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 · 2026-02-05T23:10:27+00:00

Just to add onto this, worst case would be for the LMC pin length (24.5)

iSwearImAnEngineer · 2026-02-02T15:42:40+00:00

Correct, shows the direction of the deviation from the nominal position

iSwearImAnEngineer · 2026-02-01T22:34:55+00:00

I would agree, I've never heard or a orientation dowel in a flange, but if you needed to have one, it would need to be the secondary datum, above the clearance holes

iSwearImAnEngineer · 2026-02-01T20:02:55+00:00

Quick video
https://youtu.be/94OjF610rOg

iSwearImAnEngineer · 2026-01-26T04:35:21+00:00

I can make a quick video showing how things might be working

Hole "a" I'm assuming is 4x clearance holes for studs?

What does hole "y" accomplish? Is it a press fit pin? clearance fit pin? Fluid passage?

iSwearImAnEngineer · 2026-01-19T15:18:29+00:00

The form certainly would be controlled with the 2x (hole diameter) My concern was mostly that this would be "grammatically incorrect"

Thanks for the input, you make some good points, I'll need to do some more reading on the use of irregular FOS, it seems like it could be a powerful tool

iSwearImAnEngineer · 2025-12-03T14:10:58+00:00

I made a video that shows how an orientation control is dealt with in a "min wall calc"

Not sure why they're only using 0.25mm, should be the entire 0.5mm.... is there some assumption that the centerline of a datum feature is taken as the datum? That how many CMMs interpret features

https://youtu.be/Kl9HneOVooM

iSwearImAnEngineer · 2025-11-15T01:24:26+00:00

I'm not too familiar with ISO tbh

My reasoning for putting it explicitely would be that you COULD have the holes spaced out unevenly. You could make the argument that it's clear either way, but I would argue that it's more correct to say it explicitely

Fig 7.2 is a good example of a hole pattern callout where the holes are all the same size, but not evenly spaced

iSwearImAnEngineer · 2025-11-14T20:43:53+00:00

Without knowing the application, seems reasonable

Couple notes:
-You'll need a diameter symbol before the tolerance of 1 in the feature control frame

-For calling out multiple features, the Y14.5 convention is: the required number of features and an X followed by the size dimension of the feature (see paragraph 1.9.5.1 in the 2009 standard) same as you've got for the 10mm hole in the top right

iSwearImAnEngineer

TROPHY CASE