BIZOMAT MAKEOS · Manufacturing
Milling Power and Spindle Torque
What power and torque does this milling cut need, and can the spindle deliver it?
Metal removal rate, cutting power, motor power and spindle torque for a milling cut from the specific cutting force, compared with the available spindle power.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Radial width of cut | mm | Width of the cut across the cutter. |
| Axial depth of cut | mm | Depth of the cut along the cutter axis. |
| Table feed | mm/min | Programmed table feed. |
| Cutter diameter | mm | Cutter diameter (for surface speed). |
| Spindle speed | rpm | Spindle speed. |
| Specific cutting force kc | N/mm² | Specific cutting force for the work material at your chip thickness, from the tool supplier’s data (e.g. about 1500–2100 N/mm² for steels, 500–800 N/mm² for aluminium alloys). |
| Machine efficiency | % | Overall machine efficiency from spindle motor to cutting edge (often 70–85%); check the machine manual. |
| Available spindle power | kW | Spindle power available at this speed, from the machine chart. |
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: Radial width of cut
- What it is
- How much of the cutter’s width is engaged in the work, measured across the cutter at right angles to its axis: the stepover in side milling, or the cut width in face milling.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Removal rate and cutting power rise in direct proportion.
- Get it right
- Do not confuse it with the axial depth; radial width is across the cutter, axial depth is along it.
What this means: Axial depth of cut
- What it is
- How deep the cutter goes into the work along its own axis, from the uncut surface to the floor of the cut.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 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
- Removal rate and cutting power rise in direct proportion.
- Get it right
- For a face mill this is the depth below the top surface; for an end mill on a side wall it is the length of flute engaged.
What this means: Table feed
- What it is
- The speed at which the work moves past the cutter, as programmed in the CNC or set on the machine table.
- Units and format
- Enter a number in mm/min: millimetres per minute. The form accepts 0.000001 mm/min to 1000000 mm/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 mm/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Removal rate and cutting power rise in direct proportion.
- Get it right
- This is the feed rate per minute, not the feed per tooth; multiply feed per tooth by teeth and rpm to get it.
What this means: Cutter diameter
- What it is
- The cutting diameter of the milling cutter, used here only to report the cutting speed at the cutter edge.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 63 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changes only the reported cutting speed; power and torque do not depend on it here.
- Get it right
- Use the effective cutting diameter for round inserts or ball nose cutters at shallow depth.
What this means: Spindle speed
- What it is
- The rotational speed of the milling spindle for this cut, from the program or the machine display.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 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 speed lowers spindle torque for the same power and raises the reported cutting speed.
- Get it right
- Use the speed actually run; the torque result depends on it.
What this means: Specific cutting force kc
- What it is
- The force needed to remove a chip of one square millimetre cross-section in this work material at the mean chip thickness of the cut, from the tool supplier’s cutting data.
- Units and format
- Enter a number in N/mm²: newtons per square millimetre (equal to MPa). The form accepts 0.000001 N/mm² to 10000 N/mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1800 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 power and torque rise in direct proportion.
- Get it right
- Light radial engagement gives thin chips and a higher kc; use the value for your mean chip thickness.
What this means: Machine efficiency
- What it is
- The share of motor power that reaches the cutter after losses in the spindle drive, belts and gears, from the machine maker’s data.
- 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 required.
- Get it right
- Type the percentage itself, not a fraction.
What this means: Available spindle power
- What it is
- The power the milling spindle can deliver at the speed of this cut, taken from the machine’s power and speed chart.
- 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 11 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 pass or fail line; the power required does not change.
- Get it right
- At low rpm many spindles are torque-limited and deliver less than rated power.
Method and formulas
- Removal rate Q = ae × ap × vf ÷ 1000 (cm³/min).
- Cutting power Pc = ae × ap × vf × kc ÷ (60 × 10⁶) (kW); motor power = Pc ÷ η.
- Torque = Pc × 30 000 ÷ (π n) (N·m); cutting speed = π D n ÷ 1000.
- Example: 40 × 3 mm at 800 mm/min removes 96 cm³/min and needs 3.6 kW.
Limitations
- kc must match the mean chip thickness of the cut; light radial engagement thins the chip and raises kc.
- Entry and exit shocks and tool wear raise peak power.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a face or slot milling cut against spindle power
- Find the feed a spindle can sustain
- Check torque at low spindle speed
Who it is for
- Machinists
- CNC programmers
- Manufacturing engineers
What you get
- Metal removal rate
- Cutting power and motor power
- Spindle torque
- Cutting speed
Interpreting the result
- Use the spindle power and torque curves; torque often limits large cutters at low speed.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1084–1101. Topic reference; no source tables or recommended settings are reproduced.
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