Gun Drill Handling Damage: Prevention and Inspection

Gun drills are precision tools that are surprisingly easy to damage. The long, slender shank and the exposed carbide tip are vulnerable to handling damage. I’ve seen good drills ruined by careless handling.

Here’s how to prevent damage and what to inspect for.

Common Damage Modes

Bent shank. The most common damage. A gun drill that’s dropped or handled roughly has a bent shank. Even a slight bend causes runout.

Chipped tip. The carbide tip is brittle. A drop on the floor or a knock against a metal surface chips the cutting edge.

Scratched shank. The shank surface is ground to a precise diameter. Scratches from rough handling create raised burrs that can damage the guide bushing.

Coolant hole blockage. Dirt or debris in the coolant hole blocks coolant flow.

Causes of Damage

Handling damage is usually caused by:

• Dropping. A gun drill on the floor is almost always damaged.
• Banging against machine components. Hitting the chuck or steady rest during loading.
• Improper storage. Drills stacked on top of each other.
• Transport. Drills rolling around in a drawer or toolbox.

Prevention

Preventing handling damage is straightforward:

• Always handle gun drills with both hands
• Use a drill holder or tray when moving drills between storage and machine
• Never stack drills on top of each other
• Store drills in dedicated racks
• Use protective caps on the tip

Inspection

I inspect each gun drill before use:

1. Visual check. Look for chips in the carbide tip, scratches on the shank, and signs of rust.
2. Roll check. Roll the drill on a flat surface. A bent drill doesn’t roll smoothly.
3. Runout check. Mount the drill in the machine and check the runout at the tip. Runout over 0.02mm indicates damage.
4. Coolant check. Blow air through the coolant hole to verify it’s clear.

Runout Limits

If the runout exceeds these limits, the drill is damaged and should not be used. A damaged drill produces oversized holes and may break during use.

What to Do with Damaged Drills

A damaged gun drill can sometimes be repaired:

• A chipped tip can be reground (if the chip is small)
• A scratched shank can be polished (if the scratch is shallow)
• A bent shank cannot be straightened reliably

I mark damaged drills and set them aside for inspection. A drill with a bent shank is scrapped.

Key Takeaways

Most gun drill damage is caused by careless handling. A few simple rules prevent most damage: handle with both hands, use a tray, and store properly.

I train new operators on gun drill handling as part of their onboarding. A single dropped drill costs more than the time spent on training.

Regrinding Strategy

When a gun drill tip is chipped but the shank is undamaged, regrinding can restore the cutting geometry. I send damaged drills to a specialist sharpening service that has the right equipment for gun drill geometries. A reground drill typically produces 60-80% the hole count of a new drill before needing another regrind.

I track reground drills separately from new drills in the tool inventory. A drill that has been reground twice is approaching the end of its useful life. The carbide tip thickness is reduced with each regrind, and the drill becomes more prone to breakage.

Drills with bent shanks cannot be reground. The bend causes runout that no amount of tip sharpening can fix. I scrap any drill that fails a roll check or shows runout above the limits listed earlier. For proper drill storage procedures, see How to Store Gun Drills Properly.

Key Takeaways

• Bent shanks are the most common damage mode; even a slight bend causes runout.
• Prevent damage by handling with both hands, using a tray, and never stacking drills.
• Inspect every drill before use: visual check, roll check, runout check, coolant check.
• Maximum allowable runout ranges from 0.01mm (under 3mm diameter) to 0.04mm (over 25mm).
• A chipped tip can be reground; a bent shank cannot be straightened reliably.
• Train operators on proper handling – it prevents more damage than any other measure.