Glossary

Deep Hole Drilling Glossary

A comprehensive reference of technical terms used in deep hole drilling, organized by category.

Drilling Methods & Processes

BTA Drilling — Boring and Trepanning Association drilling, also called Single-Tube System (STS). A deep hole drilling method using a multi-cutter head on a round hollow tube. Coolant is delivered externally (between the tube and hole wall), and chips evacuate internally through the hollow tube. Suitable for diameters from 7.76 mm to 700+ mm at feed rates 5–10 times faster than gun drilling. Requires a dedicated machine with a pressure head (BOZA). (UNISIG, VDI 3209, Botek) → See The Four Deep Hole Drilling Methods Explained

Chamber Boring (Internal Profiling) — Machining an internal cavity larger than the entry bore diameter within the length of a hole. Uses servo-actuated cutting edges that extend and retract under CNC control. Common in aerospace landing gear actuators and helicopter rotor shafts. (UNISIG)

Counter-Boring — Enlarging an existing hole to improve size, straightness, or surface finish. Push counter-boring uses external coolant and pilots off a finished bore. Pull boring is a variant where the tool is pulled back through the starting hole on a tensioned bar for exceptional straightness (< 0.05 mm/m). (UNISIG)

DF Drilling (Double Feeder) — A deep hole drilling system that combines features of BTA and ejector drilling, using a single tube with both push (pressure) and pull (suction) action for chip removal. Suitable for diameters above 8 mm.

Ejector Drilling — A deep hole drilling method using a dual-tube system (Double-Tube System / DTS). Coolant flows between the inner and outer tubes; a Venturi effect creates suction to evacuate chips through the inner tube. Does not require a pressure head, making it retrofittable on conventional CNC machines. Typical diameter range 18–200 mm. (UNISIG, Sandvik Coromant, Botek) → See The Four Deep Hole Drilling Methods Explained

Gun Drilling — A precision deep hole drilling method using a single-lip, asymmetrical cutting tool with internal coolant delivery. Chips evacuate through an external V-shaped groove (flute area ~22–26% of hole cross-section). Originally developed for gun barrel manufacturing. Produces the best surface finish and tightest tolerances. Typical diameter range 0.5–50 mm, L/D up to 400:1. (UNISIG, Ingersoll, ISCAR) → See What Is Deep Hole Drilling?

Cutting Fluid Flow Rate — The volume of coolant delivered per unit time. UNISIG recommends approximately 3.7–4.5 L/min per mm of tool diameter as a starting value for gun drilling (roughly 25–30 gpm per inch). Actual required flow depends on diameter, material, and L/D ratio. Flow rate is more critical for chip evacuation than pressure alone. (UNISIG Technical Reference)

Drill Drift (Deviation) — The tendency of a drill to deviate from its intended path during operation. Measured in mm/m or in/ft. Magnitude depends on operating mode: rotating tool (~1.0 mm/m), rotating workpiece (~0.17–0.50 mm/m), and counter-rotation (~0.08–0.25 mm/m). Also influenced by lip height equality, bushing condition, and material uniformity. (UNISIG)

Entry Chamfer (Starting Chamfer) — A short bevel machined at the hole entrance before drilling. Provides initial guidance for the drill, prevents entry burr formation, and reduces the risk of drill walking on inclined or irregular surfaces. Typically 1–2 mm wide at 45°.

High-Feed Twist Drilling — Deep hole drilling using modified twist drills with through-coolant capability and optimized flute geometry. Achieves L/D ratios up to approximately 40:1. Not self-piloting; does not burnish the bore wall. Faster feed rates than single-flute gun drilling but lower hole quality.

Peck Drilling — A conventional drilling method for deep holes where the drill is periodically withdrawn to clear chips. Used when high-pressure through-tool coolant is not available. Not recommended for production deep hole drilling due to cycle time penalties and work hardening risk. → See Deep Hole Drilling vs Conventional Drilling

Pull Boring — A counter-boring variant where the tool is pulled back through the starting hole on a tensioned bar, putting the material in tension rather than compression. Provides the best straightness of any deep hole method (< 0.05 mm/m). Range: 20–630 mm. (UNISIG)

Skiving & Roller Burnishing (SRB) — A combined finishing process for hydraulic cylinder tubes performed in a single pass. Skiving uses carbide blades to create a geometrically round bore (0.2–0.5 mm radial stock removal); roller burnishing cold-works the surface to a mirror finish (Ra 0.05–0.4 µm). Processing speed 8–20× faster than honing. Surface hardness increases 30–50%. (UNISIG, ECOROLL, VDI 3209 Blatt 2) → See Hydraulic Cylinders Guide

Solid Drilling — Drilling a full cross-section hole from solid material, as opposed to trepanning which cuts only an annular groove. All BTA and gun drilling of solid bars is solid drilling.

Spade Drilling — Drilling using a spade-type drill with replaceable cutting blades. Limited deep hole capability (typically up to 40:1 L/D). Not self-piloting. Higher feed rates than gun drilling but lower hole quality and straightness.

Starting Hole (Pilot Hole) — A short hole pre-drilled before the main deep hole drilling operation. Typically 1–2× diameter deep, using a drill with a larger point angle than the deep hole drill. Provides initial guidance and prevents the main drill from walking at entry.

Trepanning — A deep hole drilling method that removes only an annular groove, leaving a solid central core (slug). Used for large diameters (50–1,000+ mm) where core recovery has value or spindle power is limited. Lower accuracy than other methods (IT10–IT12); typically requires secondary finishing. (UNISIG, Coastal Metals, Sandvik Coromant) → See The Four Deep Hole Drilling Methods Explained

Tooling Types

Solid Carbide Gundrill — A gun drill made from one piece of solid carbide, combining head and shank. Maximum rigidity for small diameters. Typical range: 0.5–12 mm diameter. (UNISIG)

Brazed Gundrill — A gun drill with a carbide tip silver-brazed to a hardened alloy steel shank. The most common gun drill configuration for diameters 2–50 mm. Can be reground 5–15 times. (UNISIG)

Indexable Gundrill — A gun drill with mechanically clamped carbide inserts instead of a brazed tip. No regrinding needed; insert is simply replaced when worn. Typical range: 13.5–50 mm. (UNISIG, Silmax)

BTA Brazed Tool — A BTA drill head with brazed carbide cutting edges. Used for smaller diameters (12–65 mm) where indexable inserts cannot fit. (UNISIG, Botek)

BTA Indexable Tool — A BTA drill head with replaceable carbide inserts retained by screws or cartridges. Dominant configuration for diameters ≥ 20 mm. Allows quick grade changes between materials. (UNISIG, Botek, ISCAR)

Spade Drill — A drilling tool with a replaceable flat blade mounted in a holder. Limited deep hole capability (up to ~40:1 L/D). Not self-piloting. Lower cost per edge than gun drills but significantly lower hole quality.

Tool Geometry & Components

Asymmetrical Cutting Edge — The characteristic cutting edge design of gun drills, BTA, and ejector tools. The cutting edge is divided into inner and outer edges meeting at the tool point (apex), typically located 1/3 to 1/5 of the diameter from the outer edge. This offset design creates a self-piloting action that distinguishes deep hole drills from conventional twist drills.

Back Taper — A gradual reduction in diameter along the length of the drill, typically 1–2 µm per 10 mm of length (gun drilling) or 0.02 × diameter per 100 mm. Prevents friction between the drill body and the bore wall.

Bottle Boring Tool — A tool for machining internal profiles within the length of a bore, using a cutting insert that is extended and retracted with CNC control. Used on BTA-equipped machines for landing gear, helicopter rotor shafts, and jet engine components. (UNISIG)

Chip Breaker — A feature on the cutting edge that fragments chips into small C-shaped pieces. Essential for deep hole drilling because long chips cannot be evacuated through the tool or V-groove. Chip separators divide the cutting edge into steps (typically 2–3 depending on diameter). For chip evacuation troubleshooting, see the Troubleshooting Guide.

Chip Mouth — The opening in a BTA drill head through which chips exit into the hollow drill tube for internal evacuation.

Facet Grind (F8) — A gun drill tip configuration with distinct geometric features ground to create the cutting edge, chip clearance, and coolant relief. European standard; allows greater coolant clearance at the cutting edge compared to sweep grind.

Guide Pad — Carbide pads mounted on the tool periphery that bear against the bore wall. In BTA tools: the leading pad (typically at 178° from the cutting edge) provides main support; the trailing pad (at 276°) controls hole diameter. In gun drills, guide pads burnish the bore surface through plastic deformation — research shows pad condition affects surface finish more than cutting edge sharpness. (Weinert & Bruchhaus, Wear, 1999)

Margin — The narrow land at the outer corner of the cutting edge, typically 0.4–0.6 mm wide. The margin width affects oil film formation and friction.

Nose Grind — The set of angles ground on the carbide tip of a gun drill that controls chip formation and cutting force balance. Common types: N-8 (general purpose), N-4 (aluminum/brass), N-73 (stacked parts), Facet F8 (European standard), Sweep Grind.

Self-Piloting Tool — A deep hole drilling tool that guides itself via guide pads bearing against the bore wall, rather than being guided solely by the machine spindle. The asymmetrical cutting edge creates a radial force that presses the pads against the wall, keeping the tool centered.

Sweep Grind — A gun drill tip configuration where geometric features are blended using a cam fixture to create chip clearance and coolant relief. Adjustable for different materials.

Whip Guide — An assembly used to support long, slender deep hole drills and prevent deflection. Consists of a whip guide adapter (housing and bearing) and a whip guide insert (plastic or bronze component that supports the drill tube). Required for L/D ratios above approximately 20:1.

Machine Components & Systems

BOZA (Drilling Oil Supply Unit) — German: Bohrölzuführapparat. The pressure head assembly that seals between the workpiece and drill tube and introduces cutting fluid under pressure. Contains a conical rotating workpiece holder and a stuffing box seal. Required for all BTA drilling operations.

Coolant Filtration — Essential for deep hole drilling. Filtration to < 20 µm is standard for most applications; precision small-diameter work requires 3–10 µm. Inadequate filtration acts as an abrasive, accelerating guide pad wear and degrading surface finish. Multi-stage systems (magnetic separator + paper band + cartridge filter) are standard. (UNISIG, JimmyTool, Brinkmann) → See Coolant Pressure and Flow Rate Guide

Coolant Velocity — The flow speed of cutting fluid in the chip evacuation channel. Must be sufficient to transport chips: minimum 5–12 m/s in gun drill V-grooves, 3–6 m/s in BTA internal tubes. Coolant flow rate (volume × velocity) is more important for chip evacuation than pressure alone. (UNISIG; TU Dortmund research)

Counter-Rotation — An operating mode where the workpiece rotates in the opposite direction of the tool. The cutting speed is the sum of both rotations. Produces the best hole straightness (0.05–0.15 mm/m) and concentricity, especially at L/D ratios above 20:1. (UNISIG)

Flow-Based Chip Management — An automatic system that monitors coolant flow rate and increases pressure when a chip blockage is detected. If pressure rises above a threshold, the tool retracts to clear the blockage before drill breakage occurs. (UNISIG)

Guide Bushing (Drill Bushing) — A hardened steel sleeve mounted in the pressure head or workpiece fixture that supports the drill at the hole entry point. Provides initial guidance, seals coolant pressure, and prevents drill walking. Must be replaced when worn — an oversized or misaligned bushing causes bell-mouth holes and short tool life. (Botek, Allied Machine)

Pressure Head — A component mounted on BTA machines that introduces cutting fluid to the drilling process, precisely locates the drill bushing, and provides a seal around the drill tube. Must withstand coolant pressures of 20–100 bar. Interchangeable bushing inserts allow quick adaptation to different tool diameters.

Rotary-Union — A device mounted at the rear of a gun drilling spindle that introduces cutting fluid to the rotating tool from non-rotating fluid plumbing.

Through-Spindle Coolant (TSC) — Coolant delivery through the machine spindle centerline, enabling through-tool coolant delivery for gun drilling and BTA operations. Minimum 40 bar recommended for deep hole drilling on machining centers. (VDI 3211)

Vibration Damper — A mechanical device that supports the drill tube along its length to reduce longitudinal and torsional vibrations. Uses a pre-loaded spring pressing a damping cone against a counter-bearing. Improves surface finish and reduces cutting edge wear. Required for L/D ratios above approximately 30:1. (Botek)

Operating Modes

Rotating Tool Mode — The workpiece is held stationary while the tool rotates. Used for non-symmetrical components or off-center holes. Cutting speed is determined by spindle speed. Drill drift is most significant in this mode: approximately 0.012 in/ft (1.0 mm/m). (UNISIG)

Rotating Workpiece Mode — The tool is stationary (or rotates at a lower speed) while the workpiece rotates. Used for round parts with on-center holes. Cutting speed is determined by workpiece rotation. Drill drift is reduced: approximately 0.002–0.006 in/ft (0.17–0.50 mm/m). The workpiece must be balanced. (UNISIG)

Counter-Rotating Mode — The tool and workpiece rotate in opposite directions. Cutting speed is the sum of both rotations. Produces the best straightness and concentricity: 0.001–0.003 in/ft (0.08–0.25 mm/m). Ideal for round parts with deep on-center holes requiring maximum precision. (UNISIG)

Quality & Measurement

Concentricity — The degree to which the bore centerline coincides with the theoretical center axis. Deep hole drilling achieves concentricity of 0.01–0.05 mm depending on method, L/D ratio, and machine capability.

Cylindricity — A three-dimensional tolerance that controls the combined effect of roundness, straightness, and taper along the entire bore length. More comprehensive than roundness alone. Critical for hydraulic cylinder and landing gear applications where seal performance depends on consistent bore geometry along the full stroke.

Depth-to-Diameter Ratio (D:d or L/D) — Hole depth divided by hole diameter. The defining metric for deep hole drilling: per VDI 3210, L/D > 3 qualifies as a deep hole; industry practice sets the practical threshold at L/D > 10. Gun drilling achieves up to 400:1; BTA up to 250:1; conventional twist drills are limited to approximately 5:1. (UNISIG, VDI 3210) → See Depth-to-Diameter Ratio Guide

Exit Burr — The raised edge or protrusion formed at the hole exit when the drill breaks through. Size depends on material ductility, tool sharpness, and feed rate at breakthrough. Controlled by reducing feed by approximately 50% in the last 2 mm and using sharp tools.

Bell-Mouth Hole — The enlargement of the bore diameter at the entry point, creating a flared or bell-shaped entrance. Caused by worn or misaligned guide bushings, excessive coolant pressure at the entry face, or incorrect chamfer geometry. Once a bell mouth forms, the drill lacks proper entry guidance and straightness degrades rapidly.

Built-Up Edge (BUE) — Workpiece material that welds to the cutting edge during machining, forming a built-up layer. Alters the effective cutting geometry, increases cutting forces, and degrades surface finish. Particularly common in stainless steel and aluminum at low cutting speeds. Prevention: increase cutting speed, use coated tools (TiAlN), ensure adequate coolant flow.

Chatter (Vibration) — Self-excited vibration during drilling producing regular marks (chatter marks) on the bore surface. Caused by insufficient rigidity, excessive cutting speed, worn guide pads, or incorrect feed rate. Chatter degrades surface finish and accelerates guide pad wear. Remedy: reduce speed, increase feed slightly, check machine rigidity and guide pad condition.

Chip Compaction — The dense packing of chips in the evacuation channel, blocking the passage of subsequent chips. The most common cause of drill breakage in deep hole drilling. Warning sign: a 10% increase in coolant pressure indicates partial blockage. Prevention: optimize chip breaker geometry, maintain adequate coolant flow velocity, use real-time torque/pressure monitoring. (JimmyTool; Botek)

Chip Welding — The adhesion of chip material to the cutting edge or guide pad surface under high temperature and pressure. Causes surface finish degradation and increased friction. More common in materials with high chemical affinity to carbide (titanium, stainless steel). Prevention: use coated tools, increase coolant pressure, select appropriate cutting speeds.

Hole Oversize / Undersize — A bore that measures larger or smaller than the specified tolerance. Oversize is commonly caused by unequal drill lip heights, worn bushings, or excessive spindle RPM relative to feed. Undersize is typically caused by tool deflection, insufficient speed, or material spring-back.

IT Grade — International Tolerance grade for dimensional accuracy. Achievable values by method (note: actual values depend significantly on workpiece material):

Gun drilling achieves IT6–IT7 in soft materials (aluminum, low-carbon steel) and IT10–IT11 in difficult materials (Inconel, hardened steels). Sources: Ingersoll brazed gundrill specs; ISCAR Drilling Handbook; UNISIG Technical Reference; T2180 boring machine specs.

Roundness — The degree to which the bore cross-section approximates a perfect circle. Affected by guide pad condition, cutting force balance, and machine rigidity. BTA roundness can reach 0.036 mm per 200 mm diameter with optimized parameters.

Straightness — The deviation of the bore centerline from a straight reference line. Typical values: gun drilling with counter-rotation 0.05–0.15 mm/m, BTA 0.1–0.5 mm/m, ejector 0.3–0.8 mm/m. (UNISIG, Tiefbohrbär, HTT)

Surface Finish (Ra) — Arithmetic average roughness of the bore surface. Gun drilling achieves Ra 0.4–1.6 µm (range can extend to Ra 0.2–6.3 µm depending on material and parameters); BTA achieves Ra 0.8–3.2 µm; skiving and roller burnishing achieves Ra 0.05–0.4 µm. → See Surface Finish Guide

White Etching Layer (WEL) — A thin (≤ 12 µm), hard (up to 3× substrate) layer that forms on the bore surface at high cutting speeds and feeds. Associated with reduced fatigue life in aerospace applications. Avoided by staying within recommended parameter ranges and maintaining adequate coolant pressure.

Standards

VDI 3209 Blatt 1 — Deep hole boring systems with external supply of coolant (BTA and ejector). Provides cutting parameter guide values, tool design guidance, and diagrams for coolant quantity, pressure, and machine drive power. Latest edition: 2024-01. (VDI) → See VDI Standards Explained

VDI 3209 Blatt 2 — Deep hole boring — Approximate values for skiving and roller burnishing of bores. Provides guidance for optimal use on deep hole boring machines. (VDI)

VDI 3210 — Deep hole boring — Methods, equipment, and range of application. The umbrella standard that defines deep hole drilling processes and their classification (43 pages, confirmed 2021). (VDI, 2006) → See VDI Standards Explained

VDI 3211 — Deep hole drilling on machining centers. Presents requirements for performing deep hole drilling on standard CNC machining centers, covering tools, coolant pressure minimums (40 bar recommended), and achievable L/D limits (~15:1–20:1). (VDI, 2015)

VDI 3212 — Acceptance test requirements of deep hole drilling machines with one or more spindles. Defines terms, procedures, and methods for testing geometric accuracy in unloaded state per DIN ISO 230-1. (VDI, 2014)

German-English Terms (Tiefbohren Fachbegriffe)

Key Sources

1. UNISIG, “Glossary of Deep Hole Drilling,” “Deep Hole Processes,” and “Deep Hole Drilling Technical Reference” (February 2024) — primary terminology source
2. Ingersoll Cutting Tools, “Brazed Gundrills” — IT grade and surface finish specifications
3. ISCAR Drilling Handbook — IT grade and cutting parameter data
4. Botek, BTA catalog and “Deep Hole Drilling” — process definitions and vibration damper specifications
5. VDI 3209 Blatt 1:2024-01, VDI 3210:2006-03, VDI 3211:2015-01, VDI 3212:2014-07 — standard definitions
6. Sandvik Coromant — ejector drilling system description
7. ECOROLL AG — SRB process terminology and specifications
8. Weinert, K., Bruchhaus, T., “Tribological investigations into the operational behavior of self-piloting drilling tools,” Wear, 1999 — guide pad function research
9. Tiefbohrbär GmbH — straightness and precision specifications
10. HTT BTA machine specifications — quality data
11. JimmyTool — coolant and filtration requirements
12. Coastal Metals — trepanning description
13. Silmax — single-lip drill product catalog (tooling types)
14. TU Chemnitz BEOLINGUS — German-English technical dictionary