Fully defining 3D sketches

[Fully_Defined]

First, this is not a problem in the CAD software I normally use. I have had a Fusion 360 commercial account for years, mainly for HSMWorks access, but I don't really use Fusion much anymore.

I want to create a skeleton part for an assembly (because weldments aren't a thing) that contains exactly one 3D sketch and nothing else.

The problem is with 3D sketching in Fusion; sketch entities magically lose their fully defined status for mysterious reasons. Because I can't see 100% of the sketch relations (coincident relations are hidden for some reason by default; what else is hidden?) it's hard to figure out what happened.

In this screenshot, a simple center rectangle is fully defined, and the sketch glyph has a red lock on it. I selected the XZ "top" plane when I started the sketch.

Here, I have added a leg to the tabletop, and the whole thing goes brown. 

I selected the YZ "right" plane prior to doing this, but the results are identical in the XY plane.

You can replicate this yourself by starting a 3D sketch on the plane of your choice, sketching a center rectangle in the size of your choice (700mm x 500mm for me), and dropping the center onto the origin. It will go black.

Then, try to sketch a line from a corner to anywhere else, and presto the entire sketch becomes undefined.

It has always been an issue for me that I cannot measure to, or constrain to, a plane in Fusion 360, which would solve this weird issue. Am I missing something?

How would you 3D sketch a rectangle with four legs to make a table in Fusion 360?

For the record, I can make the sketch fully defined if I start this at the origin, but I don't want to put the origin there!

[Bunga777]

In its current state, it seems that Fusion360 may occasionally overlook the constraint of coinciding with the midpoint of a line segment, which should be considered a fully constrained condition.

Rather than defining a rectangle sketch using its diagonal, a more accurate method of determining complete constraint is to split the center line into two segments and apply the necessary constraints to each individual segment.

 

[davebYYPCU]

You add this blue line without a dimension, now it is not fully constrained.

Might help.....

[Fully_Defined]

I appreciate your response and it was genuinely helpful.

I am not a fan of construction geometry, especially not in the context you’re using it for (as a workaround). I admit that I used a center rectangle, which also includes construction geometry, but that was only because I cannot constrain midpoints to planes - or anything else.

The obvious answer here is for Autodesk to stop trying to AI us to Mars and instead fixing the places they screwed up several years ago and forgot about, and 3D sketches are a perfect example of that.

In another CAD application, I would not even be having this conversation.

[Fully_Defined]

I get what you’re getting at, but adding a dimension to the blue line would not have changed anything. The issue is that the blue line had nothing to do with whether the rectangle was constrained or not and yet it destroyed existing fully constrained geometry.

[g-andresen]

Hi,

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@Fully_Defined  schrieb:

For the record, I can make the sketch fully defined if I start this at the origin, but I don't want to put the origin there!

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This is not necessary. You only need to establish a reference to the origin, for example by measuring the horizontal and vertical distance to your object.

günther

[TrippyLighting]

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@Fully_Defined wrote:

First, this is not a problem in the CAD software I normally use. 

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The algorithms used in Fusion 360 to determine whether a sketch is fully constrained or not still have some room for improvement. That pertains to 2D sketches and particularly to 3D sketches.