Calculate spindle speed for turning

What the Spindle Speed Calculator (Turning) Works Out

In turning, the workpiece rotates and the tool stays fixed. This is why the workpiece diameter determines the cutting speed at the circumference, not the tool diameter as in milling or drilling. From the material-dependent cutting speed and the workpiece diameter, the calculator works out the suitable spindle speed in rpm for the carbide indexable insert.

The Formula

n = (v_c × 1000) / (π × d)

Here, n is the spindle speed in rpm, v_c is the material-dependent cutting speed in m/min and d is the workpiece diameter in mm. The formula is identical to the one for drilling. The decisive difference lies in the meaning of d: in turning, the workpiece rotates, so its outside diameter counts. At the same cutting speed, a larger workpiece automatically means a lower spindle speed.

Reference Values for Cutting Speed (Carbide Indexable Inserts, External Longitudinal Turning)

Important Note on the Data

The reference values above are compiled from several tool manufacturers and specialist sources and are deliberately conservative. For structural steel, the sources are good and consistent. For quenched and tempered steel, stainless steel and aluminium, the values in the technical literature vary considerably more, because they depend very precisely on the exact alloy and, for steel, the strength grade: a high-strength quenched and tempered steel (above 1000 N/mm²) needs a noticeably lower cutting speed than a softer one. For aluminium, the range in the technical literature runs from around 200 to over 2000 m/min, depending on the alloy and silicon content. The 250–500 m/min stored here is a cautious mid-range for common wrought alloys, not a universal figure. Use the results as a starting point and work up to new material and tool combinations conservatively.

How to Use the Calculator

Select the material and enter the workpiece diameter, not the diameter of the insert or the turning tool. The calculator applies a suitable cutting speed from the reference range and outputs the spindle speed. For very large workpiece diameters, the calculated spindle speed can be lower than the sensible minimum spindle speed of your machine. Check this against your machine’s speed table. You can find more calculators in the CNC calculator overview.

What to Watch For

• The workpiece diameter changes during turning: As the workpiece is turned down, it gets smaller. At a constant spindle speed, the actual cutting speed drops. Modern CNC controllers offer constant surface speed (G96) for this, which adjusts the spindle speed automatically. Always program a spindle speed limit (G50/G92) with it, otherwise the spindle speed can become dangerously high at small remaining diameters.
• Large diameters: If the calculation gives a very low spindle speed, check whether your machine can cover this range sensibly at all.
• Reference values remain reference values: Insert geometry, cooling, machine rigidity and the exact material batch also influence the optimum cutting speed.

Read background information on cutting data in the BZT Knowledge Centre.