low-pressure applications; zero wear, no leakage path |
| Split mechanical seal | Up to 150 bar | −20 to 120 °C | 6,000–15,000 hours | < 1 drop per minute | Large centrifugal pumps; replaceable without pump disassembly |
Common Pump Problems and Corrective Actions
| Problem | Symptoms | Root Cause | Diagnostic Method | Corrective Action |
|---|---|---|---|---|
| Pressure drop (> 10% below setpoint) | Tool breakage, chip packing, surface finish degradation | Worn check valves, leaking seals, pump speed reduction, air entrainment | Measure pump discharge pressure; compare to setpoint; check for pressure fluctuation | Rebuild check valves; replace plunger seals; check VFD settings; bleed air from suction |
| Pressure fluctuation (> ±5 bar at tool) | Intermittent chip packing, surface finish bands, tool chipping | Worn check valves or valve seats; air in coolant; pulsation damper failed | Install pressure transducer with data logger at pump discharge and at tool; measure ripple amplitude | Rebuild pump discharge valves; recharge or replace pulsation damper; check suction for air leaks |
| Excessive noise (cavitation) | Rattling or knocking sound from pump, pressure fluctuation | Restricted suction (clogged strainer, closed valve, undersized suction pipe); high coolant temperature | Check suction pressure (should be 0.3–0.5 bar positive); check strainer condition; measure coolant temperature | Clean suction strainer; open suction valve; increase suction pipe diameter; reduce coolant temperature |
| Seal leakage (> 20 drops/min) | Coolant puddle under pump, low tank level, pressure drop | Worn seal, incorrect seal material, seal running dry, abrasive particles in coolant | Measure leakage rate; inspect seal faces for wear or damage; check coolant particle count | Replace seal; verify seal material compatibility with coolant; ensure seal flush line is operational |
| Pump overheating | High bearing temperature, thermal shutdown, reduced flow | Incorrect oil level, wrong oil viscosity, bearing wear, internal bypass | Measure housing temperature (should be < 70 °C); check oil level and condition | Change oil; adjust oil level; replace bearings if noisy; check internal bypass valve setting |
| Flow below specification | Long cycle time, chip packing, tool overheating | Pump speed too low, internal bypass open, worn pump internals, clogged suction strainer | Measure flow with flow meter; compare to pump curve at current pressure and speed | Increase pump speed (VFD); check bypass valve setting; rebuild pump; clean suction strainer |
Filtration Requirements
Pump Protection Filtration
| Pump Type | Required Filtration Level | Filter Location | Filter Type | Consequences of Inadequate Filtration |
|---|---|---|---|---|
| Plunger pump | 10–20 µm | Pump suction (100 mesh strainer) + pump discharge (final filter) | Discharge: paper band or cartridge; Suction: wire mesh strainer | Rapid seal wear (plunger seal life reduced by 50–80% at > 20 µm); check valve seat wear; cylinder scoring |
| Piston pump | 10–25 µm | Pump suction (100 mesh) + discharge | Discharge: paper band or cartridge | Valve plate wear; piston shoe failure; control valve stiction |
| Multistage centrifugal | 25–50 µm | Pump suction (60 mesh) | Wire mesh or wedge wire; paper band | Wear ring clearance increase; impeller erosion; seal failure |
| Gear pump | 25–50 µm | Pump suction (60 mesh) | Wire mesh or wedge wire | Bearing wear; gear tip erosion; housing wear |
FAQ
What is the difference between a plunger pump and a piston pump for deep hole drilling coolant?
The fundamental difference is the sealing method. In a plunger pump, the plunger (a solid cylinder) passes through a stationary seal (packed gland or mechanical seal) — the plunger surface is hardened and ground (typically 60–65 HRC, 0.2–0.4 µm Ra surface finish) and the seal is replaceable. Plunger pumps are characterized by: high efficiency (85–92%) across the pressure range; low flow ripple with multi-plunger designs (5-plunger pumps achieve ±2–3 bar ripple at 150 bar); excellent pressure-holding capability (the seal maintains pressure when the pump is stopped); and higher sensitivity to contamination (abrasive particles damage the seal and score the plunger). In a piston pump, the piston moves within a cylinder bore with close clearance (typically 10–30 µm) but no contact seal — the seal is created by the tight clearance and the oil film. Piston pumps are characterized by: slightly lower efficiency (82–90%) due to internal leakage; higher flow capacity (up to 500 L/min versus 200 L/min for plunger pumps); lower sensitivity to contamination (the clearance gap can tolerate 25–50 µm particles); and higher internal leakage as wear increases (flow drops gradually rather than suddenly). For gun drilling applications requiring pressure above 100 bar and flow below 200 L/min, plunger pumps are the preferred choice. For BTA applications requiring high flow (200–3,000 L/min) at moderate pressure (10–100 bar), multistage centrifugal pumps are typically preferred over positive displacement pumps.
How do I size a coolant pump for a deep hole drilling machine?
Pump sizing requires simultaneous determination of pressure and flow. Flow requirement: the coolant flow must provide sufficient velocity in the chip evacuation annulus to transport chips out of the bore. For gun drilling, the minimum annulus velocity (between the drill tube OD and the bore ID) is 3–5 m/s for steel chips and 2–3 m/s for aluminum chips. For BTA drilling, the minimum velocity inside the drill tube is 3–5 m/s. The required flow is the annulus area multiplied by the minimum velocity — for a gun drill with Ø8 mm bore and Ø7.5 mm drill tube (0.5 mm annulus radius), the annulus area is 12.3 mm², and at 4 m/s the required flow is 2.95 L/min — but the actual flow must account for the coolant that exits through the drill point clearance (approximately 30–50% of total flow), so the pump must deliver 5–6 L/min. Pressure requirement: sum the pressure drops through the filter (1–5 bar), piping (1–10 bar), rotary union (1–5 bar), drill tube (20–100 bar depending on L/D), and the required pressure at the cutting zone (10–30 bar margin above the minimum for chip evacuation). For a gun drilling application with L/D 60:1, the drill tube pressure drop at 6 L/min is approximately 60–80 bar, so the pump must deliver 6 L/min at approximately 100–120 bar. Always add a 15–20% margin to both pressure and flow for filter loading, component wear, and future process changes.
How often should a high-pressure coolant pump be maintained?
The maintenance schedule depends on pump type, operating hours, and coolant cleanliness. For plunger pumps: daily inspections — check for unusual noise, vibration, or leakage; check oil level (if gearbox-driven); verify discharge pressure and flow. Monthly — check pulsation damper precharge pressure; inspect suction strainer; take oil sample for analysis (gearbox). Annually (or 4,000 hours) — inspect and replace check valves and valve seats if worn; replace plunger seals if leakage exceeds 10 drops/minute; replace gearbox oil; check coupling alignment. Every 5 years (or 15,000 hours) — complete pump rebuild: new plungers, seals, check valves, bearings, and gearbox overhaul. For multistage centrifugal pumps: daily — check seal leakage, bearing temperature, and vibration. Monthly — check coupling alignment; grease bearings (if regreasable). Annually — replace mechanical seal if leaking; check wear ring clearances; check impeller condition. Every 5–8 years — complete pump overhaul: new bearings, wear rings, seals, and impellers if eroded. Preventive maintenance costs typically represent 2–4% of the pump purchase price per year and extend pump life from 5–8 years (without preventive maintenance) to 15–20 years.
What causes coolant pump cavitation and how can it be prevented?
Cavitation occurs when the pressure at the pump suction drops below the vapor pressure of the coolant, causing vapor bubbles to form and collapse as the pressure increases in the pump. The collapsing bubbles create shock waves that erode the pump internal surfaces (particularly impeller vanes in centrifugal pumps and plunger surfaces in positive displacement pumps) and produce the characteristic rattling or knocking sound. Cavitation in deep hole drilling coolant systems is most commonly caused by: restricted suction (a clogged suction strainer or undersized suction pipe creates excessive pressure drop at the pump inlet — the suction pressure drops below the required net positive suction head (NPSH)); high coolant temperature (coolant vapor pressure increases with temperature — at 50 °C, water-miscible coolant has approximately 5× higher vapor pressure than at 20 °C, requiring 4–5× more suction pressure to prevent cavitation); low tank level (the static head from the coolant surface to the pump inlet is reduced, decreasing the available NPSH); and air entrainment (air bubbles in the coolant from tank vortexing or return line turbulence collapse in the pump, causing similar damage to cavitation). Prevention measures: maintain coolant temperature below 35 °C (use chiller if necessary); size suction piping for flow velocity below 1.5 m/s; keep suction pipe length as short as possible with minimal bends; maintain clean tank level above the minimum specified by the pump manufacturer; install a suction strainer with 3–5× the flow area of the suction pipe (not a standard pipe tee strainer); and install a pressure gauge at the pump suction to monitor for gradual restriction.
Can a variable frequency drive (VFD) be used on a deep hole drilling coolant pump?
Yes, VFDs are increasingly used on coolant pumps for deep hole drilling — particularly on multistage centrifugal pumps for BTA systems. The benefits include: energy savings (centrifugal pump power varies with the cube of speed — reducing speed to 80% reduces power consumption to 51% of full speed); process flexibility (the same pump can serve different drilling operations with different pressure and flow requirements); soft start (eliminates the pressure surge and mechanical stress of across-the-line starting); and closed-loop control (pressure transducer feedback to the VFD maintains constant pressure regardless of flow changes). For positive displacement pumps (plunger and piston pumps), VFD application is more limited because the pressure is determined by the system resistance, not the pump speed — reducing speed reduces flow but does not significantly reduce pressure in a positive displacement pump. However, VFDs on positive displacement pumps can be used for flow control (reducing flow for smaller diameter tools or pilot drilling) with pressure limiting (the VFD reduces speed when pressure approaches the setpoint). Important considerations for VFD application: ensure the pump motor is rated for VFD operation (inverter-duty motor with insulated bearings); install a pressure transducer at the pump discharge (not at the machine) for closed-loop control; set the minimum speed to maintain adequate cooling flow for the motor; and install a bypass check valve to prevent reverse flow through the pump when stopped.
Disclaimer: The pump selection guidelines, sizing calculations, and maintenance recommendations presented in this article are based on published technical literature, pump manufacturer specifications, and industry-reported experience with high-pressure coolant systems for deep hole drilling. Actual pump requirements depend on specific drilling parameters, machine configuration, coolant chemistry, and operating conditions. Pump selection should be made in consultation with pump manufacturers and machine tool builders. All maintenance procedures should be performed in accordance with the pump manufacturer's service manual and applicable safety procedures for high-pressure systems. No guarantee of specific pump life, reliability, or performance is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.