Perpendicularity

[randy64]

I am seeing a conflict on how this works.

I am in a training class right now (TCI, Al Neumann). He states that Perpendicularity can be referenced to datums that are not perpendicular to the surface being held perpendicular. He has an example in his workbook showing it that way also. He refers to Perpendicularity, Parallelism and Angularity as Orientation controls (which they are), but seems to indicate that they are really flatness controls with orientation and that Perp, Par and Ang are just our perception of the direction of orientation.

In Alex Krulikowski's book "Fundamentals of GD&T" he explicitely states that the surface being called Perpendicular must be perpendicular to the datums it is being referenced to in the feature control frame.

Which is it? I've always gone with Krulikowski's interpretation. If we didn't, we could just get rid of Perp, Ang and Par and call it all Orientation.

What says you?

Thanks.

 

[dingy2]

Perpendicularity is an angularity control but this one is set at 90 degrees. It is not strictly angularity though.

Perpendicularity on a flat surface also includes flatness. In other words, if one had a .010 perpendicularity tolerance in the feature control frame, it means that when one measured the full surface set up on 90 degrees, we should not have a dial indicator movement of more than .010. It can be all angle (indicator increases/decreases as it is moved) or all flatness (indicator is 0 at the beginning and end of the angle but movement in the centre).

Perpendicularity on a axis, such as a cylindrical pin, will include its straightness. Again, it can be all angle or all straightness but most likely a combination of both.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[randy64]

So dingy, are you saying that you would agree more with Krulikowski's take on it?

To clarify, this is about 1994. I have found an example in 2009 that does it the same way Nuemann does it.

 

[dingy2]

randy64

Sorry but I need an example before I can determine whether or not the example meets the 94 standard. There are many areas in the 2009 standard that are not applicable in the 94 standard.

Dave D.

http://www.qmsi.ca

 

[randy64]

I'm not sure an example is needed. It's a pretty simple question: When calling a surface Perpendicular, do the datums called out in the reference frame have to be perpendicular to that surface? For further clarification, I am referring to the secondary datum. They both show the primary being perpendicular.

Neumann says no, Krulikowski says yes. Which is correct?

Thanks.

 

[powerhound]

Krulikowski is correct. You can't call out perpendicularity to a datum that is not perpendicular to the feature being specified. In fact, this is so far off base that it makes me think that possibly you've misunderstood what was being said. You CAN call out perp to 2 different datums and both datums be perpendicular to the feature. Is this maybe what Neumann is saying? Neumann definitely knows his stuff.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
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[randy64]

Nope, Neumann says just what I said. He even has an example of it in his work book. His take is that the datums are setting up a framework showing how the part will be held.

I'd like to scan and attach the pages from the workbook, but I'm afraid that would be against his copyright. I will, however quote from it a bit.

********************

Perendicularity ... to two or more datums, can be very confusing if you do not understand the datum reference frame concept. It is important to remember that the datums only set up the part. The part, in this case, is set up with 3 points on A and 2 points on B. The orientation or perpendicularity tolerance zones are oriented to the datum reference frame and not the datum features. The surface must then fall within the tolerance zone.

The perpendicularity specification reads: This feature is perpendicular to the datum reference frame which is established, in order, by datum features A and B.

***********************

The above is accompanied by a picture showing a block with Perp called out to 2 datums. Datum A is perp, but datum B is actually a parallel surface.

And I'm not sure where he's getting the "perpendicularity specification" info. I can't find it in my copy of ASME Y14.5M-1994.

Powerhound, I agree with you, but this is Neumann's interpretation, and from what little bit I've seen, it's the interpretation that is used in the 2009 version.

 

[caseynick]

When calling a surface Perpendicular, do the datums called out in the reference frame have to be perpendicular to that surface?

No. The surface in question has to be oriented according to the datum reference frame. In the example you are referring to that means the perpendicularity tolerance zone is perpendicular to the primary datum and clocked by the secondary datum. The tolerance zone is, in fact, parallel to the secondary datum.
The datum features are used to set up the datum refernce frame.
The datum reference frame is used to establish the perpendicularity tolerance zone.

 

[MechNorth]

I think there's a missed detail in here. The datums must be mutually perpendicular. The datum features do NOT have to be mutually perpendicular. There is nothing in the standard that indicates that the datum plane and the surface generating it must be coplanar. See if this tip helps:

http://www.tec-ease.com/tips/january-01.htm

The original post wasn't clear to me, and it isn't fair to comment on Al's material without knowing the exact wording rather than a paraphrasing. Hope this helps.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[powerhound]

Just scan the illustration. I can't get past your assertion that a feature can be specified as perpendicular to a datum to which it is not perpendicular. If Al Neumann has somehow justified this, I have got to see it to believe it.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[randy64]

caseynick has stated Neumann's case exactly.

MechNorth, I posted a direct quote from Neumann's workbook above. No paraphrasing.

powerhound, I can't get past it either, but it's there. Check out the attachment.

Overall, I understand what Neumann is saying, but it's very counter-intuitive. To say that a surface is perpendicular to datum B, when the drawing plainly shows that they are parallel makes my head want to explode. But really, I think phrasing it that way ("a surface is perpendicular to datum B") is not what Neumann would say. I think he would say "Set the block to datum A, then datum B, and then check the surface to make sure it it in the tolerance zone," or maybe, "The surface is within the tolerance zone when held relative to datums A and B."

Even though I can wrap my brain around what Neumann is saying, I still think it is incorrect. I don't see anything in ASME Y14.5M-1994 that supports this idea. If you look at the definition of perpendicularity in 6.6.4, it states "Perpendicularity is the condition of a surface, center plane, or axis at a right angle to a datum plane or axis." It says AT A RIGHT ANGLE TO A DATUM PLANE (caps for emphasis, not yelling). To me that excludes datums that are not perpendicular.

 

[powerhound]

Wow, you weren't kidding. This is contrary to anything I've learned about perpendicularity. A datum reference frame is made up of mutually perpendicular planes by default. There should be no need to specify datum B in the illustration. I'm gonna need some help from someone here that knows what's going on. How about you Jim? Can you help me understand this too? If datum B were the surface facing us, then it would be clear. If I were in a class that tried to pass this off as legit, I think I would seriously challenge the instructor...offline of course, not during the class.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

What I see on the illustration is a confusing way of specifying "profile of a surface". While the standard specifically says that the perpendicular condition is at a right angle to a datum. It doesn't say it CAN'T be used some other way...but that's pretty lame IMO.
The standard does show conditions where 2 datums are used but the illustrations are totally different than yours (see Fig. 6-35).
Man, I'm really itching for a good explanation of this one. Thanks Randy...this is a good one (at least I think it is).

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[randy64]

powerhound, sounds like you're experiencing the same "exploding head syndrome" I'm experiencing! LOL!

The thing is, an almost identical picture is in ASME Y14.5M-2009. I think if ASME is going to do it this way, it shouldn't be called Perp or Par - it should all be Angularity and sometimes the angle happens to be 90 degrees or 180 degrees. Or they could come up with a new symbol and call it Orientation.

 

[caseynick]

ASME Y14.5M 6.6.1
"Relation to more than one datum feature is specified to stabilize the tolerance zone in more than one direction."
In the case of perpendicularity the secondary datum is solely to control a rotational degree of freedom. See fig.6-35. Datum B could just as easily be the short surface on the left side of the illustration and it would be a perfectly legal callout. The tolerance zone needs to be oriented to the primary datum and rotationally controlled by the secondary datum in either case.

 

[dingy2]

There is nothing wrong with the drawing. We have a plane that is perpendicular to datum A and also datum B using a 2 point set up.

If this plane was only perpendicular to datum A, it could still be skewed on the other axis while controlling the tilt. Having it also perpendicular to datum B orients the plane.

Dave D.

http://www.qmsi.ca

 

[randy64]

caseynick, Fig. 6-35 proves my point. It even states, "The surface must lie between two parallel planes 0.12 apart which are PERPENDICULAR TO DATUM PLANES A AND B."

You said "In the case of perpendicularity the secondary datum is solely to control a rotational degree of freedom." I don't see that anywhere in Y14.5M-1994. Everything I see says that the surface must be perp to the datums - not a word about "rotational degree of freedom." I suppose that could be derived from the use of the word "stabilize" in 6.6.1, but without explicit direction, I think we must go with what we are given, and that means in this context stabilization is attained through perpendicularity to two datums.

 

[randy64]

dingy2, are you looking the same picture I am? The one that I attached? Datum B is parallel to the surface in question, not perpendicular.

 

[dingy2]

randy64:

If you are talking about your posted picture, then yes I am. Datum B is perpendicular to datum A as I see it.

Dave D.

http://www.qmsi.ca

 

[dingy2]

randy64:

My previous statement is incorrect.

Datum B should not have been placed on the particular surface but on one of the ends. Datum B is indeed parallel to the feature but perpendicular to datum A. If datum B was on one end, then the surface would have been perpendicular to both datum A & B.

I now agree that the drawing is incorrect. I should look more closely at the drawings in the future.

I would suggest that you send a note to the author of this drawing.

Dave D.

http://www.qmsi.ca