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GD&T Application for a Plug
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Posted by: THohenberger

12/07/2006, 18:29:37

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I just finished the GD&T course given by Kelly Bramble in Houston, TX, (for Schlumberger) so low and behold, I came to work this morning and I was asked a GD&T question by a colleague. Not being an authority on GD&T, I would like to present the following design problem and get your feedback to see if the solution we have seems appropriate, or if you see a more suitable means to achieve the proper design.

I've attached a snapshot of a drawing view. The part is simply a plug. It will thread into another part and an o-ring will seal on the mating part (note the o-ring groove in the drawing). Datum A has been assigned to the planar surface into which the o-ring groove will be cut. As the drawing indicates, we are trying to maintain a perpendicular relationship between the thread axis and datum A. This is in an effort to control the o-ring squeeze around the entire diameter or the o-ring (ie, we don't want the squeeze on one side to be much greater than the diametrically opposing side). A feature control frame has been attached to the OD of the thread to establish a tolerance between the thread axis and datum A.

A primary design consideration here is that the part will be made from a low-melting metal. This brings up concerns with machining the threads because they may deform if they are turned. The current solution is to try molding the part.

Please let us know if this callout seems simple and sound or if there is something that we are overlooking, or if you see a better way to achieve our desired relationship.

In addressing this problem, can you offer any information regarding the use of the thread OD vs. the thread pitch diameter (Kelly mentioned the latter is more accurate but harder to measure)?

Thanks,
Travis
The Motorcycle Guy in the Back Row


plug.jpg (15.9 KB)  






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Re: GD&T Application for a Plug
Re: GD&T Application for a Plug -- THohenberger Post Reply Top of thread Forum
Posted by: Kelly Bramble

12/07/2006, 21:09:37

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By default, tolerances of orientation, position or datum reference applies to the axis derived from the pitch diameter for a thread feature. In your example, your perpendicularity tolerance applied to the pitch diameter is actualy pretty tight. I'm sure that applying your tolerance to the major diameter should work just fine.

I see that you do not have any specific feature defined as a datum to control the cylindrical features coaxial (concentric) relationships. I recreated the part and delineated an optional approach. The following are the principal tolerance definition changes and additions I suggest:

Qualify the flange face with a flatness tolerance.
Clarify that the primary datum is both flange surfaces.
Select a cylindrical feature for coaxial control of all diametric features (secondary datum).
Loosen the corner radii

What material are you hoping to manufacture this part from?


FLANGE.gif (11.2 KB)  





Modified by Kelly Bramble at Thu, Dec 07, 2006, 21:20:44


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Re: Re: GD&T Application for a Plug
Re: Re: GD&T Application for a Plug -- Kelly Bramble Post Reply Top of thread Forum
Posted by: traingdt

12/08/2006, 07:58:48

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I agree with you, Kelly. But since this was a section view, I don't think it's necessary to specify multiple surfaces for the flatness (there's only one surface for datum feature "A"). I think when you say flatness of 2 surfaces, it may imply to some people that there are two distinct surfaces, and they need not be coplanar.

John-Paul Belanger








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Re: Re: Re: GD&T Application for a Plug
Re: Re: Re: GD&T Application for a Plug -- traingdt Post Reply Top of thread Forum
Posted by: THohenberger

12/11/2006, 11:34:36

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Thank you both for your feedback.

To answer your question, I believe they are going to try using Indium for this plug.








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