Ellipses

[wes616]

What is the proper way to dimension a true ellipse.

Wes C.
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When they broke open molecules, they found they were only stuffed with atoms. But when they broke open atoms, they found them stuffed with explosions...

 

[ctopher]

I usually draw vert & horiz CL's thru the middle of the ellipse, then dim the side arc and the top arc stating "2X" on each dim. Use position tol if needed.

Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
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ctopher's home site (updated 06-21-05)
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[Heckler]

I would dimension the two radii, minor axis, major axis and foci distance. Then you just have to locate it and add your favorite GD&T

Best Regards,

Heckler
Sr. Mechanical Engineer
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[xgrigorix]

I draw & dimension both axes and clearly call out the fact that it is an ellipse. Seeing as how an ellipse has a precise geometric definition that should be sufficient. Add whatever tolerances and/or GD&T controls you think are necessary.

 

[CorBlimeyLimey]

A true ellipse does not have radii to define it's shape. The only way to accurately define/measure one is to give X-Y co-ordinate points.

Some NC programs can interpolate the true path required from the major & minor axes lengths, others will need multiple co-ordinates to interploate between.


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[wes616]

CbL... that's what I'm looking for, since the radi is constantly changing as you follow the chord of the elipse, what is the "standard" for drafting this....

I usually dim the major axis, and minor axis and I'll call out the degree to which the elipse lies as a reference, but today I was tasked to find the convention, and I can't find it in any of my books...

Wes C.
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When they broke open molecules, they found they were only stuffed with atoms. But when they broke open atoms, they found them stuffed with explosions...

 

[smcadman]

I draw a line to the midpoints of the major radii and also the minor radii, and then dimension the length of the lines. Some CAD software, such as Solidworks, will not let you dimension the arcs because they are spline entities (which is why I do it this way).

Flores

 

[CoryPad]

http://mathworld.wolfram.com/Ellipse.html

Regards,

Cory

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[TheTick]

The form of an ellipse is defined by size and eccentricity. Get this by any 2 of major diameter, minor diameter, and eccentricity (sometimes given as an angle, sometimes as a ratio of minor-to-major diameters).

Orientation is best defined by the angle of the major/minor axes relative to another line.

Location is best determined by ellipse center. It would also be possible to locate w.r.t. one or two points on the ellipse.

 

[MechNorth]

Dimensioning as indicated is the easiest, but alternatively, you can also specify the programming parameters used by the NC group, or (and everyone will hate this one) you can provide the mathematical formula for the ellipse.

For tolerancing the size of the ellipse, you theoretically can use formulae which increases and/or decrease the distance, and locate it using a positional tolerance on the boundary of the ellipse (ASME Y14.5M-1994 5.10.1 & Fig. 5-47)

I would just use the surface profile which provides a symmetrical offset from the theoretical (basic) geometry thereby controlling both size and location. You can even control the size & location different wrt one datum.

Jim Sykes, P.Eng, GDTP-S
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