Cutting depth (ap), S, and F are the three main cutting parameters in lathe machine

On a lathe machine, the terms cutting depth (ap), S, and F are the three main cutting parameters you must know.

1. Depth of Cut (ap)

• Meaning: How deep the tool cuts into the workpiece in one pass.

• Unit: millimeters (mm).

• Typical values:

  • Roughing (removing material fast) → 2–5 mm

  • Finishing (for smooth surface) → 0.2–0.5 mm

👉 Example: If depth of cut = 2 mm, the tool removes a 2 mm layer from the diameter of the workpiece.

2. S (Spindle Speed)

• Meaning: How fast the workpiece rotates in the spindle.

• Unit: revolutions per minute (r/min or RPM).

• Formula:

  n=1000×vπ×dn = \frac{1000 \times v}{\pi \times d}n=π×d1000×v​

  where:

  • v = cutting speed (m/min)

  • d = workpiece diameter (mm)

👉 Example: If cutting speed = 100 m/min and workpiece diameter = 50 mm:

n=1000×1003.14×50≈636 RPMn = \frac{1000 \times 100}{3.14 \times 50} \approx 636 \, \text{RPM}n=3.14×501000×100​≈636RPM

3. F (Feed Rate)

• Meaning: How far the tool moves forward along the workpiece for each spindle revolution.

• Unit: mm/rev (millimeters per revolution).

• Typical values:

  • Roughing → 0.3–0.6 mm/rev

  • Finishing → 0.05–0.2 mm/rev

👉 Example: If feed = 0.2 mm/rev and spindle speed = 600 RPM, then feed per minute =

$$0.2\times 600=120\text{mm/min}$$

✅ In summary:

• Depth of cut (ap) = how deep the tool cuts

• S (Spindle speed) = how fast the workpiece rotates

• F (Feed rate) = how much the tool moves forward each revolution

These three parameters decide the surface finish, machining time, and tool life.