Fusion
Ok so two basic approaches here. The first is a tip for your present patch loft approach. I found in order to reliably do a loft from one open *profile* spline to another, along a rail edge spline (that was coincident snapped at both ends to 'include 3d geometry' of the profile spline ends, the rail spline first changed to 3d sketch geometry via moving it off its sketch plane a little), 'chain selection,' needs to be deselected as Jeff mentioned, but also 'merge edges' should be deselected as well. As I'm sure you know you then select the two profile lines (it will prematurely draw the loft), then the rail, then change to 'convert to rails'.
The preferred approach to this whole thing however is more what you were going for originally, a contour/topological map, coupled with profile sketches/loft rail guidance sketches where needed (still using patch loft).
So to first get an idea of common topologies, I went more with this type, although others are possible, such as the more abrupt drop off at the front of the body like Jeff was doing.
Unfortunately I lost all my screenshots, so if you would like more detail on something just let me know. But basically what I did was trace the guitar with spline like you did, and create multiple offset planes from it, all defined by some user created parameter (in the change parameters window). So my user created parameter was 'spacing'. The first offset plane distance was spacing, the second was spacing*2, the third spacing*3 and so on.
I created a few "cross section" guide rails as can be seen below. Note I messed up the one since it is showing a shaded in profile, and both sides are in one sketch. There should only be one sketch per rail, and just simple splines, no closed curves, since these will act as rails. I projected the highest offset work plane (can actually do this, just project blank work plane) into a cross section rail sketch, so that I had an idea how high the guitar *hill* will be. When drawing a spline you can hold down 'Ctrl' to temporarily prevent snapping.
With a few cross section rail sketches made, I then proceeded to start making the 2D contour maps on the offset work planes. For each of these, I would do an 'Intersect' of the cross section rail sketches, and then snap to these purple points that designate where the rail sketches intersect with the contour map sketch being edited. To make really good contour maps, I even tried creating various guide offset lines and I would try and draw my contour splines to linearly deviate from one side of the guide lines to the other.
When, using various methods, you get some decent layers going for a decent contour map, can try doing a loft and selecting the multiple contour loops. Note that this was just a quick rough test, with no attention paid to actual curves for a guitar (this is likely very exaggerated). Now the rail sketches here were not made as individual spline per sketch, so I could not use as rails, but in image after this, will see how that is the way to go, to start adding guidance for where the loft is doing funny things (which particularly potentially happens if not enough contour lines/loops), in addition to the initial cross section sketches, that are done in such a way to also serve as rails.
Now this is the way to do it (note in below quick test only single offset work plane, so only two contour curves/loops). Can see how multiple additional rails were made where a problem was beginning with the loft, and so needed more guidance in places. I created new rail guide sketches via using 'plane along path' work plane, as seen in last image below. These planes are made orthogonal to the contour curve/loop selected, which is just what we want. Just use the 'Intersect' to get the purple points of where the contour curves/loops intersect the sketch/work plane being edited. If you attempt a loft to first seek out problem areas, be sure to delete it or make invisible when defining the 'plane along path', or use the "hold to select through" method to insure choosing contour curve, not loft edge.
The trick to select multiple rails is to select your contour profiles, then one of the rails, and choose the 'Convert to rails' option, then select the next rail, do a 'Convert to rails' again, and so on. Note only one cross section sketch was made in below simple test. Other rails are added later on the work planes along paths to fix loft in places.
Here is a plane along path being made to add a guidance loft rail.
Well that's it for now, let me know if it works for you.
Jesse
very nice, Jesse. That's a great method. I took a slightly different tack, using the original design, and just showing how to make a loft with rails using this data. The key, here, is accuracy. For reasons I don't completely understand, Loft is very fussy about rails having to exactly intersect the profiles. So, you have to do a lot of very precise positioning of things, including projecting endpoints of curves into your rail and profile sketches. If you do this carefully, it usually works out well. And Jesse is also right about Chaining and Merging. For this kind of loft, where the endpoints of each profile and rail actually touch each other (as opposed to the usual case where each profile is distinct from the others. Fusion tries hard to link profiles together so you can use multiple curves, but for this sort of "triangle loft" case, it just works better if you turn these flags off.
Here is my result:
and here is a screencast of making the sketches and the loft itself: http://autode.sk/1DpS7bE
again, good luck with your design. You'll get there well before version 8000...
Jeff
Jeff Strater
Engineering Director
Jesse's approach is a great one. That works fine.
But, to answer the question, what I did in my video was to constrain the green tangent handle to be horizontal. To get these handles to appear, you have to select the spline first. This will make the handles appear, then you can select them, and constrain them.
Here is another screencast showing how to do that: http://autode.sk/1NEknxX
Hope that betwee this method's and Jesse's, one will work for you.
Jeff
Jeff Strater
Engineering Director
Thanks again for sharing your design. This one is certainly a productive bug farm. 
The problem here is a strange one, all right. The reason you don't see your loft is because it fails. But, it fails in a strange way. Normally, if there is an error in a command, OK is not enabled. That is often how you know that there is a problem. But, it turns out, that, following my instructions, there is, indeed, an error. It is hard to detect, because, you first get the error when you select the rail, as expected, because Fusion thinks it is a profile. Then, when you convert it to a rail, that is supposed to 1) clear the error, and 2) enable OK. It does 2), but not 1). So, I blindly clicked OK, and was also equally confused as to why I do not see my loft. I tried it 3 times before I noticed the error:
First two profiles, no problem:
Add the third, and it fails, as expected:
Then, convert it to a rail, and the OK is enabled, but the error is still there:
OK, I will have to dig in on this one.
Thanks!
jeff
Jeff Strater
Engineering Director
Accepted solution
Well shoot that's not good. Do remember we are in new experimental frontiers here 😉
I seem to like the method of using plane along path construction planes to make rail curves where needed to guide loft (as I showed before). Be sure to scrub back timeline to before the loft so the curves will be available to the loft. I would suspect with sufficient rails very few lofts/layers are needed, perhaps even just one loft, and a top patch. It's cool to be able to adjust the rails on the fly and the loft immediately updates, as shown below (note that in below example I did not try to make the single loft go further towards the crown, which I believe though should be totally doable though):
Really hope you can figure something out.
Jesse
OK, after taking a break and trying a different approach, I finally got contours that will work for me. It isnt the smoothest tansition in some spots but will be fine when the wood is cut and a little hand sanding is done. That being said, I will try and give info on what I did.
Until then I am having some minor surface patch trimming issues.
I am trying to trim the face of the border patch (black arrow) with the curved surface of teh top (red arrow) but it wont work. Any suggestions?
You may also notice that the material pattern is not consistant on some of the lofts.
Here is the file: LP Top
OK, here is how I got my lofts to follow the shape I wanted without using rails.
I first created/traced all my contours on 1 sketch (red arrow). I made them all construction lines for no real particular reason.
I then created planes at the corresponding countour "elevations".
I went to each of the contour planes and projected the corresponding sketch segment up to the corrsepnding plane, example the highest contour projected to plane at the black arrow.
Once the projected contours were in place I selected Patch Loft and selected each green numbered line and ended up with the proper transistions.
I repeated the process for several sections until the contoured top was complete.
Whaa...I commend you for what looks like a really good method! This one might be for the text books.
Just tried to view and download your file, but A360 is giving an error. Perhaps you could try again or export the f3d file and upload it to this thread?
I'm guessing you also made vertical construction planes/sketches, each then having projected into it respective end points of the contour lines so that could then draw the connecting line segments (eg 1, 2...)? Did you try spline segments instead of line segments, for smooth transition between lofts?
I just tried this as shown below, but did need to use the two contour lines each as a rail. Did you actually do the loft somehow without using them as rails??
Jesse
Huh, that's too bad it says the file is too big. I also just tried your new link and get the same error:
Jesse