Lathe Speed RPM
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For a constant desired surface cutting speed, RPM is inversely proportional to diameter: a large-diameter workpiece must spin slower, and a small-diameter workpiece must spin faster, to keep the same edge speed where the tool actually contacts the material.

As a workpiece's effective diameter changes (for example, turning down a large blank to a narrower final diameter), the correct RPM for constant surface speed changes too, which is why some lathes have variable speed control or multiple step-pulley settings rather than a single fixed speed.

Surface speed (surface feet per minute, or metres per minute) is how fast the material's surface passes the cutting edge, which is what actually determines cutting efficiency, tool wear, and finish quality. RPM alone is meaningless without knowing the diameter it corresponds to.

Wood is far softer and more forgiving than metal, and wood lathes are typically built for much higher RPM ranges since a wood surface speed that's efficient still keeps heat and tool wear low, whereas the same RPM on metal at a comparable diameter would badly overheat the cutting tool.

Beyond poor cut quality and tool wear, a large-diameter workpiece spinning too fast carries significant rotational energy and imbalance risk; on a wood lathe this is a genuine safety hazard, which is why roughing an unbalanced blank should always start at a much lower RPM than the calculated ideal.