Symmetry on an elipse

[Karissa]

I'm pretty new to GD&T and am having difficulty trying to dimension this properly.

The goal is to keep the ellipse's major axis from tilting too much. The attached drawing is what someone came up with but I don't think that it is the correct way to dimension it. Is this correct or how should it be dimensioned?

In the picture X is the amount it can vary (hasn't been determined yet) and datum C is the centerpoint of the ellipse.

 

[powerhound]

The big issue with this drawing is the datum structure. Single segment profile of a surface should get you there but you need to change the datum reference frame. Do you have access to whatever GD&T standard you are making this print to?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Karissa]

We currently are still using the ASME Y14.5M-1994 standard which I have a copy of.

 

[drawoh]

Karissa,

The datums make no sense to me in any way whatsoever.

The symmetry tolerance would make some sense if the datum were the horizontal axis of your ellipse, but that axis is not a valid datum as per ASME Y14.5.

Are you looking for accurate tilting in an otherwise sloppy ellipse? How will your inspector decide how a sloppy ellipse is being tilted?

I do not have the standard in front of me at the moment. If an ellipse is an acceptable feature of size, you can locate it with a sloppy positional tolerance, then apply a more accurate profile tolerance with respect to the base datum. This will give you a nice looking ellipse, with rotation controlled by your positional tolerance. You can separate the positional tolerances in X and Y. Making one of these more accurate would control your rotation.

--
JHG

 

[TheTick]

Why are A and B separate datums? Wouldn't datum targets be more appropriate?

 

[Karissa]

drawoh,
The datums didn't make sense to me either, I'm glad to hear others are just as confused. The customer came back with this markup to keep the tilt from happening and stated their "GD&T experts" came up with it. I wanted to check that it wasn't just me that didn't get it before trying to come up with a better way to dimension it. I was originally thinking a profile tolerance of some sort would fix it.

TheTick,
Datums A and B are created with datum targets going around the outer cylinder spaced equally at the given heights. Surface of where A and B datums are created are slightly different diameters, there is a groove cut between the two. I was trying to simplify the drawing as the real drawing can not be shared.

 

[TheTick]

The datum targets should both be for A (they can be used for multiple datums). Change them to A1 and A2 and use just datum A in your reference frame.

 

[Karissa]

I added a top view of what we have on our prints. I will look more into that you are talking about, TheTick. I know for a fact we have errors on our prints but have a hard time getting anything corrected because thats the way it has always been done and no one wants to go to the customer with changes. Also, not many people here can correctly read GD&T which make it even harder to correct anything or learn properly how to read it.

So on the attachment you think instead of having points A1-A4 and B1-B4 that they should be points A1-A8?