Deep Hole Drilling: Comparing Gun Drilling, BTA, Ejector and Indexable Methods

Deep hole drilling, broadly defined as drilling at depth-to-diameter ratios (L/D) exceeding 5:1, represents one of the most demanding operations in metal cutting. The challenges are simultaneous: chip evacuation becomes progressively more difficult with depth, coolant must reach the cutting edge against increasing hydraulic resistance, tool deflection grows with unsupported length, and surface finish requirements typically remain constant regardless of depth. Four distinct methods have evolved to address these challenges, each optimized for specific diameter ranges, depth capabilities, and production requirements.

Method Comparison Overview

Indexable Insert Drills: The Standard CNC Solution

Indexable insert drills use two or more replaceable carbide inserts mounted on a steel body. The typical configuration employs a centre insert (positioned to cover the drill axis) and one or more peripheral inserts that cut the outer diameter. This dual-insert geometry allows each insert to be optimized independently: the centre insert handles low-speed, high-thrust conditions while the peripheral insert operates at full surface speed with primary responsibility for hole diameter accuracy.

The practical depth limit of 5-8xD results from the reliance on internal coolant channels for chip evacuation combined with the relatively large body cross-section that limits chip space. Beyond 8xD, chips begin to pack in the flutes even with through-spindle coolant. However, within their depth range, indexable drills offer the lowest cost per hole for production work because insert changes take seconds and the bodies last for thousands of holes.

Korloy’s indexable drilling lines provide extensive diameter coverage with multiple insert geometries optimized for different materials. The ability to select centre and peripheral inserts independently allows fine-tuning for specific workpiece materials without changing the drill body.

Gun Drilling: Precision at Extreme Depth

Gun drilling is the oldest dedicated deep hole method, developed originally for rifle barrel production. The distinguishing feature is a single-flute cutting head with an asymmetric geometry that creates a self-piloting action. The cutting edge is offset from the drill axis, and bearing pads on the cylindrical portion of the head ride against the freshly-cut bore wall, providing continuous guidance.

The operating principle requires high-pressure coolant (70-140 bar) fed through an internal passage to the cutting edge. Coolant enters through the kidney-shaped channel in the solid shank, exits at the tip to cool the cutting edge and lift chips, then returns to the machine through the single external V-flute along the tool length. This design enables consistent chip evacuation even at 100xD depth because the coolant continuously flushes the entire flute length.

Gun drilling produces exceptional straightness (0.02-0.05mm per 100mm) and surface finish (Ra 0.4-1.6 micron) because the bearing pads maintain constant contact with the bore wall, preventing the drill from wandering. The trade-off is low material removal rate; feed rates are typically 0.005-0.02 mm/rev for the smallest diameters, making cycle times long for large-diameter or deep holes.

Applications include hydraulic cylinder bores, fuel injector bodies, mould cooling channels, medical implant passages, and any application requiring precision bore quality at depths beyond indexable drill capability.

BTA (Boring and Trepanning Association) Drilling

BTA drilling inverts the coolant flow compared to gun drilling. Coolant is pumped at 20-60 bar through the annular space between the boring bar and the bore wall (outside-in), flooding the cutting zone. Chips are then flushed back through the centre of the hollow boring bar (inside-out) to a chip collection box at the machine rear.

This counter-flow design provides several advantages over gun drilling for larger diameters. The entire bore wall acts as the coolant delivery channel, providing massive flow volume without requiring extreme pressure. Chip evacuation through the large-diameter centre tube handles the higher chip volumes generated by larger cutting widths. The cutting head carries multiple inserts or guide pads, distributing cutting forces and enabling higher feed rates than single-point gun drills.

BTA drilling requires a pressure head (seal) at the workpiece entry point to contain the coolant and force it into the bore. This means the system requires a dedicated deep hole drilling machine or significant adaptation of standard equipment. Diameter range starts at 20mm (below which the centre tube becomes too small for reliable chip flow) and extends to 250mm or more.

Production rates are 3-6x higher than gun drilling for equivalent diameters because the multi-insert head and efficient chip evacuation allow aggressive feed rates. BTA is the method of choice for high-volume production of automotive crankshafts, landing gear components, pressure vessels, and large hydraulic cylinders.

Ejector (Twin-Tube) Drilling

The ejector system was developed specifically to enable deep hole drilling on standard CNC machines without requiring the sealed pressure head that BTA demands. It uses a twin-tube construction: coolant flows forward through the annular space between the inner and outer tubes at low pressure (5-15 bar), reaches the cutting head, and returns with chips through the inner tube.

The key innovation is the ejector effect (Venturi principle) created at the junction between the tubes. A portion of the forward-flowing coolant is diverted through small cross-holes into the return path, creating a low-pressure zone that actively draws chips into the inner tube. This eliminates the need for a sealed pressure head at the workpiece because the system is self-contained within the tool assembly.

The practical advantage is enormous: ejector drilling can be performed on any CNC machining centre or lathe with through-spindle coolant capability at modest pressure. No machine modification is required beyond the ejector tube assembly and appropriate coolant flow rate. Depth capability reaches 50xD, and diameter range spans 20-180mm.

The trade-off versus BTA is somewhat reduced bore quality (straightness and finish) because the lower coolant pressure provides less hydrodynamic stiffness, and the ejector effect is less forceful than direct pressure chip evacuation. For applications where IT9-IT10 tolerance and Ra 1.6-6.3 finish are acceptable, ejector drilling provides deep hole capability without capital investment in dedicated machinery.

Decision Criteria and Cost-Per-Hole Analysis

Decision Framework

The method selection depends on four factors ranked by priority: required depth (L/D ratio), diameter, quality requirements (tolerance, finish, straightness), and production volume. If depth is under 8xD, use indexable inserts on a standard CNC. If depth exceeds 8xD on a standard machine, ejector drilling is the practical choice. If precision bore quality is required at extreme depth, gun drilling or BTA on dedicated equipment is necessary.

Cost-Per-Hole Example

Consider a 30mm diameter hole, 300mm deep (10xD) in 42CrMo4 steel:

For low-volume production (under 500 holes/year), gun drilling on existing equipment makes sense despite higher per-hole cost because it avoids capital investment. For medium volumes (500-5000 holes/year), ejector drilling on a standard CNC provides the best balance. For high-volume production (above 5000 holes/year), the BTA system’s lower cycle time justifies dedicated equipment investment with payback typically within 12-18 months.

Selecting the appropriate deep hole drilling method requires matching the application requirements to each system’s strengths. Consult with tooling specialists to evaluate your specific diameter, depth, material, quality, and volume requirements before committing to equipment investment.