Drill Bit Length Series Explained

What it is

A length series is a family of drills that share the same proportions of flute length and overall length for every diameter. It is the first thing a buyer needs to know, because two drills of the same diameter, material and coating are still not interchangeable if one is a jobber and the other is a screw machine length. In a catalog, the series is the category level. Tool material (HSS, cobalt M35/M42, carbide) and finish (bright, black oxide, TiN, TiAlN) are attributes, or filters, applied to drills inside each series.

How to identify

Look at the drill's proportions rather than its size. A short drill whose flutes take up most of a stubby body is screw machine length. A drill that looks "normal" in a hardware-store index is jobber length. A drill noticeably longer than a jobber of the same diameter, with a straight shank, is taper length. A drill sold by its overall length (6 in, 12 in and similar) with relatively short flutes on a long body is an aircraft extension drill. A large drill with a shank smaller than its cutting diameter is a Silver & Deming (reduced shank) drill. Package labels and catalog numbers almost always name the series — read them before measuring.

Series at a glance

Key points

• Choose the shortest series that reaches the required depth. Shorter drills are stiffer, wander less and break less often.
• Taper length and taper shank are different products. Taper shank drills have a Morse taper shank and are covered in a separate sheet.
• Within any series, filter by tool material (HSS, cobalt, carbide), point (118°, 135° split) and finish (bright, black oxide, TiN, TiAlN).
• Exact flute and overall lengths for each diameter come from ANSI B94.11M or the manufacturer's catalog — confirm them when a customer has a reach or clearance limit.
• Metric DIN series do not map one-for-one onto ANSI series; treat DIN equivalents as "closest," not identical.

Common mistakes

• Building a catalog tree by coating or material ("TiN drills," "cobalt drills") and burying the length series in item descriptions.
• Quoting a jobber when the customer's chuck-to-workpiece clearance only fits a mechanics or screw machine length.
• Sending a taper shank drill when the customer asked for "taper length."
• Replacing an aircraft extension drill by diameter only, without matching overall length.

Governing standards: ANSI B94.11M (twist drills); ISO 235, ISO 494; DIN 338, DIN 1897, DIN 340. Manufacturer catalogs for exact dimensions.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.