Fig 7-39/2018 question 1

[greenimi]

In fig 7-39/ ASME Y14.5-2018

I think we all agree that the two bores should be “squiggly” (in the same way the outside diameter is shown a little “bumpy”), but my question is:
Do you think the correct way is to show the two (“squiggly” to be) holes slightly rotated clockwise? (my red adjustements)

I am comparing 7-38 with 7-39 and I am wondering why the holes in 7-39 are not rotated? Should they be? Shouldn’t they be?
Fig 7-38 looks okay, but if the intent is to show the same part on both scenarios (with and without the translation modifier) then why 7-39 does not show rotated holes?

 

[Doug Hunter]

Hi Greenimi,

Can you also share Fig. 7-38? I don't own a copy of the spec.

 

[greenimi]

Hi Greenimi,

Can you also share Fig. 7-38? I don't own a copy of the spec.

Then if you own (and I expect as a trainer you do) 2009 version of the ASME Y14.5 then figure 4-32 is the one you want to reference.

 

[3DDave]

They are showing the rotation of the datum feature . The features referring to won't rotate.

 

[greenimi]

They are showing the rotation of the datum feature . The features referring to won't rotate.

Why not rotate? Isn't it the same part as in fig 7-38?
I think it is the same intent......

 

[Doug Hunter]

Then if you own (and I expect as a trainer you do) 2009 version of the ASME Y14.5 then figure 4-32 is the one you want to reference.

Sorry, I do not. When I was directly employed the company had a copy. I never bought one for myself. I am not a GD&T trainer, just an experienced user. Are the older versions available for free download (legally)?

 

[3DDave]

Why not rotate? Isn't it the same part as in fig 7-38?
I think it is the same intent......

It is not the same part. It is showing that the datum features are different for the same result.

 

[Burunduk]

greenimi,
Had 7-39 been showing the holes rotated that would suggest the translation modifier provides some kind of additional leeway for the holes (like datum shift), but that is not the case.

 

[greenimi]

It is not the same part. It is showing that the datum features are different for the same result.

So what that does mean? That the datum reference frame does not matter? Therefore, regardless of the datum features you can get the same result?

 

[3DDave]

Same type of comparison made in Fig. 7-36 vs. Fig. 7-37 and so on. The features don't move, the datum features do.

 

[Burunduk]

The datum features are limited by the tolerances qualifying them and don't "move". The datum feature simulator for the clocking datum feature is allowed translation and is unfixed from its basic location relative to the primary datum A axis, to allow better engagement with the actual, as-produced datum feature B.

 

[greenimi]

The datum features are limited by the tolerances qualifying them and don't "move". The datum feature simulator for the clocking datum feature is allowed translation and is unfixed from its basic location relative to the primary datum A axis, to allow better engagement with the actual, as-produced datum feature B.

Well, in fig 7-39 we are NOT talking abot clocking. We are talking about aligning.
So I would say that orientation determines the alignment of the remaining axes of the datum reference frame.

I think clocking is applied when no translation modifier is used (fig 7-38).

 

[greenimi]

The datum features are limited by the tolerances qualifying them and don't "move". The datum feature simulator for the clocking datum feature is allowed translation and is unfixed from its basic location relative to the primary datum A axis, to allow better engagement with the actual, as-produced datum feature B.

So, I am saying that the physical reality of the part in fig 7-39 will make the 2 holes (the patten) to rotate slightly, clockwise.

 

[Burunduk]

Well, in fig 7-39 we are NOT talking abot clocking. We are talking about aligning.
So I would say that orientation determines the alignment of the remaining axes of the datum reference frame.

I think clocking is applied when no translation modifier is used (fig 7-38).

I disagree, 7-39 IS about clocking too. Will explain later if no one else will before me. Stay tuned.

 

[Burunduk]

greenimi,
So here is why the secondary datum feature in fig 7-39 provides clocking:
The short and trivial answer is because it constrains a rotational degree of freedom around a datum axis and that is what by convention called clocking. You could argue that it may provide some extra rotation of the part until the secondary TGC is envolved relative to the fixed-at-basic scenario, but once the relationship of the part to the secondary datum has been established, that rotation is no longer available nevertheless.

A more elaborated explanation may be in terms of how the datum reference frame is established relative to the part. Consider the cylindrical primary datum feature constraining 4 degrees of freedom. It provides a datum axis and two planes of the DRF intersecting at the axis, but those planes are free to rotate like fan blades around it. That is, until the secondary datum (center plane, in this case) is brought in. Then, the secondary datum provides a direction for one of the DRF planes to be parallel to, and for the other one to be normal to. That's when you can say that the part has a relationship to the the X and Y axes, and the holes need to conform to a pattern of 2 tolerance zones or virtual condition boundaries that is not only centered but also vertical relative to the established coordinate system.

 

[3DDave]

The tendency throughout the standard is to show parts in acceptable configuration and orientation, not to act as a tutorial showing parts that fail to meet their requirements.

 

[pmarc]

greenimi,
I agree with you that the holes should have been rotated clockwise in fig. 7-39, but only if the intent of figs. 7-38 and 7-39 was to show the same as-produced part in relation to two different DRFs.

And while I think this should be the intent from the educational point of view, I am not sure there is sufficient evidence that SC5 was thinking the same way.

 

[3DDave]

It would make more sense for Y14.5 to strip the majority of example figures from the standard and publish a help-guide of 500 examples of good and bad applications with related tolerance analysis and writeups of the reasoning. By including these it gives rise to people assuming rules that are not part of the standard, like that the point was to show rotation of the features rather than displacement of the references.

 

[greenimi]

greenimi,
I agree with you that the holes should have been rotated clockwise in fig. 7-39, but only if the intent of figs. 7-38 and 7-39 was to show the same as-produced part in relation to two different DRFs.

And while I think this should be the intent from the educational point of view, I am not sure there is sufficient evidence that SC5 was thinking the same way

pmarc,

I agree with you, but now looking at the figures equivalent from 2009, I hope you agree with me that the figures 7-38 and 7-39 / 2018 were COPIED (or kept unaltered) from figures 4-32 a.) and b.) / 2009

So, in 2009 was just ONE figure (with 2 subfigures “so to speak”, subfigure a and subfigure b)). For me, regarding 2009 standard, it is clear that the intent was to use the same “as made”/ “as produced” part in relation with two DRF’s.

Since the figures were kept unchanged when the transition to 2018 occurred then I would “safely and reasonable” assume that the INTENT stayed the same (namely here to use the same “as produced” parts in two scenarios). When they draw those figures I don’t think his intent was to change anything or use two sets of “as made” parts in 2018 figures. He just changed the titles (figures were renamed, naming from datum feature simulator to TGC), but not the concept.

Do you agree with my assessment? (I don’t have proof, facts and evidence, just using my logical –or not so logical thinking, how much I still have left)

 

[3DDave]

The 4-32 subfigures are of two different parts. I have overlayed them below.