How this calculation works
Cutting speed is the surface distance passing the tool each minute. RPM is shaft rotation. A larger workpiece needs fewer revolutions to produce the same surface speed, so the diameter is essential to converting between them.
RPM = min(1000 × cutting speed/(π × diameter), entered ceiling); feed rate = RPM × feed/rev; time = total travel/feed rateA worked example
At 100 m/min on a 50 mm diameter, uncapped RPM is 2000/pi, about 636.62. A 0.2 mm/rev feed produces about 127.324 mm/min. Two passes of 100 mm plus five millimetres approach each require 210 mm total modeled travel. Dividing by that feed rate gives about 1.64934 minutes before return and other cycle operations.
Good to know
Constant-diameter straight passes only; no material recommendation, return travel, spindle ramp, tool life or safe machining approval. The entered machine ceiling is only a numeric cap, not a verification of a chuck, workpiece, tool or machine operating rating. Confirm official equipment and tooling guidance.
Reference: Sandvik Coromant — general turning formulas (opens in a new tab) ↗
What this model answers
Cutting speed is the surface distance passing the tool each minute. RPM is shaft rotation. A larger workpiece needs fewer revolutions to produce the same surface speed, so the diameter is essential to converting between them.
The spindle ceiling is applied before calculating feed rate and time. If it binds, the requested surface speed cannot be reached at the entered diameter. The actual speed metric makes that change visible instead of continuing to use an uncapped theoretical value.
The Milling chip load feed and time calculator multiplies RPM by effective flutes and feed per tooth. Turning uses feed per revolution directly, so their input conventions should not be mixed.
Measure the right inputs
Enter a constant cutting diameter and cutting data obtained for the actual tool, material and setup. This calculator converts those inputs; it does not recommend a feed or surface speed.
Cut length and approach are added for each identical pass. Feed is millimetres per revolution, not millimetres per minute. Confusing those units can change the estimated time by orders of magnitude.
Read the result in context
The ideal travel time includes forward cutting and the entered approach allowance. It excludes returns, acceleration, tool indexing, dwell and inspection. It is therefore not a complete job-cycle quotation.
Variable-diameter work under constant-surface-speed control requires a changing RPM along the path. A single diameter here cannot represent that entire control law. Calculate individual constant sections or use a more detailed process model.
Work through the numbers
At 100 m/min on a 50 mm diameter, uncapped RPM is 2000/pi, about 636.62. A 0.2 mm/rev feed produces about 127.324 mm/min.
Two passes of 100 mm plus five millimetres approach each require 210 mm total modeled travel. Dividing by that feed rate gives about 1.64934 minutes before return and other cycle operations.
Where the model stops
Constant-diameter straight passes only; no material recommendation, return travel, spindle ramp, tool life or safe machining approval.
The entered machine ceiling is only a numeric cap, not a verification of a chuck, workpiece, tool or machine operating rating. Confirm official equipment and tooling guidance.
A few questions, answered
Why does time increase when I lower the RPM ceiling?
At fixed feed per revolution, a lower RPM also lowers feed per minute, so the same travel takes longer.
Does approach apply once or per pass?
The entered approach is added to every modeled pass.