CNC Turning Cycle Time and Metal Removal Rate Calculator
A quote starts with a cycle time, and a turning cycle starts with spindle speed, feed and the number of passes. This calculator works from surface speed to rpm, cutting time, metal removal rate and parts per hour.
Your numbers
Choose from the insert maker's data for the material. Mild steel with coated carbide is often several hundred SFM.
The lathe and chuck limit. The calculator caps rpm at this value.
Total on the radius, not the diameter.
Rapids, tool changes, drilling, grooving, threading and part handling.
Your results
First estimate only. Cutting time assumes a constant diameter and ignores acceleration and spindle speed changes on a constant surface speed control. Confirm speeds and feeds with the tool maker's recommendation for your material.
Cycle time (sec) vs surface speed
Sweeps Surface speed from 100 to 800, holding your other inputs fixed.
Compare estimated and actual cycle times
Optional: we can show how to compare routing run standards with actual cycle times from your ERP or MES.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the diameter, length of cut, surface speed and feed per revolution from the insert supplier, and the radial stock to remove with the depth of cut per pass. Enter the spindle limit so the result does not exceed what the machine can do, and an allowance for non-cutting time such as rapids, tool changes, grooving and handling. The result shows rpm, passes, cutting time, removal rate, cycle time and parts per hour. Use the chart to see how much a speed change really saves.
Formulas used
Spindle rpm equals 12 times SFM divided by pi times the diameter in inches. If the result exceeds the spindle maximum, the maximum applies and the actual surface speed is recalculated. Passes equals the radial stock divided by the radial depth of cut, rounded up. Cutting time in minutes equals passes times path length divided by feed rate, where feed rate is feed per revolution times rpm. Metal removal rate in cubic inches per minute equals 12 times SFM times feed in inches per revolution times depth of cut.
Reading the speed sweep
Doubling the speed does not halve the cycle, because the non-cutting time stays fixed and, on small diameters, the spindle cap limits rpm. The chart flattens out when either happens, and that is where more speed gains nothing. Higher speed also shortens tool life, so use it together with the tool life cost calculator before pushing speeds up. Feed and depth of cut are usually better levers than speed for roughing.
Common mistakes
Mixing diameter and radial depth of cut is the most common error: stock on the radius is half the diameter change. Leaving out the approach and overtravel makes short cuts look too fast. Using full catalog speeds with no rigidity allowance gives times a real machine cannot hold. Also do not forget that constant surface speed on facing makes rpm climb towards the centre. Feed these cycle times into your ERP routing as run standards, then compare them with actual job reports.
Frequently Asked Questions
What is the formula for rpm from SFM?
RPM equals 12 times surface feet per minute divided by pi times the diameter in inches. The commonly used shortcut of 3.82 times SFM divided by diameter is the same value rounded.
How is metal removal rate calculated for turning?
In inch units, metal removal rate in cubic inches per minute equals 12 times surface speed in SFM, times feed in inches per revolution, times depth of cut in inches. Multiply by roughly 16.4 to convert cubic inches to cubic centimetres.
Why does the calculator cap rpm?
Small diameters need very high rpm to reach the target surface speed, often more than the spindle or chuck allows. Capping rpm gives a realistic cycle time and shows the actual surface speed you get.
Does it handle facing or threading?
Not directly. Put those operations into the non-cutting and other operations time using your CAM time or past jobs. This calculator is for the main OD turning passes where most of the cutting time lives.
If your routing run times come from estimates, Netray can help compare them with actual cycle data captured in your ERP or MES.
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