Turning · Released

Threading Calculator

Calculate thread parameters for CNC lathe threading cycles: RPM, feed rate, thread depth, and approximate number of passes. Covers metric 60° threads (M profile). Helps beginners understand why threading feed equals the pitch and how thread depth is distributed across passes.

Workpiece diameter — mm

Pitch P — mm

Cutting speed Vc — m/min

Thread type

Spindle speed RPM

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Feed rate (thread feed)

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Thread depth (single depth)

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Approx. number of passes

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Minor diameter (external)

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Lead / start count

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Threading formulas (60° metric)

Feed per revolution = pitch P. On Fanuc G92/G76, the feed is programmed as F = P (mm/rev). The spindle must be synchronized so every revolution advances exactly one pitch.

RPM = 1000 × Vc / (π × D) — threading uses lower Vc (30–80 m/min) than turning
Thread depth (external) = 0.6134 × P (H/2 height of 60° triangle)
Minor diameter (external) = D − 2 × depth = D − 1.2268 × P
For internal threads, the tap drill size ≈ D − P (60% thread engagement)

Pass schedule: Threads are cut in multiple passes with decreasing depth. Typical first pass 0.3–0.5 mm, last pass 0.05 mm. Rough guides for carbide: P=1.0 → 3–4 passes; P=1.5 → 4–6; P=2.0 → 6–8; P=3.0 → 10–12. Always check the tool manufacturer's recommended infeed pattern (compound vs. radial).

Approach distance: Leave at least 2–3 mm of lead-in before the thread start (G76 Q value). This lets the spindle synchronize before cutting the full pitch.