Least bad of two options: Circular edge or Cone as tertiary datum? 2

[2JL]

Hello Everyone

I am currently dimensioning a part machined from a die casting and I find myself in a peculiar situation where I need to make a choice between two bad options. I need a tertiary datum to ensure that a pattern of threaded holes is in a correct angular alignement (no precision is needed) relative to other features. No machined features is available for this purpose so my only option is to use a conical hole or derived a point from a circular edge.

http://files.engineering.com/getfil...f86-bfd9-e17b56656398&file=Tertiary_datum.png

What do you think I should do? Thanks for your time and suggestions

2JL

 

[greenimi]

Can we see the entire print?
Sometimes tertiary datum is not even needed and simultaneuos requirement is taking care of the alignment.

 

[2JL]

Hi Greenimi,

Attached is a simplified version of the print. Basically the part is a casting with a central bore (datum B) and threaded holes. Most of the features (not shown) will be used as cast. I just dont want the threaded holes pattern to end up in a random location.

http://files.engineering.com/getfil...bd-bad2-4233ff5459a5&file=Tertiary_datum2.png

 

[greenimi]

So, if you control (position) everything to A primary and B secondary, you don't need the tertiary datum feature.

 

[CoryPad]

In the automotive industry, for parts like this it is typical to use a tertiary datum and I would select the hole not the cone.

 

[greenimi]

There is no hole for datum feature C.
What about if you are not in the automotive?
What about dimension it per the standard you are working on and not for the industry you are into.

 

[2JL]

Greenimi : "So, if you control (position) everything to A primary and B secondary, you don't need the tertiary datum feature."
Do you mean simultaneous requirement apply in this case? I don't have any other features referenced to A and B. In the final part they are all as cast but I see your point. May be if I add a general profile tolerance to relate the cast features to datum A and B, it will do the trick. And this will make sense since what we are paying for is the final part. What do you think about that?

 

[greenimi]

The other features (as cast) should be on the part right?

So, they should be somewhere in their respective --acceptable-- position in order to do their job. Therefore, IMHO they need to be positioned from some functional datum fetaures (A primary and B secondary)--they should be in a volume of space, volume defined based on their coordinate system decided.

all the features of size you position them to A and B
all the features of non size you profile them to A and B

and then the simultaneous requirements is automatically implied and all the features above will be considered as a single pattern (by defualt)

ASME Y14.5 should be implied and show on the drawing

 

[2JL]

Thanks for your input Greenimi

I didn't think this way early because we have separate drawings for cast and machined parts and we used to relate them using datum targets. Now we are checking the as-cast surfaces to machined datums in other to ensure critical clearance or mimunum material where required. That's why not having a machined tertiary datum was bothering me.

Ok enough for today!!

 

[SeasonLee]

2JL

You may choose one of the three through holes as Tertiary datum feature to keep the angular relationship between the features, most people will call out this way.

You may also remove the Tertiary, and the implied simultaneous requirement will help you to keep the interrelated to one another.

Please see the attached sketch for details. Technically, I prefer to the option two.

Season

 

[CheckerHater]

If you look closely at option 2, you'll see that you don't need secondary either!

You just call everything wrt [A] and argue Simultaneous requirement.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

CH,
I agree with you.

However, baby steps CH....baby steps
We have to walk before we can run.

 

[2JL]

SeasonLee, CheckerHater, Greenimi

I perfectly agree with you all, the simultaneous requierement rule will applied in this case but may be I didn't state clearly my question. So I hope you don't mind if I formulate it in a different way.

Let's imagine the raw part was bought from one vendor and we need to machine the base face (datum A), the centre bore (Datum B) and the 4X M8x1.5 holes. How do I ensure the threaded holes pattern wouldn't end up in a random location in the final part? My feeling was that I needed a tertiary datum to unequivocally constraint them.

This concern comes from the fact that our vendors don't always mind about (or understand) the " according to ASME Y14.5..." note, that's why I tend to avoid rules that are too specific to ASME.

2JL

 

[CheckerHater]

One thing still bothering me: where is datum feature (central bore) located? Central to what?

Is it center of the outside diameter? How far the position is allowed to vary? Should OD be referenced:
a. MMC
b. RFS
c. Other?

Or maybe it is in the center of the pattern of cast holes (same questions apply)?

Something is telling me that simultaneous requirement cannot fix everything...

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[CheckerHater]

I was typing as you posted.

Not only you will need tertiary, you may need temporary datum to locate your datum to the raw part.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

On the product drawing you don’t need the tertiary datum feature.
On the process drawing (casting) you might need one.

CH,
Datum feature B does not need location. Needs ORIENTATION only. And you locate everything else from it.


Now from Mark Foster:” AGI--Applied Geometrics--

http://www.GDandT.COM

“Mark Foster • Ok, so you gentlemen have touched on the biggest, absolutely most major issue, in all of proper GD&T implementation and usage -- datums/datum features -- manufacturing versus functional. This subject is also the most misunderstood and misapplied, and is therefore is also used as fodder for arguing against implementing GD&T. Let me preface the following with a general statement regarding datum/datum feature selection and defnition. Ultimately, function wins out in any dispute between which datums and datum features should be used to define our parts/assemblies. Having said that, in the real world, we must sometimes make some compromises for the "big picture" economics of the situation. Having said THAT, here are my thoughts on datum and datum feature design, defninition, selection, etc.:

One, everyone must realize that there are 2 major definitions of the part that must be defined, understood, and most importantly, definitively related (mathematically) to one another -- design funcitonality and manufacturing process(es) -- a.k.a. "Product" versus "Process" requirements. Two, we must all also understand that there are 2 corresponding inspections that may need to occur (notice I said, "May," not "Shall.") -- functional and process. And three, there may or may not be overlap in each of those cases. That is, in some instances, the manufacturing and the functional datum features may be one in the same, and in many other cases, they will be completely different.

Once we understand those three basic concepts, we can then begin to subdivide those into specific commodities and groups of processes -- for example, the datum structure that I would use for a machined part would be heavily focused on the "product" side, where the datum structure for a weldment may necessitate incorporating manufacturing fixturing (i.e. "process") in my datum scheme in order to define my functional requirements. The Y14.5 standard pretty explicitly states that we should not state the manufacturing process requirements on our product definitions unless those manufacturing processes are critical to the definition of the engineering requirements.

I could go on forever about this subject because I am so passionate about it, but suffice it to say that you MUST define your functional requirements somewhere, and then decide how to make the part from there.”
In your particular case, it "sounds" to me as though someone is using manufacturing fixturing/tooling/process information to determine their datum structure. I would like to see the functional datum features (and corresponding datums) defined fully before resorting to manufacturing fixturing as defining the datum structure, but welded assemblies is one place where we often must consider the manufacturing process in the development of the engineering requirements and definition. So I cannot say definitively whether the choice of datum structure in your example makes sense or not without directly seeing your particular design scenario.

My advice to all is to absolutely, positively, and completely define your FUNCTIONAL requirements up front, and then make economic decisions from there. If you don't completely define your functional requirements somewhere, there is a good chance that you will lose sight of them and end up chasing a lot of problems that never needed to exist and that you will have a tough time resolving.
I agree that it is "best" when you can have manufacturing datum features and functional datum features be the same. But the reality of life is that they rarely are. Usually manufacturing must produce the datum features, and very often, they would prefer to leave the part in some fixture that is locating the part off of features that will not, in the end product, be functional features. That is why I brought up the other issues in my previous post.

But absolutely the end product must meet its truly functional requirements from the functional datum features when the part is finished, or it will not function. What good is it if something is very easy to make and measure (according to the manufacturing datum features) if it doesn't work (or we cannot tell if it will work until we try it).
We all need to understand that there are two major considerations in the production of a part -- the product definition and the process definition -- and we can choose to use GD&T to describe either or both of these considerations. However, we must be careful to be clear when we are talking about one or the other.

Of course, the best of all worlds is when those two scenarios can make use of the same datum features in the same order of precedence, but that situation is rarely practical or even possible. So we must distinguish when we are talking about one or the other situation. I am a huge advocate of FUNCTIONAL (i.e. how the part physically interacts with its mating parts) datum features (and precedence) for PRODUCT definition -- i.e. no matter how the part is produced. Because, no matter how the part is produced, if it does not meet its functional dimensional requirements from its functional datum features, then it won't work as intended. What good does that do anyone?

But you certainly can use GD&T to define PROCESS requirements as well. However, in *MY* company (if we were a manufacturing firm), these drawings would be controlled by Manufacturing (i.e. NOT Design), and they would the words, "PROCESS DRAWING" in big red letters stamped across the drawing, so as to have no misunderstanding that what is being depicted are the PROCESS requirements, and NOT the PRODUCT requirements.

In short, PRODUCT requirements are the goals to achieve, not the instructions for how to make the part. And the PROCESS requirements are the instructions *for one particular supplier's chosen method* to achieve those goals that were stated on the PRODUCT drawing. The PRODUCT drawing is the legal requirement and obligation of the supplier. The PROCESS drawing is the chosen method for a given supplier, but another supplier could chose a different process to achieve the same PRODUCT requirements.

Having said all that, we teach that design people should begin their designs by first imagining the datums that they require for the product to function as desired, then DESIGN their datum features to be physically capable of arresting the degrees of freedom that they need arrested for the part to function as desired, and then dimensionally control features relative to those datum reference frames that they designed."

End of Mark's post

 

[CheckerHater]

Wow, that's one big post!

Could somebody point me out, where in that post I can find answer to the following questions:

1. How far datum feature is allowed to be off-center?
2. Center of what?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

CH,

Are you talking about the product drawing or about the process drawing (casting)?

I hope you are talking about the process drawing (casting) because as I stated before if you are taking about the product drawing in my opinion you don't need that info (all you need is a perpendicularity to A, primary datum feature)

Now, if you are talking about the process drawing (casting) that should be defined according to ASME 14.8 and should be developed and in agreement to the casting manufacturer (foundry) and probably they will tell you about the datum targets that needs to be used according to their fixturing equipment, parties lines, draft angles etc.

Don Day has an excellent video about the casting process on his website

http://www.tec-ease.com/

(brown bag sessions)

 

[2JL]

CheckerHater

1. How far datum feature is allowed to be off-center?
2. Center of what?

That's why I needed a complete set of datums so that the as-cast features would be checked against them (basically making sure the functional requirements are satisfied) . This way I indirectly ensure the proper location of datum B and the threaded holes.

2JL

 

[CheckerHater]

Dear greenimi,

I am talking about machined part drawing, and I am talking about purely functional requirement:

What feature(s) central bore is functionally related to???


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future