topic Re: How to find the the dimensions of an ellipse from this Drawing? in Fusion Support Forum

How to find the the dimensions of an ellipse from this Drawing?

TLTrend — Tue, 13 Jun 2023 16:05:48 GMT

Hi, I am trying to recreate this drawing in Fusion 360, but I am stuck on finding the correct dimensions of the ellipse. Instead of giving me the major and minor axis, they give the measurements to one of the foci of the ellipse ( I assume they meant foci not focus). I have no idea how to find the center of the ellipse using the dimension already given. I know that if the use those dimensions as if it's the center of the ellipse, I get a different curvature than the real-life model I have to compare to. ( I 3-D printed the part and compared it.) Does the focus of an ellipse mean the center point, or is there a way to find the center of an ellipse using the dimensions given( Foci & major axis)?

Re: How to find the the dimensions of an ellipse from this Drawing?

TheCADWhisperer — Tue, 13 Jun 2023 16:55:45 GMT

@TLTrend wrote:
( I 3-D printed the part and compared it.)

@TLTrend

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

Re: How to find the the dimensions of an ellipse from this Drawing?

TLTrend — Tue, 13 Jun 2023 16:34:30 GMT

This is pretty much what I have just to see if it is the right Curve. Finding out how to fully define the ellipse after trimming it is a different story.

Re: How to find the the dimensions of an ellipse from this Drawing?

TheCADWhisperer — Tue, 13 Jun 2023 17:08:49 GMT

@TLTrend wrote:
This is pretty much what I have just to see if it is the right Curve.

@TLTrend wrote:
( I 3-D printed the part and compared it.)

Uhmm, how did you 3D print that?

Did you Attach the correct file?

The file you Attached doesn't have any geometry that would 3D Print and looks nothing like the original drawing???

Re: How to find the the dimensions of an ellipse from this Drawing?

TLTrend — Tue, 13 Jun 2023 17:22:39 GMT

How did you get the center of the Ellipse? Did you do an overlay and find that way? My apologies I sent a pretty chopped-up version since I am just experimenting with how to get that curve right. I didn't add any of the other details.

Re: How to find the the dimensions of an ellipse from this Drawing?

TheCADWhisperer — Tue, 13 Jun 2023 20:49:22 GMT

@TLTrend

I must confess - I have no idea where you are going with this.

I don't even see these two dimensions in your sketch?

Let's solve this the way you would solve a crossword puzzle - fill in the easy stuff and work towards the hard stuff (that will probably become easy as we fill in the easy stuff).

Sketch these two construction lines from the Origin.

Attach the file here for next set of steps in the puzzle...

Re: How to find the the dimensions of an ellipse from this Drawing?

TLTrend — Tue, 13 Jun 2023 21:04:00 GMT

Alright, I understand the dimensions are there, but you're right, let's start at the beginning. I appreciate your help with this!

Re: How to find the the dimensions of an ellipse from this Drawing?

TheCADWhisperer — Tue, 13 Jun 2023 21:15:35 GMT

@TLTrend

Re: How to find the the dimensions of an ellipse from this Drawing?

TLTrend — Tue, 13 Jun 2023 21:45:54 GMT

I looked over the design and found some missing dimensions. As for the side and bottom, their length is irrelevant.

Re: How to find the the dimensions of an ellipse from this Drawing?

TheCADWhisperer — Tue, 13 Jun 2023 22:43:10 GMT

@TLTrend wrote:
…their length is irrelevant.

I’m “outta” here.

Re: How to find the the dimensions of an ellipse from this Drawing?

TLTrend — Tue, 13 Jun 2023 23:01:05 GMT

Wait, Why? Their lengths can be any given dimension, I just want to know you got the center of the ellipse, disregarding everything else... that's really all I'm going off on, and I would be grateful for any tips you can give.

Re: How to find the the dimensions of an ellipse from this Drawing?

matthew68846 — Wed, 14 Jun 2023 05:47:08 GMT

An ellipse has two focal points (foci is the plural of focus) which are located on the major axis symmetrically around the center point. The Minor axis also passes through the center, at right angles to the major. I think you already have the semi-minor dimension as 0.378 so your problem is just to find the semi-major dimension.

Here is how I solved it. Draw an arbitrary ellipse centered on the origin. Place a point on the major axis above the center to represent one focus. Place another below the center. Constrain them to be symmetrical about the minor axis. Constrain the semi-minor axis to be 0.378. Constrain the distance from the top of the major axis to the upper focal point to be 0.307. For convenience I named this dimension 'a'.

Now we can solve for the position of the focal points using a property of ellipses that the sum of the distances from any point on the circumference to the two focal points is constant. We can solve this using simple trigonometry, or we can let Fusion solve it for us.

I created a dimension line from the upper focal point to the left end of the minor axis, and I made this a driven dimension. For convenience I named this 'b'. I created a dimension line between the two foci and defined it using the formula '2*(b-a)'. This is because the sum of the distances from the point on the circumference at the left end of the minor axis to both foci (a plus a) must the the same as the sum of the distances from the point at the top of the major axis to both foci (our unknown distance + b, plus b).

Adding another driven dimension to the sketch allows us the read off the semi-major axis as 0.386.

Now that I look at this I think there might be an easier way to solve it using the property that the distance from a focus to the point where the minor axis intersects the circumference is equal to the semi-major axis (both are 0.386 in this case). That probably allows for a solution using only constraints and no formulas.

I don't think there is enough information in your sketch to fully constrain the position of the ellipse with respect to the other features. Some dimensions appear to be missing.

Re: How to find the the dimensions of an ellipse from this Drawing?

TheCADWhisperer — Wed, 14 Jun 2023 12:50:19 GMT

@TLTrend wrote:
Wait, Why? Their lengths can be any given dimension,

Might not know the dimensions - but they are not irrelevant.

Whatever you do - DO NOT trim Ellipse. Isn't needed and will cause grief. In fact, be very careful about trimming anything.

Most fillets should be added after solid as Placed Fillet Features rather than in sketch...

Exactly what is this thing that we are reverse engineering?

Do you have a picture of something similar from the real world?

Re: How to find the the dimensions of an ellipse from this Drawing?

davebYYPCU — Thu, 15 Jun 2023 09:25:44 GMT

Best guess, fuselage half former above the wing.