Blending radius-diverse fillets.

[travisbrown]

I’m trying to get a continuous fillet around the perimeter of this guitar body. I usually work in a mostly 2D space, but now poking at basic 3D modelling. Hoping someone can point me in the right direction. 

Screenshot annotation shows what I'm trying to accomplish: .375" fillet around the body, tapering to .125" around the neck block. (Trying to draw the continuous blue line with a laptop trackpad produces a drunken result.)

The sketch from which the body is extruded seems clean. I drew in Fusion. The tangent splines worked just fine for the .375" chord length fillet on the top. It is somewhat similar geometry to the back with the armrest taper, but I didn’t have to deal with transition to another radius for it. 

I first thought a variable fillet with transition points around the neck block. But Fusion just errors out saying it can’t calculate the ends of selected edges, radii too large on short segments even where I think it should be fine, etc. Lots of red lines. Also tried multiple operations, but same basic error result. Best I got was separately a .375” runaround on the body, and a .125" on the neck block, but only one or the other. 

Where am I going wrong here?

File attached. 

[TrippyLighting · Accepted solution]

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@travisbrown wrote:

I’m trying to get a continuous fillet around the perimeter of this guitar body. I usually work in a mostly 2D space,

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Even your 2D work offers a lot of room for improvement! Splines with this many control points should not capture the instrument's contour, as that creates bad curvature, as can be seen in the curvature combs. I would start with lines and arcs and only use splines when those more basic geometries cannot capture the shape.

A variable radius fillet can get you closer to your goal, but it is tricky to set up. Patience is required 😉

[travisbrown]

Thanks for the reply.

Yeah, I know there are a lot of control points. The task was to match an existing contour precisely for a jig, and didn’t leave a lot of room for approximating the major curves. Especially around the neck block.

But the problem area seems to be where I think you would need a spline: the last irregular curves into the neck block lead into straight lines tangent to simple arcs. Are you suggesting that recontouring the areas leading into the neck block will support a variable fillet? If that’s what I have to do, then I’ll revisit the legacy sketch. I can probably get away with recomposing that area.

(When I wrote “mostly 2D”, read “primitive flatpack sheet goods”; so yeah, I assume there is lots I don’t know about more complex curvature.)

[TrippyLighting]

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@travisbrown wrote:
Thanks for the reply.

Yeah, I know there are a lot of control points. The task was to match an existing contour precisely for a jig, and didn’t leave a lot of room for approximating the major curves. Especially around the neck block.

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That can and should be achieved with fewer spline fit-points, by adjusting the tangent handles 😉 

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@travisbrown wrote:
... simple arcs. Are you suggesting that recontouring the areas leading into the neck block will support a variable fillet? If that’s what I have to do, then I’ll revisit the legacy sketch. I can probably get away with recomposing that area.

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If you check the model I attached to my post, you can see I used a variable radius fillet on the geometry you created. Just be aware that the variable radius fillet can be a bit frustrating to set up. I didn't get it right the first time either 😉 

[travisbrown]

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@TrippyLighting wrote:

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@travisbrown wrote:
Thanks for the reply.

Yeah, I know there are a lot of control points. The task was to match an existing contour precisely for a jig, and didn’t leave a lot of room for approximating the major curves. Especially around the neck block.

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That can and should be achieved with fewer spline fit-points, by adjusting the tangent handles 😉 

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@travisbrown wrote:
... simple arcs. Are you suggesting that recontouring the areas leading into the neck block will support a variable fillet? If that’s what I have to do, then I’ll revisit the legacy sketch. I can probably get away with recomposing that area.

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If you check the model I attached to my post, you can see I used a variable radius fillet on the geometry you created. Just be aware that the variable radius fillet can be a bit frustrating to set up. I didn't get it right the first time either 😉 

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Thanks much for this. I got pretty close to your result when I tried also with just the planar back as you did, but including the belly cut contour is I think what was causing me the headache since I couldn’t select the whole contour as a tangent chain. (Though,  I can’t figure out how you got the -1.00 start point to work. Errors out for me.)  

I eventually figured out a workable solution using a combination of constant and variable fillets on with subsets of manually selected tangent chains. But man, you are right; is it finicky. 

(Also, I tried tracing the main profile on another sketch using tangent arcs and lines as your suggested. I could manage matching a few contours, but ended up still needing to use mostly splines to keep it on track without it getting ridiculous. The original design is perhaps just a little too lumpy for regular arcs? Dunno. I can see if I was drawing an new design where you could smooth some curves out and get a prettier comb, but not sure how I’d do it without basically fitting arcs with many more points that what I have now with splines. I’ll do a little more research.)

Again, thanks for your input.