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What is Cutting Speed?

In machining, the words “Speed”, “Cutting Speed”, “SFM” and “Surface Speed” all refer to the relative linear velocity between the tip of the cutting edge and the workpiece. The definition is the same for all machining operations turning, milling, etc. (Opposed to feedrate which has a different definition for different applications).

What is SFM?

SFM stands for “Surface feet per min”. It is the common unit to measure cutting speed in the US (But almost never used outside of the US). The speed is measure in feet/min instead of mm/min which is the common unit that is used in most countries.

  • Speed [mm/min] = Speed [SFM]⋇ 0.3048 (Commonly 0.305 is used)
  • Speed [SFM] = Speed [mm/min]⋇ 3.281 (Commly 3.3 is used)


Formula to calculate the cutting speed?

Since cutting speed is the linear velocity between the cutting tool and the workpiece it is the product of the spindle speed time the radius. on Lathe machines it the radius of the workpiece and on Milling machines the radius of the cutting tool.

In Imperial (US) units:

  • Vc [SMF] -Cutting Speed
  • n [RPM] – Spindle Speed
  • D [Inches] – Workpiece diameter in Turning and Cutter Diameter in Milling
  • Vc = (n ⋇ π ⋇ D) / 12

Metric Units:

  • Vc [mm/min] -Cutting Speed
  • n [RPM] – Spindle Speed
  • D [mm] – Workpiece diameter in Turning and Cutter Diameter in Milling
  • Vc = (n ⋇ π ⋇ D) / 1000

How to set the cutting Speed?

The correct cutting speed is determined by the combination of:

  • The Machinability of the workpiece material. (How much is the material resisting to being cut)
  • The Grade of the Cutting Tool. (How much Wear Resistant is the cutting tool)

There is a huge variety of workpiece materials and cutting tools grade.s Determining the best cutting speed that will bring a good balance between productivity and tool-life is one of the most important skills for a Machinist or a tools engineer to master.

Synonyms:
SFM
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