Can't Select Midpoint of Projected Line

[therealsamchaney]

I am having an issue in Fusion 360 where I cannot access the midpoint constraint for a projected line in a sketch. I have tried various ways that usually work, but it seems like Fusion thinks this line doesn't have a midpoint somehow. The line is a projected edge of an existing body. When I try to place a line or point at the midpoint of the line, I don't get the X and triangle appearing as usual. Also, if I create a point somewhere else and select the point and the line then right click, the midpoint constraint does not appear in the list of available constraints even though it theoretically should. I have linked a screencast showing this behavior. What is going on here?

[davebYYPCU]

We'll need the Sceencast, works here for me in Edit Sketch mode.

[HughesTooling]

As it's a projection from a body are you sure you're getting a line and not a spline? If you select the projected curve what does it say at the bottom right of the screen, Line or Spline?

Mark

[davebYYPCU]

Yeah Good point, doesn't work on a spline.

[therealsamchaney]

It's not a spline. As I stated the line is projected from an edge of existing geometry and it is a straight edge. I added the screencast link in the original post, I don't know why it isn't showing up so here is another link to it:

https://knowledge.autodesk.com/community/screencast/8797710e-31ef-4d8f-b5a4-2525d7708c33

[HughesTooling]

You can see in your screencast it's not a line but a spline. Don't know how the body's created but the edge is creating a spline when projected

Mark

[therealsamchaney]

Interesting, it seems very odd to me that Fusion would make this projected line a spline as its projected from a perfectly straight edge. Does anyone have any insight as to why or when Fusion will project a straight edge as a spline instead of a line? For more context the edge is from a shell feature, not sure if that has anything to do with it.

I attached a modified version of my design here. I ended up using a different projected edge and finishing the model from there, but in this version I've rewound the timeline to the point in time where I had the issue and created a sketch called "Fusion 360 example" showing the projected spline in question.

Thanks for the help,

Sam

[HughesTooling · Accepted solution]

The problem is the end faces are not planer but have a twist. Although the original faces have 4 edges that are made from straight line, when offset and trimmed by the shell you end up with edges that are curved.

You can see the twist if you look from the side and you'll also notice there's no option to create an offset plane because the face is not planar.

As another test, if you project a rectangle to the face all the edges of the projection are splines, this is similar to what shell is doing.. Just thought you can also use curvature analysis to show the face's not flat. You might be able to work around these problems if you can use draft instead of a loft. File's attached with the projected rectangle.

Mark

[therealsamchaney]

Thank you for the detailed analysis. I understand why that side face is not flat, but since the edge in question was at the very bottom of the shell, I thought it would have been parallel to the outer edge next to it which is in fact actually straight according to Fusion. I guess its technically "approaching straightness" like a limit in calculus but not actually truly straight. I will definitely look out for these "control point splines" that seem straight but aren't and use the curvature analysis more.

For now my workaround in this situation (if I can't find a suitable straight edge to project) will be to make a construction sketch line between the two end points of the control point spline and reference that construction line instead. This gives me almost all of the functionality of a projected line.

Thanks,
Sam