Deep Hole Drilling Glossary

The clearance gap between the outer diameter of the drill tube and the machined hole wall. In BTA / single-tube (STS) drilling, high-pressure coolant is delivered to the drill head through this annular space before chips return through the tube interior.

A gradual reduction in tool diameter from tip to tail. Gundrills are manufactured with a slight back taper (about 0.0006–0.0008 in/in of tip) so the tool body does not rub against the bore wall; after resharpening the effective diameter and flute length change slightly.

Machining an internal profile within the length of an existing bore using a CNC-extendable and retractable cutting insert on a BTA-equipped machine. Used to produce rotationally symmetric but non-cylindrical internal contours, e.g. chamber pockets in aerospace landing-gear parts.

A BTA-machine tool that finishes a profile (radii, steps, flat bottoms) into the base of a blind hole, completing the bore form after the deep hole is drilled.

The single-tube deep hole drilling system, initiated in Germany in the 1930s and named after the country’s boring and trepanning association. Coolant is fed externally between the hole wall and the drill tube; chips are evacuated internally through the tube. BTA penetration rates run roughly 5–10× faster than gundrilling because the internal chip-clearance zone exceeds 60% of the hole area.

A groove, step, or interrupted feature on the tool rake face that curls and fractures continuous chips into short, manageable segments. Essential for materials that produce long ribbon chips, such as low-carbon steel, aluminum, and copper, and for keeping chips short enough to evacuate through a narrow flute or tube.

The proportion of the hole cross-section available for chip flow. A gundrill’s V-flute area is roughly 22–26% of the hole area; the BTA internal chip zone exceeds 60%; the ejector inner tube provides about 35–40%. More clearance area means higher possible penetration rate.

The opening at the front of a BTA drill head through which chips enter the hollow interior of the drill tube. Because chips travel inside the tool, they never touch the finished bore wall, which protects surface finish.

The ideal chip form in deep hole drilling — small, regular, easily discharged, and low in power demand. C-shaped chips form when a curling chip breaks against the hole bottom; in EA4T steel BTA tests they were produced at about 80 m/min cutting speed and 0.2 mm/rev feed with high-pressure coolant.

High-pressure fluid pumped to the cutting zone in large volume to cool the cutting edge, lubricate the guide pads, and continuously flush chips away. Unlike flood drilling, deep hole coolant must reach the tool tip at depth — which is what separates the three tube systems.

Deep hole coolant pressures typically range from about 10 to 350 bar with volumes of 25–800 L/min. Chip evacuation is not controlled by pressure alone but by a dynamic balance of chip formation, tool design, and cutting parameters; exceeding the window fragments chips into micro-debris that packs between the flank and wall, doubling roughness and shortening tool life.

An operating mode in which the workpiece rotates in the opposite direction of the rotating tool. The opposing surface speeds cancel radial force components, improving straightness and concentricity. Common for hard materials and very deep holes; reference installations use it to hold landing-gear bores within 0.05 mm/m.

Enlarging an existing hole to a larger inside diameter using a counter boring tool on a BTA-equipped machine. May pilot off the finished bore for straightness or off the pre-bore for concentricity; pull counterboring (pull boring) achieves the straightest result.

A helical rather than straight V-flute ground on a gundrill shank. Because the off-axis center of gravity of a straight-fluted shank causes centrifugal whipping at speed, a convoluted flute profile mitigates vibration and improves straightness.

A broad term for drilling holes with a high depth-to-diameter ratio, typically 10:1 or greater. Per VDI Standard 3210 the drilling depth is usually greater than three times the diameter, with L/D ratios up to 100 and in special cases far beyond. Gundrilling and BTA drilling are the two main families.

The extension of deep hole drilling that creates machined features inside deep holes — counterboring, bottom forming, and bottle boring — using CNC-controlled extendable cutting tools on BTA-equipped machines.

The ratio of hole depth to hole diameter. Greater than 10:1 is generally considered deep hole drilling; greater than 100:1 is very deep. Gundrilling reaches up to about 400:1 in favorable materials, and exceptional cases reach 900:1.

Ejector drilling using two concentric tubes. Coolant flows between the outer and inner tubes to the drill head; part is diverted through Venturi slots, creating suction that returns coolant and chips through the inner tube. No pressure head and no face seal are required, so it suits conventional lathes and machining centers. Diameter range roughly 18–250 mm.

A precision bushing at the hole entry that contacts the workpiece, guiding the tool to the correct start location and start diameter. In single-tube (STS) systems it also seals against the workpiece to contain high-pressure coolant during drilling.

The ground, cylindrical tubular bar of the BTA / STS system that carries the drill head (screwed on) and evacuates chips through its inside diameter. It is smaller in diameter than the drill head, creating the annular oil room; because chip flow is internal, the tube has a fully round, rigid cross-section.

The enlarged rear cylindrical section of a gundrill that mounts to the machine spindle. It has an undercut or flat for a set screw and a concentric through-hole so coolant can pass from the machine into the shank and tip.

The double-tube deep hole method developed by Sandvik Coromant in the 1960s. Concentric inner and outer tubes deliver coolant to the drill head; a ring nozzle creates negative pressure (ejector effect) that draws coolant and chips back through the inner tube. Operates at lower coolant pressure than BTA (as low as 10–50 bar) and needs no tight workpiece seal, making it ideal for retrofitting existing machines.

Chip evacuation along the outside of the tool rather than through it. Characteristic of gundrilling, where chips and coolant travel back along the external V-shaped flute of the shank.

Coolant delivery around the outside of the tool. Characteristic of BTA / single-tube drilling, where high-pressure coolant travels down the annular space between the hole wall and the drill tube before reaching the cutting edges.

A gundrill tip configuration in which distinct geometric features are ground to create the cutting edge, chip clearance, and coolant relief. Distinct facets make resharpening straightforward and repeatable.

Carbide or PCBN strips mounted on the tool body that bear against the bore wall to pilot (self-guide) the tool and burnish the bore surface as it passes. The pads are the reason deep hole tools are “self-piloting,” and they are critical to hole straightness and surface quality.

The axial distance the guide pad must lag behind the outer cutting edge so it does not cut ahead of the edge. Lag is typically 0.5–1.2 mm, or about 2–4× the feed rate; too little lag causes unstable cutting.

A single-effective-edge, self-piloting deep hole drill with internal coolant supply and external chip removal along a V-shaped flute. It was first developed for making gun barrels — hence the name — and remains the most common deep hole tool, typically covering 0.5–50 mm diameter with L/D up to 400:1. The carbide tip is brazed to a tubular shank with a driver at the rear.

A gun-type tool variant with internal chip removal, as opposed to the standard gundrill’s external chip removal. Both share the single-lip, self-piloting geometry; they differ in which channel carries the chips back.

The two principal cutting edges of a gundrill tip. The inner cutting edge starts nearest the drill center; the outer cutting edge is the peripheral one. Their intersection is the drill point, whose radial offset (eccentricity, typically e = d/4) balances the cutting forces so the resultant force is directed onto the guide pad.

Chip evacuation through the hollow interior of the tool, characteristic of BTA / single-tube and ejector (inner-tube) drilling. Because chips never contact the bore wall, surface finish stays high and the tool cross-section can be fully round and rigid.

Coolant delivery through an internal channel in the tool. Characteristic of gundrilling (coolant exits a tip orifice and flushes chips back along the flute) and of ejector drilling (coolant flows between the inner and outer tubes).

The flank/clearance angles behind the cutting edges that prevent the tool from rubbing. Typical gundrill values: outer cutting edge primary relief 8–15° (lower for hard steels, 15° for aluminum/magnesium), secondary relief 15–25°, inner cutting edge clearance 10–15°, and a cylindrical secondary-edge clearance of about 8° with a 0.4–0.6 mm margin width.

The narrow cylindrical land at the outer diameter of a drill that maintains hole size and acts as a bearing surface. On long deep hole drills, land margins are located only at the very ends of the flutes to provide clearance and reduce drag against the bore wall.

The coolant outlet on the gundrill tip where pressurized fluid exits into the cutting zone. Common shapes are a single circular hole, a half-moon (crescent or kidney) hole, or double circular holes; the oil hole is typically about one-quarter of the tip diameter.

A drilling cycle with periodic retraction of the tool to break and evacuate chips. In deep hole drilling, full-retraction cycles (G83) are preferred over partial-retraction cycles (G73) because only a full retraction reliably clears the flute of chips.

A short pre-drilled hole that guides the long deep hole tool at entry. Best practice drills the pilot 2–5× its own diameter deep, at the same diameter as the deep hole drill, acting “almost like a bushing” to keep the long drill straight and prevent it from walking or breaking on an uneven face.

The intersection of the inner and outer cutting edges at the end of the gundrill tip. Its radial offset from the drill axis (eccentricity, typically set to about one-quarter of the diameter) balances cutting forces for stable, straight drilling.

A finishing operation that enlarges an existing hole by pulling the tool back through the workpiece on a tensioned boring bar, instead of pushing it in. Pulling the tool in tension yields exceptional straightness and uniform wall thickness; it is often used as a final sizing operation.

A reaming tool is rotated and pulled through a drilled hole to improve the straightness, size, and surface finish of long holes. Useful when a boring bar cannot be pushed through the full length without deflection.

The BTA / single-tube machine component (also called BOZA, the German drilling-oil-pressure supply unit) that introduces high-pressure coolant into the annular space between the bore wall and the drill tube, locates the drill guide bushing at the hole entry, and seals the coolant against the workpiece with a conical rotating holder. Rotating pressure heads support cylindrical workpieces; the rear is sealed by a stuffing box that also guides the tube.

The angle of the tool’s cutting face relative to the workpiece. Gundrill inner and outer cutting edges are typically ground at 0° rake to simplify manufacturing and resharpening; rake geometry strongly influences chip curling and breaking.

A finishing operation that slightly enlarges an existing hole to tighten diameter accuracy and improve surface finish, typically achieving Ra 0.10–0.80 μm and IT9–IT7. It is not a material-removal process: the hole must be pre-drilled 2–4% under size, and reaming cannot correct positional errors of the hole axis.

Re-sharpening a used drill by grinding the cutting face. Properly resharpened gundrills can perform like new up to 8–10 times, typically after 500–1,000 in of drilling; signs that a gundrill needs sharpening are higher thrust and torque, more run-out and drift, irregular chips, and coolant pressure spikes.

A device that transfers coolant from stationary piping to a rotating tool or spindle without leakage. In gundrilling it is mounted at the rear of the spindle, and it must handle high pressures (up to ~170 bar) and high spindle speeds.

The measure of a hole’s (or tool’s) deviation from perfect straightness along its length. The industry-standard straightness guarantee for deep holes is less than 0.001 in of run-out per inch of depth, given good material, equipment, and setup.

The property by which deep hole tools guide themselves against the bore wall via guide pads, requiring no external guidance or bushing once engaged. The three deep hole families (gundrill, BTA, ejector) are all self-piloting tools (SPTs), distinguished mainly by their coolant and chip-flow arrangement.

BTA-style deep hole drilling in which one drill tube serves as the chip evacuation channel. Coolant is supplied externally between the bore wall and the tube; chips exit through the tube interior. Requires a dedicated machine with a pressure head and a sealing system against the workpiece.

A combined two-operation, one-pass finishing process: carbide skiving blades true the bore round and remove stock, then roller burnishing pads cold-work the surface to a mirror-like finish (Ra < 0.2–0.8 μm). Roughly 40–50× faster than honing, it is standard for hydraulic cylinder tubes and adds a compressive residual stress layer that improves fatigue life.

Creating a shallow, square pilot exactly where the hole must begin so the long drill does not “walk” on a curved or uneven face. Spot facing is used to bell-mouth an off-square entry — an off-square start is the number-one cause of drift in long bores.

The entry bushing that guides the deep hole tool at the start of the cut and centers the hole. In ejector (DTS) drilling only a guide bush is needed because the pressure head and face seal are eliminated; in BTA the bushing is integrated into the pressure head.

How true and non-deviating the drilled hole’s centerline is along its length. The standard guarantee is less than 0.001 in run-out per inch of depth; deviation grows with depth unless counter-rotation, steady rests, and whip guides are used to hold the tool true.

A gundrill tip configuration in which the geometric features are blended (swept) to create chip clearance and coolant relief, generated with a cam fixture rather than distinct flat facets.

High-pressure coolant delivered through the machine spindle (roughly 350–1,000+ psi) so it reaches the tool tip at depth. Flood coolant fails in deep holes because less and less fluid reaches the tip as the hole deepens, causing dry cutting, chip impaction, overheating, and work hardening of the part.

Annular cutting that removes a ring of material while preserving the solid center core. It enables larger holes at the same relative power and leaves a usable core slug, which is valuable when the core is expensive material or can be reused for a smaller part. L/D is limited and blind holes are difficult.

The V-shaped external groove machined along a gundrill’s shank and tip that carries chips and coolant back out of the hole. The flute profile angle is typically 100–130° (commonly 110–120°), and the flute may be straight or helical; its area is roughly 22–26% of the hole cross-section.

The German engineers’ association (VDI) series of deep hole drilling standards. VDI 3208 covers deep hole drilling with gun drills (Einlippenbohrer, single-lip drills), including cutting values and coolant concepts; VDI 3209 covers BTA and ejector systems; VDI 3210 is the overview standard defining deep hole drilling; VDI 3211 covers deep hole drilling on machining centers.

Negative pressure created when part of the coolant is diverted through angled slots or nozzles into the inner tube, accelerating as it passes the restricted cross-section. The resulting suction draws coolant and chips back through the inner tube. In ejector drilling about 60–70% of the coolant produces this suction while the rest cools and lubricates the drill head.

A support device placed along the drill tube between the machine headstock and the workpiece to prevent the slender tube from whipping or oscillating during drilling. A whip guide adapter with a small replaceable plastic insert supports gundrills and small BTA tubes, reducing vibration and improving hole accuracy.

Vibration of a long, slender drill tube caused by centrifugal force acting on its off-axis center of gravity (inherent to a straight-fluted gundrill shank). It is countered with whip guides along the tube path and with convoluted/helical flute designs.

The nomenclature term used on gun drill diagrams for the guide pad area — the load-bearing surface that slides against the bore wall and gradually wears as it guides and burnishes the hole.

The hardening of the workpiece surface at the cutting zone. When coolant cannot reach the tool tip at depth (or feed is too low and the edge rubs), work-hardening materials such as stainless, titanium, and nickel alloys become progressively harder to cut and can fail the tool; high-pressure through-tool coolant and adequate chip load prevent it.