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Cutting speed, written as Vc, is the relative surface speed between a cutting edge and the workpiece. Choose a starting Vc from the toolmaker’s data for your material, tool and operation, then calculate spindle speed from the cutting diameter. Vc is not spindle RPM or feed rate, and no single speed is right for every CNC setup.
What is cutting speed (Vc)?
Vc describes how quickly the cutting edge moves across the workpiece surface at the point of contact. It is commonly expressed in metres per minute (m/min) or surface feet per minute (ft/min, often abbreviated SFM). It is distinct from spindle speed, which is measured in revolutions per minute (rpm).
For turning, the relevant surface is the rotating workpiece. For milling and drilling, the tool’s cutting edge moves around the workpiece. In either case, Vc describes relative motion at the cut; it does not, by itself, tell you the spindle’s rpm.
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Convert the desired cutting speed and the cutting diameter into a spindle speed using the appropriate unit system:
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- Metric: n (rpm) = [Vc (m/min) × 1000] / [π × D (mm)]
- Inch: n (rpm) = [Vc (ft/min) × 12] / [π × D (in)]
Here, n is spindle speed, Vc is cutting speed, and D is the diameter at the cutting interface. For turning, use the machined diameter; for milling or drilling, use the tool’s cutting diameter for the calculation. Sandvik Coromant gives the metric turning relationship as vc = π × Dm × n / 1000. Haas’s indexable-drill guide provides the inch-unit cutting-speed and spindle-speed formulas.
Formula example (arithmetic only)
If Vc is 100 m/min and the cutting diameter is 10 mm, then n = (100 × 1000) / (π × 10), or approximately 3,183 rpm. This demonstrates the conversion only; it does not mean 100 m/min is suitable for a particular tool, material, or machine.
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How do I choose the right cutting speed for a material?
Start with the cutting-data chart for the exact tool family and operation. Toolmakers organize recommendations around factors such as material group, hardness or condition, tool geometry and grade. A value for one steel, cutter, or operation cannot automatically be applied to another.
- Identify the workpiece material and condition. Check its description and, where relevant, hardness, then match it to the material group in the toolmaker’s chart.
- Match the tool and operation. Use data for the specific tool family, grade, geometry, and operation—turning, milling, or drilling. A solid carbide end mill and an indexable drill may have different recommendations even on the same material.
- Check diameter and engagement. Use the applicable cutting diameter in the rpm calculation. For milling, account for radial and axial engagement; toolmaker guidance may specify adjustments for side milling versus slotting.
- Account for machine and setup stability. Rigidity, tool overhang, workholding, and interrupted cuts affect whether published starting conditions are appropriate.
- Start within the published range and monitor the cut. Observe chip formation, tool wear, surface finish, sound, and machine load. Adjust within tool and machine limits using the toolmaker’s guidance rather than assuming a universal optimum.
Why published recommendations are not universal
Manufacturer figures are tied to stated tools and conditions. For example, Kennametal’s GOmill PRO metric data lists a 60–100 m/min range, with 80 m/min as the starting Vc, for regular-tool side milling in material group P5. The table also varies feed by diameter and specifies a 20% reduction in feed per tooth for slotting. Those figures apply to that product family and its stated conditions, not to every steel or end mill. See Kennametal’s GOmill PRO technical page.
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Haas’s indexable-drill guide says its listed speed conditions assume stable conditions. For that guide, Haas recommends reducing starting speed by 10% for unstable conditions and by 20% for interrupted cuts. These are Haas-specific recommendations, not universal adjustment rules; use the chart and instructions for your own tool.
When two charts give different values
Before comparing recommendations, check whether they refer to the same:
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- Workpiece material group and hardness or condition
- Tool material, coating, geometry, grade, and diameter
- Operation and radial or axial engagement
- Machine and setup stability
- Type of value: minimum, starting point, or maximum
- Unit system
If those conditions differ, the values are not directly comparable. Use the chart for the tool and operation you are actually running.
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Is cutting speed the same as feed rate?
No. Vc is the relative surface speed at the cut; feed rate describes how quickly the tool advances through the workpiece. Toolmaker data lists them separately: feed may be given per tooth for milling or per revolution for turning and drilling, while Vc is a surface-speed value.
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In milling, radial engagement can require separate adjustments to cutting speed and feed. Do not treat a feed recommendation as another way of expressing Vc, or assume that changing one automatically sets the other. Follow the toolmaker’s data for both parameters and the relevant engagement.
Quick Recap
Quick checklist for selecting Vc
- Confirm the workpiece material and its relevant condition or hardness.
- Find the matching material group in the exact toolmaker chart.
- Match tool family, grade, geometry, operation, and cutting diameter.
- Check engagement and setup stability, including interrupted cuts.
- Choose a starting value from the published range, calculate rpm with consistent units, and monitor the cut.
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