Gun Drilling Machine Product

Overview

A gun drilling machine produces deep, straight, small-diameter holes — depths of 100 diameters and beyond — that twist drills cannot reach without wandering or breaking. The process takes its name from rifle-barrel manufacture, still a core application alongside crankshaft oil galleries, injection-mould cooling channels, medical bone screws and fuel-rail bores. The tool is a single-flute gun drill: a carbide tip brazed to a long V-fluted shank with a coolant hole running through it. Oil pumped through the shank exits at the tip and flushes chips back along the external V-flute, so chips never re-enter the cut.

The machine is essentially a long, rigid alignment fixture with services attached. On the Machine Bed, the Drill Spindle Assembly, the Whip Guide System and the Workhead Assembly all sit on one rail axis held coaxial within about 0.02 mm. Get that alignment wrong and the drill bell-mouths the hole entry or snaps in the first few millimetres.

How it works

A cycle starts with the workpiece gripped in the Work Chuck and the drill tip retracted inside the carbide Guide Bushing, which is fitted only 5–10 µm over drill diameter. The bushing does what a centre drill does for a twist drill: it steers the unsupported tip square into the material. The High-Pressure Pump brings oil pressure up — 100+ bar for a 2 mm drill, falling to around 40 bar at 40 mm — before the spindle reaches speed, because a gun drill run dry destroys its bearing pads within seconds.

The Feed Carriage then advances the rotating drill at a constant servo-held feed. Unlike twist drilling there is no pecking in normal operation; the oil stream evacuates chips continuously, and feed stays in the narrow band (commonly 10–80 mm/min) where the tool produces short, well-broken chips. The carbide tip cuts slightly off-centre, so the resulting radial force presses two ground bearing pads behind the cutting edge against the freshly cut wall. The drill thus burnishes its own guide bore as it goes, which is why gun-drilled holes routinely achieve IT8–IT9 tolerance and Ra 0.4–1.6 µm straight off the tool.

Hole straightness is the process's main currency. A drill that only rotates while the work stands still drifts in whatever direction its grind biases it; counter-rotating the workpiece with the Work Spindle makes the drift axisymmetric, and the error largely cancels. Machines drilling barrels and long shafts therefore counter-rotate at a few hundred rpm against the drill's several thousand, holding drift to 0.1–0.5 mm per metre of depth.

Between spindle and workpiece, the slender shank would whip like a skipping rope at 8,000 rpm if unsupported. The Whip Steadies — spring-closed split bushings spaced along the stroke — grip the shank and snap open in sequence as the Carriage Body passes over them, so the unsupported length never exceeds a safe span.

Coolant plant

The High-Pressure Coolant System system is closer to a hydraulic power unit than a flood-coolant tank. A triple-screw pump delivers deep-hole oil (straight cutting oil or high-EP emulsion) through the Rotary Union into the hollow Spindle Shaft and down the drill. Filtration matters more than pressure: the Fine Filter holds the oil near 10 µm cleanliness, because particles recirculated through the tip scratch the bearing pads and the hole wall. The Coolant Chiller holds oil temperature within about ±2 °C; warm oil thins, the drill cuts a measurably different diameter, and bores drift out of tolerance over a shift. Returning oil and chips collect in the sealed Chip Box at the workpiece face, where a magnetic Chip Conveyor lifts out the fine chips and a Mist Extractor captures the mist that high-pressure oil inevitably generates.

Supervision and failure modes

Gun drills give little warning before breaking, so the machine watches force and pressure rather than the operator watching the tool. The Thrust Monitor load cell tracks feed force: a rising trend means a dulling tip, a spike means chip packing, and either triggers the Machine PLC to retract before the drill seizes. Twin Pressure Sensor channels compare pump output to tool-side pressure — a widening gap indicates a leaking union or cracked shank. Cycles also embed a slow "entry feed" for the first 2–3 diameters and a reduced "breakthrough feed" at exit, the two points where most drills fail.

Compared with BTA drilling, which pumps coolant around the tool and pulls chips through a tubular shank, gun drilling is slower but reaches smaller diameters (down to 1.5 mm) and needs less elaborate tooling, so it dominates below roughly 25 mm. Multi-spindle versions drill four to twelve mould plates or crankshafts in parallel from one coolant plant and one Control System cabinet.