Check my GD&T please 1

[mkubiak]

I have attached a portion of a drawing I am making at work. No one at work knows any GD&T so I am asking you to critique. My main question is: How do I use the mounting holes (4 outside corner holes) as datums, but also dimension their position using GD&T. Like most of you know, everyone is convinced they know GD&T, but you'll get 5 different answers from 5 different people. Any help is appreciated.

 

[KENAT]

What standard and what version do you work to?

For instance if working to ASME Y14.5M-1994 I'd recommend you look at section 4.5.8 & figure 4-22.

You have to consider that if you are setting the hole pattern as the datum feature then other features effectively need to be dimensioned back to the derived center planes.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[greenimi]

Can you tell us how this thing works?

 

[mkubiak]

4 outside holes are mounting holes for connecting this part to the next higher assembly. Two large holes in the middle are o-ring lands that also mate with the next level assembly. 4 inner holes mate to a sub assembly on the far side of the part. That's about all I can tell you.

 

[greenimi]

Considering you are working in Y14.5-1994 or older (for Y14.5-2009 S is not allowed)
Datum feature A: okay as is
Datum feature B: the pattern of the 4 holes 4x Ø.209±.005
Datum feature C in your picture: not needed.

Position everything else (all the other holes) to A primary and B (at MMC) secondary.
Simultaneous requirement will take care of the rest if you position everything else to A primary and B (at MMC) secondary.

A side note: in theory, you can apply B (RFS), but the standard does not tell you how you should treat that pattern and you should explain to the end user how you want the pattern to be understood---I will just stay away from that. Again, first of all because it is an uncharted territory and second because the mounting holes are assembled with clearance so it is a classical MMC (or MMB--per 2009) application.
Hope this helps more than confuse

 

[mkubiak]

That clears a lot up! Thanks for making me look smart...

 

[Tunalover]

The way you have Datum B specified, the suction cup is attached to a single hole and, as such, you are making the lower RH hole Datum B, not the entire pattern. You did the same thing with Datum C (which you don't need anyway). Lastly, you don't need the RFS symbols in the FCFs because the lack of a symbol means RFS per the 1994 standard.


Tunalover

 

[SeasonLee]

Using the 4 holes pattern as datum feature is a smart way to establish the DRF (as mentioned by greenimi), this sets the datum origin at the large center hole. Based on certain condition, you may select one of the four holes as secondary datum feature (datum B as sketched on your post) and the other hole as tertiary datum feature (datum C as sketched on your post); in this case, the datum origin will be at datum feature B, that is the difference between these two design schemes. You may find out all discussions on the recent thread regarding this issue.

http://www.eng-tips.com/viewthread.cfm?qid=373638

RFS modifier circle S is no longer used if you are using Y14.5M-1994 standard, and be sure to add the diameter symbol Ø on all position FCF callouts for the cylindrical tolerance zone.

Season

 

[Belanger]

You should keep a position tolerance on the four holes, but the only datum at the end of that feature control frame would be A. Then, as tunalover was saying, change the datum B triangle symbol so that it hangs down from that feature control frame; this will communicate that it's a group datum.

Also, add a diameter symbol before the number in each of the feature control frames for position.

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

 

[Michael854]

If we're talking about the 94 standard, the position control on the 4 hole pattern cannot be @ RFS. This would imply checking with 4 spring-loaded tapered pins...with each pin "winning." That physically does not make sense.
The design may be such that all the features of size and fasteners are identical...but it will be impossible to tune all 4 springs. So, when you gage, you will then get 1 master locating hole instead of the pattern...And you might find that since the springs fight each other, they might wear out quickly.
I'd use an MMC modifier on the control. That would imply a pattern of 4 fixed diameter pins for gaging. Besides a more-friendly gage maintenance schedule, it would simulate the function of the part a little better.

 

[greenimi]

Michael854
"If we're talking about the 94 standard, the position control on the 4 hole pattern cannot be @ RFS"

Where is the standard does it say. Please point the paragraph.
I don't think is illegal. It is not encouraged, but I willl not say CANNOT be RFS.

As I stated before:
"A side note: in theory, you can apply B (RFS), but the standard does not tell you how you should treat that pattern and you should explain to the end user how you want the pattern to be understood---I will just stay away from that. Again, first of all because it is an uncharted territory and second because the mounting holes are assembled with clearance so it is a classical MMC (or MMB--per 2009) application."

 

[Michael854]

Greenimi,
The illegality is based on a violation in Chapter 4 Datum Referencing.
If one establishes 4 equal holes all toleranced @ RFS as per the example, there is no correct way to mathematically construct the datum reference frame's secondary datum plane...unless we allow theoretically perfect datum planes to bend.
It is possible with MMC because we can construct at least 1 DRF. A single axis that is perpendicular to the primary datum can produce infinite solutions...we can see this because the part will float on the gage.
However, 4 distinct datum axes, each perpendicular to the primary...at varying rative locations does not produce any solution.
This means that the part will not fit on the gage...or the CMM will not be able to align unless the operator does a workaround...arbitrarily selecting 1 hole as B....and 1 hole as C.

 

[greenimi]

Sorry, I don't buy the fact that is ILLEGAL.
"The illegality is based on a violation in Chapter 4 Datum Referencing"

From a product definition point of view I don't think is ILLEGAL.
From the ispection point of view you could be right --there are some issues-- but remember Y14.5 is not an inspection standard.

 

[Michael854]

Greenimi,
4.2.2
...Therefore, it is necessary to establish a method of simulating the theoretical reference plane from the actual features of the part.

I guess we'll have to agree to disagree.

Best regards,

Michael Liu
ASME GDTP S-0470

 

[greenimi]

So, why 4.2.2 make it illegal?
Why you cannot establish that method?
How? Since the standard does not go there, the designer has to specify that method. But again, that does not make it illegal.
Just by me

 

[Michael854]

Greenimi,
There are quite a few topics that are not handled well in the standard.
This is one.
Perhaps this kind of confusion would be eliminated if a mathematics person were on the board.
I'll try to explain.
When there is a single hole as a datum fox @ RFS, the intent is for the mating part's corresponding fos will attach to the part such that the axes of the centerlines align. So, the fastener will expand as needed, so there is no float of the fastener. It might help to think of a rivet being crushed into place.

Remember that rivets are supposed to have zero sheer loading.

Now, picture trying to rivet together the example part. Hopefully you can see that there is no way to align the 4 axes of the holes to a mating part or a gage, even if the mating part or gage was made perfectly. One of the fos would become master. We would activate each of the 4 rivets one at a time. If the part was completely arrested after the 1st rivet, then that fos would be the locating fos, and there would be some float @ each of the other fasteners...and some sheer loading.

Chapter 4 of the standard is written with an assumption of rigidity. In other words, belief that all parts are constrained adequately by arresting 6 degrees of freedom. The part looked pretty rigid to me...and it didn't look like a kinematic assembly either.

So...looking at this for a different kind of part...if we are talking about a kinematic assembly, or flexible part it might be possible for us to align 4 axes...but I would argue that the 4 would not really function as a pattern.

I hope this helps.

Michael

 

[Belanger]

Michael... since the 2009 standard does allow for a pattern at RMB, what has changed in the "theory" to allow for this now? I'm looking at para. 4.12.4, compared to the old standard's para. 4.5.8. While RMB is now mentioned by name, was there something outside of the old 4.5.8 that made RFS "illegal"?
I agree that it's still not very practical, but you were emphasizing the theory aspect making it illegal, so I'm curious about your thoughts.

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

 

[3DDave]

JP - that's a great question. I think that the lack of supporting documentation for decisions made by the Y14.5 committee tells all that is necessary. Enough members voted to add it in; no explanation required. Remember, a similar process made symmetry disappear for a while.

 

[Michael854]

John-Paul,
I think the 2009 change must have been made for parts that are not rigid. I have not read that chapter of the 2009 standard to the level where I could speak of that as anything but a guess though.
In terms of a rigid part though...with a requirement that at least 1 datum reference frame must exist for the datum features referenced...I don't see how anyone could draw 1.
Of course, if one says float can be allowed when features locate @ RFS, theoretical planes are not necessarily flat, points can have depth, or some other axiom like that, then someone could draw a drf.
But, obeying the axioms of euclidean geometry and rules of Y14.5M, does not allow us to construct a drf if the part is rigid.

 

[Michael854]

Let's say we have a rigid rectangular solid.
Someone put 3 well-spaced target points on the largest planar surface A1, A2, and A3...3 target points on the next largest plane B1,B2,and B3...and 3 target points on the tertiary surface...C1, C2, and C3.

If the first example is legal, this one is legal too.