Speeds and Feeds


Cutting Metal

  • How fast can you cut metal?  Well, it depends on two major factors:
    • The type of metal to be cut.
    • The material the cutting tool is made from.
  • Normally harder metals are cut more slowly than softer metals; for example, aluminum.

What are speeds and feeds?

  • The speed refers to how quickly the cutting edge(s) of the tool pass through the metal.
  • The feed refers to the rate at which the tool is traversed over the work.

Why are speeds & feeds important?

  • If you try to cut metal too quickly you will:
    • Burn up the cutting edge of the tool;
    • Cause the surface of the work to become work hardened;
    • Waste time changing or sharpening tools;
    • Shorten the life of cutting tools.  The useful life of a cutting tool depends first on the cutting speed, then the feed rate, and lastly the depth of cut.

Rate of metal removal

  • The rate at which metal is removed from a workpiece is dependent upon how fast the tool cutting edges are moving through the metal (the speed) and how fast the tool is traversed over the work piece (the feed).  Let us start with the speed.  (Speed is normally expressed in Surface Feet per Minute) – sfpm or fpm

How do we know how fast to cut metal? (What speed to use.)

  • The cutting speed of all commonly used metals is freely available in manufacturers technical sheets and in the Machinery’s Handbook.
  • Some metals are so common that we remember their cutting speeds without consulting any reference.

Where can you find cutting speeds?

  • Students at the Centre may choose to consult the Machinery’s Handbook or their textbook for the cutting speeds they will require to complete their projects in the machine shop.
  • We should practice using both resources.
How do we convert the cutting speeds into RPM?
  • RPM is short for Revolutions Per Minute.
  • The cutting speed assumes the tool cutting edges are moving in a straight line.  Normally they are not.
  • nWe need to convert the cutting speed into RPM.
  • RPM = 12 x CS / π x Dia.

Applications

  • When drilling a hole, use the diameter of the drill in the formula not the size of the workpiece.
  • When milling, use the diameter of the cutter.
  • When turning on a lathe, use the diameter you are cutting.
Other factors affecting cutting speed.
  • Condition and heat treated state of the workpiece material.
  • Use of coolant, how it is mixed and how it is applied.
  • Type and grade of tooling material.
  • Operation being performed.  Feed rate and depth of cut.
  • Condition of machine and workpiece clamping.
  • Desired tool life.

Rough Shop Formula

  • RPM = 4 x CS / Dia

How do we know how fast to feed the cutter across or through the workpiece? (What feed to use.)

  • The feed rate for all commonly used metals is freely available in manufacturers technical sheets and in the Machinery’s Handbook.
  • Some cutting operations are so common that we remember the feed rates without consulting any reference.

What Determines the Feed Rate?

  • How much material each cutting edge can remove in one pass or revolution determines the feed rate.
  • In the case of a standard twist (jobber) drill, the larger the drill the greater the feed rate.  (When used in the same material.)

Help for distance calculation

  • distance traveled = time(min.) x feed/tooth x # teeth x rpm
  • Calculate how far a 2 flute drill will travel into a workpiece in 3 minutes if the feed rate is 0.002 per tooth and the rpm is 580.
  • Dist. Trav. = 2 x 0.002 x 3 x 580 = 6.96

Help with rpm question

Help with rpm question

Help with rpm question

Using Feedrates

  • Feedrates on drilling machines and lathes are set using feed per revolution.
  • Feedrates on milling machines are set using feed per minute.  To convert feed per rev. to feed per minute multiply the feed per rev. by the rpm of the cutter.

Theoretical Surface Finish

  • The feedrate has a direct impact on surface finish; so does any radius on the tool nose.  Consider the lathe example here.

 

 

 

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