Datum setup 3

[SeasonLee]

It seems there are a lot of problems on the attached print, but I will focus on the datum callout. Here is my understanding on the designer’s intent:
The bottom surface is primary datum A
Datum B is a pilot location hole
Datum C is another pilot location hole which is established base on datum B
B-C create the axis orientation
Datum B will be the original point for CMM measuring.

My questions are:
1. Can we allow two datum symbols (Datum A and Datum D) on a coplanar surface?
2. Is it a right way on Datum C callout?
3. Which one is correct on the datum reference frame callout
Primary datum A -> B-C -> Datum B
Primary datum A -> Datum B -> B-C
Primary datum A -> B-C (without tertiary datum)

Thanks for all comments

SeasonLee

 

[dingy2]

I reviewed you drawing and:
1 Each surface could be the datum but having a datum A on one surface and a datum D on the other surface does not make sense. One needs a phantom line between both surfaces and then both are datum A. Your drawing in not quite correct here.

2 Datum C should be reflected in a feature control frame referencing datum A (bottom surfaces) and datum B (hole) at MMC. The .000 tolerance is fine. It just states that the size of the MMC and the Virtual condition size are the same.

3 If you change the bottom surfaces to datum A (not A & D), the reference datums should be A| B MMC| C MMC. When one has a hole as the secondary datum, it is the intersecting point of 2 perpendicular planes. All dimensions come from datum B while datum C is only to orient the part (square it up).

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[ringster]

Dingy,

I believe that 2 planes intersect in a line, not a point.

 

[TheTick]

"Dual datums" like "B-C" are only appropriate for runout. Not appropriate for position or profile. Use |A|B|C| for primary|secondary|tertiary. The implication is that B positions the part and C clocks the part into position.

|A| and |D| can be individual datums because hey are separately identifiable features. Inspectors will need to block part to ensure part does not rest on wrong portion. Better to use datum targets to define datums with more clarity.

Why is there a |D| datum? It's not used for anything.

 

[SeasonLee]

Thanks for all comments.

When should we need to use dual datum like "B-C"? where can I find it in the standard Y14.5M ?

I believe the designer intent to specify the profile tolerance .020 is to control the flatness after forming, there is a note "two surfaces" below the profile control, so it must be a coplanar surface,I agree datum D should be deleted.

SeasonLee

 

[dingy2]

Ringster:

I agree with that statement about line rather than point. Actually, the standard states "axis".

TheTick:

One can have 2 planes as a primary datum. One surface could be datum A while the other surface is datum B. When referring to the primary datum in a positional tolerance, one would state "A-B".

It all depends on how the part mounts on the mating part. Above, it the part contacts on 2 surfaces at the same time - thus, datum A & datum B. If the part only contacts on 1 of the surfaces, then we only have a datum A.



Dave D.

http://www.qmsi.ca

 

[weavedreamer]

SeasonLee,
Multiple Datum Features is defined per 4.5.7 - 4.5.7.1 - 4.5.7.2

Fig 4-19 shows a MDF with Tolerance of Position.
Fig 6-21 illustrates a MDF with Surface Profile.
Fig 6-49 & 6-51 shows a MDF with Runout and Total Runout.

 

[SeasonLee]

I found fig 4-8 in the standard Y14.5M is quite similar as the one I posted earlier, the datum reference frame should be I A I B MMC I C MMC I for sure.

TheTick :

1. Would you please advise where I can find out the rule of “Dual datums like B-C are only appropriate for runout“ in Y14.5M.
2. Dual datums B-C not appropriate for position or profile. This makes me recall one part I inspected two years ago, all profile tolerance with dual datum B-C in the DRF as shown on the attached

All comments will be appreciated

SeasonLee

 

[weavedreamer]

The example you just posted showing Datums B & C as a feature of size taken from theoretical intersections makes complience with 4.3 quite the stretch of the imagination.

As to the illustration posted 07FEB09, the B(m)-C(m) is legal, but does not provide 'predictable' repeatability.

 

[ewh]

See thread1103-235809 for an excellent, related discussion on the use of "dual" datums.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[SeasonLee]

If dual datum B(m)-C(m) is legal in the post dated at 2-7-09, may I ask what is the difference with this DRF A I B (m) I C (m) ?

Thanks again

SeasonLee

 

[TheTick]

Please note the difference between "appropriate" and "legal".

The dual datum |B(m)-C(m)| is legal, but it does not fully convey your intent and, as previously mentioned, is not entirely repeatable.

As for stepped datums like |A-D|, use of a single datum defined by datum targets is more appropriate and much more clear.

 

[KENAT]

What drawing standard are you using? First picture appears to be 14.5 1994 edition while the second one looks like an older version from the datum designator.

1) You could keep datum D but then your FCF when you intend using a datum derived from both surfaces would be "A-D" as shown in Fig 4-20 & explained 4.5.7.1. of ASME Y14.5M-1994. All depends on end function.

Datum "B(m)-C(m)" is only a single datum. While I'm not sure the standard fully supports this my assumption is that it is intended to be the plane running through the theoretical axis of the two holes.

On the other hand, DRF A I B (m) I C (m) gives you all 3 perpendicular datums as shown in figures 4-8 & 4-9. There is no ambiguity and it fully controls location/orientation of the features.

My 2C




KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[NormCrawford]

Another interesting thread. After quick review, the only real issue I have with the drawing is datums A and D. However, it really depends on the mating part to answer why separate A and D as shown. But, if A and D are supposed to stop a translational degree of freedom together, it is appropriate to then reference A-D as co-datums is the datum reference frame. Making datum feature A multiple surfaces is ok, I guess, but I prefer using the co-datum technique so that those two datums can be appropriately toleranced in location to each other as well.
The other suggestion I have is to more correctly (imho), specify both datum features B & C using positional tolerance back to a datum of A-B. This will reduce a tolerance stack if the axis or plane intended to be functionally formed by the two holes simultaneously is the true physical reality.

The sequence of a A-D and B-C reference appears in the DRF is entirely up to the physical function. Either way could be correct. Most likely however, it is my guess that A-D primary and Bmmc-Cmmc secondary is the design intent.

Again, its difficult to judge right, wrong, or best without full information, but none the less, those are my quick thoughts.


Norm Crawford
GDTP-S
Applied Geometrics, Inc.

http://www.gdandt.com

 

[ewh]

Welcome to the fora, Norm! Another learned opinion here is always welcome.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

"Another learned opinion here is always welcome." Sure beats the likes of me making it up as I go along;-).

So just to test you out, could you explain the ASME Y14.5M-1994 explanation/implication of "Bmmc-Cmmc" I had a quick look in the standard and didn't see explicitly where it would come from. As I said I would think it came from the plane through the 'axis' of the 2 holes but am not clear on the exact derivation/definition etc.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[NormCrawford]

If I am violating any of the policy, please help me out. I did open and review the reminder and well, that was a lot of rules.
My intent is to comment and within the subject at hand.
So, with that said, an axis established by a Bmmc-Cmmc referenced datum is not explicitly written in the standard. None the less, the principles are there, and in reality, a very common physical occurance. And, I might add, most likely, in this. If the two holes (B & C) are infact clearance holes, then the datum is allowed mobility or shift. It really isn't any different then any other datum feature of size referenced with MMC. It is a pain to inspect with variable or attribute data, but is none the less the physical reality.
So, that is why it is not CLEAR on where the "target" is. It's a potentially moving target and probably is.

Norm

 

[KENAT]

Norm, that's my default sign off, it wasn't targetted at you. Although it's not a bad idea to take a look at the policies everynow & then to remind oneself.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[weavedreamer]

B(m)-C(m) have no preference between B or C. In a fixture, they could be two diametrical pins over which the work is placed. The angle of Datum A could vary by the amount of slop between the holes and the pins.

A|B(m)|C(m) establishes the orientation of A, followed by setting B(m) with a diametrical pin, followed by C(m) using a diamond pin.

Fig. 4-8 appears to be far more repeatable/predictable.

As to what is specifically on the drawing - it is the designer's responsibility to be as clear as possible. While the second example may be easier to calculate tolerance stacks from, the first example may be indicating that it is not the primary concern in this case.

KENAT, your statements have the conclusions drawn from the examples cited earlier held now in question. Thanks a lot!

 

[KENAT]

weavedreamer, it seemed the OP didn't fully grasp some of what you'd put so I was trying to clarify. Also the wording of some other replies was misleading in my opinion.

Not sure I quite follow your last line, no offence meant in any case.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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