Choosing spindle speed, feed and depth of cut, and the purpose and types of coolants.
Cutting speed and RPM
N = (1000 × V) ÷ (π × D)
N = RPM, V = cutting speed (m/min), D = job diameter (mm)
N = RPM, V = cutting speed (m/min), D = job diameter (mm)
Example: Mild steel Ø40 mm, HSS tool, V = 25 m/min. N = 25000 ÷ (3.14 × 40) ≈ 199 RPM.
Cutting speed depends on the job material, tool material (carbide tools allow 3–4 times more speed than HSS), rigidity of the set-up and the finish needed.
Feed
Distance the tool moves along the job per revolution, in mm/rev. Roughing: about 0.2–0.5 mm/rev. Finishing: about 0.05–0.1 mm/rev.
Depth of cut
Depth of cut = (D − d) ÷ 2
When the tool is moved in by 1 mm, the diameter reduces by 2 mm.
Coolants (cutting fluids)
Purpose
- Cool the tool and the job – longer tool life.
- Lubricate – less friction, better finish.
- Wash away chips; prevent rust.
Types
| Coolant | Used for |
|---|---|
| Soluble oil (oil + water emulsion, milky) | Most general turning and drilling of steel |
| Straight cutting oil | Threading, heavy cuts, gear cutting |
| Kerosene | Aluminium |
| Dry (no coolant) / compressed air | Cast iron, brass |
Quick Revision
- N = 1000V ÷ πD (D = job dia).
- Depth of cut = (D − d)/2.
- Soluble oil for steel; dry for cast iron and brass; kerosene for aluminium.
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