Reference / Materials

Materials for Large Machined Parts

On a small part the material sets cutting speed. On a large one it also sets how much the part changes shape while you cut it, and that second effect usually costs more.

Grade	Cutting	Movement risk	Typical tolerance
S355 / A36	Fast and predictable, tooling lasts	High as a weldment, from welding and flame cut edges	±0.1 to ±0.3 mm over a metre once stress relieved
42CrMo4 / 4140	Moderate, much slower once quenched and tempered	Moderate, mostly from heat treatment	±0.05 mm on bearing seats
GG25 / EN-GJL-250	Fast, and it damps vibration better than steel	Low if the casting has been aged, high if it has not	±0.05 mm on bores
GGG40 / EN-GJS-400	Moderate, tougher than grey iron	Low if aged	±0.05 mm
304 / 316 / 316L	Slow, work hardens, heavy tool wear on thick sections	Low	±0.05 to ±0.1 mm
Duplex 2205	Slower again than austenitic, high cutting forces	Low	±0.05 to ±0.1 mm
5083 / 6082	Very fast	High, rolled plate releases stress as material comes off	±0.1 mm, more on thin walls
CuSn12 and bearing bronzes	Fast	Low	±0.02 to ±0.05 mm on the finished bore
Inconel 625 / 718	Very slow, hard on tooling	Low	±0.05 mm, tighter with grinding

S355 / A36

Structural steel

Typical use: Welded frames, bases, machine structures
Cutting: Fast and predictable, tooling lasts
Movement risk: High as a weldment, from welding and flame cut edges
Typical tolerance: ±0.1 to ±0.3 mm over a metre once stress relieved
Worth knowing: The cutting is cheap. The roughing volume and the stress relief cycle are not.

42CrMo4 / 4140

Alloy steel

Typical use: Shafts, pins, gear blanks, highly loaded parts
Cutting: Moderate, much slower once quenched and tempered
Movement risk: Moderate, mostly from heat treatment
Typical tolerance: ±0.05 mm on bearing seats
Worth knowing: Most shops rough soft, heat treat, then finish, and grind the surfaces that need it.

GG25 / EN-GJL-250

Grey cast iron

Typical use: Machine bases, housings, gear cases
Cutting: Fast, and it damps vibration better than steel
Movement risk: Low if the casting has been aged, high if it has not
Typical tolerance: ±0.05 mm on bores
Worth knowing: Ask the foundry standard and the thermal history. Porosity is found on the machine, not before.

GGG40 / EN-GJS-400

Ductile iron

Typical use: Loaded housings, arms, brackets that must not be brittle
Cutting: Moderate, tougher than grey iron
Movement risk: Low if aged
Typical tolerance: ±0.05 mm
Worth knowing: Chosen over grey iron where impact or tensile load rules it out.

304 / 316 / 316L

Austenitic stainless

Typical use: Marine, food, chemical and pharmaceutical equipment
Cutting: Slow, work hardens, heavy tool wear on thick sections
Movement risk: Low
Typical tolerance: ±0.05 to ±0.1 mm
Worth knowing: Budget the hours, not the kilograms. Check the shop separates stainless from carbon steel work.

Duplex 2205

Duplex stainless

Typical use: Offshore, pressure and chloride service
Cutting: Slower again than austenitic, high cutting forces
Movement risk: Low
Typical tolerance: ±0.05 to ±0.1 mm
Worth knowing: Not every heavy shop runs it. Say the grade so shops can rule themselves out honestly.

5083 / 6082

Aluminium plate

Typical use: Large light structures, chambers, optical and tooling plate
Cutting: Very fast
Movement risk: High, rolled plate releases stress as material comes off
Typical tolerance: ±0.1 mm, more on thin walls
Worth knowing: Thermal growth is roughly twice that of steel, so measurement temperature matters more.

CuSn12 and bearing bronzes

Bronze

Typical use: Bearing shells, bushings, wear plates
Cutting: Fast
Movement risk: Low
Typical tolerance: ±0.02 to ±0.05 mm on the finished bore
Worth knowing: Usually bored in place after the housing is finished, so the bore matches its seat.

Inconel 625 / 718

Nickel alloy

Typical use: High temperature and corrosive service, turbine and process parts
Cutting: Very slow, hard on tooling
Movement risk: Low
Typical tolerance: ±0.05 mm, tighter with grinding
Worth knowing: Rare on large structural parts and expensive when it appears. Expect fewer shops to quote.

Tolerances above are typical for a competent shop on a sensible geometry, not guarantees. What a part actually holds depends on the span, the number of setups, the temperature it is measured at and whether the sequence included stress relief.

What decides how much a material matters

Blank typeWeldment, casting or forging changes the roughing volume and the stress picture more than the grade does. Read the comparison
Machining allowanceHow much has to come off, and whether it is even on opposing faces. Read the guide
Stress reliefWhether the sequence lets the part settle before the finish cuts. Read the guide
TemperatureSteel moves about 12 micrometres per metre per degree, aluminium roughly twice that. Read the guide

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