Deep-Hole Drilling Machine

Overview

A deep-hole drilling machine produces holes that are far longer than they are wide, the kind ordinary twist drills cannot make. A conventional drill wanders and clogs once a hole passes about five times its diameter; a deep-hole machine reaches depth-to-diameter ratios of 50:1, 100:1, or more while holding the hole straight to a fraction of a millimetre per metre. It does this with special single-edge tooling, very high coolant pressure to flush chips, and a rigid machine that keeps the long tool from whipping. These machines bore gun barrels, hydraulic cylinders, injection-mould cooling channels, crankshaft oil galleries, turbine shafts, and medical implant tubes.

Two tooling families dominate. Gun drilling uses a single-flute tube with coolant fed down the centre and chips flushed back along an external V-groove; it suits smaller diameters and the deepest ratios. BTA drilling (boring and trepanning association) feeds coolant around the outside of the tube and ejects chips back through the tube bore; it suits larger diameters and removes metal faster. The machine described here supports both.

Machine bed and feed

The work rides on the Machine Bed, a long and very stiff Bed Casting that may run several metres. Straightness of the bed translates directly into straightness of the hole, so the bed carries precision guideways using the shared Linear Guide Rail leaf and a Ball Screw feed. A Coolant Trough runs the length of the bed to return the large volume of coolant to the tank. The carriage is fed slowly and steadily, because deep-hole drilling rewards a constant, vibration-free feed rate.

Headstock and workpiece rotation

The Workpiece Headstock holds the workpiece in a Hydraulic Chuck on a bored Work Spindle running on bearings from the shared Ball Bearing leaf, driven by the Spindle Motor through a Helical Gear Pair. Hole straightness depends heavily on relative rotation between tool and work. The best results come from counter-rotation, the work turning one way and the drill the other, because any tendency of the tool to drift averages out as both spin. Large or non-round work is held fixed while only the tool rotates.

Drill carriage and tooling

The cutting motion comes from the Drill Spindle Carriage carriage. A Carriage Saddle saddle carries the Drill Spindle driven by the Drill Motor, and the Feed Motor, built on the shared Servo Motor leaf, advances it along the bed. The tool itself is the Drill Tube & Boring Bar: a long Drill Tube tipped with a carbide Cutting Head and fitted with Guide Pad. The single cutting edge pushes the tool sideways against the guide pads, which burnish the bore wall and hold the head centred, and this self-guiding action is what keeps a deep hole straight. The tube joins the spindle through a Tube Coupling.

High-pressure coolant

Coolant is not a luxury here, it is the mechanism that makes deep drilling possible. The High-Pressure Coolant system uses a High-Pressure Pump delivering 20-120 bar to drive cutting fluid through the tool, cool the cutting edge, and carry every chip out of the hole. Because a single chip left in a deep hole can jam and snap the tool, the coolant must stay clean and cool: a Coolant Filter removes fines to 10-25 microns, a Coolant Chiller holds the oil temperature steady so the hole does not grow as it heats, and Pressure Sensor units watch the flow. The fluid is stored in a large Coolant Tank.

Chip separation

The returning coolant carries a continuous stream of chips that must be removed before the fluid is pumped back. The Chip Separation System catches them in a Chip Box at the pressure head, screens them on a Vibrating Screen, pulls ferrous fines out with a Magnetic Separator, and carries the solids away on a Chip Conveyor. Reliable chip separation is what lets the machine run for hours unattended.

Pressure head and whip control

At the bore entry the Pressure Head & Whip Guides does two jobs. A Pressure Seal contains the high-pressure coolant at the hole mouth so it does not spray everywhere, and a Drill Bush Block starts the hole exactly true, since the first few millimetres set the direction of the whole bore. Because the drill tube can be several metres long and spins fast, Whip Guide guides close around it to suppress whip and open as the carriage passes, while Steady Rest rests support long workpieces.

Control and use

The CNC Control is a CNC built on a CNC Computer computer, a Drive Cabinet, and a Operator Panel. Beyond positioning, the control watches drilling power and coolant pressure so it can detect a clogging hole or a breaking tool and retract before the tool snaps deep in an expensive workpiece. In service the machine holds straightness of 0.1-0.5 mm per metre and a bore finish of 0.4-1.6 micron, accuracy that would be impossible with conventional drilling, which is why deep-hole machines are dedicated equipment in firearms, hydraulics, mould-making, and aerospace shafting.