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Double dimensioning, illegal, redundant, correct or other

Double dimensioning, illegal, redundant, correct or other

Double dimensioning, illegal, redundant, correct or other

(OP)
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?

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
Here is Fig 4.9 from the Casting standard for reference

RE: Double dimensioning, illegal, redundant, correct or other

There's a statement that dimensions and tolerances only apply at the drawing level they are detailed on. It should not matter if the same dimensions and tolerances are seemingly repeated because the casting acceptance is independent of the machining acceptance.

Some will complain that there's no specific machining operation and therefore the machinists should not get dinged if the cast feature is out of tolerance. If there's any sense to it they would not be and failing the part at any level should have a correct failure analysis.

There are others who point out that if the feature is that critical that it needs to be inspected that it should be machined and not left to drift up from a lower fabrication level.

I would only do this if the lower level feature was used as a datum reference and the dimensions and tolerances were needed to establish the datum.

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
The main question is how to avoid the perpetual blame between the departments:

The holes/features were good when the casting has been delivered/measured versus we didn’t touch the holes/features?

Is this a potential solution?
Looks like the agreement is not achieved yet.

What other solutions, from your experience, could be used? What else could be done to not spend much $$$ in lengthy debates an unproductive discussions ?

Again, I am looking for peoples experience in this area of expertise.

RE: Double dimensioning, illegal, redundant, correct or other

3DDave,

Is the OP question and "additional" requirement for an unmachined/ as-cast features
(aka.a. Pos 0.8(MMC) Z, Y, X
Pos 0.25 A, B, C

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

more like a CYA?

RE: Double dimensioning, illegal, redundant, correct or other

Per Don Day's article: "Geometric Dimensioning and Tolerancing, The Common Thread of a Multifunctional Design Team"

"If the casting did not clean up, they would blame the foundry for not providing sufficient material as a machining allowance. The foundry in turn would blame the machinist for not aligning the casting properly on his machine tool."

That is exactly what's happening almost every shipment ........... I would also add the stress in the part and distorsion induced by the tooling and fixturing.

Not sure myself how to circumvent that.

RE: Double dimensioning, illegal, redundant, correct or other

aniiben:

We buy small quantities of "large" castings (#3,000+ for hydro electric components. All (final) features are located relative to a datum axis. The datum feature is machined in first machining operation. The part is designed in 3D and has a model. We used a portable CMM (laser tracker or arm) and scan the all surfaces of the casting to create a point cloud. The CMM software, using the 3D model, does a best-fit of the point-loud to the model. This immediately tells us if there is insufficient machining stock in any area. It also identifies the locations of the lathe centers that establish the setup for the first machining operation that creates datum feature (datum axis). Hope this helps.

Certified Sr. GD&T Professional

RE: Double dimensioning, illegal, redundant, correct or other

Hi, aniiben:

"The hole in question (again, as–cast hole/feature) is dimensioned and toleranced from the casting datums."

This is ok.

"and ALSO from the machining datums."

This is not ok as machined datums did not exist on as-cast.

I agree that "Some design/product engineers are seeing this as double dimensioning, others as illegal or redundant".

You need to draw a very thick line between as-cast and as-machined.

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

References flow up the print structure, never down, so cast features cannot be driven by dimensions in a machining print. Regarding the holes, if you want them to remain as-cast then do not detail them on a machining print as doing so is calling for a bored hole.

RE: Double dimensioning, illegal, redundant, correct or other

Quote:

References flow up the print structure, never down
So if we're designing an airplane, we would assign tolerances to each itty bitty part based on the requirements of each at that level?
Good luck getting the plane to fit together. smile

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

RE: Double dimensioning, illegal, redundant, correct or other

Hi, Belanger:

CWB1's statement is accurate. References flow up the print structure, never down. If there is a feature that you are not sure, you can leave it to its parent component or assembly where the component is used. It does not matter whether you design a plane or a watch. The process is universal.

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

Let me clarify my point. The way to design large assemblies is a combination of bottom-up and top-down, in terms of tolerancing.

I do agree with his point that an "as-cast" hole generally shouldn't be detailed on a machined print as that might imply machining it to that size. But I was getting more at the dimensioning and tolerancing layout, not the actual manufacturing specifications. (And the purpose of a print is dim/tol layout, NOT manufacturing specs, per paragraph 1.4(e) of the Y14.5 standard.)

My bit about the airplane had to do with the tolerance stack-up effect that occurs if we focus too much on the path of "flowing up." Trust me, an aircraft can't be built if we examine the tolerances from a detail level only. I don't think that CWB1 was saying anything about tolerance stacks, but I was just cautioning against relying too much on a one-way flow of things.

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

RE: Double dimensioning, illegal, redundant, correct or other

aniiben,

The "machined drawing" is serving as the finished part drawing, correct? If so, and if there is a corresponding functional requirement, I'd say it's perfectly reasonable to apply a tolerance to a cast feature that references a machined datum feature. 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.

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?

If you have some strange drawing structure where dimensional requirements on the casting drawing somehow apply to the finished (machined) part, then much of what I said above may not apply. I've never come across such a scheme.


If I am designing a part that I intend to be manufacturable by casting and subsequent machining, I will generally design the finished part first. Based on that finished part design and the details of the planned manufacturing process, someone (who may or may not be me) will then design the casting.

Wherever practical, I'd recommend the finished part drawing allow machining on features that are generally intended to be cast. This keeps options open for manufacturing.

pylfrm

RE: Double dimensioning, illegal, redundant, correct or other

(OP)

Quote (pylfrm)

” The "machined drawing" is serving as the finished part drawing, correct?

Yes, correct.

Quote (pylfrm)

“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".

Has been decided (long before my time) that due to some economic reasons some holes/ features to be manufactured “as-cast”. (was good enough)
Then, if they were “as-cast” the decision has been made to have them toleranced from the “as-cast” datums (again, on the machining drawing). The main argument was: manufacturing is not touching them and has no intent to change their size/ location/ orientation/form. Again, “no intent”...........

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)

To show the “as-cast” holes on the machining drawing is legal (I guess) because the casting datums are (already) shown on the machining drawing (that is a standard requirement), pretty much in the same way as it is shown in the standard and also on Tec-Ease example embedded (casting DRF’s are on machining drawing).

Now, as I stated before, some will consider this approach “double dimensioning”.........(same as the title on the thread)

RE: Double dimensioning, illegal, redundant, correct or other

No, this is worse than “double dimensioning”. One should not put a dimension with tolerances on a print that no one is going to use it.

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

Belanger, if I am understanding correctly then yes I agree, designers and draftsmen may need to refer to higher level prints to determine available space claim and consider their parts total stackup during print creation. However, end-users of those prints should only have to look down, not up for necessary references. Tradesman often only have access to prints below their manufacturing level in the structure. In this case a machinist likely would have access to both the machined and casting prints, the foundry OTOH would only likely have access to the casting print.

JMO but if after machining a cast feature is suddenly out of spec then I'd doublecheck the tolerance stackup between both prints in search of a fundamental error. On the OP's machined print I'd never dimension an as-cast surface however I would use a note to clarify that its to be left as-cast and unimportant as anything other than background geometry.

RE: Double dimensioning, illegal, redundant, correct or other

Quote (aniiben)

Then, if they were “as-cast” the decision has been made to have them toleranced from the “as-cast” datums (again, on the machining drawing). The main argument was: manufacturing is not touching them and has no intent to change their size/ location/ orientation/form.

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?

What diameter tolerance is applied to the hole on each of the two drawings?

What position tolerances are applied to the hole on the casting drawing? What datum feature references are used, and how do they relate to those used on the finished part drawing?

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.

pylfrm

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
Pylfrm,

See the sketch for "conceptually" the same part as the one in question.

Here are the answers for your questions:

Imagine fig 4-9 (14.8-2009) but the part is round (not oval shape)
Datum target Z (3) as shown on the face of the part.
Datum target Y the OD
Datum target a width of a slot.

Blue holes” as cast”
Same size tolerance applied on each hole on each of the two drawings (casting and machining)
Positional tolerance on the machining drawing
Pos Ø 0.8(MMC) Z, Y, X
Pos Ø 0.25 A, B, C

Positional tolerance on the casting drawing
Pos Ø 0.8(MMC) Z, Y, X

RE: Double dimensioning, illegal, redundant, correct or other

aniiben,

You skipped the first and last questions.

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?

pylfrm

RE: Double dimensioning, illegal, redundant, correct or other

Hi, aniiben:

Are you aware that there is something wrong with the follow statement?

"Same size tolerance applied on each hole on each of the two drawings (casting and machining)."

The blue holes are not created on the machining drawing, therefore they should not have positional tolerances. If you really want, you may add reference dimensions.

No dimension with tolerance may appear more than once for any given feature.

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

(OP)

Quote (pylfrm)

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.

Quote (pylfrm)

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 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)

Quote (pylfrm)

:”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


Quote (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? Thank you


Quote (pylfrm)

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!


RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
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.

Quote (jassco)

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.

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

(OP)

Quote (jassco, )

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.

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

(OP)

Quote (mkcski )

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.bigears

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……

RE: Double dimensioning, illegal, redundant, correct or other

Anniben

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

Certified Sr. GD&T Professional

RE: Double dimensioning, illegal, redundant, correct or other

Quote (aniiben)

Quote (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|.


Quote (aniiben)

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:

Quote (pylfrm)

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?

Quote (pylfrm)

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

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
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”?


Quote (pylfrm, )

“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.

RE: Double dimensioning, illegal, redundant, correct or other

Quote (aniiben)

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.

Quote (aniiben)

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

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

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.

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

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.

RE: Double dimensioning, illegal, redundant, correct or other

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

RE: Double dimensioning, illegal, redundant, correct or other

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?

RE: Double dimensioning, illegal, redundant, correct or other

(OP)

Quote (Jassco, )

“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

RE: Double dimensioning, illegal, redundant, correct or other

Hi, aniiben:

Per ASME Y14.8, there are 3 methods to present drawings for cast item, namely:
1. Separate view drawing method;
2. Combined view drawing method;
3. End item drawing method.

Fig. 4-9 on ASME Y14.8 shows a 2nd drawing for machining features using "Separate view drawing method". The first would be a drawing for the as-cast item which Fig. 4-9 does not show it. As-cast item will be "material" to make the machined item.

The print does not tell you (and is not supposed to tell you) if this item is casted or machine. If you cast this item and can meet its requirements, then you don't need to machine it.

To answer your questions, the hole (labelled as datum feature B) and the slot (labelled as datum feature C) are machined here. Datum features X, Y, and Z are by default datum features on cast item, while datum features A, B, and C are for machined features. I think you are hung by this statement, i.e., the width dimension 8.1±0.1 is shown from both “casting” and “machining” datums. These two features (hole and slot) are located to datum Z, Y, and X. Datum A and B are merely there for qualification purpose. It basically tell us that machined datums A, B, and C need to be machined on a single setup. It is a qualification process of datum features to make sure they are of quality datums.

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
jassco,

Well, I am also hung by the fact that the length of the slot (10±1.5) (again the length and NOT the width) is shown / defined based ONLY on the casting datums.

I understand the definition of the machined datums (specially datum feature B=the hole-and datum feature C=the width of the slot), but the length of the slot remains unclear (at least in my mind).

Since it is shown from the casting datums ONLY, should I conclude that will be “as-cast” (same it is shown in my application)? Yes, no, maybe? If no, why not?

Thank you again for your patience

RE: Double dimensioning, illegal, redundant, correct or other

Hi, aniiben:

The slot is a machined feature. How do I know that? Because dimensions and tolerances for this feature is on this print for machining (Fig. 4-9). Otherwise, they will be placed on another print for as-cast part which will be "material" for this as-machined part (Fig. 4-9).

The print basically tells you to machine A, B, and C in reference to X, Y, and Z (with a single setup).

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

(OP)
Jassco,
Okay. I understood. Thank you very much.
Then the follow up question is why the length of the slot is defined in the standard only based on the casting datums (Z, Y and X) and not based on the machining datums A, B and C?
What could be a good reason to have this kind of example in the standard which suppose to be "the letter of the law" ?

RE: Double dimensioning, illegal, redundant, correct or other

The drawing is set up this way because the length of the slot is not used as a datum reference so only a coarse orientation and location relative to ZYX is required. The slot length location in the ZYX reference frame is sufficient for the intended purpose.

The width is given a coarse location and orientation relative to ZYX and then a refinement in location and orientation relative to AB.

The slot is not a cast slot. It is only controlled from cast features.

RE: Double dimensioning, illegal, redundant, correct or other

Hi, aniiben:

Nothing wrong with Fig. 4-9 on ASME Y14.8. The issue is how you understand it.

First of all, you need to understand that position tolerance of "tp 0.5 MMC | Z | Y | X " does not refer to slot length but rather center plane (a derived medium plane, technically using Y14.5 language) of the slot in vertical direction. 10.5+/-1.5 is just an FOS (feature of size) which is not supposed to have datums.

Design intents of this part dictate that the hole (8.1+/-0.1), the slot (10+/-1.5) x (8.1+/-0.1), and datum feature A are made in reference of datum features Z, Y, and X. Therefore, datum features Z, Y, and X are used.

If you make the derived medium plane of the slot (10+/-1.5) reference to datum features A, B, and C, it is essentially like a dog traces its own tail, as this derived medium plane is part of the slot itself. In another word, position of slot is created by the slot.

Is it OK to use "tp 0.5 MMC | A | B | C| " for length of the slot instead of "tp 0.5 MMC | Z | Y | X "? Yes, but not elegantly. I think John-Paul Belanger or Evan Janeshewski have better qualifications to answer this question than I do.

Best regards,

Alex

RE: Double dimensioning, illegal, redundant, correct or other

CWB1,

I am not confusing datums with surfaces. When I say "datum feature A", I am referring the surface which OP said is created by machining. How can the corresponding datum exist before this machining takes place?

Regardless, dimensional requirements applied to the raw casting do not apply to the finished part. Part deformation due to material removal, among other things, can cause drastic changes in geometry. If this is not controlled by requirements applied to the finished part, then the only option is to rely on the process. Good luck getting a foundry to guarantee their castings will exhibit a controlled amount of deformation when machined.


greenimi,

Your calculation resulting in diameter 5.8 appears to be based on perfectly cylindrical holes. It is probably more realistic, but not truly the worst case for RFS position error.

In this case it doesn't matter much matter which result is used since both are drastically larger than the finished part position requirement of diameter 0.25 RFS.


aniiben,

The slot (with width as datum feature C) in Fig. 4-9 is almost certainly intended to be machined. However, that does not mean it would be impossible to produce it by casting, even if datum features A and B were machined. An actual drawing may have requirements that rule this out, but none were included in the figure.

pylfrm

RE: Double dimensioning, illegal, redundant, correct or other

I think (but I might be wrong in my assumption so, please, feel free to correct me) that the casting standard shows casting datum features on the length of the slot 10±1.5 (again, on the machining drawing and on a machined feature) to enforce that COULD be an acceptable (read standardized/ acceptable/ agreeable) option.

Obviously, to show machined datums A, B and C for the same feature (length 10±1.5) is acceptable and fully agreed by Y14.5, so no need to be shown on Y14.8 again.
But, probably the committee wanted to emphasize that showing casting datums on a machined drawing (and the applicable features where those casting datums are shown , are NOT machined datum features) is an accepted option/ in fully agreement with the casting Y14.8 standard.

Interestingly enough, in Y14.8-1996 machining datum reference frame (4.4) wording has the word “should”, which is kept in Y14.8-2009 with one additional sentence added:

“The initial datum targets shall be shown and described on the drawing showing the machined features. See Fig. 4-9.”

The location of the length of the slot from (the casting datums) stayed unchanged.

So, again, I guess (unless I am missing something) that is a valid option (even does not looks like the most logical one).

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