The coolant pump is the heart of a deep hole drilling machine. The right pump delivers adequate pressure and flow for the drilling application. The wrong pump causes chip evacuation problems, poor surface finish, and tool breakage. I have specified pumps for eight deep hole drilling systems, and I have made both good and bad choices that taught me what matters.
Pump Types by Drilling Method
For gun drilling, a positive displacement pump is the right choice. It delivers consistent pressure regardless of flow demand. The two most common types are piston pumps and progressing cavity pumps. Piston pumps use reciprocating pistons to pressurize the coolant. They deliver pressure up to 3000 psi with good efficiency. Progressing cavity pumps use a rotating helical rotor inside a stator to move the coolant. They deliver pressure up to 2000 psi with less pulsation than piston pumps.
Positive displacement pumps have a flat pressure-flow curve. At a given motor speed, the pump delivers the same flow regardless of the system backpressure. This is critical for gun drilling because the pressure at the drill tip must remain stable to maintain the chip formation and evacuation process. A pressure variation of more than 10% during a cut can cause chip packing and tool breakage.
For BTA drilling, a centrifugal pump can work because the pressure requirement is lower at 200-500 psi. The flow requirement is higher — 200-800 L/min depending on the bore diameter. Centrifugal pumps are less expensive and more compact than positive displacement pumps for high-flow applications. However, a centrifugal pump has a drooping pressure-flow curve. As the backpressure increases, the flow decreases. This means the operator must account for pressure variations when selecting pump speed.
| Drilling Method | Pump Type | Typical Pressure | Typical Flow |
|---|---|---|---|
| Gun drilling | Piston pump | 1000-3000 psi | 20-100 L/min |
| Gun drilling | Progressing cavity | 500-2000 psi | 20-80 L/min |
| BTA drilling | Centrifugal pump | 200-500 psi | 200-800 L/min |
| BTA drilling | Multi-stage centrifugal | 500-1000 psi | 200-600 L/min |
| Micro-drilling | Gear pump | 500-1500 psi | 5-20 L/min |
Sizing Based on Drill Geometry
The pump sizing depends on the drill diameter and depth. For gun drilling, the pressure needs to overcome the pressure drop through the drill. The pressure drop through a gun drill is proportional to the length and inversely proportional to the cross-sectional area of the coolant passage. I calculate the required pressure using the drill manufacturer’s pressure curve for the specific drill diameter.
For a 10 mm gun drill, the pressure drop is typically 500-800 psi per 100 mm of depth at 20 L/min. For a 20 mm gun drill, the pressure drop is lower — about 200-400 psi per 100 mm at 50 L/min. The total pressure required is the sum of the pressure drop through the drill plus the system losses through hoses, filters, and the rotating union.
| Drill Diameter (mm) | Typical Flow (L/min) | Pressure Drop per 100 mm Depth (psi) |
|---|---|---|
| 5 | 8 | 1200-1600 |
| 10 | 20 | 500-800 |
| 15 | 35 | 300-500 |
| 20 | 50 | 200-400 |
| 30 | 80 | 100-250 |
For BTA drilling, the flow requirement is the critical factor. The coolant flow must be sufficient to fill the bore area and carry the chips back through the drill tube at a velocity of at least 3 m/s. Below this velocity, chips settle in the drill tube and cause blockages. I calculate the required flow as the bore area times the minimum chip velocity times a safety factor of 1.2.
Capacity Margin and System Design
I select the pump with 20% more capacity than the calculated requirement. The extra capacity allows for wear over time and for variations in material. A pump running at its maximum rated capacity wears faster than a pump running at 80% of capacity. The 20% margin also provides headroom for drilling larger diameters or deeper holes without swapping the pump.
The margin applies to both pressure and flow. A pump selected for 1500 psi with a 20% margin should be capable of 1800 psi at the rated flow. This margin accounts for the increased pressure drop when drilling at the maximum depth or when the filter starts loading with chips.
I also consider the motor sizing. The motor must have enough power to drive the pump at the maximum pressure and flow simultaneously. The power requirement in kilowatts is pressure (bar) times flow (L/min) divided by 600 times the pump efficiency. For a gun drilling pump delivering 100 L/min at 2000 psi (138 bar) with 85% efficiency, the motor power is 27 kW.
Material Selection for Longevity
The pump material matters for long life. I use pumps with stainless steel or hardened steel components for coolant service. Brass or aluminum pumps wear quickly with coolant. The coolant in deep hole drilling contains fine chips that act as an abrasive slurry. The pump components in contact with the coolant must be hard enough to resist this abrasion.
For piston pumps, the cylinder block and pistons should be hardened tool steel or ceramic. The valve plates should be tungsten carbide. Bronze components wear rapidly and cause pressure loss within months. I learned this when a pump with bronze valve plates lost 30% of its pressure capacity within six months of service.
For progressing cavity pumps, the stator material should be oil-resistant rubber or urethane. Standard Buna-N rubber swells and degrades in contact with coolant. I use stators made of hydrogenated nitrile butadiene rubber (HNBR) which has better chemical resistance.
| Pump Component | Preferred Material | Material to Avoid |
|---|---|---|
| Piston / plunger | Ceramic or hardened tool steel (HRC 60+) | Brass, aluminum |
| Valve plate | Tungsten carbide | Bronze, cast iron |
| Cylinder block | Ductile iron with hard chrome | Aluminum, brass |
| Seal faces | Silicon carbide vs carbon | Brass vs bronze |
| Stator (progressive cavity) | HNBR rubber | Standard Buna-N |
Maintenance Indicators and Schedule
I maintain the pump by checking the oil level, the drive coupling condition, and the seal condition monthly. A pump that loses pressure over time probably has a worn seal or impeller. I track the pressure at a standard test condition — new filter, same drill size, same material — to separate pump degradation from other system problems.
The pump oil level should be checked weekly. Low oil level causes the pump to run hot and accelerates wear. The oil should be changed at the interval specified by the pump manufacturer, typically every 2000-3000 operating hours. I use the same oil analysis service for pump oil that I use for the machine hydraulics.
| Condition | Possible Cause | Action |
|---|---|---|
| Pressure drops, flow normal | Internal seal or valve wear | Check pump components, plan rebuild |
| Pressure drops, flow drops | Pump speed issue or coupling wear | Check motor speed, inspect coupling |
| Pressure oscillates | Cavitation or air ingestion | Check coolant level, prime pump |
| Pump runs hot | Low oil level or worn bearings | Check oil, listen for bearing noise |
| External leak at shaft | Mechanical seal failure | Replace seal at next opportunity |
Key Takeaways
The pump is the most expensive single component in a coolant system, and choosing the wrong one is costly. I have learned to match the pump type to the drilling method — positive displacement for gun drilling, centrifugal for BTA. I oversize by 20% to account for wear and process variation. I specify hardened materials for all coolant-wetted components. And I track the pump performance data so I can see wear developing before it causes a failure. A good pump runs trouble-free for 5-10 years with proper maintenance. A poorly selected or undersized pump causes problems from the day it is installed.