Deep Hole Drilling Tools: BTA, Gun Drill & Trepanning Guide

Deep hole drilling is not a single process but a family of processes, each defined by the tool at its centre. The choice between a gun drill, a brazed BTA head, an indexable BTA head, an ejector system, or a trepanning cutter determines everything that follows: feed rate, surface finish, chip management, coolant pressure, and cost per hole.

Tool Categories Overview ​

The boundaries between categories overlap at the edges. A 20 mm hole can be produced by gun drill, brazed BTA, or indexable BTA depending on depth, material, and production volume.

Tip: The single most important selection criterion is whether the hole diameter requires gun drilling (under 12 mm) or BTA (over 20 mm). Between 12–20 mm, both methods are technically feasible — BTA will remove material faster, but gun drilling may achieve a finished bore without a subsequent operation.

Gun Drills ​

Design Features ​

The gun drill is the simplest deep hole drilling tool in construction but the most demanding in precision:

• Single-lip cutting edge — one carbide tip brazed to a steel shank
• Internal coolant hole — high-pressure coolant is delivered through a central bore in the tip and shank
• External V-shaped chip groove — chips travel along the outside of the shank through a groove that runs the full length
• Guide pads — two pads (or one pad and the burnished bore surface) support the drill against cutting forces

Critical limitation: The V-groove reduces the cross-sectional area of the shank, limiting torsional rigidity and maximum feed rate. This is the fundamental trade-off of gun drilling — excellent precision and depth capability at the cost of material removal rate.

Size Range and Applications ​

Typical Parameters ​

BTA Drill Heads ​

Brazed vs Indexable Construction ​

Brazed BTA heads are manufactured by brazing carbide cutting edges and guide pads onto a steel body followed by grinding to final diameter. This produces the best possible concentricity and is the standard for diameters below 20 mm where indexable insert pocket tolerances would consume too large a fraction of the diameter tolerance.

Indexable BTA heads use standard-indexable inserts mounted directly in the head or seated in replaceable cartridges. Cartridge-mounted inserts allow fine diameter adjustment to compensate for wear or to dial in a specific bore size.

Warning: An indexable BTA head is only as accurate as its insert seating surfaces. A worn or damaged insert pocket will produce an oversize or out-of-round bore regardless of the insert grade. Inspect pockets during every insert change — a pocket that shows fretting or deformation means the head body needs replacement.

BTA Head Design Geometry ​

The BTA drill head has a characteristic three-contact design:

1. Main cutting edge — does the majority of cutting, typically with a lead angle in the feed direction
2. Secondary cutting edge — positioned on the radial outside, includes a circular-ground chamfer for three-point bearing contact
3. Guide pads — first pad positioned at 40–60° from the cutting edge corner (modern design recommendation is under 70°); second pad positioned approximately 180° opposite the cutting edge corner

The guide pad angle has been the subject of significant design evolution. Modern BTA heads use a smaller guide pad angle (45–55°) than traditional designs, which reduces:

• Tilting moment on the head
• Passive cutting forces
• Friction on the secondary cutting edge
• Oscillation tendency

Thread connections vary by head size:

Ejector (Dual-Tube) Drilling ​

Ejector drilling uses a two-tube system to eliminate the pressure head seal required by conventional BTA:

• Outer tube — delivers coolant to the cutting zone through the annular space between inner and outer tubes
• Inner tube — carries chips and return coolant back through the machine spindle
• Venturi effect — a portion of the coolant is directed through ejector nozzles to create negative pressure at the chip inlet, actively pulling chips into the inner tube

Advantages Over Conventional BTA ​

The ejector system is the preferred choice for converting a conventional lathe or milling machine to deep hole drilling capability without the capital investment of a dedicated BTA machine.

Trepanning Heads ​

For diameters above approximately 350 mm, conventional BTA drilling becomes impractical due to the volume of material that must be removed as chips and the power required. Trepanning solves this by cutting only an annular groove, leaving a solid core.

Trepanning is specified when:

• The hole diameter exceeds 350 mm
• The core material has economic value (e.g., large shafts, rolls)
• Machine power is insufficient for full-face BTA drilling at the required diameter

Cutting Inserts for Deep Hole Drilling ​

Standard Insert Types ​

Insert Designation Guide ​

Reading an insert code for deep hole drilling:

TPMX 2405R-HF IC908
││││  ││││ │  │    │
││││  ││││ │  │    └── Grade (IC908 = first choice for steel)
││││  ││││ │  └── Chipbreaker (HF = High Feed)
││││  ││││ └── Hand (R = Right, L = Left)
││││  │││└── Thickness code (05 = 5.56 mm)
││││  ││└── IC/size code (24 = 24 mm inscribed circle)
││││  │└── Relief angle (M = 11°)
││││  └── Tolerance class (P = precision)
│││└── X = pressed chipbreaker (no clearance angle code needed)
││└── P = positive rake
│└── M = medium tolerance (moulded)
└── T = triangular shape

Grade Selection ​

Guide Pads ​

Guide pads are the unsung critical components of BTA drilling. They support the cutting forces, maintain hole straightness, and burnish the bore surface during drilling.

GPS Series Dimensions ​

Material Selection ​

Wear Management ​

Guide pad wear is the primary limit on BTA head life. Key indicators:

Tip: Always replace guide pads in sets. Replacing only the worn pad will create a diameter mismatch that causes the head to cut oversize. The new pad will also wear faster because it carries a disproportionate share of the load.

Tool Selection by Application ​

Drill Tubes and Accessories ​

Tube Selection ​

Tube Connection Types ​

FAQ ​

What are the main types of deep hole drilling tools? ​

Gun drills (0.2–20 mm), BTA drill heads in brazed (12–20 mm) or indexable (20–350 mm) construction, ejector (dual-tube) systems (18–100 mm), and trepanning heads (350–1,200+ mm). The selection depends primarily on hole diameter, depth, material, and production volume.

What is the difference between brazed and indexable BTA drill heads? ​

Brazed heads have carbide edges brazed to a steel body and are ground to final diameter — they offer the best precision in small diameters (12–20 mm) but must be discarded when dull. Indexable heads use replaceable screw-mounted inserts, providing grade flexibility and lower operating cost at diameters above 20 mm.

What is an ejector drilling system? ​

Ejector drilling uses two concentric tubes to deliver coolant and evacuate chips. Coolant flows through the annular space between tubes, and a Venturi effect creates negative pressure at the cutting zone to actively pull chips through the inner tube. It operates at only 0.5–3 MPa coolant pressure and does not require a pressure head seal.

When should I use trepanning instead of BTA drilling? ​

Trepanning is used for diameters above approximately 350 mm where full-face BTA drilling would require excessive power and generate more chips than necessary. Trepanning cuts an annular groove and recovers a solid core, saving material and power. It is also used when the core material has economic value.

What guide pad dimensions are available for BTA heads? ​

Guide pads follow the GPS series designation with widths from 4–14 mm and lengths from 16–250 mm. Common sizes include GPS-06-20-085 (6×20 mm) for 40–60 mm heads, GPS-10-30-200 (10×30 mm) for 100–150 mm heads, and GPS-14-40-250 (14×40 mm) for 200–300 mm heads.

What cutting speed should I use for BTA drilling of steel? ​

For carbon and alloy steels (180–300 HB), cutting speed typically ranges from 60–100 m/min for HSS-grade tooling and 80–140 m/min for carbide grades. The specific speed depends on required tool life, machine power, and coolant capability. Reduce speed by 15–25% for stainless and superalloys.

Can I use a gun drill for diameters above 20 mm? ​

Standard gun drills are limited to approximately 20 mm maximum diameter. Above this, the V-shaped chip groove would require an impractically large shank diameter relative to the hole size, and the torsional rigidity would be insufficient. BTA drilling is the standard method for diameters above 20 mm.

What insert grades are recommended for deep hole drilling? ​

IC908 is the first-choice grade for most steel and stainless steel BTA drilling applications, offering a balanced combination of wear resistance and toughness. IC806 is preferred for titanium and superalloys where fracture toughness and hot hardness are critical. IC9025 is recommended for high-speed cast iron drilling.

How do I select the correct tool for a deep hole drilling application? ​

Determine the hole diameter first: under 12 mm → gun drill; 12–20 mm → brazed BTA or gun drill (evaluate MRR vs finish requirements); 20–350 mm → indexable BTA (or ejector for retrofit); over 350 mm → trepanning. Then consider L/D ratio (gun drill excels at extreme ratios), material (grade and chipbreaker selection), and production volume (indexable for high volume, brazed for short runs).

What causes BTA drill head failure? ​

The most common failure modes are: guide pad wear (bore taper, surface degradation), insert edge chipping (incorrect grade or chipbreaker selection for the material), thread connection fatigue (overtorquing or misalignment), and chip packing (insufficient coolant pressure or flow). Regular inspection of guide pad condition and insert cutting edges prevents most catastrophic failures.

Conclusion ​

The diversity of deep hole drilling tools reflects the range of problems they solve — from 0.2 mm surgical guide wires to 1,200 mm steel mill rolls. The five primary tool categories (gun drills, brazed BTA, indexable BTA, ejector systems, and trepanning heads) cover the full diameter spectrum, with some overlap between adjacent ranges. The correct selection depends on diameter first, then depth ratio, material, production volume, and whether the hole finish requirement allows a single-pass operation or demands subsequent finishing. For the majority of production deep hole drilling (20–200 mm diameter in steels), indexable BTA heads with appropriate grade and chipbreaker selection provide the best combination of metal removal rate, tooling cost per hole, and process flexibility.