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Extra-Long & Extended-Length Drills

Extra-long drills reach past taper length, and they come in different designs from aircraft extension drills. This sheet explains the long series, how they compare and how to drill deep holes with them without breaking the drill.

Ref CT-0009ANSI B94.11MDIN 1869

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Twist drill length series run from short to long: screw machine, mechanics, jobber, taper length, then extra-long and extension types. Extra-long or extended-length drills have long flutes that run most of the body, so chips can clear from deep holes. They are sold in inch series and in metric series such as DIN 1869, which defines several length groups. Aircraft extension drills are different. They have a normal jobber-length flute on a long plain shank. They reach into tight spots but are not for deep holes. See the aircraft extension drills sheet and the drill bit length series explained sheet.

How to identify

Look at how far the flutes run. On an extra-long drill, flutes cover most of the length. On an aircraft extension drill, flutes are short and the rest is round shank. Catalogs list overall length and flute length; compare both. Some extra-long drills have parabolic flutes for chip clearance. See parabolic flute and deep-hole drills.

Long drill comparison

Type	Flute length	Best for	Watch for
Taper length	Longer than jobber	Deeper holes in machine setups	Not the longest reach
Extra-long or extended length	Most of the long body	Deep holes where chips must travel far	Flex and wander; peck often
DIN 1869 series	Long; several length groups	Metric deep holes	Confirm the series group
Aircraft extension	Jobber flute on long shank	Reaching past obstructions	Not for holes deeper than its flute
Parabolic deep-hole	Long with wide flute	Deep holes with fewer pecks	Needs rigid setup
Gun drill	Single flute with coolant through	Very deep precision holes	Special machine and coolant system

Key points

Start with a shorter drill, then follow with the long drill. A long drill started on a flat surface tends to wander.
Peck the drill to clear chips, especially past a few diameters deep.
Reduce feed when the drill first enters and when it breaks through.
Long drills flex. Use the shortest drill that reaches the bottom of the hole.
Exact flute and overall lengths come from the maker's catalog or the DIN 1869 series tables.

Common mistakes

Selling an aircraft extension drill for a deep hole.
Starting a long drill without a pilot or spot.
Running a long drill at full jobber speed and feed.
Drilling continuously without pecking until the flutes pack.

Governing standards: ANSI B94.11M (twist drills); DIN 1869 (extra-long straight shank twist drills). Manufacturer catalogs for lengths.

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.

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