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My manufacture processes teacher says susface speed in machining is not constant, even if we use CNC, but I can't find anything about it.

June7
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Luis Hoyos
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2 Answers2

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It happens when there is a varying radius to the cutter in contact with the material.

When drilling, the outer diameter is moving at the surface speed you program it to (CNC). The surface speed slows as you move along the cutting edges towards the center. At the very center it approaches 0 SFM (Surface Feet per Minute). That's why the web of a drill is thinned to make it look like a chisel. It has to blunt force its way thru the material.

mrfoulkes
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Constant surface speed (specified by G96 on most machines) is a turning center feature that automatically determines the appropriate spindle speed in revolutions per minute (rpm).

It does so based on a speed specified in surface feet per minute (sfm) in inch mode or meters per minute (MPM) in the metric mode as well as the cutting tool’s current diameter position.

It automatically applies the rpm calculating formula (rpm = sfm × 3.82 ÷ cutting diameter) and updates the spindle speed in rpm accordingly.

Constant surface speed is a great feature. It provides at least four advantages:

  1. It simplifies programming. Speed can be directly specified in sfm or MPM—units used by most cutting tool manufacturers for recommending speed. This eliminates the need to calculate rpm.
  2. It provides a consistent workpiece finish. When used in conjunction with feed per revolution feed-rate mode (G99 on most turning centers), the finish will almost always be consistent, regardless of diameters machined.
  3. It optimizes tool life. Tools will always machine at the appropriate speed.
  4. It optimizes machining time. Cutting conditions will always be properly set, which translates to minimal machining time.
NGrimberg
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