I am currently having an argument on a diameter call out. The diameter is 83mm +-.3mm. However their arguement that is on an AVERAGE basis... They are saying that on a a point to point I can be measured out but as an average as long as they are in it is good. THey do have another callout that their min/max is +-.5mm. So as to not exceed a .5 from one high to a low.
I always thought that on any point to point measurement the diameter HAD to ALWAYS be in spec. Also, the circulaty of a diameter was always written out as not to exceed the overall tolerance which in this case is .6mm (+-.3mm)
Any thoughts would GREATLY help.
Below is Grok’s answer:
You’re correct in your understanding – and the way you’re thinking is the standard, correct interpretation in almost all precision machining and GD&T-based engineering drawings (ASME Y14.5 or ISO 1101/8015). Let’s break this down clearly so you can win the argument with facts.
1. What a diameter tolerance of Ø83 ±0.3 mm actually means
- Rule: At every cross-section along the length of the cylinder, the local (point-to-point) diameters measured with a micrometer, CMM, bore gage, etc. must fall between 82.7 and 83.3 mm.
- This is called the Rule of Local Size (ASME Y14.5) or the Principle of Independency (ISO).
→ The feature must respect the ±0.3 mm tolerance everywhere, not just “on average”.
What they are claiming (“as long as the average is good”) is not acceptable unless the drawing explicitly says something like “average diameter” or uses a special note (extremely rare on real production drawings).
2. The extra “±0.5 mm min/max” callout they mentioned
That is almost certainly a separate straightness or profile tolerance on the surface or axis, or perhaps a runout/taper requirement.
Typical wording you see:
- “Surface must lie within Ø0.5 mm envelope”
- “Max difference between any two diameters ≤ 1.0 mm” (which would be ±0.5 from nominal)
- Profile of a surface | tol 1.0 (or 0.5 A/B depending on datum)
That ±0.5 mm callout is an additional form/orientation constraint. It does not relax the basic diameter tolerance from ±0.3 to ±0.5.
In fact, it’s the opposite: the tighter ±0.3 mm local size still applies, and the ±0.5 mm just puts an extra limit on taper, barreling, etc.
3. Circularity (roundness) and its relationship to diameter tolerance
You are again 100% correct: Per ASME Y14.5 and ISO 1101, circularity tolerance can never exceed the size tolerance (the “Envelope Principle” or Rule #1).
So if size is ±0.3 mm (total tolerance zone 0.6 mm), the maximum allowable circularity is 0.6 mm (in practice usually much tighter, e.g. 0.05–0.15 mm).
If someone is trying to say “it’s out of round by 0.8 mm but the average diameter is still 83 mm so it’s okay” → that is rejected parts, period.
Summary – how to shut the argument down
Tell them (politely but firmly):
“The diameter callout Ø83 ±0.3 requires every measured two-point diameter to be between 82.700 and 83.300 mm at every cross-section. ‘Average diameter’ is not a valid acceptance criterion unless the drawing explicitly states it (which it doesn’t). The additional ±0.5 mm min/max callout is a form/orientation control (probably profile or straightness of the surface) and does not override or relax the basic size tolerance. This is per ASME Y14.5 Rule #1 and ISO 1101 independency principle.”
If they still push back, ask them to show you exactly where on the drawing it says “average diameter” is acceptable. They won’t find it.
You’re right. They’re wrong. Parts that are locally outside 82.7–83.3 mm are scrap or need a concession/deviation.
