The Problem: Holes That Measure Over Nominal

In precision metalworking, a 10.00 mm hole that comes out 10.15 mm is scrap. Oversized holes ruin press fits, loosen fasteners and force rework. Yet the drill bit is often blamed when the real cause sits in the setup, the sharpening or the machine.

Drilling is a roughing process by nature, but with the right controls it holds size reliably enough for most tolerance bands. The drill is only one part of a system: the holder, spindle, feed and sharpening all shape the final diameter.

Why It Happens: Runout, Symmetry and Rigidity

Every source of oversize can be traced to a mismatch between where the bit points and where the material actually is. When those two agree, the hole comes out on size; when they do not, the bit cuts a larger circle than its nominal diameter.

• Spindle or chuck runout. If the bit orbits rather than rotates on its axis, it cuts a larger circle. Even a few hundredths of runout shows up in the hole.
• Unequal lips. After hand sharpening, one cutting lip longer than the other makes the bit drill off-centre and oversize.
• Excessive point angle or web thickness. A blunt or thick web pushes rather than cuts, and the bit wanders.
• Insufficient rigidity. A long, thin bit or a loose setup flexes and reams the hole open.
• Wrong feed and speed. Too light a feed lets the bit rub and polish, expanding the hole; too high a speed overheats and dulls the edges.

The Solution: Control the Variables That Add Size

• Measure runout first. A dial indicator on the shank exposes a bent bit or worn chuck before you blame the tooling.
• Use precision-ground bits. A fully ground DIN338 P6M5 M2 bit holds size far better than a roll-forged economy bit because its lips are symmetrical.
• Spot the hole first. A cobalt centre drill creates a true starting point so the twist bit does not wander.
• Keep the bit short and rigid. Use the shortest bit that reaches, and reduce the length of unsupported flute.
• Set a positive feed. A constant, slightly heavier feed keeps the cutting edge slicing rather than rubbing.

Application: When Size Really Matters

Bearing seats, dowel holes and tapped holes that must hold thread class all depend on hole size. Where a tight band is essential, a DIN340 M2 precision bit plus a rigid collet chuck and a spot drill will out-perform any hand-ground bit. For critical diameters, drill undersize and ream or bore to final size.

Technical Reference: Troubleshooting Hole Size

Typical HSS twist drilling holds IT9 to IT11 tolerance on a rigid machine; ream for IT7 or better.

Key Takeaways

Hole size is a system result, not a bit-only result. Check runout before anything else, regrind or replace bits with unequal lips, and keep the setup as rigid as the job allows. Where the tolerance is tight, treat drilling as a preparation step and finish with a reamer or boring head.

FAQ

Q: How much oversize is normal for HSS drilling?
A: On a rigid machine with a sharp, symmetric bit, 0.05 to 0.10 mm is common; more than that signals a setup problem.

Q: Can a bent bit be straightened?
A: No, replace it. A bent shank cannot be corrected reliably and will always cut oversize.

Q: Does coolant affect hole size?
A: Indirectly. Good cooling prevents thermal growth and edge wear that drift the size.

Q: Should I peck drill for accurate holes?
A: Yes, pecking clears chips and reduces the rub that enlarges deep holes.

Q: Does tool holder type really matter?
A: It does. A collet holder has far less runout than a worn three-jaw chuck, and less runout means less oversize.

Making HSS Drill Bits Repeatable

Predictable hole size is a property of how HSS drill bits are used as much as how they are made. Check runout, keep HSS drill bits sharp and symmetric, and hold the setup rigid so the cutting geometry does the work. Treated as precision tools rather than consumables, decent bits hold tolerances with ease.

Get Holes Back to Size

Precision starts with symmetric, fully ground geometry. Compare HSS Drill Bits and precision centre drills to tighten your hole tolerances.