Double dimensioning, illegal, redundant, correct or other

[aniiben]

A part similar to the one shown ASME Y14.8-2009 Fig 4-9 (page 28). (Casting Standard)

The one and important difference is that there is an “as –cast” hole thru the part that is not used as a datum feature on the machined drawing.

On the machined drawing: Casting datums and machined datums are shown similar to the datum scheme above. The hole in question (again, as–cast hole/feature) is dimensioned and toleranced from the casting datums and ALSO from the machining datums.

Pos 0.8(MMC) Z, Y, X
Pos 0.25 A, B, C

Z, Y, X- casting datum targets
A, B, C machined datums


Some design/product engineers are seeing this as double dimensioning, others as illegal or redundant and others are just in agreement with this scheme.

The intent is to avoid the “as-cast” hole/feature to be out of “functional specification” before the part is used in the assembly. Since the hole is “as-cast” and does not get machined Z, Y X datum scheme is needed, but deformation of the part during the removal of the material has also been noticed.

I have no much experience with casting and plastic parts, but learning…..

On Tec-Ease I’ve seen a similar example, but again, no “as-cast” hole / feature dimensioned from the casting datums and also from the machined datums (except the initial datum targets that are shown and described /become on the drawing machined datums feature). See attachment for Tec-Ease example.

On Tec-Ease example, lets say Ø15 holes is "as-cast" and is also dimensioned WRT to Z, y and X in addition the the current position Ø 0.4 to A, B(M) and C(M) shown. How you you see it? Double dimensioning, illegal (per the standard), redundant, correct or other?

 

[pylfrm]

jassco,

A position tolerance applied to cast holes and referencing machined datum features is a requirement for the relationship between those two feature sets. The holes may not be created by machining, but the relationship certainly is.


pylfrm

 

[aniiben]

You skipped the first and last questions.

The first question was: ”The "machined drawing" is serving as the finished part drawing, correct?”
I did answer that. I will answer it again: correct.

I don't understand the purpose of the 0.8(M) position tolerance that is applied to the same feature but references the cast datum features on the "machined drawing". Is that intended to cover some separate functional requirement of the finished part?”

The “as cast” holes are not “altered” for size [highlight #CC0000][/highlight]by the manufacturing/machining (they don’t touch these holes) so, as I stated before, has been decided (wrongfully or not) to be dimensioned/locate them from the casting datums. But then, ….rest of the story in my previous replay.
(copy-paste: Well, all was good until after the machining process has been noticed that these “as-cast” holes and features are no longer where they need to be, based on the casting datums (Z, Y and X).
Solution: Another ECN has been run and a positional callout (for the holes) has been added on the machining drawing with tighter tolerance from the machined datums A,B and C. And the argument was the deflection of the part during manufacturing. (please note that the position from the casting DRF was not removed or changed)

:”It seems quite strange that the holes have a size tolerance of 5, but a position tolerance of 0.25 RFS. What's the story there?”

The ”as cast” holes could vary in size more than in their location as a rubber seal and three conical pins are mating in the holes. We want uniform/ pressure around the circumference of those rubber seals to avoid premature and uneven wear and negatively compromising and impacting the cycle life of those dumpers used as anti vibration isolators. The conical pins are tight/ precise for their location, but can move in and out of the plate in question to accommodate the size variation in the “as-cast” holes. Again, the rubber seal goes into the ”as-cast” holes and the conical pins into the rubber seal. The conical pins can move axially, but not radially. Uniform stress around the rubber seal/ conical anti-vibration mount is the goal to be achieved. LT =Transition Clearance or Interference Fit or LN =Locational Interference Fit between the conical pins and rubber dampers/ shock absorbers

“Ideally, the casting drawing would have a tighter tolerance for that same feature, referencing cast datum features that will eventually be used to establish the relationship between the casting and the machined datum features.”

Why is that? Could you, please, explain it a little more? Thank you

A position tolerance applied to cast holes and referencing machined datum features is a requirement for the relationship between those two feature sets. The holes may not be created by machining, but the relationship certainly is.”

So, are you implying that the shown datum scheme for the “as-cast” holes is not “double dimensioning / redundant or illegal” ? Not the values by themselves (those should be the functional ones), but the datum scheme concept/ approach.


THANK YOU AGAIN FOR YOUR INPUT!

 

[jassco]

Hi, pylfrm:

No, a position tolerance applied to cast holes and may not reference machined datum features which did not exist.

OP has a print for as-cast part telling vendor requirements of this as-cast part;
The position tolerance of 3 blue holes goes to this as-cast print. There are no machined features available on this print to be used as datum.

OP has another print for as-machined telling his machine shop requirements of this as-machined part. This print tells vendor not to touch those 3 blue holes.

I hope this makes sense to you.

Best regards,

Alex

 

[aniiben]

Jassco, (Alex),
The print shown is the machined print.
We are not talking about the "as cast" print or part or casting drawing. We are only talking about the "as cast" holes on the machined drawing.

The position tolerance of 3 blue holes goes to this as-cast print.

Yes, but also goes to the machined drawing as "as-cast" holes.

 

[jassco]

Hi, aniiben:

There are only two things you do to "as cast" holes on the machined drawing:

1) Select them as datum features or dimension machined features to them;
2) Add reference dimensions to show their sizes and locations to other "as cast" features. (Not recommended)

Best regards,

Alex

 

[aniiben]

There are only two things you do to "as cast" holes on the machined drawing:

1) Select them as datum features or dimension machined features to them;
2) Add reference dimensions to show their sizes and locations to other "as cast" features. (Not recommended)

Well, in the picture embedded Fig. 4-9, (which by the way it is from the casting standard) it is shown a feature (on a machined drawing) that is neither of your two options.

On the machined drawing, the length of the oval slot 10±1.5 is shown from the casting datums Z, Y and X.
I guess the standard does not fit your theory or your theory is not in agreement with the current standard. The length of the slot is NOT a datum feature (the width is) nor is shown as reference.

 

[jassco]

Hi, aniiben:

There are only two things you can do to "as cast" holes or features on the machined drawing:

1) Select them as datum features or dimension machined features to them;
2) Add reference dimensions to show their sizes and locations to other "as cast" features. (Not recommended)

I see Fig. 4-9 does neither of the items above.

Please pay attention to "two things you can do to as-cast holes or features".

On the machining drawing, you (cutter) do not touch the 3 blue holes.

Best regards,

Alex

 

[mkcski]

anniben:

Why not machine the 'as-cast" holes with the other features and eliminate the cast-machined relationship problem; who's disposition appears to cost more than cost of machining.

Certified Sr. GD&T Professional

 

[aniiben]

Why not machine the 'as-cast" holes with the other features and eliminate the cast-machined relationship problem; who's disposition appears to cost more than cost of machining.

That would be part of a long term solution, maybe, but for now let’s answer, if all possible, the initial question.

As I said before, could be economic (setup, fixtures, tooling, cycle time,) reasons on why they went that route……..before my time…….
On the other hand the rough surface (sand casting 40-15 µm) might help retaining the rubber seal in place in case of misalignment. We don’t want to solve one problem and create two more, you know unintended consequences……

 

[mkcski]

Anniben

Just a thought. I understand - the irresistible force and an immovable object scenario. Oh well.

Certified Sr. GD&T Professional

 

[pylfrm]

Ideally, the casting drawing would have a tighter tolerance for that same feature, referencing cast datum features that will eventually be used to establish the relationship between the casting and the machined datum features.

Why is that? Could you, please, explain it a little more?

Your casting drawing hole size tolerance of 5 and position tolerance of 0.8(M)|Z|Y|X| combine to create an upper limit of 10.8 on the RFS position errors of the holes with respect to |Z|Y|X|. Now imagine the machining process always creates datum features A, B, and C such that the RFS position errors of the cast holes with respect to |A|B|C| are never more than 0.1 larger than the corresponding errors with respect to |Z|Y|X|. This ensures the holes will always meet a position tolerance of 10.9|A|B|C|, but the actual functional requirement is apparently 0.25|A|B|C|. To ensure the functional requirement is met by the finished part, the casting drawing would need to specify a position tolerance of 0.15|Z|Y|X|.

So, are you implying that the shown datum scheme for the “as-cast” holes is not “double dimensioning / redundant or illegal” ?

The 0.25(RFS) position tolerance on the finished part is certainly not illegal, and doesn't appear to be redundant either. Whether or not it is "double dimensioning" is a pointless argument, so I will skip that.

The 0.8(MMC) position tolerance doesn't make any sense based on the information you have provided.


When I said you skipped the first and last questions, I was referring to these from my 11 Oct 17 22:50 post:

It sounds like this tolerance probably does not reflect any functional requirement of the finished part. If so, I'd say the tolerance should be eliminated. Do you agree?

Does the machining process use datum features X, Y, and Z (as defined on the finished part drawing) to establish the relationship between the casting and the machined datum features? If some other method is used, please explain.

jassco,

I understand what you've said, but I disagree with most of it. A position tolerance applied to cast holes and referencing machined datum features simply means those datum features must be created such that the holes fall within tolerance.


pylfrm

 

[aniiben]

pylfrm,

Okay. So, you think that Pos Ø 0.8(MMC) Z, Y, X –for “as-cast” holes, should be eliminated from the machining drawing?
Again, to be kept on the casting drawing (maybe with even tighter tolerance than the current specification of Ø0.8 (MMC) to make sure the functional tolerances at the machining level are setup for success… I know we did not talk about the casting drawing at all), but it is useless of the machining drawing even the holes are “as-cast”?

“Does the machining process use datum features X, Y, and Z (as defined on the finished part drawing) to establish the relationship between the casting and the machined datum features?”

And to answer your last question:

As far as how the machining process goes, I don’t have much experience in that area, but all I can tell (if make any sense for the people with manufacturing experience) is the following:
- First machining step: datum feature A (bottom flat surface). Part is held/centered on some “as-cast” surfaces (three small ribs/ protrusions) and the chuck is contacting datum feature Z in three area.
- Then, the second step is the OD, datum feature Y (same setup as step 1). Please don’t ask me for speeds and feeds as I have no idea.
- Third step, different setup/machine: Drilling datum feature B and machining C, with the part sitting on A, centered on Y and clocked on X.

pylfrm,

Does my explanation help? Did I answer all of your questions? Thank you again for your help.

 

[pylfrm]

So, you think that Pos Ø 0.8(MMC) Z, Y, X –for “as-cast” holes, should be eliminated from the machining drawing?

Yes, unless it corresponds to some finished part functional requirement you haven't mentioned.

Again, to be kept on the casting drawing (maybe with even tighter tolerance than the current specification of Ø0.8 (MMC) to make sure the functional tolerances at the machining level are setup for success… I know we did not talk about the casting drawing at all), but it is useless of the machining drawing even the holes are “as-cast”?

Even with a position tolerance of 0 (MMC), the RFS position error could be as large as 10. Without accounting for the additional errors introduced by machining, this is already 40 times larger than the finished part tolerance. The casting position tolerance needs to be RFS to provide any meaningful control.


I can't be sure without a lot more information, but it sounds like there's a decent chance that |A|B|C| can be created with an accurate relationship to |Z|Y|X|. That should allow the scheme I described at the start of my previous post to work, assuming the individual process can meet the necessary tolerances.


pylfrm

 

[greenimi]

pylfrm,

I am trying to follow the thread and I would like to ask you how you came up with 10.8 upper limit on the RFS?
Basically, I am lost on the following sentences:
“Your casting drawing hole size tolerance of 5 and position tolerance of 0.8(M)|Z|Y|X| combine to create an upper limit of 10.8 on the RFS position errors of the holes with respect to |Z|Y|X|. Now imagine the machining process always creates datum features A, B, and C such that the RFS position errors of the cast holes with respect to |A|B|C| are never more than 0.1 larger than the corresponding errors with respect to |Z|Y|X|. This ensures the holes will always meet a position tolerance of 10.9|A|B|C|, but the actual functional requirement is apparently 0.25|A|B|C|. To ensure the functional requirement is met by the finished part, the casting drawing would need to specify a position tolerance of 0.15|Z|Y|X|.”
.............
“Even with a position tolerance of 0 (MMC), the RFS position error could be as large as 10.”

Would you mind explain this approach/concept in the layman terms?

Thank you pylfrm

 

[CWB1]

JMO but this is getting unnecessarily deep.

The relationship between datum A and the cast hole should be clearly established on the casting print, therefore calling out the cast feature on the machining print is somewhere between unnecessary and lousy drafting. Changing the tolerance or applying a stack between the two levels for this relationship is simply wrong as two features have only one relationship.

 

[pylfrm]

greenimi,

No special concept or approach beyond just imagining geometry that would lead to the largest possible RFS position error while still meeting the tolerances. In this case, the hole would not be anywhere near round.


CWB1,

Datum feature A does not exist on the casting.


pylfrm

 

[CWB1]

You're confusing datums and surfaces. On the machining print and finished part the OP provided the surface and datum coincide, on the casting print they should not, datum A should be within the casting and above the bottom as-cast surface by whatever casting allowance the OP's comfortable with. The relationship between major datums and major features (A & the hole) should never change on a simple part like this, layer the two prints on top of another and you should see the part within the casting print. Keeping the major datums common helps communicate design intent and prevents design and drafting mistakes. It also allows you to add the usual optional machining, allowing the foundry to machine these datums for both casting and dimensional checks without having to provide them your finished part print.

 

[jassco]

Hi, CWB1:

Two thumbs up for you! I like your comment below:

"therefore calling out the cast feature on the machining print is somewhere between unnecessary and lousy drafting."
Note: I would not call it "lousy". I would call it "illogical".

"Changing the tolerance or applying a stack between the two levels for this relationship is simply wrong as two features have only one relationship."

Hi, aniiben:

You need to draw a VERY THICK LINE between cast features and machining features. You can't size a cast feature (holes in this case) on your machined print if you decide not to machine them (the 3 holes). You may want to consult with ASME Y14.8.

Best regards,

Alex

 

[greenimi]

pylfrm,

So, the requirement is Ø 0.8 (at MMC) for the casting holes at the machining level.
Then at LMC (biggest holes) the max pos. is Ø 5.8 (2x 2.5 +0.8) , correct? (per the OP drawing: holes size Ø7.8 ± 2.5)
Where 10.8 is coming from?

On the RFS should be 0.25 RFS to A, B and C. How I can relate both callouts with 10.8, I do not know yet. Could you, please, give me a hint?

 

[aniiben]

“You can't size a cast feature (holes in this case) on your machined print if you decide not to machine them (the 3 holes). You may want to consult with ASME Y14.8.”

Jassco,

I did consult 14.8 and in the picture embedded Fig. 4-9, (from Y14.8 casting standard) it is shown a feature (on a machined drawing) 10±1.5 from the casting datums Z, Y and X.
Can you conclude then, that the length 10±1.5 is “as-cast” and its applicable width 8.1±0.1 is machined? Or both features (length and width) are machined? Or both are "as-cast" ?

On the machined drawing shown:
- the length of the oval slot 10±1.5 is shown from the casting datums Z, Y and X and
- in the meantime the width 8.1±0.1 is shown from both “casting” and “machining” datums

Could you, please explain your point a little bit better with no disagreement with what is currently shown in the standard you mentioned Y14.8.

Thank you