topic Re: Likely bug: distorted sketch pattern element in Fusion Support Forum

Likely bug: distorted sketch pattern element

GRSnyder — Wed, 16 Feb 2022 23:45:17 GMT

I recently discovered that the sketch solver can solve for pattern parameters such as spacing, center of rotation, or direction. Wow, this is a wonderful and really powerful feature!

I've seen a couple of anomalous behaviors, though. Here's one that's consistently reproducible and clearly wrong. Start with this sketch of a fully-constrained hexagon:

The goal is to create a pattern of three hexagons along the construction line with spacing such that the corner points are in contact. Previously I would have thought it necessary to calculate the corner-to-corner diameter of the hexagon in order to set the spacing appropriately. But evidently this isn't actually needed! You can put in a dummy spacing value, then constrain two adjacent corners to be coincident, and Fusion 360 should adjust the spacing appropriately:

Then constrain these two points together:

Or at least, that's the way it seems to work in many sketches. But in this case, Fusion 360 just distorts the second pattern element to satisfy the constraint.

Clearly that's wrong. Note that the pattern is still active within the sketch and remains editable. But you can't, for example, change the side-to-side diameter of the hexagon to 9mm; the solver just complains.

(For anyone tempted to go down the "hey, you know you can just dimension the vertex-to-vertex distance on the hexagon..." road: yes, yes, of course. This is just an example to demonstrate the pattern+constraint issue. I don't have an actual need to make rows of hexagons.)

Re: Likely bug: distorted sketch pattern element

jhackney1972 — Thu, 17 Feb 2022 02:01:54 GMT

I cannot replicate this issue. Could you post your model with the sketch BEFORE you add the Coincident sketch constraint? I would like to play with it a bit.

Re: Likely bug: distorted sketch pattern element

hamid.sh. — Thu, 17 Feb 2022 03:54:12 GMT

... You can put in a dummy spacing value, then constrain two adjacent corners to be coincident, and Fusion 360 should adjust the spacing appropriately...

...Or at least, that's the way it seems to work in many sketches.

@GRSnyder Wait is this actually possible!? This is one thing I always missed from SolidWorks. Could you please share a file?

Re: Likely bug: distorted sketch pattern element

benjamintoews — Thu, 17 Feb 2022 15:20:22 GMT

I came to Fusion from AutoCAD LT and have always been disappointed that there isn't an option to edit Patterns. I suppose this means more programming and their focusing on fancy stuff like AM and Circuit Board design?

Re: Likely bug: distorted sketch pattern element

benjamintoews — Thu, 17 Feb 2022 15:24:25 GMT

Oops. Sorry. I just noticed that you can edit a pattern!

I just never took the time to figure it out!

Re: Likely bug: distorted sketch pattern element

GRSnyder — Thu, 17 Feb 2022 20:12:26 GMT

@jhackney1972 wrote: I cannot replicate this issue. Could you post your model with the sketch BEFORE you add the Coincident sketch constraint? I would like to play with it a bit.

Hmm, interesting! I thought this might be one of those Fusion issues that creeps up after the app has been open and in use for a while (in this case, several days), so I restarted the app and in consequence received an app update, which I accepted by restarting a second time. It still behaves the same way for me - I'm on macOS Monterey 12.1.

Here's the file prior to applying the constraint. I have also verified that I can open this exported file and apply the constraint to reproduce the issue.

Re: Likely bug: distorted sketch pattern element

GRSnyder — Thu, 17 Feb 2022 20:20:52 GMT

@jhackney1972 wrote: I cannot replicate this issue.

Ah, here's a possibility... Is your initial hexagon centered at the origin as mine was? If I just draw a free-floating hexagon and go through the same procedure, I get an "unable to solve" error instead of a distorted pattern element when applying the constraint.

Re: Likely bug: distorted sketch pattern element

GRSnyder — Thu, 17 Feb 2022 21:40:15 GMT

@hamid.sh. wrote: Wait is this actually possible!? This is one thing I always missed from SolidWorks. Could you please share a file?

The more I look at this, the more I think I was mistaken.

There were two specific cases in which Fusion 360 seemed to be solving for spacing. The first was when rectangular-patterning rows of hexagons to form a grid, kind of like the example above but with more hexes. After reacting much like @hamid.sh. did above ("Wait?! You can actually do that?"), I made a separate little test document which seemed to confirm it. Unfortunately, I didn't save either state and I now can't reproduce them.

I think the underlying rule is probably that patterns are fairly analogous to mirror constraints. Anything that is in the sketch, such as a center point or direction line, can be solved bidirectionally. Anything you enter into a panel (or have F360 enter into a panel by using manipulators) is fixed.

For example, the case outlined above works fine if you drop the size constraint on the hexagon. But it's not adjusting spacing, just the patterned shape.

Another example is attached. Here, the top chevron is drawn and dimensioned. I wanted a rotated copy of the chevron pointing to the midpoint of the vertical line at right, but because the two lines are of different lengths, the center point needed to achieve the proper rotation for that is not immediately obvious. But you can circular-pattern around a randomly-placed point and then constrain the tip of the chevron to the midpoint, and the pattern center moves to the right place.

Re: Likely bug: distorted sketch pattern element

jhackney1972 — Thu, 17 Feb 2022 21:47:22 GMT

First of all, I am running Windows 10.

Trials:

Opened your supplied file, I could replicate the issue.
Save your file by a new name, removed the pattern but left the original polygon, I could replicate your issue.
Open your file, removed the complete sketch, recreated same size polygon on the origin, added the pattern and could not replicate your issue.
In a new file, created the same size polygon on the origin, did not use the default radius of the circumscribed circle but applied a 10mm dimension across the faces like @GRSnyder did, added the pattern and I COULD replicate your issue every time. The only difference is the way the polygon is dimensioned. Screencast is attached. @jeff_strater this may be of interest to you.