Watch Where You Put That Triangle - Discusion 1

psiwak · Nov 20, 2012

I had discussion about what's mean datum target on extension line.
My opinion is that according to ASME Y14.5 1994 3.3.2 there is no difference where to put and which view (outline, extension line, align with dimension, top view, plain view) but my adversary claim that according to TEC-EASE GD&T Tips when I put on extension line than Datum is tangent to cylinder. Unfortunately English is not my native language, but I understand from description of second figure that this is accordance with Y14.5 but for clarity and to avoid doubts shouldn't be use. I know that Y14.5:1994 doesn't show example with datum target on cylindrical feature in side view but also 3.3.2 (and whole standard) no mention about tangent datum plane.

My questions are.
Datum A from Fig 2 produce datum axis?
Author of this tip wants to introduce good standard (from what I completely agree) of drawings and not allow to show datum target symbol which is not align with dimension on plain view of cylindrical feature

Regards
Pawel

CheckerHater · Nov 20, 2012

No, datum on fig.2 does not produce datum axis.
When you think of different ways to apply your datum think of different ways to fixture your part.
Look at the enclosed picture.

powerhound · Nov 20, 2012

Fig 2 does not produce a datum axis. The only way to produce a datum axis from a cylinder is to do it according to the figures in the first set of examples in the tip. If you attach the triangle offset from the dimension line then the meaning is ambiguous according to the ASME standard but could be interpreted as a tangent line to the cylinder arc. If that is your intent, then use a datum target line to remove any ambiguity. If your intent is to indicate the cylinder axis, then do it as shown in the first examples.

Powerhound, GDTP S-0731
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Belanger · Nov 20, 2012

I agree with PowerHound, and I don't necessarily agree with the Tec-Ease tip. The standard tells us clearly how to indicate a datum to be an axis, and how to designate a datum derived from a surface. But the one shown with the datum symbol on the bottom of a cylinder -- not associated with a feature of size -- is ambiguous, in my opinion. It doesn't fit any of the prescribed methods to identify a datum feature.

Paragraph 3.3.2 tells us that we can place the datum feature identification symbol on an extension line "when the datum feature is the surface itself." But a single tangent line at the bottom of that cylinder is not "the surface itself." A datum target symbol could be used, but I don't see the standard indicating that a regular datum symbol on that tangent line equates with a target line.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

pmarc · Nov 20, 2012

Well, 3.3.2(c) of Y14.5M-1994 clearly says that if datum feature symbol is placed on a extension line of a cylindrical feature separated from size dimension, the datum is considered to be an axis. In the light of that Tec-Ease tip is not in accordance with the letter of the standard.

CheckerHater · Nov 20, 2012

One might think that Para.3.3.2(c) of 2009 would make things more clear, but unfortunately it only references illustrations where “cylinder outline” is a circle.
Considering that Tec-Ease tip clearly suggests showing target line (adding second view?) to avoid ambiguity, I don’t see it being in direct violation of the standard.

pmarc · Nov 20, 2012

The tip says: "In the view below, the datum may be interpreted as a line lying in a plane tangent to the feature indicated."
For me this is in direct violation of the standard. If one knows what 3.3.2(c) says about interpretation of such placement of datum feature symbol, the datum may not be interpreted as Tec-Ease suggests.

However I agree that the only way to indicate that tangent plane (or tangent line as a matter of fact) is a datum is to use datum target line.

psiwak · Nov 20, 2012

The problem started when I interpret the old drawings (about 20 years and more) where datum symbol is attached to cylindrical surface to extension line or to outline in plain view (similar case to Fig2)
According to 3.3.2 (c) I interpret it as a Datum Axis not as a tanged plane because in many cases Rounout, Cylindricity is measure with ref to this Datum.
Can I refer with interpretation in FIG2 to 3.3.2 (C)? If not the reason is the lack of proper drawing in Y14.5:1994?

pmarc · Nov 20, 2012

If the drawings are indeed more than 20 years old, I would search for the answer in ANSI Y14.5M-1982 not in '94 edition.
I do not have '82 version with me, but if there is no difference in interpretation of this particular problem between '82 & '94, I would say the drawings are correct and the datum is axis (especially if it is referenced in runout tolerances).

P.S.: Cylindricity tolerance cannot use any datum(s) as reference.

powerhound · Nov 20, 2012

I think when Don said it "may be interpreted", the context was "might be interpreted." not "is allowed to be interpreted."

Powerhound, GDTP S-0731
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CheckerHater · Nov 20, 2012

Then he adds “If line contact is desired a datum target line should be indicated”.
So, if you actually show datum target (say, in second view) your intent will be clear.

pmarc · Nov 20, 2012

powerhound,
Each print, even the perfect one, is a potential subject of different interpretations. It is the role of GD&T standard invoked on the print to reduce these multiple options into a single one.

I agree, this way of datum feature symbol location is not the most common option, and not the one I would use if wanted to define datum axis, but Y14.5 clearly says how it should be interpreted. I will not even try to guess what Don meant in his tip - I only see interpretation that is against official Y14.5 GD&T law and adds confusion (if we look at OP's question).

powerhound · Nov 20, 2012

Okay, let me amend my original supposition. Maybe what Don meant was that it "might be misinterpreted..."

Don't get me wrong, I vehemently disagree with this way if the intent was to indicate the axis of the cylinder as the datum. It's way too easy to do it the right way and eliminate ambiguity. Either use a datum target line OR move the triangle to the OD of the cylinder or in line with the dimension line. As it is, the drawing is incomplete and has no correct interpretation.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
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pmarc · Nov 20, 2012

And I keep saying that the drawing shown in left upper corner of fig. 2 from initial post is complete if we are talking about datum feature identification per Y14.5M-1994 and has got one and the only correct interpretation.
Why do you think it is incomplete and ambiguous? Because the method used is not as common as the other methods shown in the tip?

powerhound · Nov 20, 2012

Maybe you should point me to where the interpretation of Fig. 2 in the OP is clearly stated in the standard. I can't find it.

Powerhound, GDTP S-0731
Engineering Technician
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Belanger · Nov 20, 2012

The left-hand picture in the OP's "Fig 2" does not comply with the ASME standard (I guess we are sticking to 1994 for now). The closest thing that it comes to is paragraph 3.3.2(a), but that is allowed if "the datum feature is the surface itself."

However, that doesn't really apply here, because we are left hanging with this question: if the "surface" is the datum feature, what is the real datum? One could argue it's the axis, since this surface wraps all around into a cylinder; or you could argue that it's a plane, since some folks might think you're just identifying a tangent line, not the actual surface. See the confusion?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

MechNorth · Nov 20, 2012

PowerHound nailed it when he said "may be misinterpreted as". My first GD&T instructor gave me the tangent plane explanation, which gave me heartburn because you can't specify where on the perimeter the tangent plane acts. Don Day (Tec-Ease) was the one who corrected that particular error in interpretation. If you want a tangent plane, use a datum target line.

The perceived ambiguity comes from the reality that many people won't recognize that section 3.3.2 (c) exists and gives explicit instructions on interpretation.

Jim Sykes, P.Eng, GDTP-S
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pmarc · Nov 20, 2012

powerhound,
Doesn't 3.3.2(c) in '94 and '09 editions of Y14.5 describe what is shown on the left-hand picture in fig. 2?

Jim,
Many people do not recognize that a lot of different rules exist within Y14.5 realm. Take simultaneous requirement concept just as one example. Does it mean I should put SIM REQT note under each positional/profile callout referencing to the same DRF because otherwise the simultaneous requirement may not be recognized at all? If I wanted to follow "may be misinterpreted as" logic of the tip, this is exactly what I should do in order to avoid misinterpretations, despite the fact that certain rules have been already implied by invoking Y14.5 on the print.

But coming back to the original issue, I miss one clear statement from you, therefore could you answer to following question: "Knowing that Y14.5 is in charge, what is the datum in fig. 2 and is it clearly defined by the standard?"

powerhound · Nov 21, 2012

pmarc,

No it doesn't. At the end it gives the figures which to refer to for an illustration of the description (3-4(d) and (f)). Neither of them show what is in Fig 2 of the OP. The extension line is not of the feature outline.

Regarding your last sentence. I'm pretty sure it was intended for Jim but I'll chime in. There is no valid datum in the OP. One could assume but whatever the assumption, it is unsupported by either standard.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
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Belanger · Nov 21, 2012

The funny thing is -- if you ignore the pictures and simply read 3.3.2(c), it could actually go with the Fig 2 given in the OP. Two problems, though: if we try to apply 3.3.2(c) to Fig. 2 above, then the datum is the axis, not a tangent line. Secondly, the standard really meant to say, "if the datum feature symbol is placed on the outline where the feature appears as a cylinder..."
That's different than saying "placed on the outline of a cylindrical feature." A picture of a rectangle could be a cylindrical feature!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

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Watch Where You Put That Triangle - Discusion 1

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