Quick Math #4

Surface feet per minute (SFPM or SFM) is a unit of velocity used in machining to identify the machinability ratings of a material.

SFM is the velocity around the tool or the material measured in feet-per-minute as the spindle of a lathe or screw machine turns. The faster the spindle turns, the higher the SFM. The goal is to tool a job to run the SFM as high as possible to increase hourly part production. When SFM is known for a specific material (ex 303 Stainless Steel = 120 SFM for High Speed Steel (HSS) Tooling), a formula can be used to determine spindle speed for live tools or spindle speeds for turning materials.

Tool diameter should be substituted for stock diameter when the cutter is revolving and the stock is stationary.

The spindle speed can be calculated using the following equation:

RPM= (12 x SFM)/(π x Dia), or more simply (4 x SFM)/ Dia.

We will interchange cutting speed (CS) with SFM in this class a lot.

Answer the following questions for the spindle RPM.

  1. What is the rpm of a 3/16” hss dia drill on 1018 crs? SFM=100
  2. What is the rpm of a ½” dia hss endmill on aluminum? SFM=200
  3. What is the rpm of a ¾” dia carbide endmill on 1018 crs? SFM=300
  4. What is the rpm of a letter “C” dia hss drill on 1018 crs? SFM=100
  5. What is the rpm of a 3/8” dia hss drill on 1018 crs? SFM=100
  6. What is the rpm of a 5/8” dia hss drill on 1018 crs? SFM=100
  7. What is the rpm of a .53” dia c/bore tool on 1018 crs? SFM=50
  8. What is the rpm of a ¼” dia hss reamer on 1018 crs? SFM=50
  9. What is the rpm of a 5/8” dia hss endmill on 1018 crs? SFM=100
  10. What is the rpm of a 27/64” dia hss drill on 1018 crs? SFM=100

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