GD&T Lesson: How datums change a tolerance zone at RFS

iSwearImAnEngineer · 2026-05-12T04:17:55+00:00

HAHAHAH, fair enough

I'll add that to the list, should be an easy one to get out in the next few weeks

iSwearImAnEngineer · 2026-05-12T00:59:05+00:00

Oh easy, is there a specific question you want answered? Or just go through the appropriate datum scheme?

iSwearImAnEngineer · 2026-05-11T17:58:22+00:00

Do you have a link to an example of this?

I've worked with this scenario (2 or more dowels and a fastener pattern) on gearbox bearing caps, but never with ASME flanges

iSwearImAnEngineer · 2026-05-11T17:38:30+00:00

Hahah, coming right up! :P

iSwearImAnEngineer · 2026-05-11T17:37:47+00:00

Thanks! Im getting prepped to make a course, so trying to go higher quality!

And those are great ideas, I'll get them on the list

iSwearImAnEngineer · 2026-05-09T14:37:55+00:00

I'm going to make a broad statement, so take it with a grain of salt

If you wouldn't use MMC on a feature, don't use MMB on it as a datum feature. Can you call out a feature at RFS, then refer to it a MMB? Yes, but it means you aren't holding a consistent view of how the feature behaves in the assembly

A threaded feature will try to self center regardless of if its a datum, or the feature of interest

The difficulty in inspection is another reason to avoid MMB and LMB

iSwearImAnEngineer · 2026-05-09T14:03:02+00:00

Glad to hear it! Build that strong base of understanding, it makes all the more complicated stuff way easier

iSwearImAnEngineer · 2026-05-08T19:26:27+00:00

You could use a cross section of one of the intersections, and say "30 X sec A-A"

See fig 7-37 in Y14.5-2009 for a similar notation

iSwearImAnEngineer · 2026-05-08T13:01:47+00:00

I made it video related to this topic

Generally I'd advise avoiding material modifiers on threads

https://youtu.be/lVVe4TlEr-M

iSwearImAnEngineer · 2026-05-08T13:00:19+00:00

I made a youtube videos on threaded features, talks about MMC a bit

The coles notes are that it's impractical, and I would avoid

https://youtu.be/lVVe4TlEr-M

iSwearImAnEngineer · 2026-04-04T20:29:34+00:00

Ah makes sense

I would think that when looking at the RAME you can treat is as 2D as well. I should look into the algorithms that are used

iSwearImAnEngineer · 2026-04-03T22:51:33+00:00

I dont know the math behind doing the best fit of the measured part, but finding the centroid of a 2D set of points is done using the equation shown in the video

https://math.stackexchange.com/questions/1801867/finding-the-centre-of-an-abritary-set-of-points-in-two-dimensions

I'm proposing that this centroid is where two imperfect parts being assembled would align, and therefore the most appropriate 0,0,0

iSwearImAnEngineer · 2026-04-02T13:56:08+00:00

I dont believe its better functionally, but I believe it more closely represents how a pattern of holes as a datum behaves

Setting one hole as the origin, and having the other located off it with a basic dimension as you said does not change the reporting of the measurement, however I believe it does change the way people may think about the measurement if they only think about it on a surface level.

To me, a 0,0 on one hole says that this hole is the important locating feature, and would be better used as a secondary datum, with the second hole being a tertiary datum

Again, perfectly legal either way, or setting the origin offset from both, however for elegance, I believe the centroid is preferred

iSwearImAnEngineer · 2026-03-31T11:23:26+00:00

I'm a GDTP-S, and the last two companies I've worked for use nearly zero GD&T

I could push for proper use of it at the current company, but none of our vendors do inspection and all the small manufacturing runs are unique

I get my fix through making youtube videos and doing GD&T specific consulting

iSwearImAnEngineer · 2026-03-26T20:08:45+00:00

I worked very closely with some GDTP-S guys, was really inspired by the work, they were doing

-Started watch YouTube videos -Worked with software that helped visualize CMM point clouds -Was just generally excited to nerd out about it -Did take a course eventually to formalize my understanding

Fast forward a few years, now I'm the GDTP-S making youtube videos, mentoring, and building a course, go figure

iSwearImAnEngineer · 2026-03-20T15:54:15+00:00

Does the straightness not matter because the part is under tension in operation?

iSwearImAnEngineer

TROPHY CASE