KEVOS® MAKEOS · Manufacturing
Parting-Off Plan (Spindle Speed Limit, Cutting Force and Power, Parting Time)
When I part off this bar or tube, where does the spindle hit its speed limit, what force and power does the blade need, how long does the cut take, and does it fit my machine and cycle time?
Plan for parting off a bar or tube on a lathe at constant cutting speed: starting spindle speed, the diameter at which the spindle reaches its maximum speed, the cutting force from the specific cutting force, blade width and feed (as the turning power tool), cutting and motor power and torque at the start, and the parting time, integrated at constant surface speed out to the speed limit and at the limit speed after that, against the available power and your cycle time.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bar outside diameter | mm | Outside diameter of the bar or tube. |
| Bore of a tube (0 = solid bar) | mm | Bore of a tube; 0 for solid bar. |
| Blade (insert) width | mm | Width of the parting blade or insert. |
| Radial feed | mm/rev | Radial feed per revolution while parting. |
| Cutting speed | m/min | Programmed constant cutting speed. |
| Spindle speed limit | rpm | Spindle speed limit set in the program or the machine’s maximum. |
| Specific cutting force | N/mm² | Specific cutting force for the material at this chip thickness, from the tool maker. |
| Machine and cycle | ||
| Spindle drive efficiency | % | Efficiency of the spindle drive. |
| Spindle power available | kW | Spindle power available at the speeds used. |
| Parting time allowed | s | Time the cycle allows for parting. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Bar outside diameter
- What it is
- The outside diameter of the bar or tube being parted, where the cut starts at the largest radius.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bar takes longer to part and needs more starting torque.
- Get it right
- Use the actual stock diameter, not the finished turned diameter, if parting is the first cut.
What this means: Bore of a tube (0 = solid bar)
- What it is
- The bore of a tube, which ends the cut before the centre; a solid bar has no bore.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bore shortens the cut and keeps the cutting speed above zero.
- Get it right
- Enter 0 for solid bar; the cut then runs to the centre, where the cutting speed falls to zero.
What this means: Blade (insert) width
- What it is
- The width of the parting blade or insert, which is the width of the groove and of the chip.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force and power rise in proportion to the width.
- Get it right
- Use the insert’s cutting width; narrower blades waste less material but are less rigid.
What this means: Radial feed
- What it is
- How far the blade advances radially for each revolution of the spindle while parting.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 2 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.08 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force rises in proportion; parting time falls in proportion.
- Get it right
- Use the programmed parting feed; it is usually reduced near the centre, which this tool does not model.
What this means: Cutting speed
- What it is
- The constant surface speed programmed for parting, held until the spindle reaches its speed limit.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 2000 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 m/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises cutting power and shortens the constant-speed part of the cut.
- Get it right
- Use the speed recommended for the insert grade and material by the tool maker.
What this means: Spindle speed limit
- What it is
- The highest spindle speed allowed, set by the program or the machine, which caps the speed as the diameter shrinks.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher limit lets constant surface speed continue to a smaller diameter, shortening the end of the cut.
- Get it right
- Include any limit set for the chuck or workholding, not just the machine maximum.
What this means: Specific cutting force
- What it is
- The cutting force per square millimetre of chip cross-section for the material, used with the blade width and feed to find the cutting force.
- Units and format
- Enter a number in N/mm²: newtons per square millimetre (equal to MPa). The form accepts 0.000001 N/mm² to 20000 N/mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N/mm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force, power and torque rise in proportion.
- Get it right
- Take it from the tool maker for the material and the small chip thickness used in parting.
Machine and cycle
What this means: Spindle drive efficiency
- What it is
- The efficiency of the spindle drive, turning cutting power into the power the spindle motor must supply.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower efficiency raises the motor power needed.
- Get it right
- Use the machine maker’s figure; belts and gearboxes lower it.
What this means: Spindle power available
- What it is
- The spindle power the machine can deliver at the speeds used for parting, compared with the motor power needed.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 10000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the power check; the power needed does not change.
- Get it right
- Use the power at the starting spindle speed from the machine’s power curve, which can be below the rated power.
What this means: Parting time allowed
- What it is
- The time the cycle allows for the parting operation, compared with the calculated parting time.
- Units and format
- Enter a number in s: seconds. The form accepts 0.000001 s to 100000 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the time check; the parting time does not change.
- Get it right
- Take it from the cycle-time budget or quote for the part.
Method and formulas
- n = 1000 V_c ÷ (π d), limited to n_max; the limit is reached at d_lim = 1000 V_c ÷ (π n_max).
- Cutting force F = k_c × width × feed; cutting power = F V_c ÷ 60 000 kW at constant speed; motor power = cutting power ÷ η (as the turning power tool).
- Time at constant speed from radius R to r₁ = max(d_lim ÷ 2, bore radius): π (R² − r₁²) ÷ (1000 V_c f) min; then at n_max from r₁ to the bore: (r₁ − r₀) ÷ (f n_max) min.
- Example: the spindle reaches 3000 rpm at 15.915 mm diameter; 1.5 kW and 15.132 s to part off.
Limitations
- Constant specific cutting force; at the small feeds used near the centre the real specific force rises, and the cutting speed falls to zero on solid bar, which wears the insert edge.
- Feed reduction near the centre, chip control and pip removal are not modelled.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a parting operation before running a new bar job
- Find the parting time for a cycle-time quote
- See where the cutting speed starts to fall towards the centre
Who it is for
- CNC programmers
- Machinists and setters
- Production engineers
What you get
- Starting speed and the diameter where the speed limit is reached
- Cutting force, power and torque at the start
- Parting time
Interpreting the result
- On solid bar most of the parting time is spent on the outer diameter at constant speed; raising the spindle limit only speeds up the last few millimetres.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1084–1101. Topic reference; no source tables or recommended settings are reproduced.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Parting-Off Plan (Spindle Speed Limit, Cutting Force and Power, Parting Time) calculator