Gundrilling Process

30-Second Summary: Gundrilling is the oldest deep hole machining method, originating from gun barrel manufacturing. It features a single cutting edge, V-shaped groove for external chip removal, and self-piloting action. Suitable for small diameters (0.5–50 mm), extremely high depth-to-diameter ratios (up to 300:1, with modern tools exceeding 400:1 in special cases), and precision holes requiring IT5–IT7 tolerances under optimal conditions.

How It Works

A gundrill consists of a carbide drill head (brazed or indexable), a V-shaped groove drill tube, and a shank. High-pressure coolant is injected through the central hole of the drill tube, exits through an outlet at the drill head bottom, and carries chips away along the V-shaped groove. The chip flows along the external groove between the tool and the hole wall — unlike BTA where chips pass internally.

• Self-piloting principle: The outer edge of the drill head's cutting edge has guide pads (support pads) that press against the hole wall during machining to provide guidance. The drill point is offset from the axis, generating a radial force that keeps the pads in contact with the bore surface.
• Single-lip cutting: Unlike conventional drills with two cutting edges, a gundrill has one cutting lip. This creates a balanced force couple between the cutting forces and the guide pad reaction forces, enabling straight hole generation.

Three Motion Methods

Method	Application	Straightness
Tool rotates + workpiece stationary	Irregular-shaped workpieces, eccentric holes, large non-rotating parts	~0.001 in/in (0.03 mm/m)
Workpiece rotates + tool stationary	Round bars, center holes in shaft parts (best for round stock)	~0.001 in/ft (0.08 mm/m)
Counter-rotation of tool and workpiece	High precision requirements, large diameters, when straightness is critical	Best straightness (0.0005 in/ft or better)

Counter-rotation minimizes the effects of rotational runout and cutting speed variation, producing the finest straightness and surface finish. It also reduces the effective speed differential for large-diameter work.

Pilot Hole and Guide Bushing Requirements

Gundrills are not self-starting — they always require either a pilot hole (on CNC machines) or a guide bushing (on dedicated gundrilling machines). Proper preparation is essential for hole straightness and tool life.

Parameter	Specification
Pilot hole diameter	Drill diameter +0.02 mm (+0.0005" to +0.001")
Pilot hole depth	1.5–3.0 × D (minimum 0.5 × D)
Pilot drill tolerance	m7 fit
Pilot hole bottom	Flat bottom preferred (conical for difficult materials like Inconel 718)
Guide bushing clearance	Drill diameter +0.02 mm (max 20 μm clearance)
Guide bushing standard	DIN 179A (medium series)
Bushing material	Hardened steel; carbide-lined or solid carbide for high production

Critical: Too large a clearance between tool and bushing/pilot hole causes the "walking phenomenon" and bell-mouth formation. Too tight a fit can over-constrain the drill and cause catastrophic breakage at entry.

Cutting Parameter Reference

Material	Vc (m/min)	Feed (mm/rev) D=5mm	Feed (mm/rev) D=10mm	Feed (mm/rev) D=20mm
Low-carbon steel (<900 N/mm²)	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 (>900 N/mm²)	60–70 (solid carbide) 70–80 (carbide-tipped)	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
Stainless steel (ferritic/martensitic)	40–80	0.010–0.020	0.020–0.035	0.050–0.075
Aluminum alloy	80–160	0.030–0.080	0.070–0.130	0.150–0.250
Copper alloys (brass/bronze)	80–140	0.020–0.060	0.050–0.100	0.100–0.200
Titanium alloys	15–30	0.006–0.015	0.010–0.025	0.030–0.060
High-temp alloys (Inconel, Hastelloy)	10–20	0.004–0.012	0.008–0.020	0.020–0.050
Cast iron (gray/ductile)	60–100	0.015–0.040	0.035–0.070	0.080–0.150

Start-of-Cut Procedure

1. Drill pilot hole (flat bottom, 1.5–3 × D deep, reamed to drill diameter +0.02 mm)
2. Feed gundrill to within ~1.5 mm of pilot hole bottom at ≤50 RPM with coolant OFF
3. Turn on coolant to full pressure, wait approximately 1 second
4. Increase speed to machining RPM during initial 0.5–1.0 mm of engagement
5. Ramp feed rate to 80% of target for the first 2–3 mm of cut, then full feed
6. Drill continuously to depth — no peck cycle required
7. Reduce speed to ≤50 RPM before retracting; maintain coolant flow during retraction

Whip Guide Supports

Support the drill tube every ~40 × D to prevent whipping and vibration. For a 10 mm drill, place supports every 400 mm. As L/D exceeds 100:1, closer spacing may be needed. Vibration dampers (bearingized type) are recommended for L/D > 200:1.

Modern Indexable Insert Gundrills

Recent advances include indexable insert gundrills with triangular inserts featuring chip-splitting geometry and wiper technology (e.g., Iscar GD-DH/GD-DHL, Tungaloy DeepTri-Drill). These offer:

• Up to 4× productivity improvement over traditional brazed carbide gundrills (75 m/min vs. 41 m/min, feed 0.06 vs. 0.04 mm/rev — recorded on AISI 4140)
• Replaceable inserts eliminate regrinding — simply index to a fresh cutting edge
• Wiper flat geometry improves surface finish to as-drilled Ra 0.4–0.8 μm
• Available in diameters from ~12 mm to 28 mm, with maximum lengths up to 2400 mm

Advantages and Limitations

Advantages: Highest hole quality among deep hole methods, relatively low equipment cost, regrindable solid carbide tools or indexable inserts, self-piloting eliminates need for guide bushings on short holes, suitable for small batches and multiple varieties, widest diameter range starting at 0.5 mm.

Limitations: Lower machining efficiency than BTA (typically 1/5 to 1/10 the feed rate), limited torque capacity (weak V-groove cross-section — the tube wall is thin), requires high-pressure coolant system (50–150+ bar), extremely small feeds for sub-3 mm diameters.

Coolant Specifications

• Recommended: Deep hole drilling oil (neat oil) for best lubrication and chip evacuation
• Acceptable: High-performance emulsion at limited degree
• Emerging: MQL (Minimum Quantity Lubrication) suitable at limited degree for certain materials
• Coolant filtration to 20–30 μm is critical — particles block the small coolant passage in the drill head