Please explain the points used to draw the elliptical arc

[JennyB]

I have been playing around with the elliptical arc tool which is clearly very powerful, but I can't get my head around the relationship between the second point and the first and third points that define the curve. I'd like to understand this better so that I can select a second point in a way that will let me create the arcs I need to create without so much trial and error.

The description given in the help page says "the endpoints of the arc are exactly parallel with a line from the endpoints to the middle point used when drawing the arc." This doesn't make sense to me and there is nothing to illustrate it.

Could someone illustrate this for me in a way that would help me understand where to put that 2nd point to achieve a certain shape of curve. Thanks! This site has already answered quite a few of the questions that I couldn't find through reading the help and manual.

--Jenny

[i44troll]

Jenny, Welcome to the Group!

Your right! Horrible explanation. Let me try to illustrate it plainly .....
You quoted them saying,"the endpoints of the arc are exactly parallel with a line from the endpoints to the middle point used when drawing the arc."
Below is a drawing that shows what they are attempting to say....
   
(Notice that the "second point" is the intersection of both lines which is used to interpolate the curve.)
   
What they did not fail to convey was their recommended application for elliptical arcs. They said that an elliptical arc was "useful when drawing
3-D isometric drawings". Absolutely! Try drawing an arc or circle in an isometric drawing any other way; it's nearly impossible!
Applying an arc from a called-out radius fails miserably as well when applied to isometric drawings. Here's why...
   
Notice that the arc is much more pronounced when the angle is smaller creating a non-proportional, somewhat distorted drawing.
However, when using the elliptical arc in your iso drawings your sure to have more proportional arcs that do not grow or shrink with the size of the angle.
   
This shows why an elliptical arc is the preferred method when creating fillets and round-overs in isometric drawings.
Hope this helps...

Chris "i44troll"

[JennyB]

Thanks! That's much more helpful.