Machinability of industrial materials
Machinability tells how easily a material can be cut to shape. A material with good machinability gives a long tool life, a smooth surface, low cutting force and neat chips.
- Soft, ductile metals (aluminium, brass, free-cutting steel): cut easily. Brass and special free-cutting steels break into short chips.
- Hard or tough metals (hardened steel, stainless steel, titanium): the tool gets hot, wears fast, and the metal can harden as it is cut. Poor machinability.
- Cast iron: brittle, so it makes short broken chips; the graphite inside helps lubricate.
- Plastics and wood: cut easily but can melt or tear if the tool is hot or blunt.
- Ceramics: too hard for a normal tool. They are shaped by grinding with diamond or before firing.
To cut better we choose a harder tool, set a good cutting speed, feed and depth of cut, and use a coolant.
Main processing methods
Processing methods fall in four families.
- Casting: pour liquid material into a mould (engine blocks, bells, plastic parts).
- Forming (reshaping, nothing removed): forging (hammer or press a hot block), rolling (squeeze between rollers to make sheets), drawing (pull through a die to make wire), extrusion (push through a die to make a long section), sheet pressing and bending.
- Cutting (machining): turning (work spins, tool moves), milling (cutter spins), drilling (holes), grinding (fine finishing with an abrasive wheel). CNC machines do this under computer control.
- Joining: welding (melt and fuse), brazing and soldering (a filler metal melts but the parts do not), bolts, rivets and adhesives.
Also: heat treatment (annealing, hardening, tempering), surface finishing (polishing, painting, plating) and additive manufacturing (3D printing, layer by layer).
Choosing: forming wastes little material; cutting gives the most exact size; casting suits complex shapes and large numbers; joining makes big structures from small parts.
Try it: squeeze a ball of clay between your palms (forming), then cut slices off it with a plastic knife (cutting). The mass is the same after forming but smaller after cutting.
Key formulas and definitions
- Cutting speed v = π × D × N ÷ 1000 (m/min; D in mm, N in rev/min)
- Turning time T = L ÷ (f × N) (min; L length, f feed per rev)
- Material removal rate = v × f × d (mm³/min with v in mm/min)
- Forging: volume stays the same, so area × height = constant
- Rolling reduction % = (t₀ − t₁) ÷ t₀ × 100
Worked examples
1. A bar of diameter 50 mm turns at 400 rev/min. Find the cutting speed.
v = π × 50 × 400 ÷ 1000 = 62.8 m/min.
2. A 120 mm long cut is made with feed 0.2 mm/rev at 300 rev/min. How long does it take?
T = L ÷ (f × N) = 120 ÷ (0.2 × 300) = 120 ÷ 60 = 2 min.
3. A forging block 100 mm × 100 mm × 200 mm high is pressed down to 100 mm high. Find the new base area.
Volume = 100 × 100 × 200 = 2 000 000 mm³. New area = 2 000 000 ÷ 100 = 20 000 mm² (the base grows).
4. A strip is rolled from 20 mm to 15 mm thickness. Find the reduction %.
(20 − 15) ÷ 20 × 100 = 25%.
5. A turning cut has v = 100 m/min, feed 0.25 mm/rev and depth 2 mm. Find the material removal rate.
v = 100 m/min = 100 000 mm/min. MRR = 100 000 × 0.25 × 2 = 50 000 mm³/min.
Common mistakes
- Thinking harder metal is always easier to machine. Harder metals usually wear the tool faster.
- Using millimetres for D and forgetting the ÷ 1000 in the cutting speed formula.
- Thinking forging cuts away material. It only reshapes, and the volume stays the same.
- Mixing up brazing and welding. In brazing the base metals do not melt.