Real problems, real solutions, no fluff. I have spent years working across multiple shops running gun drilling, BTA drilling, and ejector drilling machines — setting up jobs, troubleshooting coolant pressure and chip evacuation issues, drilling deep holes in everything from hydraulic cylinders to aerospace titanium. This site is where I share what I have learned.
I use gun drilling alone when possible and add reaming for tight tolerances. The decision between the two processes comes down to surface finish, tolerance, cycle time, and cost per hole. Here is how I break down the comparison on my shop floor. Reaming is traditionally required after drilling to improve finish and tolerance. But gun drilling can achieve similar results in a single pass. The trade-offs are not always obvious, so I keep a systematic comparison. ...
The Real Danger of High Pressure Coolant High pressure coolant systems in deep hole drilling run at 1000-2000 psi. At those pressures, a pinhole leak creates a jet that cuts through skin like a scalpel. The entry wound looks small and harmless. The coolant carries bacteria and metal fines deep into the tissue, and the infection that follows is what causes permanent damage. I have seen a maintenance technician get a coolant injection injury from a pinhole leak in a hose that looked perfectly sound. The leak was invisible at idle but opened up under full pressure. He needed two surgeries to clean out the infection. The hose had been in service for three years. ...
Running coolant at higher pressure than necessary wastes energy without improving hole quality. I’ve learned to match the coolant pressure to the job requirements and save energy in the process. Pressure Requirements by Process Different deep hole drilling processes need different coolant pressures: Process Typical Pressure Pump Type Flow Rate Energy Consumption (kW) Gun drilling, small diameter (2-8 mm) 1500-3000 psi Positive displacement 2-8 GPM 5-15 kW Gun drilling, large diameter (8-30 mm) 800-1500 psi Positive displacement 8-30 GPM 10-30 kW BTA drilling 200-500 psi Centrifugal or PD 30-100 GPM 7-20 kW Ejector drilling 200-400 psi Centrifugal 20-60 GPM 5-10 kW The pressure requirement depends mainly on the drill diameter and the depth of the hole. Smaller-diameter drills need higher pressure because the coolant passage through the drill is smaller. The flow rate requirement depends more on the hole diameter — larger holes need more coolant volume to flush the chips. ...
Understanding Hole Deviation Hole deviation is one of the most frustrating problems in deep hole drilling. The drill starts on center and slowly drifts off course as it goes deeper. By the time it reaches full depth, the hole may be out of position by several millimeters. I have seen deviation ruin parts at depths as shallow as 50mm and as deep as 2 meters. The financial impact adds up fast when you scrap a part that already has hours of machining invested. ...
A cold machine does not drill straight holes. The thermal expansion of the spindle, bed, and coolant changes the machine geometry during the first few hours of operation. I’ve learned to account for thermal effects in every job, and I’ve developed a warm-up procedure that eliminates the guesswork. Why Warm-Up Matters As a machine warms up, several changes happen simultaneously: The spindle bearings expand, changing the spindle position by 0.01-0.03mm The bed expands, changing the alignment between headstock and tailstock The coolant warms up, changing its viscosity and flow characteristics The ball screws expand, changing the positioning accuracy These changes are not small. I measured a 0.15mm drift in hole position on a large BTA machine over the first 90 minutes of operation. The first hole of the day was in a different location than the holes drilled two hours later. The operator had been chasing this by adjusting offsets — compensating for a problem that thermal stability would have solved. ...
I calculate power requirements before selecting a machine for any deep hole drilling job. The machine spindle must have enough power to drive the drill at the required speed and feed without stalling or bogging down in tough material. Underpowered machines produce inconsistent hole quality and short tool life. The Basic Power Formula The standard formula I use every time is Power (kW) = Torque (Nm) x RPM / 9550. For a 10 mm gun drill in mild steel running at 3000 RPM with 10 Nm of torque: Power = 10 x 3000 / 9550 = 3.1 kW. That gives me the power at the tool tip, not at the motor. ...
Why Pilot Hole Geometry Matters The pilot hole is the only thing guiding the gun drill during the first few millimeters of entry. If the pilot geometry is wrong, the drill starts off-center or at an angle, and the rest of the hole follows that bad start. I have measured the effect of pilot geometry on hole straightness across dozens of jobs. A pilot hole that is 0.1mm oversized produces a starting hole that is 0.05-0.08mm oversized at entry and the error propagates as the drill goes deeper. ...
I have lost count of how many times I have seen a shop chase tool breakage or surface finish problems for weeks, only to discover the real culprit was a coolant pressure issue. The machine gauge reads fine, but the pressure at the tool tells a completely different story. Coolant pressure problems are the most common root cause of deep hole drilling issues I encounter. Here is my systematic approach to diagnosing them. ...
Problem Description Feed marks are visible helical lines that follow the tool advance pattern on the bore surface. They look like thread crests running along the hole wall. I have measured feed marks that were 0.02 mm deep on a 15 mm bore — enough to fail a 1.6 Ra surface finish callout by a wide margin. Scratches are different. They run axially along the bore and look like the surface was dragged across something rough. Scratches come from chip drag or recirculating debris. I have seen both problems on the same hole, and each needs a different approach. Understanding the distinction is the first step to fixing surface defects in gun drilled holes. ...
What Happens at an Interruption Interrupted cuts in deep hole drilling happen when the drill passes through a cavity, cross hole, keyway, or any gap in the material. The cutting edge hits the far wall of the cavity, loses contact, and then slams back into the material on the other side. That shock loads the edge and chips it. I have seen a sharp C2 carbide edge chip on the first interruption of a 10mm hole through a valve body with a 6mm cross hole. The chip happened in the first 0.1 seconds of re-entry. After that, the drill was cutting with a damaged edge and the bore finish was shot. ...