🕹 SINGLE-LIP · EXTERNAL CHIP REMOVAL

Gundrilling Process

Single cutting edge, V-shaped external groove for chip removal, internal coolant channel, and self-piloting guide pads. Originated for gun barrel manufacturing — now used across aerospace, automotive, medical, and mold & die for holes from 0.5mm to 50mm at extreme L/D ratios.

0.5–50mmDiameterOptimal 1–25mm

Up to 300:1L/D RatioUp to 400:1 extreme

50–150+ barCoolantHigh pressure required

IT5–IT7ToleranceBest precision of all methods

PRINCIPLE

How Gundrilling Works

💡 Self-piloting: The drill point is offset from the axis, generating a radial force that keeps the guide pads pressed against the bore wall. This force couple between the single cutting lip and the pads enables straight hole generation without external guidance. Guide pads also burnish the hole wall, reducing surface roughness by up to 70%.

Motion Method	Application	Straightness
Tool rotates, workpiece stationary	Irregular shapes, eccentric holes, large parts	~0.001 in/in (0.03 mm/m)
Workpiece rotates, tool stationary	Round bars, shaft center holes	~0.001 in/ft (0.08 mm/m)
Counter-rotation (both rotate opposite)	Highest precision, large diameters, critical straightness	~0.0005 in/ft (best)

SPECIFICATIONS

Technical Data

0.5–50

Diameter range

300:1

L/D

Max depth ratio

50–150+

bar

Coolant pressure

10–60

L/min

Coolant flow rate

Ra 0.4–6.3

μm

Surface finish

IT6–IT11

Tolerance

Optimal IT5–IT7

SETUP

Pilot Hole & Guide Bushing

Gundrills are NOT self-starting — they always require a pilot hole (CNC) or guide bushing (dedicated machine).

Parameter	Requirement
Pilot hole dia	Drill dia +0.02mm
Pilot hole depth	1.5–3.0 × D
Bushing clearance	Max 20 μm
Bushing standard	DIN 179A

⚠️ Critical: Too much clearance → drill walking and bell-mouth. Too tight → over-constrains drill, risk of catastrophic breakage at entry.

CUTTING DATA

Cutting Parameters

Material	Vc (m/min)	Feed D=5mm	Feed D=10mm	Feed D=20mm
Low-carbon steel	90–120	0.010–0.025	0.020–0.040	0.050–0.080
Alloy steel (annealed)	60–80	0.010–0.025	0.020–0.040	0.050–0.080
Alloy steel (hardened)	50–80	0.008–0.020	0.015–0.030	0.040–0.070
Stainless steel (austenitic)	30–60	0.008–0.018	0.020–0.030	0.050–0.070
Aluminum alloy	80–160	0.030–0.080	0.070–0.130	0.150–0.250
Titanium alloys	15–30	0.006–0.015	0.010–0.025	0.030–0.060
Superalloys (Inconel)	10–20	0.004–0.012	0.008–0.020	0.020–0.050
Cast iron	60–100	0.015–0.040	0.035–0.070	0.080–0.150

💡 Coolant: Use neat oil for best results. Filtration to 20–30μm is critical. MQL suitable for certain materials at limited capacity. Coolant pressure/flow per Allied Machine TG-SFC guidelines.

PROCEDURE

Start-of-Cut Sequence

Pilot hole

Drill flat-bottom pilot, 1.5–3×D deep, reamed to drill dia +0.02mm

Position tool

Feed gundrill to within ~1.5mm of pilot hole bottom at ≤50 RPM, coolant OFF

Coolant ON

Turn on coolant to full pressure, wait ~1 second for stabilization

Ramp speed

Increase to machining RPM during initial 0.5–1.0mm of engagement

Full cut

Ramp feed to 80% target for first 2–3mm, then full feed. Drill continuously — no peck cycle

Retract

Reduce speed to ≤50 RPM before retracting. Maintain coolant flow during retraction

PROS & CONS

Advantages & Limitations

✅ Advantages

Highest hole quality among deep hole methods
Widest diameter range (from 0.5mm)
Regrindable or indexable tooling options
Self-piloting — no bushings for short holes
Excellent as-drilled surface finish

⚠ Limitations

Feed rate 1/5 to 1/10 of BTA
Limited torque — thin V-groove tube wall
Requires 50–150+ bar coolant system
Not self-starting — pilot hole required
Coolant filtration to 20μm mandatory

APPLICATIONS

Where Gundrilling Is Used

🎯 Firearms BarrelsThe original application. Ø5–20mm, L/D 50:1–100:1+, straightness ≤0.05mm/m

✈ AerospaceLanding gear, actuators, turbine shafts. High-strength alloys, Ra ≤0.8μm as-drilled

🚗 AutomotiveFuel injectors, crankshaft oil passages, transmission valve bodies. High volume, small diameters

🏯 Mold & DieCooling channels Ø6–16mm, L/D 20:1–50:1. Positioning accuracy critical for thermal performance

️‍⚖ MedicalBone screws, surgical instruments, dental implants. Small diameters, tight tolerances, Ti/stainless

📝 Oil & GasDownhole tool components. Inconel, stainless steel, straightness <0.1mm over full depth

ADVANCED TOOLING

Indexable Insert Gundrills

💡 botek Type 01 / Iscar GD-DH: Indexable insert gundrills offer approximately 2× the penetration rate of brazed carbide tools (TriumphTool, 2024). Recorded on AISI 4140: 75 m/min vs 41 m/min, feed 0.06 vs 0.04 mm/rev. Wiper flat geometry achieves as-drilled Ra 0.4–0.8μm. Inserts and guide pads index on-machine with no re-adjustment (setting within ±0.01 mm); botek’s 225S-91 universal inserts add productivity on steel, and Iscar GD-DH covers 9.9–43.99 mm to 2400 mm length.

SUPPORT

Whip Guide Requirements

Support the drill tube every ~40×D. For a 10mm drill: supports every 400mm. For L/D > 200:1, use vibration dampers (bearingized type) from botek, UNISIG, or Precihole. Closer spacing required as L/D increases.

SAFETY

Key Safety Points

⚠️ Coolant pressure: 50–150+ bar — never disconnect fittings while pressurized. Relieve at pump before maintenance. Use whip-checks on all high-pressure hoses.

⚠️ Fine chip hazard: Gundrilling produces fine, sharp V-groove chips. Use dedicated hooks for chip removal — never pull chips by hand.

⚠️ RPM limit: Do not exceed 50 RPM unless the tool is engaged with the workpiece. Free-spinning a long gundrill at high RPM creates a dangerous whip condition.

Gundrilling Process

How Gundrilling Works

Technical Data

Pilot Hole & Guide Bushing

Cutting Parameters

Start-of-Cut Sequence

Advantages & Limitations

✅ Advantages

⚠ Limitations

Where Gundrilling Is Used

Indexable Insert Gundrills

Whip Guide Requirements

Key Safety Points

Keep Reading