CNC Machining: What Cutting Speed (Vc) Means and How to Select It

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Cutting speed (Vc) describes how fast a cutting edge moves across a workpiece surface, measured in meters per minute (m/min) or surface feet per minute (SFM). It is distinct from spindle speed (RPM) — two tools rotating at the same RPM can have very different cutting speeds depending on their diameters. The relationship is expressed as Vc = (π × D × n) / 1000, meaning a 20 mm tool at 2,000 RPM yields approximately 125.7 m/min. Selecting the correct Vc before calculating RPM is critical, as the wrong cutting speed can produce unsuitable machining parameters even if the spindle speed formula is applied correctly. Material type, cutting tool material, coatings, and the machining process — whether milling or turning — all influence the appropriate cutting speed to use.

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