Trouble Adding Threads to STL

[changsc]

I'm trying to add threads (7/8-14 ANSI Unified Screw) to several holes in an STL (one hole is already threaded in the original file). The surfaces of the holes are not cylinders. I tried patching the holes and creating a cylindrical hole through the patches, but still am unable to thread the new cylinders. File is attached. Any help is appreciated.

[wmhazzard]

I would use the mesh as a template and create a new solid body. 

[changsc]

Thanks. I did Mesh to BRep and reduced the number of facets before trying to thread. Is that what you mean?

[etfrench · Accepted solution]

You have the right start.

1. Next create a sketch on the bottom or top face. 
2. Project three points around each of the holes.
3. Draw 3 point circle for each of the holes.
4. Draw a larger circle around each hole.
5. Close sketch.
6. Extrude both the larger and smaller circles to fill the holes.
7. Extrude the smaller circles to create new holes.
8. Use the thread tool to complete the task.

p.s.  As @wmhazzard says: It would be better to just do the entire model in Fusion 360.

[changsc]

Thanks for both replies. Is there a way to use the file I have as a guide to create a new model in F360?

[wmhazzard · Accepted solution]

Yes, there is a way. Use the steps 1-8 that @etfrench outlined in his post. Basically you project the corners and centers of the BRep to sketches, draw new arcs and lines to complete the sketch and use them to extrude a new body. The problem with the BRep is that it is still faceted like the STL so you have to make sure you project the correct points on the sketch.

[changsc]

Thanks again for the help. I'll give it a try.