Datums coaxial A-B

Question

Datums coaxial A-B

rje2019 (Mechanical)

(OP)

28 Mar 19 15:55

Hi I'm trying to clean up Gtol prints for different engineers but I don't recommend changes until I have confirmed that the changes I recommend follow standard. I couldn't find this in the book specialty

Ok I have an assembly situation

this is what Eng1 has listed
two bushings space 24plus inches apart welded in place (Intended to be coaxial). the left bushing is Dia x.xxx position .xxx mmc A, creating Datum -A1- (attached to feature frame). Right bushing is Dia x.xxx position .xxx mmc A creating Datum -A2- (attached to feature frame). he then uses these two bushings as Datum -A- in other feature frames.
From my understanding the left side should be dia x.xxx creating datum -A- and right side should be dia x.xxx position .xxx mmc A, creating datum -B- and all future feature frames should call out A-B to establish the two bushing as a datum.

my assumption is Eng1 is trying to use the datum target concept A1, A2, Etc using the datum flag to create a datum thru the two bushings. I believe this is incorrect application. plus he is referencing the datum to itself

Let me know if I'm off on this

rje2019 (Mechanical) · Answer 1 · 2019-03-28 16:18:34

I created a sketch to present this better

https://files.engineering.com/getfile.aspx?folder=c6624f88-c957-4ef2-a541-1

chez311 (Automotive) · Answer 2 · 2019-03-28 17:00:50

rje2019,

What standard are you working to? Y14.5-2009 or an ISO standard?

rje2019 (Mechanical) · Answer 3 · 2019-03-28 17:28:09

RE: Datums coaxial A-B

rje2019 (Mechanical)

(OP)

28 Mar 19 17:28

yes Y14.5-2009

Belanger (Automotive) · Answer 4 · 2019-03-28 17:40:58

Per ASME Y14.5-2009 (paragraph 3.3.2), those are not valid datum feature symbols. If you're using the datum feature symbol (the triangle type), then only letters can be used (except for I, O, Q).
If using datum targets, then you can get into the letter/number combination.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

rje2019 (Mechanical) · Answer 5 · 2019-03-28 18:39:51

Yes I agree. and I would also say datum targets would not apply to a hole because datum targets needs line from balloon to phantom line, point or center line. this would suggest to me that a, b, etc ahould be used and feature frames should be listed A-B

chez311 (Automotive) · Answer 6 · 2019-03-28 19:07:53

I agree with JP, regardless if the intent was to utilize datum targets or multiple datum features they have been improperly applied.

If the top 4x diameters are one size and the bottom 2x are another size the simplest thing to do would be reference the bottom 2x pattern as datum A (no datum references in position tolerance), top 4x pattern as datum B (reference datum A in position tolerance) and reference it as A|B in the FCF on the controlled feature in the center. Is there any reason you wouldn't want to do this?

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