Gun Drill Manufacturing Process: From Raw Material to Finished Tool

Gun Drill Manufacturing Process

A gun drill is a precision cutting tool — its performance depends not only on correct selection and use, but on how it was manufactured. The manufacturing process for a quality gun drill involves many steps, each requiring tight tolerances and specialized equipment.

This guide covers the complete manufacturing process from raw materials to the finished product.

Raw Materials

Carbide Tips

Grade	Grain Size	Cobalt Content (%)	Hardness (HRA)	Application
Standard (K20)	1.0–1.5 µm	6–8	90.5	General-purpose steel, cast iron
Fine-grain (K10-K15)	0.8–1.0 µm	5–6	91.5	Stainless steel, alloy steel
Micrograin (K05-K10)	0.5–0.8 µm	4–5	92.0–93.0	Titanium, superalloys, precision
Ultra-fine (0.2–0.5 µm)	0.2–0.5 µm	3–4	93.5+	Hardened steel, high precision

Steel Shanks

Shank Type	Material	Hardness	Application
Standard	AISI 4140 / 4142	280–320 HB	Most gun drills up to 40 mm dia
Heavy-duty	AISI 4340	320–360 HB	High-torque applications
Stainless	17-4PH	350–400 HB	Corrosive environments, medical
High-speed	M2 / M35 HSS	62–65 HRC	Brazed tips on HSS shanks

Manufacturing Steps

Step 1: Carbide Powder Processing

Tungsten carbide powder (WC) + Cobalt powder (Co) + Binders
  → Ball milling (24–72 hours for uniform mixing)
    → Spray drying (granulation to free-flowing powder)
      → CIP (Cold Isostatic Pressing) or die pressing
        → Pre-sintering (debinding at 400–600°C)
          → Sintering (1400–1500°C in vacuum or HIP)
            → Hot Isostatic Pressing (optional — for premium grades)
              → Finished carbide blank

Sintering parameters:

Parameter	Typical Value	Effect
Sintering temperature	1,400–1,500°C	Higher temp = larger grain size, lower hardness
Sintering time	30–90 min	Longer time = more complete densification
Atmosphere	Vacuum or Ar/H₂	Prevents oxidation during sintering
HIP pressure	100–200 MPa	Removes residual porosity

Step 2: Shank Machining

Operation	Equipment	Tolerance
Centerless grinding (OD)	Centerless grinder	±0.01 mm
Coolant hole drilling	Gun drill (ironically) or gundrill-style machine	±0.05 mm position
Flute milling	CNC flute milling machine or specialized gun drill flute mill	±0.05 mm depth
Shank end preparation	CNC lathe	±0.02 mm

Step 3: Coolant Hole Drilling

This is a critical step — the gun drill must have a precisely positioned internal coolant hole:

Drill Diameter	Coolant Hole Diameter	Position (from center)
3–6 mm	0.5–1.0 mm	0.2–0.4 mm offset
6–15 mm	1.0–2.5 mm	0.5–1.0 mm offset
15–30 mm	2.5–5.0 mm	1.0–2.0 mm offset
30–50 mm	5.0–8.0 mm	2.0–4.0 mm offset

The coolant hole is positioned off-center — closer to the cutting edge tip — so that coolant exits directly at the cutting zone.

Step 4: Flute Milling

The V-shaped flute is the chip evacuation channel:

Flute Parameter	Typical Value
Flute depth	15–25% of shank diameter
Flute width	40–60% of circumference
Flute surface finish	Ra 0.4–0.8 µm (polished)
Flute helix angle	0° (straight flute for gun drills)

Flute polishing is critical — a rough flute surface increases friction for chip evacuation, leading to packing.

Step 5: Brazing

Brazing attaches the carbide tip to the steel shank:

Brazing Parameter	Typical Value
Brazing alloy	Silver-based (Ag-Cu-Zn-Cd) or copper-based (Cu-Zn)
Brazing temperature	650–750°C (silver), 900–1,000°C (copper)
Heating method	Induction (preferred), torch, or furnace
Joint gap	0.05–0.15 mm
Cooling	Controlled cooling to prevent cracking

Common brazing defects:

Defect	Cause	Prevention
Cracking at braze joint	Too-rapid cooling, wrong alloy	Controlled cooling, correct alloy selection
Void in braze joint	Insufficient alloy or heating	Adequate alloy amount, even heating
Tip misalignment	Movement during brazing	Fixturing accuracy during heating
Carbide cracking	Thermal shock	Preheating, controlled cooling rate

Step 6: Tip Grinding

This is the most precision-critical step:

Operation	Equipment	Tolerance
Facing (nose grind)	5-axis CNC tool grinder (Christen, Ewag, Walter)	±0.005 mm lip height
Primary relief	CNC tool grinder	±0.5°
Secondary relief	CNC tool grinder	±0.5°
Outer corner radius	CNC tool grinder	±0.01 mm

The most critical measurement: lip height — the height difference between the cutting edge and the tool centerline.

Lip Height Tolerance	Effect on Hole Quality
±0.005 mm	Excellent — consistent diameter, straightness
±0.010 mm	Good — acceptable for most applications
±0.015 mm	Marginal — hole oversize 0.02–0.05 mm
> ±0.020 mm	Unacceptable — drill wanders, poor quality

Step 7: Coating Application

Coating	Process	Thickness (µm)	Deposition Temperature
TiN	PVD (arc evaporation)	2–4	450–500°C
TiAlN	PVD (sputtering or arc)	2–5	500–550°C
AlTiN	PVD (nano-layer)	2–4	500–550°C
DLC	PACVD	1–3	150–300°C

Step 8: Quality Inspection

Inspection	Method	Acceptance
Lip height	Optical comparator	±0.005 mm
Cutting diameter	Micrometer	±0.01 mm
Shank diameter	Micrometer	±0.01 mm
Nose grind angles	Optical comparator or tool presetter	±1°
Relief angles	Optical comparator	±0.5°
Flute surface finish	Profilometer	Ra < 0.8 µm
Brazed joint quality	Visual (10× magnification)	No voids, cracks, or misalignment
Runout	Dial indicator at tip	< 0.01 mm TIR

Manufacturing Tolerances Summary

Feature	Standard Quality	Premium Quality
Diameter	±0.015 mm	±0.005 mm
Lip height	±0.010 mm	±0.005 mm
Nose grind angle	±1.5°	±0.5°
Relief angle	±1.0°	±0.5°
Shank runout	< 0.015 mm	< 0.008 mm
Flute surface	Ra < 1.6 µm	Ra < 0.4 µm (polished)

Major Gun Drill Manufacturers

Manufacturer	Location	Specialty	Offerings
Guhring	Germany	Complete range	EB 100, EB 80, EB 800 series; regrind services
Hartner	Germany	Precision small diameters	E 100, E 800 series; custom geometries
Star SU	USA	Large diameters, custom	Brazed, solid carbide, indexable
Botek	Germany	Premium quality	Precision gun drills for difficult materials
UNISIG	USA/ Switzerland	Deep hole drilling systems	Manufactures both tools and machines

Summary

Gun drill manufacturing involves eight main steps — carbide powder processing, shank machining, coolant hole drilling, flute milling, brazing, tip grinding, coating, and inspection. The most critical quality parameter is lip height accuracy (required: ±0.005 mm for premium tools). Brazing quality and flute surface finish are the next most important factors affecting tool performance. Properly manufactured gun drills can be reground 5–7 times (solid carbide) or 3–5 times (brazed tip) before replacement. For selection and application guidance, see gun drill geometry and tool types. For regrinding, see gun drill regrinding best practices.