Guides / Process
Why a Four Metre Steel Part Changes Size During the Shift
September 5, 2026
Steel expands when it warms. Everybody knows this and almost nobody accounts for it, because on a normal machined part the effect is smaller than the tolerance.
On a large part it is not. It is often a large fraction of the tolerance, and it is the reason a part that measured correctly in the machine measures differently at goods inwards.
The arithmetic
Linear expansion coefficients, per metre per degree Celsius:
| Material | Approximate expansion |
|---|---|
| Carbon and low alloy steel | 11 to 12 micrometres |
| Stainless steel, austenitic | 16 to 17 micrometres |
| Grey cast iron | 10 to 11 micrometres |
| Aluminium | 23 to 24 micrometres |
Work it through on a four metre carbon steel frame. A three degree rise over a shift gives about 0.14 mm of growth along its length. If your positional tolerance across that length is 0.1 mm, the temperature has already spent the whole budget before any machining error is counted.
Now do the same for a four metre aluminium structure. The same three degrees gives about 0.28 mm.
Where the heat comes from
The cut. Roughing puts real energy into a part. A heavy roughing pass can leave a large steel part noticeably warm, and it does not cool evenly.
The machine. Spindle and drives warm up over the first hours of a shift, and the machine’s own structure grows with them. This is why the first part of the day and the fourth can measure differently on the same programme.
The workshop. Doors opening, sun on a roof, night and day. A hall that swings eight degrees between night and afternoon will swing your part with it.
Coolant. Flood coolant both removes heat and introduces its own temperature, which may not be the same as the air.
How shops manage it
Let the part stabilise. The simplest method. After heavy roughing, leave the part on the table overnight so it reaches room temperature before finish cuts. It costs a shift of machine time and it removes most of the problem. On a tight part this is normal practice, not a luxury.
Probe in process. Touch off the datum features immediately before finishing, so the programme works from where the part actually is rather than where it was this morning.
Finish in one pass, quickly. If a long face has to be flat, cutting it in one continuous operation means the whole surface is machined in roughly the same thermal state. Splitting it across a break introduces a step.
Measure at a stated temperature. Record what it was. A measurement without a temperature is not repeatable on a long part.
Control what can be controlled. Coolant temperature, machine warm-up cycles, and keeping the part out of a draught or direct sun.
The dispute this prevents
This is worth spelling out, because it causes real arguments.
A shop machines a four metre frame, measures it at 26 degrees on the machine, and reports it in tolerance. The part ships, arrives at a plant at 15 degrees, is measured and comes back 0.13 mm short.
Nobody is wrong. Both measurements are correct for the condition they were taken in. The only way to settle it is to have agreed the reference temperature in advance, which is what the twenty degree convention exists for.
Put it on the drawing and the argument never happens.
What to write
On any part with a tight requirement over more than a metre:
All dimensions apply at 20 degrees Celsius. Part to be stabilised at measurement temperature before final inspection.
And if the part has to be measured somewhere the temperature cannot be controlled, add how the correction is handled:
Measurements taken outside 20 ± 2 degrees Celsius to be temperature compensated using the nominal expansion coefficient for the material, and the measured temperature recorded on the report.
What this means for lead time
Thermal stabilisation is time on the machine or time in the shop, and it shows up in the lead time rather than in the cutting hours. A quote that includes an overnight stabilisation before finishing is a more considered quote than one that does not, and it will be longer. That is the trade.
What to do next
Work out how much of your tolerance the temperature swing in your own plant would consume. If it is more than about a fifth, put a reference temperature on the drawing before you send it out.
Related: tolerance across a large part and inspecting large machined parts.
Frequently asked questions
How much does a large steel part actually move?
Carbon steel expands roughly 11 to 12 micrometres per metre for each degree Celsius. A four metre part that warms three degrees has grown about 0.14 mm. Aluminium is roughly twice that.
Do shops control workshop temperature?
Some do for precision work, but a fully controlled hall for a four metre part is rare. More often shops manage it by letting the part stabilise, measuring at a stated temperature and probing in process.
Should I put a measurement temperature on the drawing?
Yes, on anything with a tight requirement over a long span. Twenty degrees Celsius is the usual reference. Without it, the shop and your inspector can both be right and still disagree.
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