Gun drills
Gundrill coolant and chip paths, entry guidance, tool construction and support, with a bounded depth and feed-time example.
Tags: tooling, drilling, deep-holes, gundrilling
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A 6.00 mm hole to 300 mm cylindrical depth is 50:1. With a 6 mm pilot already cut, 294 mm remains. At 3,000 relative rpm and 0.020 mm per relative revolution, 60 mm/min gives 4.90 min of ideal cutting. Check entry support, chip removal, coolant and excluded machine motions.
Process and chip path
A single-flute gun drill carries coolant internally and returns chips through an external V-flute. Botek distinguishes bushing-guided and pilot-hole entry. Botek, Gundrilling process
Gundrilling and BTA drilling have different fluid paths. UNISIG describes gundrilling as internal coolant delivery with chip return along the outside flute; in its BTA description, coolant travels outside the tool and chips return through the drill tube. Trepanning is a further distinction because it leaves a central core. These are process differences, rather than interchangeable names for every long hole. UNISIG, Oil and Gas deep-hole tooling overview.
Tool construction and guidance
The cutting diameter, cutting length, overall tool length and holding interface describe different constraints. Botek's Type 110 offering is explicitly a single-flute design with a brazed drill head; its ordering grid separates diameter from overall length. The listed overall length is not automatically usable hole depth after allowances for the driver, support and machine arrangement. Botek, Type 110 single-flute gundrills.
Star Cutter's single-flute description identifies a cutting edge and guide pads, internal coolant, and a starting bushing or pilot hole. Once supported in the hole, that tool is self-piloting. The relationship between pilot/bushing size and tool size remains consequential. Its solid-carbide version has a one-piece carbide head and body, potentially joined to a steel shank, and still requires guided entry. Thus, a stiffer construction does not eliminate the entry-guidance requirement. Star-SU, Deep Hole Drills, single-flute and solid-carbide sections.
The same page also describes double-crimp, two-flute and four-flute variants. Consequently, “gundrill” does not establish flute count, coolant-hole arrangement or workpiece-material suitability without a named tool family. Claims about one construction's productivity or unsupported length cannot simply be transferred to another. Star-SU, named deep-hole tool variants.
Support and coolant form a complete system
UNISIG's dedicated-machine overview describes rotation of the tool, workpiece or both, plus axial feed. A starting bushing guides entry and helps contain coolant. External whip guides stabilize long tools outside the workpiece; a chip box maintains alignment and directs returning chips and fluid. Its coolant system includes pumping, filtration and temperature management. Its continuous chip evacuation description explains how gundrilling can avoid repeated peck withdrawals in the described arrangement; it does not establish that any long tool on any machine can use uninterrupted feed. UNISIG, Gundrilling Machine Overview.
Explanatory synthesis: pressure and flow are different quantities. A pressure specification alone does not establish delivered coolant volume at the cutting edge, while a nominal pump flow does not establish pressure after losses through the tool and connections. Filtration and fluid temperature also describe different system properties. The tool's coolant passage and return space must function with the surrounding machine system.
A depth-to-diameter ratio describes the hole, while unsupported tool length describes a different mechanical span. A 300 mm hole can progressively support an entering drill even though a substantial external length initially remains. Entry guidance, external support and guidance inside the bore therefore answer different physical questions.
Worked example: depth, removal volume and feed time
Use the hypothetical 6.00 mm diameter and 300 mm cylindrical depth, with a 6 mm-deep pilot of equal diameter. Exclude point shapes, breakthrough and finish stock.
Final depth-to-diameter ratio = 300 / 6 = 50:1
Remaining new cutting distance = 300 − 6 = 294 mm
Newly removed volume = π × 6² / 4 × 294
= 8,312.65 mm³ ≈ 8.313 cm³
The 50:1 ratio is neither an allowable unsupported length nor a guaranteed capability. The volume excludes the material already removed for the pilot.
For a purely illustrative relative rotational speed of 3,000 revolutions/minute and axial feed of 0.020 mm per relative revolution:
Axial feed speed = 3,000 × 0.020 = 60 mm/min
Ideal cutting time = 294 / 60 = 4.90 min
The rotational speed here is relative tool-to-workpiece motion; counter-rotation can make it differ from either spindle's individual speed. The arithmetic excludes pilot preparation, tool entry, acceleration, withdrawal, handling and any interruptions. It therefore does not provide complete cycle time.
If a further hypothetical constant coolant flow of 8 L/min reached the process throughout those 4.90 minutes, 39.2 L would pass through. That is throughput, not purchased or consumed coolant: a recirculating system can reuse fluid. Neither the assumed flow nor removed solid volume predicts chip packing, coolant losses, required pressure or reservoir capacity.
Finished-hole and source limits
Diameter, roundness, straightness, position and surface texture are separate outcomes. A tool's nominal cutting diameter does not establish all of them. Tool geometry, alignment, support, chip control, wear and workpiece conditions influence the result; intersecting holes, an irregular entry or an angled exit introduce additional application constraints.
The live primary pages reviewed on November 2, 2026 PST display no separately verified technical issue dates. This reference establishes process distinctions and explicitly assumed arithmetic. It adopts no universal pilot clearance, coolant pressure, concentration, cutting speed, feed, achievable tolerance or tool-life guarantee.