EACH RADIAL ELEMENT note for orientation control

[bxbzq]

What is the difference between fig.6-16 and fig.6.17 in '09 standard? I don't see the difference from product definition standpoint. I don't even see the difference if remove the datum B from the FCF in fig. 6.17. Can anyone enlighten me?

 

[Belanger]

Sure .. the difference is that the pin may not be perfectly perpendicular to the flat surface. So in Fig. 6-16, the designer wants the radial elements to follow what the simulated axis of the pin is doing. In Fig. 6-17, the designer wants the radial elements to follow what the datum plane from the back surface is doing.

Make a sketch for yourself of a part where the pin is a little angled, and I think you'll see the difference.

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

http://www.gdtseminars.com

 

[bxbzq]

In fig. 6-17, is datum B in FCF necessary? If remove datum B from FCF, would it be a different control?

 

[pmarc]

Yes, it would be a different control.
As explanation in fig. 6-17 says that two parallel lines creating the tolerance zone are parallel to A and oriented radially from datum axis B, it is the datum axis B from which tolerance zones for all radial elements emanate. If there was no B referenced in parallelism FCF, one would not know at which axis of the part all the tolerance zones meet. Is it at the axis of smaller diameter? Or of the bigger one? Now if you consider that both diameters will always have a certain coaxiality error, it is quite easy to imagine that depending on the datum axis referenced in the FCF, "the origin" of parallelism tolerance zones will differ, so the inspection results may be different accordingly.

Side question: the title of fig. 6-17 is: "Specifying Peprendicularity for a Radial Element of a Surface". Since the figure focuses on Parallelism, is it a typo or is there a hidden logic behind it? Thanks.

 

[Belanger]

Right -- datum B is needed because if we are taking radial elements, that naturally means we need a reference to establish the "radial."

Good catch, pmarc. That title seems to be a typo.

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

http://www.gdtseminars.com

 

[bxbzq]

Very clear. Thank you both.