Spindle RPM calculator converts a target cutting speed (SFM) and tool or workpiece diameter into the spindle speed to run.
Solve for spindle speed (RPM) from a target surface speed (SFM) and tool diameter, the reverse of the SFM calculation.
Formula: RPM = (SFM times 3.82) divided by D, with D in inches.
How it works
This rearranges the surface-speed formula to solve for RPM directly, so the SFM from a tooling chart and the tool diameter give the spindle speed to dial in.
FAQ
What if the RPM comes out higher than the spindle max? Use the highest RPM the machine supports; running below the ideal SFM is safer than exceeding it.
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How it works
Cutting speed (SFM) is a property of the tool material and workpiece material combination, independent of tool size, so the same SFM target requires a higher RPM for a smaller-diameter tool and a lower RPM for a larger one. This is the starting calculation behind nearly every other milling speeds-and-feeds calculation.
FAQ
Where do I find recommended SFM for my tool and material? Tool manufacturer catalogs and speed and feed charts list recommended SFM ranges by tool material (HSS, carbide, coated) and workpiece material; these are the best starting point, adjusted down for poor rigidity or up for optimized, well-supported setups.
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