Confusion over Datum choice on Positional Tol.
Confusion over Datum choice on Positional Tol.
(OP)
Hi All,
Perhaps people can help me with a debate we are having at work.
See picture.
My colleagues are questioning what Datum A brings to the table for the positional call-out on the ∅37. They argue that referencing Datum B should suffice for controlling the concentric relationship between the two bores.
From a functional perspective, the Datum B bore houses one end of a shaft bearing. The datum A face is a spigot/location diameter that abuts to a mating part in the main housing. I want to ensure all bores are concentric. I'm also trying to convince them that using an actual concentric tolerance is not necessary and inspecting it is pure evil..
I thought that having datum A introduces a perpendicularity control. A separate thread here: Link describes something similar and I can follow the train of thought in that post, but i'm having a hard time applying that logic to my example.
I've seen it many times where a positional tolerance for a bore references a perpendicular mating flange face.
We work to ASME.
Thanks all.

Perhaps people can help me with a debate we are having at work.
See picture.
My colleagues are questioning what Datum A brings to the table for the positional call-out on the ∅37. They argue that referencing Datum B should suffice for controlling the concentric relationship between the two bores.
From a functional perspective, the Datum B bore houses one end of a shaft bearing. The datum A face is a spigot/location diameter that abuts to a mating part in the main housing. I want to ensure all bores are concentric. I'm also trying to convince them that using an actual concentric tolerance is not necessary and inspecting it is pure evil..
I thought that having datum A introduces a perpendicularity control. A separate thread here: Link describes something similar and I can follow the train of thought in that post, but i'm having a hard time applying that logic to my example.
I've seen it many times where a positional tolerance for a bore references a perpendicular mating flange face.
We work to ASME.
Thanks all.






RE: Confusion over Datum choice on Positional Tol.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Confusion over Datum choice on Positional Tol.
Here's one alternative - this part is mounted onto a shaft using an interference fit on the 22mm diameter identified with B and the 37mm diameter is the sole interference fit guide for another part. The goal is to limit the location and orientation variation between the shaft and the other part. This alternative is one where only B would be of importance to the position tolerance for the 37mm diameter.
Any other surfaces that orient the part or any loop of mechanical connections should be included as datum references.
RE: Confusion over Datum choice on Positional Tol.
Thanks for the response.
From a functional point of view the part locates into a housing bore on the ∅78 until it abuts onto an internal shoulder on Datum A face. A shaft runs through the housing on a bearing located Datum B.
I'm still struggling to see what Datum A brings to the party.
RE: Confusion over Datum choice on Positional Tol.
I'm merely restating Dave's point: If it's an interference fit with one stop point and no flattening onto the shoulder, then your colleagues are correct that datum A is not to be referenced.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Confusion over Datum choice on Positional Tol.
RE: Confusion over Datum choice on Positional Tol.
Are you able to post a sketch showing interfaces for the part in question? The best would be if the sketch not only showed the assembly, but also a bit of information on interface tolerances.
RE: Confusion over Datum choice on Positional Tol.
Belanger, 3D Dave,
The ∅78 is a sliding fit into the mating housing. So yes it is contacting on the single highest point. Therefore any out-of-perpendicularity of that Datum face A has minimal impact on the orientation. However, Datum Face A does perform an important functional purpose of clamping onto the shoulder of an internal part.
pmarc,
I'll post a picture later today showing the assembly, I'm at home at the moment.
3DDave
I;m beginning to think this might be the better approach.
RE: Confusion over Datum choice on Positional Tol.
See shot below of part in context.
Bearings are in red. Housing in question on the left.
RE: Confusion over Datum choice on Positional Tol.
Ø78mm feature is driven so locate/position this to DRF (B RFS promary and A secondary)
RE: Confusion over Datum choice on Positional Tol.
It would probably help if you explain your goal in more detail. Your context image doesn't show anything mating with the diameter 37 bore in question, and it's not clear what else may have been left out as well.
If there are screws clamping the flange down all around the circumference, I'd be very surprised if datum feature A only makes contact on the single highest point.
pylfrm
RE: Confusion over Datum choice on Positional Tol.
To clarify, the external flange does not clamp down and contact the external face.
The 37 bore houses a brushless resolver stator. The resolver rotor mounts on the shaft.
Mounted between the bearings is a Motor, the rotor being on the shaft and stator clamped in the housing. I want these and the resolver running concentric.
RE: Confusion over Datum choice on Positional Tol.
Certified Sr. GD&T Professional
RE: Confusion over Datum choice on Positional Tol.
So why A primary?
Will the part orients/ be centered in the assebly based on A or based on B?
RE: Confusion over Datum choice on Positional Tol.
If the bearing is sufficient to fix the orientation of the part, then it would negate the ability to clamp the middle piece.
If the latter is true, then this is a problem for the stress analysts to determine if the tolerance stack produces unacceptable forces at assembly and unacceptable internal stresses.
RE: Confusion over Datum choice on Positional Tol.
3DDave got to it first. My thinking is: Datum A will provide the primary orientation between the OP part and the "center" part when the bolts are torqued up and Datum A goes "tight". The bearing will "follow" this first interface.
Certified Sr. GD&T Professional
RE: Confusion over Datum choice on Positional Tol.
What is the physical reality of this assembly?
Not necessarily what the designer wish for, but what is actually happening.
I still say that the datum feature A is small "enough" to NOT do a proper job.
Datum feature B looks much bigger.
RE: Confusion over Datum choice on Positional Tol.
What would be the effect of removing datum A altogether and just referencing Datum B for positional? Maybe I just need a flatness control on the Datum A face as mentioned by mkcSki?
The more I read about GD&T the less I think i understand....
RE: Confusion over Datum choice on Positional Tol.
RE: Confusion over Datum choice on Positional Tol.
Re:"The more I read about GD&T the less I think i understand...." --just read the standard and will be much easier:):)
RE: Confusion over Datum choice on Positional Tol.
Do not be frustrated. Your OP is the MAIN purpose of GD&T: to force you to think and understand how parts "function and fit-up" and then use the symbols to communicate the concept the best you can. After 35 years I still "fight" to applying the controls, hoping the interpretation will be as I intended.
Keep at it. Don't work in a vacuum, use the Forum to expand your understanding.
Certified Sr. GD&T Professional
RE: Confusion over Datum choice on Positional Tol.
Another aid is to build a directed graph that has each surface of the part joined by the applied tolerances. This can be applied to the entire assembly, but need not include surfaces that will not participate. With the graph done, you can look at the tolerance path between surfaces that don't have explicit applied tolerances.
In the first image one would immediately notice that there is no relation between A and B, so there isn't a way to know how much variation in the orientation of A with respect to B is acceptable.
RE: Confusion over Datum choice on Positional Tol.
With regard to the question: "What would be the effect of removing datum A altogether and just referencing Datum B for positional?", I think that attached graphic may help you a little bit to understand the effect of removing A from the positional callout applied to dia. 37 cylinder.
http://files.engineering.com/getfile.aspx?folder=6...
This (especially the picture on the right) and the fact that the part is clamped to the housing should also slightly help you to find an answer to the question whether functionally dia. 23 bore is/should be the key player in orienting the part in the entire assembly or not.
RE: Confusion over Datum choice on Positional Tol.
pmarc,
Thanks for taking the time to draw that out, it really helped. I had that scenario imaged in my mind so I’m happy to see I wasn’t thinking total nonsense (or at least not the only one
So looking at you diagram, the first thing that strikes me is that the part on the far right would have failed inspection, the part in the middle would have passed. This was purely down to how datum's have been applied – correct?
Now, IMHO the scheme in the middle is more appropriate to the design intent as that is how the part is orientated when assembled. However, the bearing when installed into the housing looks like the picture on the far right – is doesn’t care about datum face A and will adopt whatever orientation it can from the bore. So to ensure alignment of its axis with the dia.37 the picture on the far right seems more appropriate?
RE: Confusion over Datum choice on Positional Tol.
I just wanted to recognize your efforts.
You are the dude! You captured the concepts perfectly - in color too. Geez. I am not CAD literate and would have had to use hand sketches. Great support for the GD&T community.
Certified Sr. GD&T Professional
RE: Confusion over Datum choice on Positional Tol.
But in general, yes, the way the datums are applied/defined will almost always have impact on inspection results. Unfortunately, not everyone realizes this and that is why we often see a lot of "creative", but incorrect, ways of simulating datum reference frames that may and most likely do lead to false conclusions about part's conformance against dimensional requirements specified on a drawing.
The bearing may indeed not care about datum face A when installed into the housing, but the question is whether it will also not care about it when the housing is clamped with bolts to the other housing?
The other question is - functionally, does the dia. 37 need to be oriented to the datum face A in the first place? I am afraid that based on: "The 37 bore houses a brushless resolver stator. The resolver rotor mounts on the shaft. " I am not able to offer any meaningful answer.
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Thanks. I just didn't know that Paint and Powerpoint qualify as CAD tools
RE: Confusion over Datum choice on Positional Tol.
Your secret is out. Hahaha
Certified Sr. GD&T Professional