Is the print correct?

[greenimi]

1.) Am I right saying this print is not correct?
Missing basic dimension for the positional callout.


"7.2.1.1 Dimensions for True Position.
Dimensions
used to locate true position shall be basic and defined in
accordance with para. 2.1.1.2."


2.) If they have used perpendicularity instead of position I guess would be okay.

Just please confirm.



Thank you

 

[ewh]

You are correct.

“Know the rules well, so you can break them effectively.”
? Dalai Lama XIV

 

[KENAT]

You're right on point 1.

Not sure on point 2 as position tol should be used to locate features of size per 14.5M-1994.

Unequal bilateral position tolerance might be what they really want but can't be sure from the info available.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fsincox]

Also, if it was not a cylindrical zone it would be OK.
Frank

 

[ewh]

Ah, but point 2 was a guess; thus I confirmed it as such.
(ducking)

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[pmarc]

Positional callout itself is absolutely OK, because in addition to perpendicularity it controls offset between datum axis A and toleranced hole's axis. It would be much better visible if you had a cross-section in a plane perpendicular to section A-A going through the center of the hole. Implied basic 0 dimension defines true location of the hole relative to datum axis A.

Perpendicularity callout would be OK too, though such callout would only control orientation of hole's axis to A but not its location.

The other side of the story is +/- dimension in axial direction. Some would say it is acceptable. I am saying basic 0.750 dimension should be used, side of the rod assigned as datum feature B, and B reference added to the positional callout.

 

[fsincox]

If it was a unidirectional position it would need to be oriented in the other direction, also.
Frank

 

[fsincox]

Let me clarify, I mean that the dimension lines need to be going in the other direction. Per our recient orientation discussion, we do need a location dinension to be located, if referencing the current ASME standard.
Frank

 

[ewh]

As usual, the pro's are correct. Rushed as usual, I did not pay enough attention to the example; if the end surface were a datum which was being referenced in the FCF, then yes, the locating dimension should be basic. As it is, the only basic dimension you could conceivably add would be "0.000" to locate the hole vertically to the shaft centerline.
mea culpa!

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[MechNorth]

The +/- tolerance locating the hole wrt the end of the rod is not legal; the hole is a FOS and therefore must be located using a position control at the basic location, hence a basic dimension is needed. If you make the end Datum-Feature-B, then you can stack two single segment position controls; the first one within DIA .03 wrt A & B, and the second within DIA .005 wrt A. I don't think that a composite control would be valid in this case, to achieve what you want.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[greenimi]

First of all thank you.
I have changed the print per Pmarc and Jim's input. See revised sketch.
Since the intent is to control the hole more relaxed in the left-right direction and more tight 90 degrees from this direction, do I need to have diametral symbol Ø ?
On both callouts?
On neither callouts?

Another question would be: if I use perpendicularity instead of the SECOND positional callout (first callout stay as is) the effect is the same or not?

Thank you again

 

[Belanger]

A diameter symbol is needed in both callouts. Also, the datum feature symbol for A (the triangle) should really be on the horizontal portion of the leader line, or even the angled portion.

Yes, perpendicularity in the 2nd callout is the same -- and actually it is the correct symbol to use there. (Don't start me on this, guys!)

Finally, an aside to the group here: While it is legal, I often wonder if it's impractical to have a primary datum referenced with the "M" modifier when the secondary datum is a plane. This is because any looseness around A will allow the part to flatten out on B, thus making it the primary datum at times. I know it's OK to do it -- the new standard made a point of showing this option in Fig. 4-21(d) as well as Fig. 7-59 (although no text is given to explain it), but I'm just thinking about how the contact points affects the datum precedence.

Frankly, Greenimi, in many ways I vote for your first picture.

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

http://www.gdtseminars.com

 

[fsincox]

I agree with the cylindrical zones, the two position callouts would be OK if your intent is to tighten the location to the centerplane of datum A, IMO. This would give you a functional reason not to just replace it with an orientation callout.
Frank

 

[bxbzq]

You are missing the perpendicularity relationship between datum A and B.

 

[pmarc]

J-P,
Perpendicularity to A in the second callout is not the same as position to A. Of course tt can be used as long as one realizes that the perpendicularity will not control the intersect of the axis of the toleranced hole and the datum axis A (shown in section B-B) but only their angular relationship. Position will do both.

 

[pmarc]

greenimi,
The way you have it now, you need diameter symbols for both callouts. This will create one cylindrical tolerance zone (wrt A) floating withing larger cylindrical tolerance zone (wrt A & B).

There is also another way you may want to consider which is shown in fig. 7-28 of Y14.5-2009 - bidirectional positional tolerancing, means 2 separate callouts without diameter symbol, creating rather rectangular tolerance zone. It is up to you and your functional requirements to choose the right method.

 

[MechNorth]

The diameter symbols are needed. Datum A Feature Symbol needs to be relocated. Datum Feature B needs to be controlled wrt Datum A.

Perpendicularity could be used in the second FCF - ONLY IF - you don't want the location of the hole wrt the Datum-A axis refined tigher than the .030 tolerance. It was my understanding that you wanted it tightened in this direction.

J-P, we (Tec-Ease) discuss with our students the issue of datum modifiers wrt datum precedence. It seems that it is a major eye-opener even to many long-certified GDTP's. We don't tell people not to do it, but do advise them to check the potential for datum precedence violation if the tolerances are generous.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

I think sometimes we talk in generalities to simplify and in the end the numbers do matter and become the final determining factor. This kind of goes back to the over simplification for ease of understanding thing.
Frank

 

[Belanger]

Jim ... glad that we're on the same page (regarding datum precedence). I say the same thing in my classes: here's the possible options, but be aware of the effect of having datum shift on a primary datum.

pmarc (and Jim) ... thanks for setting me straight on the lower frame and perpendicularity. I wasn't paying attention to the location of the hole across the shaft (doh!) since I was so preoccupied with the perpendicular aspect. My bad.

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

http://www.gdtseminars.com

 

[ewh]

I'm curious; why are diameter symbols required if a rectangular tolerance zone meets the functional requirements of the part, as noted by pmarc?

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV