How Gun Drilling Works: A Step-by-Step Guide

How Gun Drilling Works

Gun drilling is a continuous-process machining method that produces deep, straight, precision holes in a single pass. Unlike conventional drilling—which must peck and retract to clear chips—gun drilling uses a specialized tool and high-pressure coolant system to evacuate chips continuously, enabling depth-to-diameter ratios up to 300:1 in a single uninterrupted operation.

This guide walks through the process step by step, from workpiece setup to final inspection.

The Key Components

Before examining the process sequence, it helps to understand the three elements that make gun drilling work:

The Gun Drill Tool. A single-lip carbide cutting tip on a long steel shank with a V-shaped flute. Coolant flows through an internal hole in the shank and exits at the cutting tip.

Guide Pads. Carbide pads immediately behind the cutting tip that press against the bore wall. These provide self-piloting action, continuously steering the tool on-axis.

High-Pressure Coolant System. Delivers cutting fluid at 300–2,000+ PSI through the tool, cooling the cut and flushing chips back through the V-flute.

Step-by-Step Process

Step 1: Workpiece Setup and Guide Bushing Alignment

The workpiece is securely clamped in the machine. A guide bushing (drill sleeve) is positioned at the entry point of the workpiece and precisely aligned with the spindle axis.

The guide bushing serves three critical purposes: it provides an exact starting point for the drill, seals the workpiece to contain high-pressure coolant, and prevents the drill from wandering at entry. The bushing must be accurately aligned—misalignment is a common cause of hole deviation and tool breakage.

Important: The gun drill should never rotate when entering the guide bushing. Coolant flow is started first, then the spindle is engaged.

Step 2: Pilot Hole Preparation

A short pilot hole (typically 1–2 diameters deep) is pre-drilled at the hole location. The pilot hole is slightly larger in diameter than the gun drill itself—usually 0.0005–0.001" (0.013–0.025 mm) oversized.

The pilot hole accomplishes two things: it guides the gun drill during the first moments of cutting when the tool is most susceptible to wandering, and it allows the high-pressure coolant to establish flow before the drill engages the full workpiece cross-section.

Step 3: Coolant Initiation

Before the spindle starts, high-pressure coolant is turned on and flows through the internal coolant channel of the gun drill, exiting at the cutting tip. This ensures the cutting zone is lubricated and chip evacuation begins immediately when cutting starts.

Coolant pressure is diameter-dependent. For small diameters (0.125" / 3.2 mm), pressures of 1,000–1,500 PSI are typical. For larger diameters (1.0" / 25 mm), 300–500 PSI is sufficient.

Step 4: Cutting Action Begins

The spindle is started, and the rotating gun drill is fed into the workpiece at a controlled feed rate—typically 0.010 to 0.025 mm per revolution for standard gun drilling.

As the carbide tip engages the material, the single cutting edge removes material along the full radius of the hole. The geometry of the nose grind creates an unbalanced cutting force that pushes the drill toward the side of the hole. This force is immediately countered by the guide pads, which bear against the freshly cut bore wall and keep the drill centered.

This self-piloting action is the heart of the gun drilling process. The guide pads continuously steer the tool, maintaining straightness typically within 0.05 mm per 300 mm (0.001" per foot) of depth.

Step 5: Continuous Chip Evacuation

High-pressure coolant, exiting at the cutting tip, immediately flushes the chips backward through the V-shaped external flute on the drill shank. The coolant velocity carries the chips continuously out of the hole.

Chip control is critical. Short, well-broken chips (C-shaped segments) evacuate reliably. Long, stringy chips can clog the flute, leading to heat buildup, tool damage, and scrap parts. Experienced operators monitor chip color and shape as a real-time indicator of process health:

• Silver or light straw chips indicate proper cutting conditions
• Blue or burned chips signal excessive heat—reduce speed or increase coolant pressure
• Long stringy chips suggest feed is too low—increase feed rate slightly
• Powdered chips indicate feed is too high or tool is dull

Step 6: Whip Guide Support for Deep Holes

As the drill advances beyond approximately 30–40× the drill diameter, the long, unsupported shank becomes susceptible to deflection and whipping—a dangerous condition where the rotating tool bends and vibrates, causing poor hole quality and potential tool breakage.

Whip guides (also called tool supports or steady rests) are positioned along the drill shaft to provide support. These devices use rotating bearings or carbide blades to contact and stabilize the tool without damaging it.

For very deep holes (100× diameter or more), multiple whip guides are placed along the tool path. Longer gun drilling machines may have six or more whip guides that fold over each other as the drill advances.

Step 7: Contra-Rotation (Counter-Rotation)

In dedicated gun drilling machines, both the workpiece and the drill rotate simultaneously in opposite directions. This contra-rotation significantly improves hole straightness, especially at extreme depth ratios.

Contra-rotation works by canceling out the rotational forces that cause the tool to drift. With contra-rotation, straightness deviations can be held to within 0.03 mm over 150 mm—a dramatic improvement over single-rotation setups.

Standard CNC lathes can only rotate the workpiece. This is one reason dedicated gun drilling machines achieve better straightness on very deep holes.

Step 8: Depth Reached — Tool Withdrawal

When the drill reaches the programmed depth (or exits the workpiece for through-holes), the spindle is stopped before the tool begins to withdraw. This is important—retracting a rotating tool can damage the bore surface.

The drill is withdrawn slowly while coolant continues to flow briefly to flush any remaining chips from the hole.

Process Sequence Summary

Gun Drilling on Different Machine Types

Dedicated gun drilling machines offer the best results. They provide contra-rotation, high-pressure coolant systems, whip guide support, and process monitoring. Depth ratios of 100:1 or more are routine.

CNC lathes can perform gun drilling with the right setup: high-pressure coolant through the live tooling, a guide bushing mounted on the turret or tailstock, and proper pilot hole preparation. Depth ratios are typically limited to 40:1 without whip guide support.

CNC machining centers can also be retrofitted for gun drilling, but are generally limited to shorter depths (20:1 to 30:1) due to tool overhang limitations.

Common Setup Mistakes to Avoid

• Rotating the drill while entering the guide bushing. This wears the bushing and can damage the tool tip.
• Insufficient coolant pressure. The most common cause of poor hole quality and tool breakage.
• Misaligned guide bushing. Even slight misalignment causes hole drift.
• Incorrect pilot hole depth. Too shallow doesn’t guide the drill; too deep traps chips.
• Excessive feed rate. Causes tool overload, chip packing, and breakage.

Summary

Gun drilling works through a carefully orchestrated interaction of tool geometry, coolant hydraulics, and machine mechanics. The single-lip cutting edge removes material efficiently, the guide pads maintain straightness, the internal coolant delivery lubricates and evacuates chips, and whip guides support the long tool at extreme depths. When properly set up, the process produces deep, straight, precision holes in a single pass—capabilities that conventional drilling cannot match.

For detailed parameter recommendations, see our gun drilling speeds and feeds guide. For tool geometry details, see gun drill types and geometry explained. For a complete overview, visit the gun drilling guide.