Datum surfaces not perpendicular?

[ImNotSure]

Ive been talking with the customer & they assure me its correct. Their datum surfaces are not perpendicular to each other & correct me if im wrong, but couldnt you "slide" along datum "A" and have the nominals change for the holes on datum D & E?

Or are the basics coming from the intersection of datum D/E & datum A?

Im sorry for the bad picture/drawing, Im a bit busy. Datum "A" is the top surface, where the basic 0.00 is called out in zone B8

 

[btrueblood]

It's legal. The locations of the holes are controlled by datums A and C, with the angle of the hole axes taken as perpendicular to E (or D). You measure the locations of the holes where a line perpendicular to -A- meets datum -E-, the inside surface, for instance. Might be tough to make a gage for this, as the angularity is not specified or controlled...but a simple plug fixture to pick off hole centers and measure back to the datums is do-able...and with a CMM all things are possible.

 

[ImNotSure]

Great, thanks. I had it in my head that I needed datum points for datum "A".

So those basics are coming from the intersection of D/E & A. My CMM was spitting out numbers, now I know they are correct numbers.

 

[btrueblood]

Umm...

I see the basic dimensions coming from the A and C datums, but no basic dimensions specifically coming from the D/E datums...

But I didn't spend a long time looking at the drawing, either, nor writing a CMM program for it.

If I were writing a program, I would

1. Establish datum A
2. Probe the surface of datum D/E and establish it (or its averaged plane) to be perpendicular in the one axis, relative to A, and floating in terms of angularity in the other axis.
3. Establish datum C as the hole called out on the print.

You can now locate holes, by measuring from datum C in a direction parallel to plane A and to plane(s) D/E, and by measuring perpendicular from datum A, with the center of the hole lying on datum E...

Hope that makes sense?

 

[ImNotSure]

Sounds right to me. Ive got another question in regards to that drawing. See in zone E2, the position call out:

#1: That would be interpreted independently, with having two position callouts, correct? (they are trying to get at controlling the features to themselves with that bottom segment)

#2: If they had it correctly drawn with a composite FCF, they dont want that second datum reference in there (B @ MMC), right? all they are trying to do is control the features to them selves....?

Oye, when assembled, all of these faces are independent to each other and nothing ever goes to datum C when they have everything together, its a pilot hole to drill a mounting hole on a beam...

 

[btrueblood]

Um, I don't see datum -B- called out anywhere. Datum C is the one hole on a flange ear where the coordinate dimensions are "0.00"...

...assuming -B- is one of the "W" hole pattern holes, then what you said in #1 is correct.

#2 - yes, but you meant B not C: they reference A and B datums for the tighter position control, relative to all the holes back to the "C" hole, which has a looser tolerance. Again, assuming B is one or part of the W pattern?

 

[ImNotSure]

I should have clarified,

"A" as you know is the top plane
"B" is the hole that has 2 0.00 basics going to it (zone D4)
"C" is the hole thats on the same 0.00 north/south but is at the complete right side of the part (right edge of D2)

So for my question #2, the lower tier of the FCF should just be:
0.2 to "A"
and not
0.2 to "A" & "B"

 

[fsincox]

How are "B" & "C" called out themselves, how are the holes similar to them called out then? This is an interesting applicational example.
Frank

 

[fsincox]

It may give us some insight to how important relations to "C" are. I would not say it is wrong without more info.
Frank

 

[ImNotSure]

Alrighty, Heres page 1 of 2, page 2 is in my first post...

 

[ImNotSure]

Crap, sorry, Im not sure how to edit posts (or if you can)

That link above for the first page of the drawing is broken somehow... Heres the real 1st page

Again, Im new, sorry everyone...

 

[fsincox]

ASME or ISO?
Frank

 

[ImNotSure]

ASME

 

[fsincox]

They may be intending on the "simultaneous requirement" to "hold things together" so to speak, it would not, in the same way with a composite. All A (primary), B (secondary, at MMC) positioned features are to be treated as a single simultaneous pattern.
Frank

 

[fsincox]

ImNotSure,
First, Is there a basic angle to get from datums D and/or E to datum A somewhere? One of the common issues I see on complicated (real) part applications is simple violations of the basic rules, like, with a position callout the feature must be defined with BASIC dimensions from the datums to the features. The holes in E & D reference A & C but we have a break in the BASIC chain to establish that relationship.
Second, In situations like this: where I have not seen the assembly condition, I assume the drawing is correct unless there is an obvious violation of the rules, to me the position callouts referencing A (primary), B (at MMC, secondary) do not violate any rules, so I would not say the guy is "wrong" or it is "illegal" to do so.
Third, I would love to know why they switch from B to C for the datums of the D/E holes.

BlueBlood,
In the D/E holes the datum D or E is the primary datum, it sounds a little to me like you would be making it a secondary datum?

Frank

 

[btrueblood]

Wups. No, Frank I've got them in the wrong order...that is a confusing drawing, I was thinking I was seeing projected views, and missed that they are labelled views. I.e. I thought the datums D/E were at 90 degrees (approx) to the plane the holes were penetrating.

I have to agree with Frank's answer above also, I don't know why they want tighter tolerance to the single datum B hole, but it may be a functional requirement (i.e. it is okay for the pattern to pivot on axis -B- within the 0.5 limits of the first FCF to datum C). But, if they really do just care about the hole patterns relative to themselves, then having the second FCF referencing datum A only would be correct.

 

[fsincox]

btrueblood,
I agree it is not the typical ASME book example, but, it is an example of what lots of us out here in the real world must deal with, real parts and describing complex relations between features. It is for that reason I applauded it alone. I really hate to say it has issues without really knowing the function, as I stated, I prefer to give the benefit of the doubt to the engineer who should know this.
I think part drawings like this are really a compromise of the basic principles when applied as shown. I see this kind of leeway taken all the time with parts. The book says “must be defined basic” and it is just ignored. Then people wonder why no one takes it seriously and it is not being used “correctly”.
Parts like this are my candidates for: “all dimensions basic” applied to a master framework, with possible temporary substitute datums for deriving a temporary cast part framework, general profile all surfaces to the loosest process tolerances and when it must be tighter specify the tighter requirements, very ISO “ish” as I currently understand their tolerancing philosophy. Admittedly this is more complicated on parts with big features and small ones, which is also why I like the tolerance based on size increments like they do.
Frank

 

[ImNotSure]

The GD&T on this drawing is there for fun. When I first got this drawing they were using perpendicularity callouts for the location of heights & just this morning I got them to change their FCFs to composite FCFs and the lower tier just to reference datum "A".

Their basics were all rounded (some to >0.5mm) and they had their true position tolerance @ 0.2mm

TL;DR: These guys dont understand GD&T and dont need it on their drawing, they just put it there to look good. For example, the holes on datum D/E are independent wired connectors (so they are loose and the relationship between them makes no difference)

Now for future reference, having datums not perpendicular to eachother is legal, & 0.00 is @ the intersection of these datums?

 

[fsincox]

I did not say datums features must be perpendicular, datums are by definition. Also feature locations must be defined with basic dimensions from the datum references to the features.

"These guys dont understand GD&T and DON'T NEED IT ON THEIR DRAWING, they just put it there to look good"
This is an example of what is wrong with the current thinking on GD&T, IMHO
Frank

 

[ImNotSure]

Wait, so datum "A" being at an angle to "D/E" is illegal?