Most shops choose drill length by habit: grab the jobber drill; it's what's in the drawer. But drill length is one of the biggest levers you have over hole accuracy, tool life, and cycle time. The shortest drill that reaches the bottom of the hole is almost always the right drill—here is why and when to break that rule.
Why Length Matters More Than You Think
A twist drill is a slender, twisted beam being pushed hard against a surface. The longer that beam, the more it flexes—and flex is the root cause of most drilling problems:
- Wandering and oversize holes: A flexing drill walks off location at entry and cuts oversize, bell-mouthed holes
- Poor straightness: Deflection compounds with depth—a long drill that starts 0.002" off finishes much worse
- Vibration and chipping: A long, unsupported drill chatters, and carbide edges chip under vibration
- Breakage: Rigidity falls with the cube of length—doubling stickout makes a drill roughly eight times easier to deflect
Stub Length: The Production Workhorse
Stub (screw-machine length) drills are the shortest standard series—and the first choice for CNC production work.
Advantages: Maximum rigidity, best hole location, highest feed rates, longest tool life.
Rule of Thumb: If a stub drill reaches it, use the stub drill.
Jobber Length: The Versatile Standard
Jobber drills (roughly 9-10x diameter flute length) are the general-purpose standard—long enough for most through-holes in plate and bar work and short enough to survive everyday abuse.
Trade-off: Noticeably less rigid than a stub—expect more wander at entry. Spot drill first for location-critical holes.
Extra-Long: For When There Is No Choice
Extra-length and aircraft-extension drills exist for deep holes and reach-around situations—drilling through a web, inside a cavity, or past a fixture. They demand technique:
- Start the hole with a stub or spot drill so the long drill enters an existing, accurate location
- Reduce speed 25-50% versus standard recommendations—long drills amplify every vibration
- Peck to clear chips: Packed chips in a deep hole are the number-one killer of long drills
- Support the shank: Use the shortest stickout the job allows, even on a long-series drill
Material Changes the Equation
- Aluminum: Chips are the challenge, not hardness—polished flutes and aggressive helix evacuate the long, soft chips; peck less and feed more
- Steel: Standard 118°-135° split-point carbide or coated HSS; watch chip color for heat
- Stainless: Firm, constant feed—let a stainless drill dwell, and the work-hardened layer will destroy it. Rigidity (short drills) matters double here
- Cast iron: Abrasive but forgiving—wear-resistant coatings extend life dramatically
The Selection Checklist
- Measure the true required depth—including fixture and clamp clearance—and pick the shortest series that reaches
- Spot drill (or use a stub pilot) whenever position tolerance is tighter than ±0.005"
- Match point geometry to material: split points resist walking, 135° for hard materials
- Use through-coolant drills for anything past 4x diameter in production
- Track drill life by holes drilled—drills fail by accumulated wear, and the failure usually takes the hole with it
Conclusion
Drill selection is rigidity management. Choose the shortest drill that does the job; add technique (spotting, pecking, and correct parameters) as length grows; and match geometry and coating to the material. The reward is straighter holes, tighter positions, and drills that last until you retire them on schedule—not drills that retire themselves in the middle of a part.
Keep Learning
Continue with these related articles from the FM Carbide engineering team:
- Selecting the Right End Mill for Your Material
- Troubleshooting Guide: Common Machining Issues
- Carbide End Mill Feeds and Speeds Explained
Need help choosing the right drill? Browse our carbide tooling catalog or talk to our engineering team — FM Carbide helps you machine better!