Gun Drilling Aircraft Hinge Pins: Ø2.45mm × 1,100mm in One Pass

Customer Background

Every aircraft carries dozens of hinges that most people never notice. Control surfaces — ailerons, elevators, rudders, flaps, slats, spoilers — attach to the wing and tail structure through hinge assemblies. Each hinge is a row of lugs, one set on the fixed structure and one on the moving surface, joined by a long cylindrical pin. The pin is the pivot axis: it carries the aerodynamic loads transmitted through the hinge, and it has to let the surface rotate freely and predictably for the life of the aircraft.

Because of that job, a hinge pin is a long, slender, precision part. The customer is a manufacturer of these hinge assemblies, machining pins in aerospace-grade pin material that rewards stable, well-supported cutting. Each pin carries a fine longitudinal bore running its full length — drilled through for weight and for the pin’s assembly and locking hardware. That bore is the detail that decided the whole process, and the reason this job became a gun drilling project.

The Challenge

The bore the drawing demanded was Ø2.45mm, 1,100mm deep — a depth-to-diameter ratio of roughly 450:1. That number places the job at the very edge of what deep hole drilling is documented to do, and far beyond what a conventional twist drill can attempt:

A drill thinner than a needle. At Ø2.45mm, the gun drill is about the diameter of a matchstick. Over 1,100mm of depth, any runout, vibration, or deflection transfers directly to the tool — a machine that would pass for stable on ordinary work is unusable here.

Chip evacuation through a 2.45mm channel. The coolant and chip path inside a tool this size is minuscule. If the chips do not leave continuously, they recut, clog, and snap the tool.

No room for drift. At 450:1 there is no “close enough.” A hole that wanders by a fraction of a degree at the start is centimeters off by the bottom.

The previous process could not hold any of this. The customer was drilling these pins with a conventional twist drill. It meant repeated pecking to clear chips, constant drift and rework, and a production rate of about six pins a day.

The Solution

RuidCNC engineered a purpose-built horizontal long-stroke gun drilling machine and the process around it — because at this diameter-to-depth ratio, machine, tool, and fixturing all have to be designed together:

• Single-pass gun drilling at Ø2.45mm to 1,100mm — one continuous cut, no pecking, with a ~450:1 depth-to-diameter ratio
• A 4,000 rpm spindle with minimal runout — at this tool diameter, spindle precision is process stability
• High-pressure through-tool coolant — continuous chip evacuation and cooling through the narrowest of paths
• A Ø2.45mm × 1,860mm gun drill designed for the job — single-lip geometry and guide pads sized and ground for the pin material, so the tool self-pilots instead of wandering
• Machine rigidity and whip-guide support — a stiff bed and tool support along the full 1.8-meter drill stroke keep the slender tool cutting true
• Fixturing engineered around a long, slender pin — steady support along the workpiece so the pin cannot whip or deflect under the cut

The parameters were set deliberately. At 4,000 rpm and a 30 mm/min feed — a conservative 0.0075 mm/rev — the priority is tool stability and straightness, not speed. That is the correct trade at a 450:1 ratio.

Key Process Parameters

Parameter	Value
Hole diameter	Ø2.45 mm
Drilling depth	1,100 mm
Depth-to-diameter ratio	~450:1
Gun drill length	1,860 mm
Spindle speed	4,000 rpm
Feed rate	30 mm/min
Drilling passes	1 (no pecking)

Results

With the custom process in place, the customer went from about six hinge pins a day with twist drills to one finished hinge pin every 30 minutes — and with the cycle refined further, that is moving toward 20 minutes per pin.

Metric	Twist Drill (Previous)	RuidCNC Gun Drilling	Improvement
Production rate	~6 pins per day	1 pin / 30 min (20 min optimized)	~3× faster
Drilling passes	Multiple pecking passes	Single pass	No pecking
Depth-to-diameter ratio	Not achievable	~450:1	Single pass
Hole quality	Drift, chip-clogging, rework	Straight, clean, repeatable	Process-stable

Process Validation

The process was validated before and after delivery:

• Trial pins drilled on the completed machine to verify straightness and roundness over the full 1,100mm depth
• Tool-life tracking across the production run to confirm the process is repeatable, not just once-accurate
• Full dimensional check of the bore at entry, mid-depth, and exit

Conclusion

The hole could not be made with a twist drill, and it could not be made on a machine that was merely adequate. By engineering the machine, the Ø2.45mm gun drill, and the fixturing as one system, RuidCNC took the hinge pin bore at a ~450:1 depth-to-diameter ratio from six pins a day to one finished pin every 30 minutes — with the optimized cycle heading toward 20 minutes. For work at the edge of what deep hole drilling can do, the machine and the process have to be designed together. That is what this was.

Figures are representative of typical applications.