In its annealed condition (~200 HB), AISI 4140 has fair to good machinability, rated at about 65% of a 100% free-machining steel standard (e.g., 1212). Practices include:
Tooling: Use sharp, positive-rake geometry carbide inserts or premium HSS tools.
Speeds & Feeds: Medium surface speeds with consistent, moderate feeds. Avoid light feeds that cause rubbing and work hardening.
Coolant: Use ample, high-quality coolant to dissipate heat, improve tool life, and break chips.
Chip Control: The alloy content can lead to stringy chips; chip breakers are essential.
Hard State Machining: If machining in a pre-hardened (QT) state (>30 HRC), use rigid setups, reduced speeds, and carbide or CBN tooling. Grinding is often preferred for finishing hardened 4140.