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SFM Calculator

Calculate machining surface speed, spindle RPM, or cutting diameter from the other two values, with a metres-per-minute result.

Result

157.08 SFM

A 2in tool at 300 RPM cuts at about 157.08 SFM

Quick Answer

An SFM calculator links diameter and spindle speed to surface feet per minute. Multiply pi by diameter in inches, divide by 12, then multiply by RPM. A 2-inch diameter at 300 RPM gives 157.08 SFM, or 47.88 metres per minute. The same formula can solve for RPM or diameter.

How An SFM Calculator Connects Speed, RPM And Size

An SFM calculator finds surface speed, spindle RPM, or diameter. The tool needs the other two values. SFM tells how fast the work meets the active tool point. In turning, the work spins. In milling or drilling, the tool often spins. The right diameter is the point where the cut takes place. A machinist may start with a speed from a tool or material chart. The calculator then finds an RPM setting. The page also shows metres per minute. The final setup must still fit the tool, stock, feed, coolant, and machine limit. A chart value is a start point, not a fixed rule. Watch chips, heat, sound, and finish during the first cut. Check the first part with care.

The Surface Feet Per Minute Formula Used In Machining

Metric conversion: Metres Per Minute = SFM × 0.3048. Surface speed relates linear velocity at the outer edge of a tool or part to the rotational speed.

  • SFM = π × (Diameter ÷ 12) × RPM.
  • RPM = (SFM × 12) ÷ (π × Diameter).
  • Diameter = (SFM × 12) ÷ (π × RPM).

SFM: surface feet per minute.

Diameter: active diameter in inches.

RPM: spindle turns per minute.

How To Use An SFM Calculator In Five Simple Shop Steps

Inputs

  • Solve for: choose SFM, RPM, or diameter.
  • Diameter: active cut diameter in inches.
  • RPM: spindle turns per minute.
  • Surface speed: known or target surface speed.

Steps

  1. Choose the unknown value to solve for.
  2. Enter the two known values.
  3. Check that you are using the active cut diameter.
  4. Run the calculation.
  5. Compare the result with tool and machine limits.

Machining Surface Speed Example With All Steps Shown

A cut diameter of 2 in and a spindle speed of 300 RPM.

  1. Convert diameter to feet: 2 in ÷ 12 = 0.1667 ft.
  2. Multiply by pi and RPM: 3.14159 × 0.1667 × 300 = 157.08 SFM.
  3. Solve for RPM in reverse: (157.08 × 12) ÷ (3.14159 × 2) = 300 RPM.
  4. Convert SFM to metric: 157.08 × 0.3048 = 47.88 m/min.

Surface speed is 157.08 SFM (or 47.88 m/min) and reverse spindle speed is 300 RPM.

When Surface Feet Per Minute Helps Set Spindle RPM

Use SFM when a tool or stock chart gives a target speed and the machine needs RPM. Check that the chart fits the tool, stock, and cut.

For other workshop speeds and machining ratios, see the engineering calculators hub and the glossary.

Assumptions

  • Diameter is in inches.
  • RPM is steady.
  • The active diameter is used.
  • The target SFM fits the job.
  • Rounding occurs after the math.

Limitations

  • Feed and chip load are not set.
  • Machine power is not checked.
  • Max tool or holder RPM is not checked.
  • Speed charts differ by tool and cut.

In Practice

Machinists often use the full tool size when the active diameter is smaller. Use the diameter at the cut, then check the RPM limit.

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Frequently Asked Questions About SFM And Spindle RPM

What Is The Difference Between SFM And RPM?

SFM is the linear speed at the cut surface, while RPM is the number of spindle turns in one minute. The same RPM gives more SFM as diameter grows. Both values are needed to match a speed chart to the job.

Which Diameter Should I Enter For An SFM Check?

Enter the active diameter where the tool meets the stock during the cut. In turning, that value may change as the part gets smaller. For some milling tools, the effective cut diameter can differ from the full tool size.

Can I Use One SFM Value For Every Material?

One SFM value does not fit every stock, tool material, insert, or cut type. Use data for the exact setup from a sound shop source or toolmaker. Adjust within the stated range when the machine or cut calls for care.

Does An SFM Result Also Set The Feed Rate?

SFM does not set feed rate by itself because feed uses other job data. Chip load, active edge count, stock, tool type, and cut style all matter. Use the speed and feed rules from the same source when possible.

How Do I Convert SFM To Metres Per Minute?

Multiply SFM by 0.3048 to convert feet per minute to metres per minute. A speed of 157.08 SFM is about 47.88 m/min. The conversion changes the unit only and does not change the physical cut speed.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.