Deep hole drilling leans on the coolant harder than any other operation. The fluid has to reach the cutting edge through a small channel, carry every chip back out through the flute, and hold a lubricating film under the guide pads — all at once. When flow, pressure or viscosity drift, heat, chip packing and tool wear show up within a few holes. Most gun drilling runs on a neat cutting oil rather than a water-soluble coolant, because the pads need lubrication more than the cut needs cooling. Below are the eight problems we hear about most, with how to read and fix them.
Symptoms: blue discolouration, loss of dimensional accuracy, or microcracks on the workpiece or the tool tip.
Probable causes: not enough flow or pressure, an oil too thin to hold a stable film, or delivery lost to a leak.
Solutions: check pump output, pressure regulation and oil temperature. Match viscosity to the drill: smaller diameters need a thinner oil to get through the coolant channel, larger and deeper holes can take a heavier one. For severe loads, a dedicated gun drill oil with higher film strength stabilises temperature and protects the guide pads.
Symptoms: irregular cutting sound, torque spikes, broken chips compacted in the flute.
Probable causes: pressure too low to evacuate chips, wrong viscosity, or contaminated oil that no longer flushes well.
Solutions: clean the filters, confirm the system reaches its rated flow, and go down a viscosity grade if chips are not clearing. Hold the oil at 30–50 °C (86–122 °F) so it flows the same way all shift.
Symptoms: crater wear on the rake face, or small chips broken out of the cutting edge.
Probable causes: thermal overload, an oil that has oxidised, or a lubricating film that breaks down at temperature.
Solutions: replace degraded oil, keep the supply temperature stable, and use an oil with anti-wear and oxidation inhibitors. Do not top up old oil with new without filtering first.
Symptoms: metal welded to the cutting edge or the margin pads, poor surface finish, vibration.
Probable causes: cutting speed too low for the material, or an oil without enough boundary lubrication — typical on stainless, aluminium and other sticky alloys.
Solutions: raise cutting speed in small steps while keeping pressure constant, and move to an oil with stronger extreme-pressure additives for the sticky materials.
Symptoms: uneven bore size, vibration, visible wear on the pads.
Probable causes: pressure above the drill's range, or abrasive fines circulating because filtration is not keeping up.
Solutions: bring pressure back within the safe operating range and shorten the filter maintenance interval. Replace worn pads promptly to restore concentricity.
Symptoms: the drill fractures or bends.
Probable causes: intermittent delivery, air in the line, pressure loss, or a chip blockage that overloads the torque.
Solutions: purge trapped air, hold steady back pressure, and watch the return line. A drill that loses flow for a second can break in that second.
Symptoms: the hole measures larger or smaller than nominal.
Probable causes: temperature swings, the wrong viscosity, or tool deflection from uneven lubrication.
Solutions: keep oil temperature uniform, use a viscosity grade that suits the depth-to-diameter ratio, and make sure pressure regulation is stable.
Symptoms: frequent tool changes, premature wear.
Probable causes: usually several of the above at once — some overheating, some poor filtration, an oil that does not suit the material.
Solutions: go through flow, pressure, filtration and oil selection in that order. Tool life is the sum of all four.
Oils for this job are in Gun Drill Oils; inserts for deep hole drilling are in Deep Hole Drilling Inserts. For the difference between neat oils and water-soluble coolants, see Types of Coolant, or browse all metalworking fluids.
Not sure which oil fits your drill, material and depth? Send us the details and we will suggest one.
