Coolant System Guide

30-Second Summary: In deep hole drilling, the role of coolant goes far beyond cooling — it performs three critical functions: chip evacuation, guide pad lubrication, and heat dissipation. Pressure and flow rate are both essential; neglecting either will cause process failure. The coolant system is often the most expensive auxiliary system in a deep hole drilling installation.

Three Functions of Coolant

• Chip evacuation: High-pressure coolant flushes chips away from the cutting zone and carries them out of the hole. In gundrilling, chips travel along the external V-groove; in BTA, chips travel inside the drill tube; in ejector drilling, the Venturi effect creates suction to assist evacuation.
• Guide pad lubrication: Forms a hydrostatic oil film between the guide pads and the hole wall, reducing friction and preventing galling. This oil film is approximately 4 μm thick — finer filtration directly improves pad life.
• Heat dissipation: Removes cutting heat from the tool-workpiece interface, preventing thermal softening of the cutting edge and thermal expansion of the workpiece.

Pressure and Flow Rate by Method

Method	Pressure Range	Flow Rate Range	Notes
Gundrilling (D < 5 mm)	100–200 bar	2–8 L/min	Smaller diameters require both higher pressure and finer filtration
Gundrilling (D 5–20 mm)	50–120 bar	8–60 L/min	Most common gundrilling range
Gundrilling (D > 20 mm)	30–80 bar	60–200 L/min	Larger diameters need less pressure but much more flow
BTA Drilling	15–100 bar	50–500+ L/min	High flow rate is critical; pressure depends on depth and chip size
Ejector Drilling	10–50 bar	25–200+ L/min	Venturi effect requires adequate volume flow more than pressure
Trepanning	10–40 bar	200–2200 L/min	Very high flow needed for large annular cutting area
Skive & Roller Burnishing	10–30 bar	50–200 L/min	Mainly lubrication; chip load is low

Quick Pressure Estimation

Coolant Type Selection

Type	Advantages	Disadvantages	Best Application
Neat cutting oil (EP additives)	Excellent lubrication, rust protection, high film strength	High cost, requires degreasing step, fire risk, poor cooling	Gundrilling, BTA finishing, precision machining
Water-based emulsion (3-10%)	Good cooling, low cost, fire-safe	Inferior lubrication vs. oil, can promote rust, bacterial growth	BTA roughing, general production, aluminum
Synthetic fluid (water miscible)	Good cleanliness, excellent anti-bacterial, good cooling	Average lubrication, may foam at high pressure, residue issues	Aluminum alloys, cast iron, general machining
Semi-synthetic fluid	Balance of lubrication and cooling, cleaner than emulsion	Moderate cost, may still foam	Most deep hole applications, good all-rounder

Filtration Requirements

Coolant filtration precision directly affects guide pad life, hole surface quality, and pump reliability. Insufficient filtration is the most common cause of premature guide pad failure.

Application	Recommended Filtration	Consequence of Poor Filtration
Gundrilling (precision)	≤ 20 μm (15 μm recommended)	Guide pad scoring, short tool life, surface finish degradation
Gundrilling (small dia., D < 5 mm)	≤ 10 μm	Coolant passage blockage, catastrophic tool failure
BTA Drilling	≤ 30 μm (20 μm recommended)	Guide pad wear, chip evacuation issues
Ejector Drilling	≤ 50 μm	Nozzle blockage, loss of Venturi effect
Skive & Roller Burnishing	≤ 10 μm	Roller surface damage, poor surface finish
High-pressure pumps (>100 bar)	≤ 25 μm minimum	Pump wear, seal failure, pressure loss

Pump Sizing and System Design

System Size	Working Pressure	Flow Rate	Motor Power	Typical Application
Small CNC retrofit	20–70 bar	33–41 L/min	3.0–5.6 kW	Single-spindle ejector or small gundrill
Medium dedicated machine	50–100 bar	60–200 L/min	11–22 kW	Production gundrilling or BTA
Large BTA system	15–80 bar	200–500 L/min	22–45 kW	High-volume BTA production
Heavy trepanning	10–40 bar	500–2200 L/min	45–90 kW	Large diameter trepanning

Sump capacity: The coolant tank should hold at least 3-10 times the maximum pump output per minute (5× for steel, 7× for cast iron/aluminum, 10× for grinding/high stock removal). This allows chips to settle and the coolant to cool between circulations.

Pump types:

• Centrifugal pumps: Suitable for low pressure (10–30 bar), high flow — ejector drilling, trepanning
• Screw pumps: Medium pressure (20–80 bar), moderate flow — general gundrilling and BTA
• Ceramic plunger / piston pumps: High pressure (50–200+ bar), lower flow — small diameter gundrilling

Coolant Temperature Management

Coolant temperature directly affects machining accuracy and process stability:

• Ideal operating range: 20–35°C (oil) / 15–30°C (emulsion)
• Maximum recommended: 40°C — above this, drill tube thermal expansion reduces accuracy, and oil viscosity drops, compromising the guide pad oil film
• Temperature change effect: 10°C temperature change in a 1000 mm steel workpiece causes approximately 0.12 mm length change — significant for tight tolerances
• Chiller requirement: Systems above 100 bar operating continuously generally require a heat exchanger or chiller to maintain stable temperature

System Components Overview

A complete deep hole drilling coolant system includes:

1. Coolant tank — with baffles for chip settling and foam control; tangential return lines minimize aeration
2. Filter system — drum-type paper filter (15-50 μm grade), magnetic separator for ferrous chips, or multi-stage bag/cartridge filter
3. High-pressure pump — centrifugal, screw, or plunger type as dictated by pressure/flow requirements
4. Pressure relief and regulation valves — to protect the system and allow adjustment
5. Accumulator / pulsation damper — smooths pressure fluctuations from plunger pumps
6. Coolant delivery system — rotary union (for rotating tools), pressure head (for BTA), or connector block (for ejector)
7. Temperature control — heat exchanger or chiller for continuous operation
8. Return / filtration loop — chip conveyor, settling tank, return pump with secondary filtration