Need assistance with a loft (I think)

greg_duncan · October 2020

I am new to this forum and fairly new to OnShape, and I was hoping perhaps to get some assistance from some of the experts here.

I am trying to model a surfboard fin, which has a cross-sectional profile like an airplane wing or an airfoil. However it needs to end in what is essentially a sharp edged curve. Most foil examples that I have seen don't end that way.

I tried several methods including making a loft with several profiles as a solid, but really could not get it to work unless I did a loft, as surfaces, and then merged the surfaces into a solid. But I am struggling to get the loft feature to do what I want. Perhaps I should be using a different feature instead? The central part is pretty easy and works fine, but the top portion above the upper profile is where I am running into problems. Unfortunately, due to the inability of OnShape to duplicate a sketch (as far as I can tell), I have to keep sketching the same thing over and over to test out various ways of modeling the part and it is getting tedious and time consuming and I an getting close to giving up on it. I totally admit that I am a noob with Onshape (my background is ProE/Creo and Inventor) and a noob with surfacing in general, as most of the parts that I have done for work are not done with splines, surfaces, lofts, etc. So please be kind if I am missing something really obvious.

So far, my best result has been using the outer shape as guidelines, but there is a small defect at the top where everything comes together.

It is a public doc and I think this link will take you to it.
https://cad.onshape.com/documents/82e3b92a87c08409cf7f2e9b/w/40754e450ad07f104d1eb694/e/92ff8b550985917fbcf7e9b6

Any assistance would be greatly appreciated.

MichaelPascoe · October 2020

@greg_duncan

Hi Greg,

Try reversing the profile & guide selection.

Image: https://us.v-cdn.net/5022071/uploads/editor/tb/766hjga29ksr.gif

matthew_stacy · October 2020

@greg_duncan, consider only lofting to the upper foil. I believe that this will give you much better control over the sharp edge that you are seeking to create at the top of the fin. At any rate, it will eliminate the cowlick anomaly that was in there. "Split" the main profile sketch to accomplish this (so the loft guide curves will end at the Plane for Upper Foil.

Image: https://us.v-cdn.net/5022071/uploads/editor/h1/acle5sr6w5ul.png

Then fill the tip of the fin. Note that you could probably add some guide curves to control the 'sharpness' of that top edge. Also note that you that specifically using the EDGE OF LOFT 1 (rather than the upper foil sketch) will allow you to specify tangency there. I was not able to curvature continuity, but you might be able to improve that by applying guide curves.

Image: https://us.v-cdn.net/5022071/uploads/editor/2s/tos6h31hxwcb.png

Fill the back face.

Image: https://us.v-cdn.net/5022071/uploads/editor/zb/x9w9diydoz2l.png

Finally, filling the bottom face will automatically enclose the fin thus converting it to a solid which can then be boolean unioned to the face.

Image: https://us.v-cdn.net/5022071/uploads/editor/a4/ugkso69d5rzn.png

I'm sure there are other ways to approach this geometry, but there's my $0.02 worth. My changes are in the "B" workspace (original in MAIN): https://cad.onshape.com/documents/59869f46e37ed320f1c6cf54/w/5353a4c55a597d05036faf70/e/8ecef67ac2991552e9dae04a

Cheers,

-Matt

Need assistance with a loft (I think)

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