Can't project sketch origin

[GRSnyder]

Your mission, should you choose to accept it, is to move the elements in Sketch1 (within the Capsule Bottom component) to the sketch origin. For me, the sketch origin will not project into the sketch no matter what I do.

Restarting Fusion 360 does not help. It seems to be some kind of corruption within the document (and more specifically, within this specific sketch).

I think I had the origin projected at an earlier point, but I deleted it.

[jhackney1972]

Here you go.

[GRSnyder]

You turned on the coordinate origin and projected that in, if I understand correctly. That works fine for me too. But is the sketch origin not supposed to project even when the visibility of all coordinate systems is turned off?

If you create a blank document and sketch on one of the default planes, the origin is still projectable even though the axes are off by default.

Just to be clear, this is not a question about how to project a coordinate origin into a sketch. This document does not seem to have standard behavior on my system.

[jhackney1972]

I have never really noticed.  If I begin a new sketch in a new file on the default plane, the origin is available to snap to.  If I edit your sketch, I have to project the origin point of the component to add the Coincident constraint.  I feel this as it has always been but I will not swear to it.

[jhackney1972]

If I turn on the Origin, of the component, but do not project it into the sketch, the origin is available to snap to automatically.  The Origin must be made visible.  This is what I see in your document, not a new file.

On the other hand, if I create a new sketch, in the file you supplied, sketch a figure off of the origin, finish the sketch and then edit it, the origin is available without being visible or projected.  You say you deleted the original projected origin point in the original sketch, maybe that is what is causing it.

[GRSnyder]

OK, I think I have a simpler test case and a better idea of what the actual problem is. 

If you project the sketch origin into a sketch, I don't think you're supposed to be able to delete it. You're stuck with it forever. Not every impossible-to-delete point is a sketch origin, but all sketch origins should be impossible-to-delete points.

However, it seems you can de-magic the sketch origin so that it's neither undeletable nor constrained to be at the actual origin. Here's one way to do it.

1) In a new sketch, draw a line. Anywhere, any orientation.

2) Apply a Coincident constraint between the midpoint of the line and the sketch origin.

3) Draw another line that starts at the midpoint/origin. You can end it anywhere.

4) Use the Trim tool to remove the two halves of the original line.

At this point, you can drag the second line away to show an underlying point at the origin. I'm not sure if it's actually a de-magicked sketch origin or just a remnant of the first line's midpoint. However, you can move and delete that point freely and there is no other sketch origin. And you can no longer reference the real sketch origin in any way, short of explicitly turning on a coordinate system and projecting that in (which seems like a completely separate mechanism).

[Parenthetically, and probably unrelatedly, I notice that you cannot manually Project the sketch origin, which seems somewhat peculiar now that I think about it. You can only activate the sketch origin by referencing it implicitly. It doesn't seem to matter whether "Automatically project edges on reference" is turned on or off - the sketch origin always seems to behave as if that option were enabled.]

[jeff_strater · Accepted solution]

I think there is a bit of confusion here.  "Project the sketch origin into the sketch" is not an operation that should be available.  You should never have to, or be able to "project" the sketch origin of a sketch into itself.  The fixed origin point should just be part of the sketch from its creation.  However, what you have uncovered is a bug in sketch - there should never be any set of operations that result in the deletion of the sketch origin.  We will investigate that and try to fix that bug.

[edit] I created bug FUS-112352 for this issue.

[HughesTooling]

I've reported this a long time ago and still see it occasionally, @jeff_strater can you take a look?

@jhackney1972 one problem is not all sketches have the origin at the component origin, if you delete it's magic origin point you can be left with a sketch you can not fully constrain! Had this the other day and had to delete the sketch and rectreate.

Mark

[GRSnyder]

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@jeff_strater wrote: I think there is a bit of confusion here.  "Project the sketch origin into the sketch" is not an operation that should be available.

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Egad! You're quite right - my mental model of the origin is completely wrong. The origin rosette is not a Point, it highlights differently from a Point, and it is always there.

In my defense, though, I think Fusion 360 actively directs you toward the idea of an origin point as something "projected" into the sketch, for the simple reason that the origin itself apparently can't participate in constraint relationships. Whenever a constraint against the origin is involved (that is, pretty much any time you actually reference the origin), Fusion 360 adds an extraneous, undeletable Point to the sketch. Presumably, that Point has an internal "at the origin" constraint/property which creates this unique behavior.

For example, draw a line to the origin. Then draw another line that starts at the origin. You end up with two end Points and one Point at the origin/joint. If you drew this same figure without involving the origin, there would be no Point shown at the joint. So clearly, operations involving the origin are different and they can indeed involve "projecting the origin into the sketch as a Point." (What is "projecting" if not "creating a local proxy that is tied to a reference object?" In this case the reference object--the origin--remains directly accessible as well, but the Point is still its projection.)

Even drawing a simple line to the origin creates the phantom Point, because the origin then has to absorb the constraint of being on the line. As near as I can tell, the only thing you can actually do with the origin that doesn't involve projecting it as a Point is to use it as a dimension reference point.

Not to mention (🙂) the fact that if you hover the mouse cursor over the bare origin rosette, it highlights as a Point. Why not a box?

Thanks for bugging this, @jeff_strater, and thanks also for the more detailed explanation. I always find your comments helpful and enlightening.