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Screw Machine (Stub) Length Drills

Screw machine (stub) length is the shortest standard straight-shank drill series. It is chosen for rigidity and hole position accuracy, at the cost of reach.

Ref CT-0004ANSI B94.11MISO 235DIN 1897

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

What it is

Screw machine length drills are defined in ANSI B94.11M. They have the shortest flute and overall length of the standard straight-shank series. The name comes from automatic screw machines, where tool overhang must be short. Today the series is widely used in CNC machining centers and lathes. The closest metric series is DIN 1897, often called "stub" or "extra short" in metric catalogs. ISO 235 also covers a stub series.

How to identify

A screw machine drill looks short and thick for its diameter. The flutes are short and the shank takes up a larger share of the body than on a jobber. Packaging may say "screw machine," "stub," "SM" or "DIN 1897." Carbide drills for CNC work are often sold in stub proportions, but carbide drills also come in many maker-specific lengths, so read the label.

Choosing a short series

Need	Screw machine (stub)	Mechanics	Jobber
Rigidity	Highest	High	Moderate
Position accuracy on entry	Best	Good	Fair without a spot drill
Reach and hole depth	Least	Moderate	Most of the three
Typical holder	Collet, hydraulic or drill chuck on CNC	Hand or cordless drill	Drill press chuck
Often used as	Spot or start drill, short holes, tough materials	Field and maintenance drilling	General shop drilling

Key points

Use the shortest drill that reaches the full hole depth plus breakthrough. Every extra bit of length adds flex and runout.
Stub drills are often used to start a hole that a longer drill then finishes. The short drill sets the position.
Short flutes clear chips over a short distance only. Peck drill or step up a series if the hole is deep.
Exact flute and overall lengths by diameter come from ANSI B94.11M, DIN 1897 or the maker's catalog.

Common mistakes

Assuming "stub" always means the same length. Inch screw machine and DIN 1897 lengths are close, not identical.
Ordering a stub drill for a hole deeper than its flutes.
Confusing a stub drill with a spotting or center drill. Those are separate tools with their own point angles.
Replacing a CNC stub drill with a jobber and changing the tool length offset without telling the operator.

Governing standards: ANSI B94.11M (twist drills); ISO 235; DIN 1897 (stub series). Manufacturer catalogs for exact 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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