Guides / Process
Proving a Four Metre Part When It Will Not Fit on a CMM
September 4, 2026
Measuring a 300 mm part is a solved problem. Put it on a coordinate measuring machine and read the numbers.
Measuring a four metre welded frame is not, because nothing that size fits on a CMM and moving the part to the measuring room is often harder than machining it. Heavy shops use a different toolkit, and it is worth knowing what it can and cannot do before you write an inspection requirement.
In-process probing
A probe in the machine spindle touches the part and reports where features actually are.
What it is for: finding the part after clamping, confirming the datum before finishing, and checking a feature before the part comes off the table. It is the cheapest insurance in heavy machining, because a feature found wrong while the part is still located can be re-cut. Once the part is on the floor it cannot.
The limit: it measures in the machine’s own coordinate system. If the machine’s geometry is off over a long travel, the probe agrees with the machine rather than with reality. It proves the machining, not the part.
Laser tracker
A tracker sends a beam to a reflector that an operator moves over the part. It measures angle and distance to work out the reflector position in three dimensions.
What it is for: overall geometry on large parts. Bore positions metres apart, flatness across a long face, squareness between structures, alignment of an assembled machine.
Why it suits heavy work: the instrument goes to the part rather than the part going to the instrument, and it holds useful accuracy over ranges of tens of metres. On anything past a couple of metres it is normally the most accurate practical option.
The limit: it needs line of sight to the reflector, so deep bores and shadowed features are awkward. Setup and operator skill matter more than with a CMM.
Portable arm
An articulated arm with a probe or scanner on the end, mounted near the part.
What it is for: features within the arm’s reach, typically a couple of metres. Good for bores, edges and local geometry, and for scanning a surface.
The limit: reach. On a large part it usually means leapfrogging the arm to new positions and tying the measurements together, which adds uncertainty at each move.
Conventional methods that still earn their place
Do not dismiss these. On heavy work they are often the right answer.
- Bore gauges and micrometers for the bore sizes themselves, which is where the tightest tolerances usually are
- Straight edges and feeler gauges for local flatness on a mounting strip
- A precision level for long straightness, which is how machine beds have been checked for a century and still works
- Optical or laser alignment for shaft and bore alignment over distance
A sensible inspection plan mixes methods: a tracker for the geometry across the part, hand metrology for the feature sizes.
Temperature, again
Everything measured on a large part depends on temperature. A report without a recorded temperature is not comparable with a second report taken elsewhere. See thermal growth in large part machining.
Ask for the temperature on the report. It costs nothing and it prevents the most common inspection dispute in heavy machining.
What to ask for in the RFQ
Which features are inspected. Not all of them. The two or three that carry the function, as identified in tolerance across a large part.
By what method. Say the requirement, let the shop choose the tool, but ask them to state it.
In what condition. Clamped in the fixture, free on supports, or in the service orientation. On a large part all three give different numbers.
At what temperature. Twenty degrees Celsius is the convention.
In what format. A dimensional report against a numbered drawing, a first article inspection form, or a tracker report with a point cloud. Say which you need.
Whether it gates the batch. If piece one has to be approved before piece two starts, say so, because it changes the shop’s schedule and the lead time.
What good looks like
A useful report on a heavy part is short. It lists the features you asked for, gives measured values against nominal and tolerance, names the instrument, records the temperature and the condition, and is signed. Anything longer is usually padding, and anything shorter is not evidence.
What to do next
Write the inspection requirement at the same time as the tolerances, not afterwards. A tolerance nobody has worked out how to measure is a dispute waiting to happen, and on a part this size the dispute is expensive for both sides.
Related: large bore machining and coaxiality and how to choose a large part machining shop.
Frequently asked questions
How accurate is a laser tracker compared with a CMM?
A bridge CMM is more accurate on a part that fits inside it. A laser tracker holds accuracy in the region of hundredths of a millimetre over tens of metres, which no CMM can reach at all. On a four metre part the tracker is the more accurate option in practice.
Can the part be measured while it is still on the machine?
Yes, with a spindle probe, and on large parts it is often the only practical way to check before the part is released. It measures in the machine's own coordinate system, so it proves the machining rather than independently verifying the part.
Who should pay for the first article report?
It is a real cost in hours and it belongs in the quote. Ask for it explicitly in the RFQ so every quote includes it, otherwise the cheapest quote is simply the one that left inspection out.
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