How to Choose and Order Carbide Gun Drill Bits
A gun drill is a precision tool with a narrow window: the wrong geometry, coating, or shank fails fast no matter how good the carbide. Buying one is a specification decision, not a price decision. This guide walks through the selection in order — tip type, diameter, geometry, coating, shank, pads — and ends with the spec sheet to hand a supplier and the checks to make when it arrives.
Step 1: Choose the Tip Type
| Type | Best for | Notes |
|---|---|---|
| Brazed tip | General production, repairable tooling | Carbide tip brazed to a steel shank; regrind-friendly and economical per hole |
| Solid carbide | Small diameters, precision work | One piece of carbide; best stiffness and accuracy, replaced rather than often reground |
| Indexable insert | Large diameters, high-volume | Replaceable insert, no regrind; common in multi-spindle production |
For the full detail, see gun drill geometry and tool types.
Step 2: Diameter and Tolerance
- Order the drill at the finished hole size you need — a gun drill produces its own diameter.
- The drill’s diameter tolerance is usually specified in micrometers over nominal (e.g., +0/-0.005 mm), and the hole follows the drill closely.
- If the part needs H7 or better, confirm the drill’s grinding tolerance with the supplier and plan for the tolerance the tolerance guide describes.
Step 3: Nose Grind Geometry
The nose grind is the chip-breaking and edge-relief geometry ground into the tip. Different grinds suit different materials:
| Condition | Typical grind |
|---|---|
| General steels, cast iron | N-8 or R1 relief |
| Hard / prehardened steels | Reinforced edge, facet grind |
| Stringy, ductile materials | Facet or R2 for aggressive chip breaking |
Get the grind wrong for the material and you get stringy chips, packed flutes, and short tool life. Nose grinds are covered in gun drill geometry; the material-specific picks appear in gun drilling by material.
Step 4: Coating
Coatings extend edge life and control chip flow:
| Material class | Typical coating |
|---|---|
| Alloy and tool steels | TiAlN or AlTiN (nano for hardened) |
| Stainless, stringy alloys | AlTiN, sometimes DLC |
| Non-ferrous, composites | Uncoated or DLC |
| Extremely abrasive | CVD diamond (special) |
For the full selection logic, see gun drill coatings and cutting tool materials.
Step 5: Shank, Flute, and Coolant
- Shank. Must match your collet, bushing, or machine holder — straight, fluted, or threaded shanks are common. Confirm the shank diameter and length against your machine’s tooling.
- Flute. Single-flute is standard; double-flute (two V-grooves) improves chip evacuation at high feed and is common at larger diameters.
- Coolant. One or more coolant holes in the tip; the holes and the coolant pressure rating must match your system (see coolant systems). Hole size scales with drill diameter.
Step 6: Guide Pads
Guide pads size the hole and keep the drill on-axis. Confirm the pad width and position for your diameter, and the pad material (carbide, sometimes with a chip-breaking edge). Pads wear, so the regrinding guide applies to pads as much as to the tip.
The Order Spec Sheet
Hand the supplier a spec sheet, not a description:
Gun drill order
Diameter: 12.00 +0/-0.005 mm
Overall length: 500 mm | Flute length: 380 mm
Shank: straight, Ø12.7 mm, 40 mm long
Tip type: brazed | Nose grind: N-8
Coating: TiAlN
Coolant holes: 2, pressure-rated 100 bar
Guide pads: 2, carbide, chipbreaker edge
Rotation: right-hand
Workpiece: 4140HT, 28-35 HRC
Machine: dedicated gun drilling machine
What to Verify on Delivery
- Diameter — micrometer the drill; confirm it matches the order within tolerance.
- Nose grind — check the grind matches the order (a loupe or scope); the wrong grind is the most common silent mismatch.
- Coolant holes — confirm they are clear and correctly located.
- Shank fit — the shank must seat cleanly in the collet or bushing without play.
- Marking/traceability — an ID or serial marking helps track regrind count and life. See tool lifecycle management.
How Many Drills to Order
Order spares around your expected tool life and regrind cycle. A brazed-tip drill can be reground many times; solid carbide less so; inserts are replaced outright. Budget for the regrind count rather than assuming one drill per job — the regrinding guide gives the break-even logic.
FAQ
How do I spec a gun drill? Diameter, length, shank, tip type, nose grind, coating, coolant holes, guide pads, rotation, and the workpiece material/condition. Use the order spec sheet above.
Brazed tip vs solid carbide vs insert — which should I buy? Brazed tip for repairable general production, solid carbide for small-diameter precision, indexable insert for large diameters and high volume. The geometry page compares them in detail.
What coating should I get for hardened steel? AlTiN nano or TiAlN on fine-grain carbide, with a reinforced edge grind. See the coatings guide for the full material map.
Single-flute or double-flute? Single-flute is the standard for gun drilling; double-flute improves chip evacuation at high feed and larger diameters.
How do I know the drill I received is right? Mic the diameter, check the nose grind, confirm coolant holes and shank fit, and verify the ID marking before it goes into a machine.
Summary
Buying a gun drill is a specification exercise. Choose the tip type, confirm the diameter tolerance, select the nose grind and coating for your material, match the shank to the machine and the coolant holes to your pressure, and specify the guide pads. Hand the supplier a complete spec sheet and verify the delivery — diameter, grind, coolant holes, shank — before it touches a spindle. The spec that takes five minutes to write saves the machine time and scrap that a wrong drill costs.
For the fundamentals, see gun drill geometry and tool types and gun drill coatings. For setup with the new tool, see pilot holes and guide bushings.