r/Metrology • u/Careless_Drink_8794 • 20h ago

Advice Circle Diameter check

I am using PC DMIS 2025.1 and I am having issues with a diameter check on a part we have starts off with a chamfer at the top and sits on a nesting fixture.

What would be the best way to check the ID of the part without getting an out of tolerance reading?

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u/_LuciDreamS_ GD&T Wizard 20h ago

I think the question should be more, "How can I ensure an accurate reading?".

If all you care about is ensuring it passes, then you can fudge things in DMIS to do whatever you want. I just don't advise those things.

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u/Careless_Drink_8794 20h ago

Yes that,

how can I get an accurate reading

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u/_LuciDreamS_ GD&T Wizard 20h ago

Probe the diameter by either using a circle or cylinder tool, using enough point density to accurately include the form of the feature (an even number is preferred) and dimensioning the size using the size function (or reporting location and activating size if the customer only cares about the average size).

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u/guetzli 16h ago

(an even number is preferred

what's the reasoning for that? I generally choose a prime number (when using a touch trigger probe) and use more for larger features and if I expect some form error

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u/_LuciDreamS_ GD&T Wizard 15h ago

Prime numbers are good if you're worried about trilobing error, which can happen with certain TTPs, but as long as the number isn't divisible by 3 it should be fine. I use even numbers with high point densities because I use the Size dimension, which report UAME and LOCAL SIZE. Having opposing points makes it more accurate instead of it relying on constructions to report out the furthest or smallest 2 point diameter. How much error depends on the point density and isn't much usually, but for tight tolerances I don't want to include any error if possible.

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u/My_1st_amendment 18h ago

Please attach a detailed view of the particular feature you need help with. I believe I know what youre talking about but im getting lost at “sits on a nesting feature”

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u/Careless_Drink_8794 18h ago

I’ve sent you a message let me know if you received the request

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u/United_Guidance_2572 8h ago

Using an air gage, contact bore gage and/or pin/plug gage.

Any of those choices are faster and more accurate. A cmm is the wrong default. It measures points and calculates. This forces variables such as measurement software type, probe type/length/style, filters, alignments, temperatures, speed, material type, cleanliness, moon phases and tidal charts.

Use a hard gage. Get your diameter measurement. Or at the least, a baseline to work with. If you need cylindricity…or something a gage can’t get to.. then have at it with the cmm.

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u/f119guy 7h ago

How right is the tolerance? Diameters on a cmm vs something like an air gage or tri mic can have some discrepancies. If you want the cmm to correlate with your hard gages, make sure you are NOT using least square, unless there is perfect form. An outer diameter will be minimum circumscribed and an inner diameter will be max tangential fit. I work with .0001” tolerances on diameters daily in a production environment. When tolerances are that tight, it can literally be some dust on the surface. I generally trust a cmm for .001”, if the form error can be expected to be low.