Least bad of two options: Circular edge or Cone as tertiary datum?
Least bad of two options: Circular edge or Cone as tertiary datum?
(OP)
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/getfile.aspx?folder=1...
What do you think I should do? Thanks for your time and suggestions
2JL
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/getfile.aspx?folder=1...
What do you think I should do? Thanks for your time and suggestions
2JL





RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Sometimes tertiary datum is not even needed and simultaneuos requirement is taking care of the alignment.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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/getfile.aspx?folder=9...
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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!!
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
I agree with you.
However, baby steps CH....baby steps
We have to walk before we can run.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Not only you will need tertiary, you may need temporary datum to locate your datum [B] to the raw part.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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-- 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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Could somebody point me out, where in that post I can find answer to the following questions:
1. How far datum feature [B] 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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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)
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
1. How far datum feature [B] 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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
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
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Therefore, datum feature A (primary) should be the face and datum feature B should be the central bore. If these two datum features are NOT the functional datum feature, please kindly advise. Are they or not?
If they are not, which feature are?
If they are, then the secondary datum feature B must be oriented to the first datum feature A (so, perpendicularity is the correct control).
“A” is the first thing in the universe (the universe for this part = the coordinate system for this part).
“B” is the second thing in the universe and is not located, but is ORIENTED to the first daum (primary) datum feature.
THEN, you are going to locate anything else to “A” primary and “B” secondary. So, you are going to locate the pattern of the holes to A primary and B secondary, and are going to locate the OD to A and B and are going to profile the feature of non-size to A and B, etc.
That would be my opinion.
I am 100% sure CH, you already know this, but you are just trying to have fun. I am perfectly fine with that. Hopefully, the OP will not get confused.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Why?
Should I understand that you agree with me?
Anybody else?
Belanger, pmarc, Evan, 3Dave, just to name a few of the MVP's of this forum.........
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
My idea is to describe the functional requirements of the final part then the raw material (ie the remaining as-cast features) will be checked against the machined datums. Since we aren't producing the parts in-house, the goal is to let the process expertise to the supplier. My only concern is good parts no matter how they are handle during manufacturing.
2JL
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Could you please reference standard that describes distinction between product drawing and process drawing?
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
If the overly used “functional” word/ functional requirements in Y14.5 and Mark’s statements (above) are not enough for you, I will rest my case.
The question is: are you seeking solutions or are you in love with your problems?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
I suggest you keep personal insults to yourself.
With all due respect to Mark Foster, he appears to live in imaginary world where there is no need to make parts from his drawings (see his statement: "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")
Still, I would like to see your own personal opinion, as deep, as Mark Foster's.
In real world there is only one drawing - "drawing", and there is only requirement - to provide all the information necessary to make good part, otherwise it's a "bad drawing".
ASME Y14.8 clearly prefers "Separate views drawing method", that is, casting/molding and machining requirements are shown on separate views / drawings.
OP was very clear saying that "we have separate drawings for cast and machined parts"
Now, open Y14.8-2009 and look at Fig.2-1
You will see that cast features are defined using their own, separate set of datum features (A, B, and C on Fig.2-1 (a))
The machined features are defined using their own, separate set of datum features (D, E, and F on Fig.2-1(b))
Accompanying paragraph 2.2 states importance of specifying cast-to-machining relationship. You can see that on Fig. 2-1(b)dimensions are provided to relate cast datum features to machined datum features.
All this is in letter and spirit of ASME Y14.5-2009 Para. 4.8.1 Temprary and Permanent Datum features - cast datum(s) serve as temporary datums for establishing machining datums.
Now, OP has no control over cast features. He is asking for advise how to best relate machined features to cast (datum)features. Your answer is "Simultaneous requirement".
How possibly simultaneous requirement can control both cast and machined features if THEY BELONG TO TWO DIFFERENT DRAWINGS?! And you say I have problems.
Simultaneous requirement is a powerful tool and shouldn't be used to cover sloppy design work. And I agree - the word "functional" IS overused.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
CH,
"How possibly simultaneous requirement can control both cast and machined features"
Because both features (machined and cast) are on the same piece of meat regardless how you make them (casting, forging, machining, etc). They have to work together. They have to have a relationship to one another.
With all due respect:
How would you do it? So far, just dismantling someone else “potential solution” without offering other to replace it does not help CH.
And also offering conflict statements does not help either:
Statement 1
“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.”
I am not sure you were sarcastic here or not
Versus statement 2
“Not only you will need tertiary, you may need temporary datum to locate your datum [B] to the raw part.”
Are these two statements in conflict to each other or it is just me? If it’s just me again the question, how would YOU do the drawing (the good drawing)
Your quote: “In real world there is only one drawing - "drawing", and there is only requirement - to provide all the information necessary to make good part, otherwise it's a "bad drawing".”
And rest assured I have a ton of respect for you for the time spent here on this forum.
As for “Still, I would like to see your own personal opinion, as deep, as Mark Foster's.” CH, you cannot see that because…….. is very simple…. I don’t have it as deep as Mark’s. Sorry about that. I am still learning. But that does not keep me from getting, from time to time, in conflict with the MVP’s of this forum.
Therefore CH, how would you do it?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
How would I do it?
First I would produce drawing of the casting. Here everything could be subject to Sim Req, because, after all, it's all being cast together in one chunk.
Second, I would produce final (machined) drawing, paying attention to:
- Concentricity of central bore. Concentricity is never implied, so two questions: how concentric and concentric to what (I think you already guessed it)
- Position of tapped holes. Techniques may vary, but clocking wrt cast holes is making sense.
I have to go to do some of my actual job, but I will try to produce couple of pictures, if you are still interested.And suggesting that you are not deep enough, was unacceptable. Sorry
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
The case is simplified (maybe oversimplified), and I tried to use (maybe overuse) Simultaneous requirement.
Casting is shown mostly for illustration as OP has no control over casting. Suggested tolerance scheme is not the only one possible, but the one using Sim. Req.
I added central hole to casting, as it would reduce amount of subsequent machining.
I like simple elegant solution as much as the next guy, but when there is possibility of misinterpretation, it’s better to explain yourself in finer detail, maybe even verbal note if necessary.
On machining drawing dimension 1.900 ties casting datum to machining datum.
It would be easy for machinist to center the bore with the existing central hole, but after central hole being removed, there will nothing to check against. So, we center the bore wrt cast hole pattern.
Then we position tapped holes wrt the same hole pattern. This creates Simultaneous requirement – the bore and tapped holes are located loosely to casting (per OP’s “no precision is needed”), but the bore and the tapped holes now form one single pattern.
Like I mentioned, this is not the only possible solution, but it uses Sim. Req., takes in consideration both Y14.5 and Y14.8, agrees with common sense and principles of machining and QC.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Okay. Agreed.
Just a couple of comments I still have. Please see if you agree with them or not.
On your casting drawing: I do not know if using position for the central hole just to force the simultaneous requirement is the right (or even legal for that matter) thing to do or not. Personally, I would use perpendicularity and that is because orientation is the only thing you CAN do for that hole. The central hole cannot be located to anything else.
Another thing is when defining castings / forgings I prefer to use the last letters of the alphabets. So, in our discussion I would use Z as the primary datum for the casting (I know that is a minor point, but just to make sure). Probably it is an industry best practice to use Z, Y, X, etc. for castings and A, B, C, D, etc for final product.
Sorry, I do not have any capabilities to change/update your pdf at work.
On your machining drawing (and I will call it product drawing) I would have a couple of corrections (again, IT IS my personal opinion, since you asked for it):
- Your shown datum feature A to become datum feature Z
- Datum feature C to be moved from the lower/ smaller hole to the central hole (big change versus what you proposed)
- Remove the positional callout for Ø.2.250 hole and add perpendicularity to B (datum feature for machining) and another perpendicularity to Z (datum feature for casting)
- All the other applicable holes will be controlled by the following two single segments positional (not composite) callouts: upper line position to B primary and C (maybe at MMC) secondary; lower segment position (maybe at LMC since it is casting) to Z.
- Since you made the OD basic, I have no problem with that and I would just add another profile (for machining) to B primary and C secondary.
Thank you for your time and cooperation. Any comments or concerns with “my revised drawing” please let me know.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
Hope helps more than confuse.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
I am glad you agree that there is more than one way to skin the cat.
I am a bit uneasy with your way of using perpendicularity, because it still orientation control. I said it before, just because it is shown in the center, doesn't always say HOW central it is. So, concentricity, position, runout - something.
On my machining drawing datum C is actually pattern of three holes, which may be used to derive central axis - I don't know how much that will change the way you see it.
If you still insist on making central bore secondary datum for machined part, I would simply use cast hole for clocking as OP suggested.
I really don't want to start another fight over every tiny detail. My example was to show general idea of separating cast and machined features.
Just like your attachment shows, cast part may see heavy use of datum targets - this is something we both did not even consider yet, but it could be interesting.
So, what will we suggest to OP?
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
CH,
I don’t know. If the OP will tell us more about how this product functions and also tell us more how this product is made (datums / datum targets used by their foundry/ casting guys, parting lines, fillet radii, gate requirements, draft angles, die closure, flash, etc.) then probably we will be able to compromise and find an acceptable solution.
There are probably 2-3 good ways to define a product. Not 50 good ways. But there are 200 ways to wrongfully define the same product.
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
For this particular case, the bore and the flat bottom face are the functionally relevant features. The threaded holes are for mounting an accesory part for which the orientation is not very important relative to the other features. The as-cast taper holes are big enough to provide sufficient clearance for mounting bolts.
3 months ago I would have done it as per Tec. Ease document but now we are exploring different avenues to make our drawings the most process-agnostic as possible. The goal is to shift the focus to the final part functional requirement and let the process expertise to the suppliers. But we are not just shipping them drawings and expect good parts. Before production is started, we have a series of meeting during which we explained the design to the selected supplier(s) and hightligh critical safety and functional features. Then we go through each step they are going to take to manufacture the parts and if needed, make some change to the final design before starting production.
2JL
RE: Least bad of two options: Circular edge or Cone as tertiary datum?
So, in my opinion, I will go with the option I have proposed.
Re:
CH,
"I am a bit uneasy with your way of using perpendicularity, because it still orientation control. I said it before, just because it is shown in the center, doesn't always say HOW central it is. So, concentricity, position, runout - something."
I am not sure why, ON THE MACHINING drawing, you have to locate the center bore? Why you cannot just orient it?
again, why locate, in your opinion, is a must (concentricity, position runout - something)
Why we cannot live only with orientation control and locate everything else from it?