Parellelism Against A Complex Surface

[brwalker145]

Hello all, long time reader, first time poster...

I have a customer-supplied print that calls out a Parallelism inspection against Datum D. However, the feature they have defined as Datum D is a non-planar complex surface (it looks flat on the print, but it's not). I have been arguing that we cannot measure Parallelism as currently defined, but the customer insists that the print is correct.

Am I wrong here? I just don't see how this could possibly be measured. I'll be the first to admit that I am far from a GD&T expert, but unless I've completely missed it, I don't see anything in Y14.5-2018 that would support their argument.

 

[greenimi]

3DDave,

Perhaps we can get greenimi on a flight to explain to the customer that the situation is made much simpler if datum feature D is perfectly flat

Really 3DDave? Where did I say that a DATUM FEATURE is perfectly flat?
Don't put words in my mouth.....

Again, I am asking AGAIN, what makes you think, BASED ON THE DRAWING, that datum feature D is not designed (intent) as a nominally flat feature? What part of the drawing indicates just that?

And the OP still hasn't answer my question......If parallelism (0.1 wrt D) is changed to profile (with or without tangent plane T modifier) could you now measure it?

I understand that "the feature they have defined as Datum D is a non-planar complex surface", but the print (incomplete posting maybe) does not show that

 

[3DDave]

The title of the thread and the original problem statement makes me think it is not a nominally flat feature.

It is worth trusting the OP and not someone who doesn't have a copy of the complete drawing. Perhaps you have a copy of the print and not just a tiny clipped image? If so, can you post the entire print?

 

[greenimi]

Perhaps you have a copy of the print and not just a tiny clipped image? If so, can you post the entire print?

That's a good question and inquiry for the OP.

 

[greenimi]

It is worth trusting the OP and not someone who doesn't have a copy of the complete drawing.

Agreed. With only one caveat:

The OP is biased. We might not have a full story. Just the story to fit the agenda.

Per the old saying on this forum(I crossed out what is not applicable)

OP:
1. We don’t follow any standard in our company
2. Recently I’ve had an argument with my co-workers
3. Please tell me that I was right

Poster 1: Is it ISO or ASME you do not follow?

Poster 2: You don’t have to do anything; the Simultaneous Requirement will take care of it

Poster 3: Use Profile

 

[axym]

Hi All,

There are a lot of difficulties with this.

Y14.5's treatment of orientation tolerances is quite limited. It only deals with orientation of single planar surfaces or single cylinders, with the datum features being simple combinations of single planar surfaces and single cylinders. There is no mention of other cases, so we have to make judgments on extension of principle.

The Parallelism tolerance on the OP customer's drawing is not valid, because the toleranced feature is not nominally parallel to the datum feature (because the datum feature is not planar). However, I would say that it would be technically valid if the tolerance was Angularity. The underlying principle of orientation tolerances is that the tolerance zone is allowed to translate relative to the DRF. Y14.5 does not mention complex datum features in the context of orientation tolerances, but it doesn't prohibit them either. I would say that this would be a valid extension of principle.

Having said that, defining a datum feature on a nearly-planar-but-not-quite feature is problematic. This would invoke a TGC (True Geometric Counterpart) that would be a perfect-form opposing version of the basic datum feature. So if the basic datum feature is very slightly convex, the TGC would be correspondingly very slightly concave. This TGC would define the basic orientation of the toleranced feature, for the Angularity tolerance zone. There is also the issue that the complex datum feature needs to be referenced at a particular boundary condition (RMB, MMB, LMB, or BSC). I'm not sure that it's clear which boundary condition would apply in the OP's case.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

There is no evidence whatsoever that datum feature D is a complex surface. All proof is pointing that it is a nominally flat feature

 

[brwalker145]

Greenimi,

I'm not sure why you've turned such a simple question into such an ordeal. You're extrapolating an opinion based on a single section view while making a conscious effort to ignore the context I provided. This is an internet forum, not court of law; if you want proof, print out the image and use a straight edge.

To everyone else, I appreciate the concise feedback and the opportunity to learn. I consider this thread answered and closed.

 

[greenimi]

I'm not sure why you've turned such a simple question into such an ordeal. You're extrapolating an opinion based on a single section view while making a conscious effort to ignore the context I provided. This is an internet forum, not court of law; if you want proof, print out the image and use a straight edge.

I agree with you. I have done it intentionally. The point was (and still is) to show how easy is to be argumentative when a drawing is not clear. Otherwise stated can be argued both ways.
Are you buying or are you selling? Depending which side of the fance you are initially placing yourself (and you choose already one) there are people who can argue the other side with fair chances of succeeding.

Gibson's law holds that "For every PhD there is an equal and opposite PhD."

 

[3DDave]

When the goal is to argue rather than achieve then that's when you stop trusting the other party.

 

[greenimi]

When the goal is to argue rather than achieve then that's when you stop trusting the other party.

You are correct.
And I did (stop trusting the other party) when I saw that answering my questions has been done in a very selective way. Only questions what fit the answers the OP wants to hear have been answered. The others ignored.

 

[axym]

brwalker145,

I'm glad that you got some useful information from the discussion. One other concern I forgot to mention. The profile tolerance on datum feature D is 0.4, which means its form error could be 0.4 or at least close to it. Then the toleranced surface needs to be oriented within 0.1 to datum D. This doesn't violate any rules, and it's possible to achieve that tolerance, but it seems like a "likely to fail" scenario.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca