Coplanar Widths As Datum Feature

optomist1

A Sea of Statistics
Trusted Information Resource
#1
Good Day To All,

I have a question regarding the definition of a Coplanar Width Used As A Datum. I have attached some text and several drawings. Based on the attached my conclusion is that the 2X 16.8 - 16.4 surfaces is the only Coplanar Width, Datum Feature, Datum B in this case.

Is this correct or is Datum A also considered a Coplanar Widths Datum Feature?

Thank you for your assistance

Regards,
Marty
 

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D

David DeLong

#2
Wow, is this ugly!! I am going to use ASME Y14.5-2009 page 49 as a reference.

First of all, datum A is coplanar since all four pads share the same plane. This is the primary datum. Using ASME Y14.5-2009 standard, datum A constrains along the Z axis (translational) and also u (rotational) and also v (rotational). In other words, it constrains 3 DOF. We have 3 DOF left.

The secondary datum is the overall width between the pads and it should not be shown as "2X" since there is no break in the feature. The width is simply 16.6 - 16.8 mm and is perpendicular to datum. This datum constrains the X axis (translational) and also w (rotational). We now only have 1 DOF left.

The tertiary datum (3rd) is shown as the width of 2 pads that are in alignment. I really don't know where the term "coplanar widths" comes from since it is not in the 82, 94 or 2009 standard. We could have shown 1 pad but 2?? There is a conflict with the secondary datum which controls the rotation of the part. The 3rd datum could be 1 pad but not 2. Just imagine if the secondary and tertiary datums were in RFS, both the secondary and tertiary datums would try to control the rotation of the part. The secondary controls the rotation and not the tertiary.

GD&T is a complex subject and using degrees of freedom (DOF) doesn't help.

Sorry but this drawing is not correct.
 
D

Dave Dunn

#3
Good Day To All,

I have a question regarding the definition of a Coplanar Width Used As A Datum. I have attached some text and several drawings. Based on the attached my conclusion is that the 2X 16.8 - 16.4 surfaces is the only Coplanar Width, Datum Feature, Datum B in this case.

Is this correct or is Datum A also considered a Coplanar Widths Datum Feature?

Thank you for your assistance

Regards,
Marty

By definition, "coplanar" means that the features are on the same plane, not just parallel to each other, so "coplanar width" is an oxymoron and not physically possible - it can't be a width and coplanar at the same time.

You have two types of datum features shown:
* datum A is a coplanar datum formed by the four surfaces;
* datum B and C are features of size defined by two parallel planes, one an inside width, one an outside width.
 

optomist1

A Sea of Statistics
Trusted Information Resource
#4
Wow, is this ugly!! I am going to use ASME Y14.5-2009 page 49 as a reference.

First of all, datum A is coplanar since all four pads share the same plane. This is the primary datum. Using ASME Y14.5-2009 standard, datum A constrains along the Z axis (translational) and also u (rotational) and also v (rotational). In other words, it constrains 3 DOF. We have 3 DOF left.

The secondary datum is the overall width between the pads and it should not be shown as "2X" since there is no break in the feature. The width is simply 16.6 - 16.8 mm and is perpendicular to datum. This datum constrains the X axis (translational) and also w (rotational). We now only have 1 DOF left.

The tertiary datum (3rd) is shown as the width of 2 pads that are in alignment. I really don't know where the term "coplanar widths" comes from since it is not in the 82, 94 or 2009 standard. We could have shown 1 pad but 2?? There is a conflict with the secondary datum which controls the rotation of the part. The 3rd datum could be 1 pad but not 2. Just imagine if the secondary and tertiary datums were in RFS, both the secondary and tertiary datums would try to control the rotation of the part. The secondary controls the rotation and not the tertiary.

GD&T is a complex subject and using degrees of freedom (DOF) doesn't help.

Sorry but this drawing is not correct.
Well this posting will be almost as confusing as the interpretation of the attachments; not just two Daves but two Dave Ds.


David DeLong thanks for your quick response. Using the previously attached text as my guide I;


Eventhough the four pads are not dimensioned, merely shown with a Profile Control 0.1, this could be my error as the drawing is shown as a (Partial Drawing), One can assume that in another view these Coplanar Surfaces are properly dimensioned. The coplanarity of the surfaces being controlled by the Profile control, but dimensioned elsewhere on the drawing. The text is not clear regarding designating or establishing a Datum Feature for widths in the same plane.


I interpreted the 16.8 - 16.4 feature of size as the Coplanar Width because it has the Datum ID applied to it, all of the other Datum IDs are applied to dimensions/geometric tolerances rather than to a coplanar width dimension. Again this may be due to the (Partial Drawing) status.


Regarding the "2X" designation, could this designation be due to the interrupted surface, i.e the 12.8 - 12.6 hole? Are Interrupted and Broken Surfaces different?


If not Coplanar Widths, then what term is used to describe this situation?
and more importantly I have ordered my copy of Y14.5M 1994.


So, my intial read (five days ago) was correct, although not fully dimensioned the four pads shown constitute one Datum Plane, the Primary or Datum Plane A; the 16.8 - 16.4 feature of size establishes Datum Plane B and the two pads or one establishes Datum Plane C.

Regarding the term degrees of freedom, what is the proper terminology for movement along or around an axis?

Thank you both for your responses, insight and patience with a GD&T "green horn".

Regards,
Marty

"morals are meant for mere mortals, Not for Gods or Wizards of Wall Street
:thanx:
 
D

David DeLong

#5
Even though the four pads are not dimensioned, merely shown with a Profile Control 0.1, this could be my error as the drawing is shown as a (Partial Drawing), One can assume that in another view these Coplanar Surfaces are properly dimensioned. The coplanarity of the surfaces being controlled by the Profile control, but dimensioned elsewhere on the drawing.

I interpreted the 16.8 - 16.4 feature of size as the Coplanar Width because it has the Datum ID applied to it, all of the other Datum IDs are applied to dimensions/geometric tolerances rather than to a coplanar width dimension. Again this may be due to the (Partial Drawing) status. dk


Regarding the "2X" designation, could this designation be due to the interrupted surface, i.e the 12.8 - 12.6 hole? Are Interrupted and Broken Surfaces different?


If not Coplanar Widths, then what term is used to describe this situation?
and more importantly I have ordered my copy of Y14.5M 1994.


So, my intial read (five days ago) was correct, although not fully dimensioned the four pads shown constitute one Datum Plane, the Primary or Datum Plane A; the 16.8 - 16.4 feature of size establishes Datum Plane B and the two pads or one establishes Datum Plane C.

Regarding the term degrees of freedom, what is the proper terminology for movement along or around an axis?

Thank you both for your responses, insight and patience with a GD&T "green horn".

Regards,
Marty

"morals are meant for mere mortals, Not for Gods or Wizards of Wall Street
:thanx:
Sorry but the last post was complex although the explanation referenced DOF and blows a lot of people away. The 94 edition of ASME Y14.5M does not get into this so disregard it. I don't even train using DOF unless one takes an advanced subject where I use the 2009 edition.

There is no such thing as "Coplanar Width".

Datum A is coplanar since the surfaces of all 4 pads share the same plane. The profile requirement is correct and is the same as requiring a flatness tolerance on the 4 surfaces together. All vertical dimensions should come from Datum A.

The 16.4 - 16.8 inside width is of 1 continuous feature that is not interrupted. A hole does not interrupt this feature. The "2X" does not make sense. The term is "inside width".

A tertiary is a 1 point contact or 1 slice and the width of 2 pads is not correct application for a tertiary datum.

Hope I am clearer this time.
 

optomist1

A Sea of Statistics
Trusted Information Resource
#6
Dave,

yes, much clearer but I am good with the DOF references and use....presently, I am taking an Advanced Concepts of GDT. I recently completed a basic GD&T course.

Your last post clears up a lot.....I assume in your explanation of Tertiary Datum you were referencing the 3-2-1 guideline.

Thanks again for your assistance!!

Regards,
Marty
 
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