Can't draft beyond certain angle

[Anonymous]

I am having trouble using the draft tool to do some v-groove carving. Before I even get to the CAM part, I'm having trouble getting the model right in CAD. Depending on which outline I select, there seems to be some kind of maximum angle the draft tool will accept, beyond which it gives an error I don't understand:

The operation could not create a valid result. Try adjusting the values or changing the inputs.

It tells me what I can try, but it doesn't explain why what I entered doesn't work. I also get:

Could not taper surface as requested.

sometimes.

I'm attaching some screenshots showing the draft tool perfectly happy with 29 degrees but error's on 30 degrees. There's nothing special about 30 degrees, other outlines won't accept 1 degree, some accept 45.

In case it's not clear from the screenshots, expanding the draft angle another degree isn't going to run the draft geometry into anything else. That said, there are situations where I'd like to be able to do that too.

Any help would be greatly appreciated. Thanks!

[kate.raskauskas]

Hi @Anonymous,

Thanks for posting in the forums! 

From what I can see of your screenshots, it looks like you're trying to draft a shape with a base outline made with splines. Drafting the walls on a spline-based extrusion like that may not work because the changes in the "top" outline can make it difficult for Fusion to calculate; you may need to make your part with Loft instead, like in the screencast I made-

<iframe width="640" height="650" src="https://screencast.autodesk.com/Embed/Timeline/60fe324f-7bf2-4358-a9c7-e984a86290b5" frameborder="0" allowfullscreen webkitallowfullscreen></iframe>

Does that help at all?

[Anonymous]

Hi Kate, thanks for the response.

From a mathematical standpoint, I don't see why the faces being extruded from splines would make a difference. It can draft at 29 degrees when they are splines, so 30 degrees shouldn't be out of reach. The math used to do the geometry calculations doesn't suddenly become invalid. Why would the tool suddenly decide that 30 degrees is too much? Is there a setting I can change that would affect where this arbitrary line is drawn?

While I understand that lofting could be used, that doesn't seem to be the logical way of approaching the problem. I have existing geometry, and I have an angle to draft the sides at. In the context of the CAD operation, drafting is what I want to do, not lofting. Using lofting for this purpose seems like a bit of a hack to get around what appears to be a bug. I say bug because in your screencast, vs what I've been able to attempt on my own, we've got 3 different error messages for the same attempted operation, and none of them accurately explain why the attempted operation can't be completed.

Additionally, the loft workaround requires many additional steps:

• Create an offset plane from the base sketch
• Copy the contents of the original sketch into the new sketch
• Using trigonometry, calculate the necessary offset distance based on the extrusion height and the desired angle of the draft
• Offset all of the profiles in the new sketch using the value calculated
• Create a loft between the two sketches
• Delete the original body

Compared to adding a draft, this is quite a bit of extra overhead. There's no way to get the draft tool to behave correctly?

Again, thanks for your response, and your patience with my newbieness. 🙂

[Anonymous]

Hello again. I decided to try your suggestion of using offsets and lofting as a temporary workaround while trying to find a way to fix the issue with draf. Unfortunately, there's apparently a similar issue with offset. After doing the trig to find the necessary offset distance for my desired amount, entering it into the offset command generates an error:

You can see that the offset works on either side of my desired value, but a nonsensical error is presented for the value itself. As such, your suggested workaround for the bug in draft is in fact bugged and not usable.

Any other suggestions?

[innovatenate]

Could you just sketch out some profiles to use for the toolpaths in the CAM environment? Is it even necessary to model "draft" angles? 

Is the sketch geometry in this case all Fusion natively drawn spline information? Or did you insert this from a dxf/dwg/svg file? The reason I ask this, is you could use curvature combs in the sketch too look at the curvature of your sketches. You could try to smooth out sketch spline to help with either increasing the draft angle or by creating an offset sketch. 

I hope this suggestion helps. Please let me know if you have any questions.

Thanks,

[Anonymous]

@innovatenate wrote:

Could you just sketch out some profiles to use for the toolpaths in the CAM environment? Is it even necessary to model "draft" angles? 

That was my original intention, but I found I was unable to achieve the toolpaths I needed with a less than accurate model. The number of issues encountered trying to CAM, all with cryptic and useless error messages, could fill its own forum post. In any case, there's no reason the model shouldn't be accurate.

Is the sketch geometry in this case all Fusion natively drawn spline information? Or did you insert this from a dxf/dwg/svg file? The reason I ask this, is you could use curvature combs in the sketch too look at the curvature of your sketches. You could try to smooth out sketch spline to help with either increasing the draft angle or by creating an offset sketch. 

I imported it from a dxf file. I don't see how modifying the spline would change it's mathematical behavior. The spline is draftable at 29 degrees, so it doesn't seem like the spline is the problem here. As for creating an offset sketch, as noted above that works for some offset distances and not others. So again, the spline doesn't appear to be the problem. Even if it did work, creating an offset in a separate sketch and using lofting is absolutely the wrong way to achieve drafting in any case.

I appreciate you taking the time to reply. Unfortunately, all of the suggested workarounds to this point have hit more bugs than they avoid, and I can't wait 8 more days for another reply. I'm going to have to switch to another software with fewer bugs.

[innovatenate]

I wouldn't discourage from using whatever tools you have available at your disposal. It does seem like this feature could us a bit of resiliency. I can describe the challenge here as I understand, perhaps it will help.

As you draft or offset a sketch, the curvature of the spline/edge is going to increase/decrease. In the case where the curvature decreases, eventually becoming extremely tiny, it can be hard to generate geometry. Think BREP geometry with a radius out to the E-7 decimal place. This is why looking at the curvature comb can be a helpful task. It's not always a solution...

Often tiny curvatures will cause the body to self-intersect in other areas which can also be problematic for the geometry kernel and cause failures. 

I'd love to see more for these types of issue. However, if you're importing the sketch geometry, there could be just one little hiccup (that isn't even visible to the eye) that causes the curvature to get hairy at a very small draft angle/extrusion distance (offset distance).

If you have a chance to share the design with us, I would be happy to take a look. I can't make any promises for a solution, but I'm happy to try. 

I hope that helps.

Thanks,