ISO 2768 General Tolerances: What the Title Block Means
ISO 2768 general tolerances set limits for every untoleranced dimension by size. What class m allows, why mK needs a look, and how to word the title block.
A drawing lands with 40 dimensions and tolerances on six of them. The title block decides the other 34, and ISO 2768 general tolerances are one of the two usual ways to fill it in.
Should a Title Block Say ISO 2768 or a Plain ± Tolerance?
Either works if it's written out in full. ISO 2768-m scales the tolerance with feature size and plenty of shops know it, while a flat ± note is easier for a supplier who doesn't own the standard. A bare "ISO 2768" with no class is the one version that doesn't work.
That was the question in an r/engineering thread from a US team switching its drawings to metric. Their inch title blocks set tolerances by decimal places, and that breaks once trailing zeros are dropped. A reply from Germany said "general tolerances ISO 2768-mK" in the title block was what they'd always used. An engineer in Australia said his team prints the actual ISO 2768 numbers in a small table on the sheet, so nobody needs a copy of the standard to read the drawing.
What ISO 2768 general tolerances cover
ISO's page for ISO 2768-1 says it sets general tolerances for linear and angular dimensions in four classes, for parts made by metal removal or formed from sheet metal. The classes are f (fine), m (medium), c (coarse) and v (very coarse).
The allowance grows with the feature. Class m gives ±0.1 mm from 0.5 to 6 mm, ±0.2 mm from 6 to 30 mm, ±0.3 mm from 30 to 120 mm and ±0.5 mm from 120 to 400 mm. That's the real difference from a flat ±0.12 mm note. A 300 mm plate gets room to move, and a 4 mm boss stays tight without anyone writing a number on it.
The "K" in mK is the other half. It calls up ISO 2768-2 for geometric tolerances like flatness and straightness, and ISO lists that part as withdrawn and replaced by ISO 22081:2021. If your template still says mK, it's worth a review. Our post on straightness vs flatness callouts explains what those controls do.
How we read it
Our CNC page lists "ISO 2768-m as standard, down to ±0.01 mm where the drawing calls for it", and says tighter tolerances carry a price uplift shown in the quote. If your drawing names a different class, or still carries the K, ask us when you quote how we'll read it. Either way, let ISO 2768 general tolerances cover the rest and put tight numbers only on the features that need them. Each one adds machining and inspection time, which is a big part of what drives a CNC quote, and each is one more thing a first article inspection has to measure.
One more thing before you update the template. ISO's page says Part 1 is expected to be replaced by a new ISO 2768 within months, so check which edition you're citing.
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