topic Re: Sketch a line tangent to a model edge in Fusion Support Forum

Sketch a line tangent to a model edge

Bug.Robotics — Tue, 12 Mar 2019 16:47:43 GMT

Please let me know if I'm losing my mind here as this should be very simple. I have a model and I'm sketching on top of a plane referenced from the model. I'd like to draw a line tangent to the model. The tangent constraint is not available when trying to select the model edge (even when projected). Screencast is below.

I even tried defining a plane tangent to the model's edges and then using that to align my sketched line...no dice.

Re: Sketch a line tangent to a model edge

Bug.Robotics — Tue, 12 Mar 2019 17:25:20 GMT

I'm still hoping that I'm incorrectly selecting references or something similar. Workaround is below.

Create a plane tangent to the models edges of interest;
Start sketch and create a projection from the plane;
Now you can reference the model edges.

This is a fundamental sketch tool that needs to be implemented. At the very least let the user reference planes without having to project the plane.

Re: Sketch a line tangent to a model edge

karina.harper — Tue, 12 Mar 2019 17:29:55 GMT

Hi @Bug.Robotics

It's hard to say without looking at the file. Can you share it here?

Cheers,

Karina

Re: Sketch a line tangent to a model edge

Bug.Robotics — Tue, 12 Mar 2019 17:48:30 GMT

Hi Karina,

Thanks for the reply. So there is hope then? It appears as though Fusion cannot make a line tangent to a spline.

I can't upload that model but made another that shows the issue.

Re: Sketch a line tangent to a model edge

karina.harper — Tue, 12 Mar 2019 18:21:00 GMT

Hi @Bug.Robotics

Thanks for that. If this was made from a spline then no. If this is made with arcs or circles then yes. As the part is a base feature in the file you provided, I can't say for sure.

@jeff_strater provided a good workaround for making splines tangent in this thread here.

This body could easily be remade with circles/arcs though. See here:

Let me know what you think,

Karina

Re: Sketch a line tangent to a model edge

Bug.Robotics — Tue, 12 Mar 2019 18:53:53 GMT

Hi Karina,

The file was created with arcs and circles. What causes the spline to be projected is the added chamfer. I thought that by creating a midplane and then projecting the radiused edge I could get a proper arc but Fusion outputs it as a single point spline for whatever reason.

If all parts were as simple as the uploaded file it would be trivial to do what you did there by adding the circles. If you couldn't select the outer edge of the part to make the circle equal and concentric you would be forced to do more work. This is the case with my model I showed in the screencast and the reason I posted at all. Comparing this to other sketch environments, it's an awful lot of work to get a sketched line to be tangent. One time, no biggie. If you were attempting to use this package daily with this type of sketching the extra clicks/sketches/planes/projections add up quickly.

Re: Sketch a line tangent to a model edge

karina.harper — Wed, 13 Mar 2019 18:49:38 GMT

The file was created with arcs and circles. What causes the spline to be projected is the added chamfer. I thought that by creating a midplane and then projecting the radiused edge I could get a proper arc but Fusion

outputs it as a single point spline for whatever reason.

@Bug.Robotics

I found that I was able to reproduce this from scratch if there's a chamfer on both sides of the body. This has been logged internally as FUS-47980.

Thanks for bringing it to our attention!

Cheers,

Karina

Re: Sketch a line tangent to a model edge

Bug.Robotics — Wed, 13 Mar 2019 15:30:19 GMT

I appreciate your diligence Karina. Happy to be able to contribute.

Re: Sketch a line tangent to a model edge

jeff_strater — Wed, 13 Mar 2019 22:21:28 GMT

@Bug.Robotics , I worked with Karina on this issue yesterday, and I understand better now what is going on, and I think I can explain it. I still agree that the result is undesirable, but thought you'd appreciate a detailed explanation.

Projecting a body into a sketch is something we call "silhouette projection" internally. The term is pretty self-explanatory - we want to take the apparent edge of the model as seen from the direction of the sketch normal. In the case of this model, the side faces of the model are all parallel to the sketch normal. When possible, Fusion looks for edges that it can project to represent the silhouette (there are some cases, like a sphere, where there are no edges, so we have to generate the curve differently, but if there are edges, these are used first). So, there are two choices for each edge when projecting into the sketch. Let's look at a couple of these edges. The first one looks like this:

I've selected the projected curve and the two edges which are equal candidates for the representative silhouette projection. Both of those are circular edges, so the result is a circle in the sketch.

The second set, though, is where the issue occurs:

either of those edges would work, and are equivalent as far as silhouette representatives. But, both of these are actually spline edges in the model, as you noted, because of the chamfer. So, the result is also a spline (even though, technically it could be a circle, but that is not how the modeler works)

The third one is similar as well:

both edges are equivalent projections, and both are spline edges because of the chamfer.

The interesting case is when we only chamfer one of the edges:

again, there are two candidate edge sets here (one single edge, the other a chain of two) that could be used. But, in this case, Fusion picks the front one, which does result in a circle.

The last thing is another type of workaround you can use: If you put the sketch before the chamfer, then this is much cleaner:

because you can then project the silhouette without the geometric complexity that the chamfer adds.

Re: Sketch a line tangent to a model edge

Bug.Robotics — Wed, 13 Mar 2019 22:41:36 GMT

Thanks for the details and potential workarounds Jeff. Issue is I only use Fusion for generating 3D toolpaths (still holding my breath for Autodesk to release HSMWorks from the Product Design and Manufacturing Collection death grip) and so unless I defeature in SolidWorks, upload the model, then add the features back (which I'm not going to do) that won't work.

In theory, should a midplane in the part project arcs, because there isn't a chamfer at the midplane, or does Fusion only look at the top face?

Re: Sketch a line tangent to a model edge

jeff_strater — Wed, 13 Mar 2019 23:17:26 GMT

that's a great question. The short answer is no. If you use the same Project command, you get the same results, because the utilities we use are the same. The projection doesn't know that this is a midplane:

however, for a midplane, you can use the Project -> Intersect command, which does a plane cut of the body, and will generate arcs, because the intersection of a plane and a cylinder is an arc: