Well I encounter this problems in many cases - arc with 3 points always works but arctangent in many cases not

Hi Tom,

re:
> Why can I draw an arctangent on the red curve and not on the blue one ??

Which particular arc drawing tool are you trying to use? There is a command in MoI called arctangent but it's for creating an arc between 2 curves which is tangent to both curves. It sounds like you are trying to use a different tool than that though.

- Michael

I used Arc > Tan

Hi Tom, the problem is that the ArcTangent command will only work on 2 coplanar curves, and your curves here are not on the same plane.

If you switch the view to split view mode and then look in the Right side viewport, you can see the blue curve is kind of swooping down at an angle there, while the red curve is horizontal in that view.

In order to get ArcTangent to work on these you will need to squish them down so they are both on a common plane. You can do that by selecting the 2 curves and then in the Right side viewport, drag the edit frame corner grip and drag it down until you get "flat" snap as shown here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4

Hope that helps explain it! Thanks, - Michael

Hi Michael, I would not have found the solution without your tip - thank you !