Quick question about a LOFT dilemma for a Covid project

peter_bloom

Attached are 3 screen shots of a Covid mask I'm designing. I need to loft from the front of the contour profile to a circle 10 mm in front of the contour. For some reason, there is a section in the loft that results in a weirdly twisted curve. I've added a center line 3D fit spline but that made no difference. I assume there is some way to guide or to create a path so the problematic section untwists, but I'm not sure how to make that happen. Any suggestions will be greatly appreciated.

John_P_Desilets

Thanks for sharing @peter_bloom!

@bruce_williams and @romeograham are right on point for this. Lofting to a sharp edge can be tricky, and most often a loft needs to have an equal number of points to match. The attempt I made was slightly different but I think it will do the trick. I used the edges of the mask as the loft guides instead of the profiles. Below is a break out.

Onshape Document

https://cad.onshape.com/documents/1bd1cc0d8d697101e8d7a763/w/c6c1cac17073a5c989e02d2a/e/0d852aaeaeab13a6897a383f

First, I used a delete face operation to clean up this face. This can cause issues down the road. 



Using the sketch intersection tool and 2D spline, I created 6 profiles along the exterior and interior of the mask. Roughly 1 mm spacing. I guessed what the offset distance should be but this can be adjusted. 



Next I split the face of the mask. This is so the loft guide stops at the sketch profile.



First loft, using the edge of the mask as the lower guide, and the circular sketch profile as the top guide. 



Created an offset surface to retain the face geometry of the mask and to use as a guide for the next loft.






Second loft adding to the offset surface. 





Next, I took the circular sketch profiles and used offset surface of zero.




Enclosed the surfaces to form a solid part.









Mirrored with the add option to add all solids together. Please let us know how you make out. Stay safe and thank you for everything you are doing!

John_P_Desilets

Hello @peter_bloom,

Would it be possible to share the link to your document? 

romeograham

Lofts behave best if there is the same number of elements in all profiles. You may try splitting your circle sketch so that it has the same number of segments as the front edge of the mask part. You can use the "split" sketch tool.

peter_bloom

Thanks for both of your responses! Here is a link to the document.

https://cad.onshape.com/documents/6270c88489393a17b2c5361a/w/cf204938e12789b796fbd823/e/98253f690708572a4b911461

I have tried every combination of splitting, vertex matching, path, guides, etc. and can;t get rid of the twist. I even tried spl;itting the contour and circle in half so that I can more easily fix the problem and mirror the result. I'm sure I'm missing something obvious but I am stumped.

bruce_williams

As @John_P_Desilets said - avoid sharp points on lofts.  I was able to loft a surface around the outside without twist; although your splits on circle probably need to be changed for best surface..  Then thicken for wall.

bruce_williams

here is link to my attempt. 

John_P_Desilets

Thanks for sharing @peter_bloom!

@bruce_williams and @romeograham are right on point for this. Lofting to a sharp edge can be tricky, and most often a loft needs to have an equal number of points to match. The attempt I made was slightly different but I think it will do the trick. I used the edges of the mask as the loft guides instead of the profiles. Below is a break out.

Onshape Document

https://cad.onshape.com/documents/1bd1cc0d8d697101e8d7a763/w/c6c1cac17073a5c989e02d2a/e/0d852aaeaeab13a6897a383f

First, I used a delete face operation to clean up this face. This can cause issues down the road. 



Using the sketch intersection tool and 2D spline, I created 6 profiles along the exterior and interior of the mask. Roughly 1 mm spacing. I guessed what the offset distance should be but this can be adjusted. 



Next I split the face of the mask. This is so the loft guide stops at the sketch profile.



First loft, using the edge of the mask as the lower guide, and the circular sketch profile as the top guide. 



Created an offset surface to retain the face geometry of the mask and to use as a guide for the next loft.






Second loft adding to the offset surface. 





Next, I took the circular sketch profiles and used offset surface of zero.




Enclosed the surfaces to form a solid part.









Mirrored with the add option to add all solids together. Please let us know how you make out. Stay safe and thank you for everything you are doing!

peter_bloom

I REALLY appreciate the work you put into helping me with this. Definitely answers my question and I learned a lot at the same time. Thank-you very much!

John_P_Desilets

@peter_bloom

Don't mention it. Please feel free to post some  pictures of your mask when it is done. Good luck!