Deep hole drilling is one of the most demanding machining operations. Success depends heavily on coolant flow, pressure, and lubricant characteristics. When any of these parameters drift from optimal, heat, chip evacuation, and tool wear problems appear quickly. This guide summarizes the most frequent issues encountered in deep hole drilling and explains how to diagnose and correct them before costly failures occur.
Symptoms: Blue discolouration, loss of dimensional accuracy, or microcracks on the workpiece or tool tip.
Probable causes: Insufficient coolant flow or pressure, low-viscosity coolant unable to form a stable film, or a drop in delivery due to leaks.
Solutions: Verify pump performance, pressure regulation, and temperature control. Ensure that the coolant viscosity matches the tool diameter and depth. For severe loads, a dedicated gun drill oil with higher film strength can stabilize temperature and protect guide pads.
Symptoms: Irregular cutting sound, torque spikes, or broken chips compacted in the flute channel.
Probable causes: Coolant pressure too low to evacuate chips, inappropriate viscosity, or contaminated fluid reducing flushing capacity.
Solutions: Clean filters, verify that the coolant reaches the required flow rate, and adjust viscosity downward for finer flushing. Maintain the system temperature in the 30–50 °C range to keep fluid mobility consistent.
Symptoms: Pitting or crater-like wear on the rake face of the tool.
Probable causes: Thermal overload, chemical instability of the coolant, or loss of lubricating film at high temperature.
Solutions: Replace degraded coolant, stabilize flow temperature, and use fluids with anti-wear and oxidation inhibitors. Avoid mixing old and new fluids without filtration.
Symptoms: Metal deposits sticking to the cutting edge or margin pads, degraded surface finish, tool vibration.
Probable causes: Insufficient lubrication, low coolant temperature, or excessive feed rate.
Solutions: Increase feed gradually while maintaining flow pressure. Verify coolant composition and use a grade that provides stronger boundary lubrication when drilling sticky alloys.
Symptoms: Uneven bore size, vibration, or visible wear on the pads.
Probable causes: Excessive coolant pressure or entrained abrasives due to inadequate filtration.
Solutions: Reduce pressure within the safe operating range and check filter maintenance intervals. Replace worn pads promptly to restore concentricity.
Symptoms: Complete tool fracture or bending.
Probable causes: Intermittent coolant delivery, air pockets, or pressure loss. Chip blockage causing torque overload.
Solutions: Purge the system to eliminate trapped air, maintain constant back pressure, and monitor coolant return lines. Consistent flow is essential for tool stability.
Symptoms: The drilled hole measures larger or smaller than nominal.
Probable causes: Temperature imbalance, viscosity mismatch, or tool deflection from uneven lubrication.
Solutions: Maintain uniform temperature, use a viscosity grade appropriate for the depth ratio, and ensure stable pressure regulation.
Symptoms: Frequent tool changes or premature wear.
Probable causes: Combination of overheating, poor filtration, and unsuitable coolant.
Solutions: Review system parameters: flow, pressure, and coolant selection. Small improvements in lubricant quality often yield major increases in productivity.
For details on selecting viscosity grades and maintaining stable deep hole drilling lubrication, see our Gun Drill Oil category, featuring ISO 12, 24, and 36 products engineered for precision drilling.
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