EngineerBenchCalculating lathe speed and feed
Turning speed is based on the diameter of the workpiece at the cut, not the diameter of the tool. At a fixed RPM, cutting speed falls as the work diameter gets smaller. Enter the cutting diameter and the appropriate surface speed to calculate RPM, then use feed per revolution to obtain a linear feed rate.
Use the diameter where the cut happens
For a straight turning pass, use the diameter being machined. Facing crosses a range of diameters, so one fixed RPM does not maintain constant surface speed across the face. The calculator does not create a constant-surface-speed program or check chuck and workholding speed limits.
Allow for more than cutting travel
Travel, feed, and pass count estimate time in the cut. Add approach, retract, tool changes, inspection, and loading separately. Supplier cutting data and the machine's speed limit take precedence over a generic material starting point.
Checking changing workpiece diameter
At 100 m/min surface speed, a 50 mm diameter corresponds to about 637 RPM. A 25 mm diameter requires about 1,273 RPM for the same surface speed. This is why a fixed spindle setting does not describe the same cutting condition throughout a facing operation. The workpiece, chuck, and setup must all permit the chosen speed; a calculated value does not override their limits.
Feed per revolution is multiplied by actual RPM to obtain feed per minute. For example, 0.2 mm per revolution at 500 RPM gives 100 mm/min. A 50 mm cutting travel then takes half a minute for one steady pass. Count each pass and any extra travel that belongs in the estimate. Roughing and finishing can use different settings, so calculate them separately when a single average would conceal the process. For a programmed constant-surface-speed operation, check the controller's spindle cap and the diameter range explicitly; the calculator does not generate or validate that program.
Formula
RPM = 1000Vc/(πD). Feed = RPM × feed/rev; milling multiplies chip load by flute count.