Error or Artifact?

[Anonymous]

I'm currently navigating the wonderful world of lofts, and being perpetually new at this I'm not sure what to do with an all-to-frequent error message that the rails don't intersect the profiles on my 3D sketch. I don't know what to do to find out exactly where the disconnect is, and this has led me on many a not-so-merry chase in the last week or so.

I have tried to use approaches such as magnifying the intersection to the maximum to see if a disconnect is visually obvious. In doing so, I have found on many occasions that a line I snapped to an arc (for example) goes past the arc, which makes me think, "AHA! There you are you little rascal!!" But the overshoot can't be trimmed, and if I try to place a point on what appears to be the intersection, it won't go anywhere except at the top of the overshot line.

I'm beginning to think that this isn't really an error, but a display artifact. Is this a known issue? Just need a little clarification on this so I can go sniffing around somewhere else.

[laughingcreek]

If you zoom way in, then yes, you will get artifacts where things that are actually coincident look like they aren't.  they only way to be sure is to use projections and constraints.  if you have a coincedent constraint between 2 points, then they are.

If you struggling with a loft question, why not post your model along with some screen shots and a question?

I have in the past seen loft error messages that where mis-leading in what the problem was

[g-andresen]

Hi,

Unfortunately (fortunately) we can't see the pictures in your head if you describe the process with words.
Therefore, it is better to set the image material or even better the file in question.
With the "overshooting" lines I suspect like you that it is a Display issue.

günther

[davebYYPCU]

Attach the file, prevents rabbit holes.

You mentioned 3d sketch, and I just answered your question about Projected articles.

Loft is finicky, will not tolerate Self Intersections, or sloppy construction.

Loft has a few Rules, mostly to do with Rails.

Loft cannot do hollow, donut style, pipe style work, in one hit, must be outside solid, inside cut in Two Lofts.

Rails must be Tangent connection if Built from segments, Profiles don’t have to be tangential.

Rails must connect to all profiles, and if you get that error, it will be true.

Rails must run from 1st to last Profile.

Project > Intersect, to bring into the current sketch, the article you wish to connect to, then hide that projected article, to confirm there is a purple point result, and that, you are connecting to the right point.

Use the eyeballs to reduce clutter, and increase accuracy.

Happy to check the file...

[jeff_strater]

@Anonymous - yes, it can be hard to find the source of the missing rail/profile intersections.  The best way to avoid this particular error is by constructing the profiles and rails correctly from the beginning.  I put together this post a while ago to help show how to create sketches for Loft to avoid this error:  introduction-to-loft-using-rail-curves-in-fusion 

The key is the Project -> Intersect command.

[Anonymous]

I’ve attached the file for my latest drawing. It is very similar to the previous one, but I’ve tried to make it much simpler because @TrippyLighting  suggested that I could achieve an excellent loft with only three rails, one at the widest part of the upper bout, one at the widest part of the lower bout, and one at the narrowest part of the center bout (plus the terminating rails at either end of the arch).

My approach has been to draw a construction line across these intersections. The one at M, for instance, will allow me to place aligned drawing points on both sides of the inner profile and at the centerline of the drawing. After that I’ve been thinking that I want to trim the construction line away from the sketch geometry, and THEN project the point. My problem is that I am not sure how to place the drawing point on the profile and not the construction line, since both need to be present. I suspect that’s my problem at A, and why the line is blue instead of black.

Thanks to all who have been helping me with this. Thanks @jeff_strater  for linking me to your loft videos. I found them very helpful, but I do have one additional question. Previously, lofts have worked for me when the end of the arc rail and the end of the profile are at exactly the same coordinates. If the profile and the rails intersect, but the profile is extends farther, will the loft fail?

OK, two questions. 😐 When the points are projected, does the intersecting plane have to be enabled? For example, the points at M are present in the main sketch on the XY plane. Do I project them at that time, or do I create the intersecting plane at M, hit create sketch, and then project the points?

Think I’ll stop now . . . 🙂

[laughingcreek]

I don't have a clear understanding of your questions.  To answer one of your questions though, rail don't have to terminate on the profile at the ends. they can pass through, as long as they DO connect.

Also noticed (and I think this answers the question of the original post) that in sketch "arc at D" the wrong type of projection was used.  You projected the endpoint onto the sketch plane, which is why it doesn't allign.  should be and "intersect" type. 

Also, rail segments must be at least tangent to each other.  Your as aren't

[TrippyLighting]

@Anonymous the things @laughingcreek is pointing out have been pointe4d out in previous threads. You really are better off starting from scratch with a new, fully constrained sketch.

Another recommendation would be to replace arcs with control point splines and closely watch the curvature on those. 

In previous threads I did not simply "suggest" that this can be done with a start, end and three intermediate profiles, I provided proof beyond and reasonable doubt 😉

I thought my instructions in this thread were pretty clear.

[Anonymous]

That's terrific, thanks. As for the wrong type of projection, just chalk it up to wandering mouse pointer syndrome. If you will, would you give me a brief explanation of the tools you used to reveal the errors? I'm still a little confused why the connection between the spline and the arc was not tangent. I applied a tangent constraint, or at least I thought I did, and Fusion didn't complain. What a drag! But at least it's nice to know how to look for those kinds of things down the road and not have to bother you guys so much.

[Anonymous]

First and foremost, I'm really grateful for your advice. I would never want you to think you had to prove anything to me about Fusion. After all, you can make this program fly and I can't even get on the runway. Nor would I reject your advice and blow you off by not even trying it. All the problems are at my end. I'm really still new at this, and there's a lot of stuff coming at me that sometimes doesn't get processed at the right time or in the right order.

The attached file shows my basic first step in laying out the design for a violin (although this particular one is of a bass). This is an important file. Once it's finished and constrained, I never touch it again and work from a copy. This might or might not be part of the problem. First thing I need to know is how when I work in the copy is how to eliminate the construction lines in such a way that the sketch remains fully constrained. I know Fusion has the tools, but the process isn't always obvious or intuitive.

I also need some additional clarification on the use of splines. Splines have already caused me some problems when I have tried to make them tangent to arcs, but one thing at a time. And thanks again.

[TrippyLighting]

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@Anonymous wrote:

... and there's a lot of stuff coming at me ...

---

Yes, I totally agree. This is NOT at all a beginner project and it will take you a while to get more comfortable with this. But you clearly have a goal and in my experience these "Instrument threads" are some of the most satisfying if not for the folks who start them but for me personally. At the end usually there is a beautifu instrument I cannot wait to see yours come to life!

I know @jeff_strater likes these threads too.

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@Anonymous wrote:

I know Fusion has the tools, but the process isn't always obvious or intuitive.

I also need some additional clarification on the use of splines. Splines have already caused me some problems when I have tried to make them tangent to arcs, but one thing at a time. And thanks again.

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Intuition comes with practice and some knowledge of underlying principles. 

I'l see if I find some time to explain this in a screencast.IN the meantime you can go through the timeline of the attached model step and see what you can take away form this. 

[Anonymous]

Thanks for your generous and supporting message. Sad to say, my attempt to download your file failed twice. Unfortunately, I have had the same problem when attempting to download and open other files from the forum. Not sure what or where the problem is. If I can grab and copy the error message, to whom should I send it?

I wanted to ask you for a little clarification about the use of splines. My arcs across the centerline are defined in the sketch by three points, so a three-point arc seemed like the best way to go. In experimenting with fit-point splines, I think I end up with exactly the same arc. Since these arcs are used as rails when lofting, is there any advantage to using one over the other?

I haven't tried control point splines for these arcs, but I have used them in the corners of the attached sketch. They work pretty well here, but I have not been able to figure out how to constrain them. I'd think that this would be done by the points, but when I try Fusion tells me the drawing would be over-constrained. I am also not able to extend the lines when I'm trying to find spots in the sketch that are not fully enclosed. Is this their normal behavior?

Lastly, I think you said to use splines for the profiles rather than multiple arc segments. I need to use arcs at first to set correct offsets and curvatures from a common center point. Did you mean to impose the splines over the arcs and go completely from one end of the drawing to the other so there would only be two profiles in the entire sketch? I'm certainly willing to give it a try, but how do I then eliminate the underlying arcs if the spline is exactly above them?

Slightly off-topic, but I also appreciated your comments about the resulting musical instrument. If I live long enough to see this through, there could be some amazing results I think you'd appreciate. Your encouragement goes a long way here and makes the challenges easier to overcome.

[TrippyLighting]

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@Anonymous wrote:

I wanted to ask you for a little clarification about the use of splines. My arcs across the centerline are defined in the sketch by three points, so a three-point arc seemed like the best way to go. In experimenting with fit-point splines, I think I end up with exactly the same arc. Since these arcs are used as rails when lofting, is there any advantage to using one over the other?

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The advantage of splines over arcs is curvature control. An arc has constant curvature.

With a spline the curvature can vary along it's  length. The wavy line is the spline. 

To see that red  curvature comb, when in the sketch, select the curve, right click and select "Toggle curvature display".

Here is a link to the design I posted earlier in this thread https://a360.co/2rm5g8o

[jeff_strater]

@Anonymous - following up on some probably already-answered questions:

"I found them very helpful, but I do have one additional question. Previously, lofts have worked for me when the end of the arc rail and the end of the profile are at exactly the same coordinates. If the profile and the rails intersect, but the profile is extends farther, will the loft fail?"

Yes, rails and profiles can definitely extend beyond the ends of the intersection point.  for instance, this setup:

produces a successful loft:

then, you ask:  "When the points are projected, does the intersecting plane have to be enabled? For example, the points at M are present in the main sketch on the XY plane. Do I project them at that time, or do I create the intersecting plane at M, hit create sketch, and then project the points?"

If I understand the question correctly, then your last statement is correct.  The sketch into which the intersection point is to be created must be active when the intersection is created.

And yes, @TrippyLighting is correct - I do love the musical instrument-related threads.  So cool to see people using Fusion to create beautiful instruments...

[Anonymous]

You did indeed understand my question correctly, and your answer removes another of those obstinate little barriers I've been dealing with. It always amazes me how sometimes a very small point can make a huge difference. In many of the YouTube videos I watched (imagine my eyes becoming increasingly rectangular as the weeks wore on . . .) the advice will usually say that one needs to project something. This uses the term generically, and almost no one says that, well, you know, there are actually two choices under that menu. And you have to have the intersecting plane active before you can actually intersect anything with it. And so on.

While I still have many questions about lofts, rails, and profiles, I thought you might like to know that this morning I had my best and cleanest lofts EVER. Not as good as @TrippyLighting does it, but then, no one else does, either. Happy Thanksgiving!

[Anonymous]

I just mentioned to @jeff_strater that this morning I had the best and cleanest lofts EVER in my never ending quest to create violin arching. I even managed to do it with five rails instead of the three you used. It's better than the 12 or 13 I started with,  so I'm definitely getting closer. The additional midpoint rails were required because the loft began to flatten out if the rails were too widely spaced, and at that point I understood why you used a "helper" along the long arc. Since I can't seem to open your files for some reason, may I ask how you created this geometry? I suspect that you offset the arc, closed it with lines on either end, and then trimmed off any excess before doing your loft. I'd like to try it when I next go for three, but I'm not feeling totally secure about this yet.

Thanks again, and have a happy Thanksgiving.

[TrippyLighting]

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@Anonymous wrote:

 and at that point I understood why you used a "helper" along the long arc.

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The purpose of the helper surface is to create tangency across the center line.

The "flattening" you described is the reason I m advocating for the use of splines instead of arcs. Control!

That flattening can be controlled by carefully adjusting splines, tangent weights and as a last resort, adding another rail.

[Anonymous]

I thought you might be interested in learning why I've clung to using arcs when I had other possible options available. The centuries-old method of geometric violin design was lost for hundreds of years. In all the trials it takes to work one's way backward using the tools and techniques then available, certain things become obvious. Spirals and spline-like curves are very difficult to draw with a compass and a straightedge, although with painful care and the use of many tangent portions of a circle, it can be closely approximated.

Arcs, on the other hand, are easily done with a compass, so I think the old geometers used arcs whenever they could, and I decide to do the same. After all, the point is to produce an object that is indistinguishable from the original, and in my profession there are people who can tell the difference just by looking.

Once I get to that point, I'll be free to try other approaches (and I will). As many suggestions as I can remember are in the back of what's left of my mind, and I hope to try them using the tools and techniques currently available. 😊

[TrippyLighting]

That's a fair point!

If arcs accurately represent the geometry of the instruments then they are certainly easier to tame then any form of spline, and that's a good thing!

However, you'll still need the helper surface to create tangency across that center section.

Und the hood, regardless whether you use analytical geometry (lines, arcs) or spline curves, Fusion 360 will create a NURBS surface from this, which mathematically is simply a 3D version of a spline. Thus you still need tangency control through that helper surface.

I would not argue that spline and spiral like curves are difficult to draw with a compass. Those simply aren't the right tools to do that. Spline like curves, however, are easy to draw with easily available tools even hundreds of years ago 😉

[Anonymous]

I've attached my latest and almost final model in hopes that the gurus here can help me figure out what I haven't done correctly. The sketch shown as "purfling groove" in the browser was done on the model face, and the two lines for the purfling  were positioned by offsetting from the edge profile.

At the very top the purfling deviates from the outline, and those repositioned lines are blue because Fusion will not let me dimension them. Looks like I have two problems; the first is to constrain the sketch, and second is to put the lines together so that the purfling groove forms a profile I can extrude 3mm down into the body.