Appearance
A production engineer notices that a deep hole drilling machine that consistently produced holes within tolerance for two years has started producing 0.015 mm oversize holes. The coolant pressure gauge at the pump reads 65 bar — the same as it has always been. The cutting parameters have not changed. The tool is new. After three days of troubleshooting, the engineer installs a pressure gauge at the drill shank connection instead of relying on the pump gauge. The reading is 38 bar. The pump delivers 65 bar, but only 38 bar reaches the tool. The 27 bar difference is a pressure drop somewhere between the pump and the drill shank. Investigation reveals that the rotary union seals have worn, creating an internal leak path that bypasses coolant from the high-pressure side back to the return before it reaches the drill shank. The leak is invisible — no external dripping — because the coolant returns internally to the tank. The fix is a rotary union rebuild. After replacement, the pressure at the drill shank returns to 62 bar and hole diameter returns to nominal. This case illustrates a principle that applies to every deep hole drilling coolant system: what the pump delivers and what reaches the cutting edge are never the same, and the difference determines process capability.
The Role of Coolant Hydraulics in Deep Hole Drilling
The coolant system in deep hole drilling is not an accessory — it is a precision hydraulic circuit that must deliver the correct pressure and flow to the cutting edge. The hydraulics of this circuit determine:
- Chip evacuation: The coolant must have sufficient velocity to transport chips out of the hole. Below a threshold velocity of approximately 8–12 m/s, chips settle in the flute or tube and cause packing.
- Cutting edge cooling: The coolant must reach the cutting edge at sufficient pressure to penetrate the chip-tool interface and remove heat.
- Guide pad lubrication: The guide pads slide against the hole wall under high pressure. Inadequate coolant at the pad interface causes galling, increased torque, and surface finish degradation.
- Chip formation: Coolant jet impact at the cutting edge affects chip curling and fracture through hydraulic pressure.
The hydraulic circuit consists of the pump, pressure regulation valve, flow control, filtration system, rotary union (or pressure head for BTA), drill shank coolant channel, drill tip coolant hole, the gap between the drill and hole wall (for gun drilling) or the annular space (for BTA), and the chip return path.
Every component in this circuit adds resistance. The total pressure drop from pump to discharge determines the actual conditions at the cutting edge.
Orifice, Nozzle and Venturi Fundamentals
The flow of coolant through a deep hole drilling system is governed by the same physics that apply to orifice plates, nozzles, and venturi tubes used in industrial flow measurement. Understanding these fundamental devices helps in analyzing coolant system performance.
Orifice Plate
An orifice plate is a sharp-edged restriction in a pipe. When fluid passes through the orifice, it accelerates to maintain continuity, and the pressure drops. The flow rate is proportional to the square root of the pressure difference.
Q ∝ √(ΔP)
This relationship is fundamental to understanding all coolant flow in deep hole drilling. Doubling the flow rate requires four times the pressure differential. This means that small increases in flow demand — from a worn pump or a partially blocked channel — produce disproportionately large increases in system pressure.
The key limitation of orifice-type restrictions in coolant systems is permanent pressure loss. An orifice plate loses 50–70% of the measured differential pressure as unrecoverable loss. In a drill shank coolant channel, every sharp change in cross-section — the transition from the shank bore to the tip coolant hole, for example — acts as an orifice restriction.
Flow Nozzle
A flow nozzle has a smooth, contoured entry that reduces turbulence compared to an orifice plate. The permanent pressure loss is 30–50% of the measured ΔP — better than an orifice but still significant. Nozzle-type geometries appear in drill tip coolant holes that have a radiused or chamfered entry rather than a sharp edge.
Venturi Tube
A venturi tube has a converging section, a throat, and a diverging section. The gradual expansion downstream allows much of the pressure to be recovered. Permanent pressure loss is only 10–15% of the measured ΔP.
Venturi effects appear in two critical locations in deep hole drilling:
- The ejector drill system: A nozzle at the rear of the drill head creates a venturi that draws chips and coolant from the cutting zone.
- Tapered coolant channels: A converging coolant hole accelerates the flow before it reaches the cutting edge, increasing jet velocity by 15–25%.
| Device | Permanent Pressure Loss | Deep Hole Drilling Application |
|---|---|---|
| Orifice-plate geometry | 50–70% | Sharp-edge coolant hole entries |
| Nozzle geometry | 30–50% | Radiused coolant hole entries |
| Venturi geometry | 10–15% | Ejector systems, tapered channels |
Pressure Drop in Drill Shank Coolant Channels
The coolant channel in a deep hole drilling tool is a long, small-diameter passage that creates significant pressure drop before the coolant reaches the cutting edge.
Darcy-Weisbach Equation
The pressure drop in a straight coolant channel is calculated using the Darcy-Weisbach equation:
ΔP = f × (L / D) × (ρ × V² / 2)
Where:
- ΔP = pressure drop (Pa)
- f = Darcy friction factor (dimensionless)
- L = channel length (m)
- D = hydraulic diameter (m)
- ρ = coolant density (kg/m³)
- V = mean flow velocity (m/s)
For a typical gun drilling application — a 500 mm long, 3 mm diameter coolant channel carrying oil at 10 m/s — the pressure drop through the channel alone is approximately 30–50 bar, depending on the oil viscosity and temperature.
Factors Affecting Coolant Channel Pressure Drop
Channel length: Pressure drop is directly proportional to length. A 1000 mm gun drill has twice the coolant channel pressure drop of a 500 mm drill at the same flow rate.
Channel diameter: Pressure drop is inversely proportional to the fourth power of the diameter for laminar flow. Reducing the coolant hole diameter from 4 mm to 3 mm — a 25% reduction — increases the pressure drop by approximately 200%.
Coolant viscosity: Higher viscosity oils create higher pressure drops. A 10°C temperature rise reduces oil viscosity by approximately 40%, which reduces coolant channel pressure drop by a corresponding amount.
Flow velocity: Pressure drop is proportional to velocity squared. Doubling the flow rate quadruples the pressure drop through the channel.
Implications for Process Design
The pressure drop through the drill shank coolant channel means that the pressure reaching the cutting edge is always significantly lower than the pump discharge pressure. For long, small-diameter drills, the drill shank pressure drop can consume 50–70% of the total pump output pressure.
A pressure gauge installed at the pump reading 80 bar may correspond to only 30 bar at the cutting edge of a 6 mm diameter gun drill that is 800 mm long. This is why measuring pressure at the drill shank — not the pump — is essential for process control.
Coolant Delivery System Design
The coolant delivery system must be designed as an integrated hydraulic circuit, not as a collection of components.
System Pressure Budget
The total pump pressure is distributed across the system components:
| Component | Typical Pressure Drop (bar) |
|---|---|
| Filter | 1 – 5 |
| Piping and hoses | 2 – 10 |
| Rotary union or pressure head | 3 – 15 |
| Drill shank coolant channel | 20 – 60 |
| Drill tip coolant hole | 5 – 15 |
| Bottom clearance area | 2 – 5 |
| Total | 33 – 110 |
The drill shank coolant channel is the largest single contributor to system pressure drop. Any improvement in channel design — larger diameter, smoother surface finish, reduced length — produces the greatest benefit in delivered coolant pressure.
Rotary Union Maintenance
The rotary union that transfers coolant from the stationary supply line to the rotating drill shank is the highest-maintenance component in the coolant delivery system. Internal seal wear is the most common cause of gradual pressure loss at the tool.
Signs of rotary union wear:
- Pressure at the drill shank decreasing by more than 5 bar per month
- Visible coolant leakage (external wear)
- No visible leakage but pressure drops when the spindle rotates (internal seal bypass)
- Fluctuating pressure readings at constant pump output
Rotary union seals should be inspected and replaced at scheduled intervals — typically every 2000–4000 operating hours depending on coolant type and operating pressure.
WARNING
A rotary union with worn seals can bypass 40–50% of the coolant flow internally without any external leakage. The only way to detect this condition is to compare the flow rate entering the rotary union with the flow rate exiting the drill shank. Install test ports at both locations to enable this measurement during routine maintenance.
Coolant Filtration and Pressure Drop
Filters contribute to the system pressure budget and their condition must be monitored. A clean filter adds 1–3 bar pressure drop. A partially blocked filter can add 10 bar or more, reducing the pressure available at the tool.
Monitor filter differential pressure (pressure before the filter minus pressure after the filter). When the differential reaches 5 bar above the clean filter baseline, replace or clean the filter element.
Ejector Drill Hydraulics
The ejector drill system — also called the double-tube system (DTS) — is the most sophisticated application of venturi hydraulics in deep hole drilling.
Operating Principle
The ejector drill uses two concentric tubes. Coolant is pumped between the outer and inner tubes. At the drill head, approximately two-thirds of the coolant flows through passages in the head to the cutting edges for cooling and chip flushing. The remaining one-third is directed through a nozzle that creates a high-speed jet in the inner tube.
The jet creates a venturi effect — the high velocity of the jet reduces the static pressure in the inner tube below atmospheric pressure. This pressure differential draws the coolant and chips from the cutting zone into the inner tube and propels them back to the machine's chip collection system.
Hydraulic Design Parameters
The venturi nozzle is the critical design element in an ejector drill. Key parameters:
- Nozzle cross-sectional area: The ratio of the nozzle area to the inner tube area determines the suction pressure. A ratio of 0.20–0.35 is typical.
- Nozzle position: The distance between the nozzle exit and the inner tube entry affects the efficiency of the venturi. The optimal distance is 3–5 nozzle diameters.
- Flow split ratio: The ratio of coolant directed to the cutting zone versus the venturi nozzle. A 65:35 split (cutting zone:venturi) is a common starting point.
- Coolant pressure requirement: Ejector systems typically operate at 15–40 bar — lower than equivalent BTA/STS systems because the venturi assists chip evacuation.
Advantages and Limitations
| Advantage | Limitation |
|---|---|
| No pressure head seal needed | Reduced suction at depths exceeding 60:1 |
| Compatible with conventional CNC machines | Higher total coolant flow requirement |
| Lower operating pressure than BTA/STS | Less effective at very small diameters (under 18 mm) |
| Suitable for retrofitting to existing equipment | More complex drill head design |
The venturi effect in ejector drilling provides the most efficient chip evacuation of any deep hole drilling system for medium depth-to-diameter ratios (20:1 to 60:1) in diameters above 18 mm.
Nozzle and Coolant Hole Optimization
The geometry of the coolant hole at the drill tip determines how effectively the coolant jet reaches the cutting edge and removes chips.
Coolant Hole Configuration
Research using CFD modeling (Woon et al., 2017) has identified the optimal coolant hole configuration for gun drills:
- Kidney-shaped coolant channel: Provides higher flow and better directed coolant delivery compared to round holes of the same cross-sectional area.
- Inclined outlet angle: The coolant hole should exit at a 30–45° angle to the drill axis, directed toward the cutting edge. A 90° outlet (perpendicular to the axis) wastes coolant energy by directing the jet away from the cutting zone.
- Offset position: The coolant hole exit should be positioned as close as possible to the cutting edge corner to minimize the distance the coolant must travel across the rake face.
Tapered Channels
A converging coolant channel accelerates the flow before it exits at the cutting edge. A channel that tapers from a larger diameter at the shank end to a smaller diameter at the tip increases the exit velocity by 15–25% without increasing pump pressure. This venturi-like effect improves both cooling and chip evacuation at no additional energy cost.
Micro-Grooves
Micro-grooves on the internal surface of the coolant channel promote turbulent flow, which has 2–3 times higher heat transfer coefficient than laminar flow. The grooves create local turbulence without significantly increasing the overall pressure drop. This technique is particularly effective for small-diameter drills where the coolant channel Reynolds number is in the transitional or laminar range.
Dual Coolant Holes
For drills above 10 mm diameter, dual coolant holes provide better flow distribution than a single hole. The two holes can be positioned to direct coolant to both the cutting edge and the leading guide pad, ensuring that the guide pad contact surface is adequately lubricated.
Flow Measurement and Monitoring
Continuous monitoring of coolant flow parameters provides the earliest warning of process degradation.
What to Measure
| Parameter | Sensor | Alarm Threshold |
|---|---|---|
| Pump discharge pressure | Pressure transducer | ±5% of setpoint |
| Drill shank pressure | Pressure transducer (at tool connection) | ±10% of baseline |
| Flow rate | Turbine or magnetic flow meter | ±10% of setpoint |
| Coolant temperature | Thermocouple or RTD | ±3°C from setpoint |
| Filter differential pressure | Differential pressure gauge | 5 bar above clean baseline |
Flow Rate vs Pressure Monitoring
Pressure monitoring alone is insufficient. A system with a partially blocked coolant channel will show normal pressure but reduced flow. The combination of low flow and normal pressure indicates a restriction. The combination of low flow and low pressure indicates a pump or drive problem.
Flow rate is measured with an inline flow meter installed between the pump and the rotary union. The flow reading should be recorded on every production cycle. A gradual downward trend — even 1–2% per week — indicates progressive fouling of the coolant channels or filter.
Troubleshooting Coolant Hydraulic Problems
Systematic troubleshooting of coolant hydraulic problems follows the pressure budget.
Procedure
Verify pump output. Install a pressure gauge at the pump discharge. If the pressure is below the setpoint, check the pump drive, pressure regulation valve, and fluid level.
Measure pressure at the drill shank. If the pressure drop from pump to shank exceeds 20 bar, check the rotary union seals, hoses for kinking or collapse, and filter condition.
Measure flow rate. If flow is below the required minimum for the drill diameter, the issue is a restriction (if pressure is normal) or a pump problem (if pressure is also low).
Check the drill coolant hole. Remove the drill and flush the coolant hole with compressed air. If flow is restricted, the hole may be partially blocked with debris from the coolant system.
Verify coolant temperature. If the temperature exceeds 40°C, the viscosity is reduced, which changes the pressure-flow relationship. The pump may need adjustment to compensate.
Common Issues
| Symptom | Likely Cause | Check |
|---|---|---|
| Low pressure at shank, normal pump pressure | Worn rotary union seals | Internal bypass test |
| Normal pressure, low flow | Partially blocked coolant channel | Flush drill and check |
| Fluctuating pressure | Air entrainment in coolant | Check return line for foaming |
| Pressure drops during drilling | Chip plug in flute or tube | Listen for chip blockage sounds |
| Pressure rises during drilling | Chip congestion at inlet | Check chip box or pressure head |
| Low pressure and low flow | Worn pump or drive fault | Measure pump RPM |
FAQ
Why is the pressure at the drill shank lower than the pump pressure?
Pressure drops through the filter, piping, rotary union, and the drill shank coolant channel. In long, small-diameter drills, the coolant channel alone can consume 50–70% of the pump output pressure. A pressure drop of 20–40 bar from pump to shank is normal.
How do I know if my rotary union is leaking internally?
Install a flow meter at the rotary union inlet and compare the flow rate to the flow rate exiting the drill shank. If the shank flow is significantly lower than the inlet flow, the union is bypassing coolant internally even if no external leak is visible.
What is the minimum coolant velocity for chip evacuation?
The minimum required velocity is 8–12 m/s at the coolant hole exit, depending on the material and chip volume. Higher velocities provide more reliable evacuation. Below 8 m/s, chips settle in the flute and cause packing.
Does coolant viscosity affect hole quality?
Yes. Higher viscosity oils provide better lubrication and maintain higher pressure at the cutting edge but increase the pressure drop through the coolant channel. Lower viscosity oils reduce pressure drop but may not provide adequate lubrication at the guide pads.
What is the optimal coolant temperature for deep hole drilling?
20–30°C with variation of no more than ±5°C during production. Temperatures above 40°C reduce viscosity enough to affect both the pressure delivered to the tool and the lubricity at the cutting edge.
How does the venturi effect work in an ejector drill?
A portion of the coolant is directed through a nozzle at the rear of the drill head. The high-speed jet creates a low-pressure zone in the inner tube that draws coolant and chips from the cutting zone, assisting evacuation without requiring a pressure head seal at the workpiece entry.
What is the most common cause of coolant pressure loss in deep hole drilling?
Worn rotary union seals. The seals wear gradually, so the pressure loss develops over weeks or months and is not noticed until it affects hole quality. Regular comparison of pump pressure and shank pressure detects this condition early.
How do I calculate the flow rate needed for a given drill diameter?
The minimum flow rate is determined by the cross-sectional area of the coolant channel and the required velocity. A general guideline: for a 10 mm gun drill, minimum 30 L/min; for a 25 mm BTA drill, minimum 120 L/min.
Can I use a standard coolant pump for deep hole drilling?
Only if it delivers the required pressure (minimum 40 bar for most deep hole drilling, up to 150 bar for small diameters) and the flow rate is adjustable to match the drill diameter. Standard machining center coolant pumps (10–20 bar) are insufficient for deep hole drilling except at very shallow depths.
Why does coolant pressure fluctuate during the drilling cycle?
Pressure fluctuation typically indicates intermittent chip evacuation — chips are building up and being flushed in cycles rather than flowing continuously. This can be caused by coolant velocity at the low end of the required range, chip shape that is too large for smooth evacuation, or a partially blocked chip return path.
Summary
Coolant hydraulics is the most technically complex subsystem in deep hole drilling and the most common source of process variation.
The key principles:
Pressure drops are additive and significant. The pressure delivered to the pump must account for losses through the filter, piping, rotary union, drill shank coolant channel, and drill tip coolant hole. For small-diameter, long drills, the shank channel alone can consume the majority of the pump output.
Measure at the tool, not the pump. A gauge at the pump shows what the pump delivers, not what the tool receives. Install a pressure transducer at the drill shank connection for process monitoring. The difference between the two readings tells you the condition of the delivery system.
The venturi effect enables ejector drilling. By directing a portion of the coolant through a nozzle, ejector drills create suction that assists chip evacuation without requiring a pressure head seal. This makes deep hole drilling possible on conventional CNC machine tools.
Coolant hole geometry matters. The shape, angle, and position of the coolant hole at the drill tip determine how effectively the coolant jet reaches the cutting edge. Tapered channels, inclined outlet angles, and kidney-shaped holes all improve coolant delivery efficiency.
Monitor flow, not just pressure. Pressure alone does not tell you if the coolant is reaching the cutting edge in sufficient quantity. An inline flow meter detects restrictions and internal leaks that pressure monitoring misses. The combination of flow and pressure data provides a complete picture of coolant system health.