another one, holes on angled surfaces 1

[bxbzq]

In this sketch, datum feature A and B are equal important. Each of them sits on a surface at same time, then bolted down through the two clearance holes. My question is:
1. Is the datum setup OK?
2. Given the datum hole pattern C and D, how to define their interrelationship?
3. Is the position callouts for the two dia 28mm holes and 30mm thick bar correct?

Thanks.

 

[MikeHalloran]

C can't have a true position wrt A. It could have 'half' of a true position wrt B.
Similarly, D can't have a true position wrt B, but it could have half of a true position wrt A.
The thickness dimension of the diagonal bar cannot have a true position wrt to anything.



Mike Halloran
Pembroke Pines, FL, USA

 

[Belanger]

Mike, those position callouts (C wrt A and D wrt B) are legal because they are applied to 2 holes each. However, there's nothing to line up those 4 holes; the two 2-hole patterns could be anywhere on the datum planes. So I think more is needed to get the OP's intent.

I wouldn't hyphenate datums A and B (co-datums, or whatever you want to call it) because they don't act in the same direction of space.

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

http://www.gdtseminars.com

 

[pmarc]

bxbzq,
By applying perpendicularity callout to datum feature B wrt A, you somehow made A of higher importance than B. I would recommend profile of surface callout without any datum reference applied to both surfaces simultaneously. Mutual relationship between profile tolerance zones would be defined through basic 100 degrees dimension.

Why not to assign all 4 holes as datum feature C through position callout? This would solve the problem of interrelationship between 2 pairs of holes.

For position of 2 dia. 28 holes basic dimension is (are) missing. Either one between the axes or two identical from center plane of the part to each axis.

For position of 30 mm thick bar another basic dimension is missing from the center line passing all 4 holes to the center of the bar.

 

[bxbzq]

Something like this?
There are more questions:
1. When no datum referenced in profile control, it is form control. Specifically, a flatness control. So how are the datum feature A and B related to each other by form control?
2. How does the datum reference frame look like?
3. How to locate the 2 ø28mm hole and the bar vertically in the bottom view?

 

[pmarc]

bxbzq,
1. No, if you do something like on the attached picture. I mean form separately as well as mutual orientation of both surfaces will be controlled.

2. DRF is a 3 mutually perpendicular planes (see the picture):
- #1 - plane passing through the center of V;
- #2 - plane passing through the vertex of V, parallel to the bar, perpendicular to #1;
- #3 - plane passing through the center of 4 holes, perpendicular to #1 & #2.

3. Locate them from vertex of V.

I am just wondering if my initial idea of positioning all 4 holes together was functionally correct. It rather seems that each pair of holes should be controlled separately wrt to proper planar primary datum.

http://files.engineering.com/getfile.aspx?folder=9397f605-fdb7-44fb-9590-e24b1a6db1b4&file=drf.JPG

 

[bxbzq]

Wow... How come adding 2X to profile callout will magically control mutual orientation of the two surface? This is like play big game relying on little trick.
About the 2 pairs of holes, I think would I go as my first sketch shows, add interrelationship, which is not easy.
Something more.
Functionally I think using A-B as one datum makes excellent sense, but in the std I don't find something supports the usage of two angles surfaces as ONE datum. The '09 std only shows coplanarity application. I think this is also Belanger's doubt. Similarly, use the 2 non-parallel pairs of holes as ONE datum.

 

[Belanger]

Using A-B isn't the way to go because you can't sit on both of them with equal precedence. They are at 90 degrees, so it's like the generic rectangular part that are given in a basic GD&T example such as Fig. 4-2 in the 2009 std. If your two surfaces A and B are guaranteed to always be 90 degrees exactly, then there's no issue. But if those two surfaces are at an angle of 89.99992 degrees, then one of them must take precedence (primary vs. secondary, or secondary vs. tertiary)in order to have repeatability and an unambiguous drawing.

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

http://www.gdtseminars.com

 

[pmarc]

bxbzq,
It is not only that "2X" does the trick. It is also the Profile of surface callout that makes those two surfaces tied together. That is why in standard coplanarity example (fig. 4-23) Profile of surface is used and not Flatness (which is reserved for single unrelated features). I agree that my idea is not standard-like, but I do not think there is anything in Y14.5 that prohibits it. It is just an extention of principles. Maybe this will sound too absolute, but in my opinion there is no other way to define a relationship between those inclined surfaces without making one of them of higher importance.

As for interrelationship between 2 pairs of holes, you can always apply additional positional callout to all 4 holes without any datum reference. This should do the thing.


J-P,
I am having troubles in picturing of what you said. Why are you saying that the surfaces are at right angle? Aren't they at basic 100 deg or am I missing something? I also do not really get why those features cannot contact datum feature simulators at the same time, regardless of actual value of the angle between them - especially that they are primary datum features. And I would rather compare it to the situation with datum targets B1 and B2 in fig. 4-47 of Y14.5-2009, with the difference that the targets are planes and not lines.

 

[bxbzq]

Belanger,
Attached sketch shows the difference between A-B and A|B to my understanding. In my case, datum feature A and B are of equal importance and make contact with mating surfaces simultaneously.

 

[Belanger]

Sorry pmarc -- yes it was 100º (I was writing that post from memory). But I don't know if Fig 4-47 is the best example for this situation either, because in that case the targets are being used to create a datum plane B running across the middle of the part.

So we have to ask: what is the true, theoretical datum that you are after? A single vertical plane?

Also, (to bxbzq): I think you're relying on gravity to be what equalizes those two hyphenated features. But suppose we rotate the picture 40º in either direction. Should we still assume that the part will equally hit both surfaces? That was the issue I was getting at (despite my goof about the 90º).

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

http://www.gdtseminars.com

 

[MechNorth]

J-P, you raise a frequent concern about precedence of multiple datum features in practice vs theory, but that is how it is in reality. We set up our datum feature simulator(s) to represent the basic case, and we are allowed to "seat" the parts to provide minimum rock, or to pass the parts. To suggest that the use of non-coplanar multiple datum features is inappropriate because of the fixturing issues is not reflective of legal and common practice. I have done this (successfully) on many parts and use datum targets similarly. The reality is that you NEVER have 100% contact with simulators, even on a single surface, and it is ALWAYS a best-fit/seat situation...for better or for worse.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Jim -- therefore your are saying that we can take Fig 4-2 in the standard and change the feature control frame to have D primary, and E-F as secondary.
I agree with the stuff about never having 100% contact with the simulators, of course, but I'm trying to consider what the true theoretical datum is once this is done, and whether that's really what we want to dimension everything from.

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

http://www.gdtseminars.com

 

[MechNorth]

Don't really see value to that, J-P; you already have mutually perpendicular datums established by E and F. In the op, that isn't the case. Consider instead a case of two planar / parallel datum surfaces that are offset. I would still make them a multiple datum feature and would still have the datum precedence issue, correct? Yet, it is commonly (and validly) done. From your posts, I suspect that you would not use multiple datum features; is that right? If not, then how would you reflect that functionality in the datuming?
Also essentially the same as using two coaxial datum features to establish a single datum; which one has precedence? Are you guaranteed stability of setup?
I believe we've also debated the idea of a piping system with end datum features aligned at some basic compound angle wrt each other; same issue, and same validity.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[bxbzq]

Will this do the work? I'm not sure the difference between the 4 options for positional callouts to the 2 pairs of holes.

Jim, do you mean this thread?

http://www.eng-tips.com/viewthread.cfm?qid=237732

 

[MechNorth]

Yup, that's the thread.
I agree with the A-B, but the C-D isn't necessary. The only dof not constrained by A-B is translation into the depth of the sheet; only one of the two holes is necessary to constrain that dof, so adding another feature would add complication without value (that I can see). For datum feature C, use option #2. For all other holes, use A-B/C or A-B/C(M). For profile controls, A-B/C (no (M) on the C because it's not used in a profile simulation anyway).

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

No Jim, I did not have a problem with the coplanar or offset planar examples. And I didn't mean that multiple datum features must always be in the same plane or offset planes.
You say that in Fig. 4-2 the datums are already mutually perpendicular. Of course, but I'm posing that the datum features might not be mutually perpendicular. That's where we need to know the precedence. Thus, if we hyphenate E-F, they become multiple datum features -- why is that suddenly not a good way to go?
Same with the coaxial examples. Sure, two coaxial diameters can form multiple datum features. Now suppose those diameters are designed to be at a 20º angle -- still OK to make them multiple datum features? Then what about two diameters at 90º?

I didn't mean to divert the OP's stuff. The other suggestions are just fine and I think he's on the way to a solid GD&T scheme.

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

http://www.gdtseminars.com

 

[bxbzq]

J-P,
No problem. I'm here to learn more than just solving specific problems.

By reading your posts, I'm actually a bit lost as to whether to go A-B or A|B.
My idea was even the two surface were at right angle, the primary datum should still be A-B. But now I would think it over.

 

[MechNorth]

J-P, I was considering the graphic in the orientation shown, thus no value in an A-B datum. If the function was to have the part rotated 45-deg, then I would go with the A-B datum. The difference is that in the first case the perpendicular features are not intended to act simultaneously (gravitational effects - an assumption, but there it is). Second scenario, gravity will force the two faces to be engaged simultaneously. The selection of datums again is based on functionality.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

If it matters, I am with Jim on this one.
Gravity, or actually assembly configuration, makes the difference.