Machinist exam: Speeds and feeds

Key points

• RPM (imperial) = (CS x 4) / diameter in inches, a common shop shortcut.
• RPM (metric) = (CS x 1000) / (pi x diameter in mm).
• Feed rate = RPM x feed per tooth x number of teeth.
• Harder materials need lower cutting speeds. Carbide runs faster than HSS.
• Larger diameters need lower RPM for the same cutting speed.

Study tip

Check the units: metres per minute for metric, surface feet per minute for imperial.

Try these

1. A 1 in HSS drill in steel at 100 SFM runs at about:

   1. 100 RPM
   2. 400 RPM
   3. 1000 RPM
   4. 4000 RPM
   Show answer

   B. 400 RPM 100 x 4 / 1 = 400.

2. Doubling the cutter diameter at the same cutting speed:

   1. Doubles RPM
   2. Halves RPM
   3. Keeps RPM
   4. Quadruples RPM
   Show answer

   B. Halves RPM RPM is inversely proportional to diameter.

3. Feed rate for 1000 RPM, 4 teeth, 0.002 in/tooth is:

   1. 2 ipm
   2. 4 ipm
   3. 8 ipm
   4. 20 ipm
   Show answer

   C. 8 ipm 1000 x 4 x 0.002 = 8.

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