Guides / Materials
How Each Common Heavy Material Behaves While You Machine It
September 13, 2026
On a small part the material decides cutting speed. On a large part it also decides how much the part changes shape while you cut it, and that second effect usually costs more than the first.
Here is what each of the common heavy materials does in practice.
S355, A36 and similar structural steel
The default for welded frames, bases and machine structures. Cheap, widely available, easy to cut and easy to weld.
On the machine: predictable, tooling lasts, high material removal rates are possible with enough spindle power.
What catches people out: it almost always arrives as a fabrication, which means welding stress, flame cut edges with a hardened skin and a generous machining allowance. The cutting is cheap. The roughing volume and the stress relief cycle are not.
Typical expectation: ±0.1 to ±0.3 mm over a metre on a stress relieved fabrication, far tighter on individual features.
42CrMo4, 4140 and similar alloy steel
Shafts, pins, gear blanks and anything that carries load in bending or torsion.
On the machine: slower than structural steel, and much slower once quenched and tempered. Higher cutting forces, so the setup has to be rigid.
The sequence question: machine soft and heat treat afterwards, and the part moves in the furnace. Heat treat first and the machining is slower and tool wear is higher. Most heavy shops rough soft, heat treat, then finish, and leave grinding for the surfaces that need it.
Typical expectation: ±0.05 mm on bearing seats is routine when the sequence is right.
Grey iron GG25 and ductile iron GGG40
Machine bases, housings, gear cases and anything that has to damp vibration.
On the machine: grey iron cuts easily and damps beautifully, which is why it remains the material of choice for machine tool structures. Ductile iron is tougher and more demanding but far less brittle.
What catches people out: casting quality. Porosity and sand inclusions do not appear until you cut into them, sometimes during finishing, and by then the machine time is spent. Ask the foundry standard and the inspection level before committing.
Thermal history matters. An aged casting is predictable. An un-aged one moves like a weldment.
Typical expectation: ±0.05 mm on bores.
Stainless 304, 316 and duplex
Marine, offshore, food, pharmaceutical and chemical equipment.
On the machine: slow. Work hardening means the tool must keep cutting rather than rubbing, heat does not leave the cut easily, and tool wear on thick sections is heavy. Duplex grades are harder again.
What catches people out: the price gap to a heavy shop is larger than the material price gap. Budget the hours, not just the kilograms. Also check that the shop separates stainless from carbon steel work, because iron contamination on a marine or food part is a real rejection cause.
Typical expectation: ±0.05 to ±0.1 mm.
Aluminium 5083 and 6082 plate
Large light structures, vacuum chambers, optical benches, tooling plate.
On the machine: fast and cheap to cut, and a welcome change after steel.
What catches people out: movement. Rolled plate carries residual stress and releases it as soon as metal comes off. Large thin aluminium parts often need roughing, a rest or a stress relief cycle, re-clamping with lighter pressure, then finishing. Thermal growth is roughly twice that of steel, so the temperature at which the part is measured becomes a real question.
Typical expectation: ±0.1 mm, and more on thin walls.
Bronze and lined bearing parts
Bearing shells, bushings and plain bearing housings.
On the machine: easy to cut, but usually bored in place after the housing is finished so the bore matches its seat rather than a drawing.
Typical expectation: ±0.02 to ±0.05 mm on the finished bore.
A summary you can send with the drawing
| Material | Cutting speed | Movement risk | What to watch |
|---|---|---|---|
| S355 / A36 | High | High as a weldment | Allowance and stress relief |
| 42CrMo4 / 4140 | Moderate | Moderate | Heat treat sequence |
| Cast iron | High | Low if aged | Casting soundness and ageing |
| Stainless | Low | Low | Hours and contamination control |
| Aluminium plate | Very high | High | Rolling stress and temperature |
| Bronze | High | Low | Bore in place after the housing |
All of these are typical ranges rather than promises. The shop that quotes you confirms what it holds on your geometry.
What to do next
Put the grade and its condition in the RFQ, not just the family name. “42CrMo4 quenched and tempered to 280 HB” tells a shop everything. “Alloy steel” tells it nothing, and you will get a quote priced for the worst case.
Related: weldment or casting and machining allowance on large blanks.
Frequently asked questions
Which material is cheapest to machine on a large part?
Structural steel and grey iron, by a wide margin. They cut predictably and tooling lasts. The saving does not always survive, because structural steel usually arrives as a weldment and brings stress relief with it.
Does material choice change which shops can quote?
Yes. Thick stainless and high nickel alloys need more spindle power and better tooling, and not every heavy shop runs them routinely. Say the grade in the RFQ so shops can rule themselves out honestly.
Why do aluminium parts move so much?
Rolled plate carries residual stress from the mill. As soon as material comes off one face the plate relaxes. On large thin aluminium structures this is often the hardest part of the job.
Need a quote for a part like this?
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