Mixed modeling: Cannot extrude to an exact point/face in a STL

matti_suuronen

I am trying to modify an existing design that only comes in STL. I know I could re-model the STL, and probably will do that, but since this something I counter quite frequently, I would hope there could be an easier way. 
Here is the design: https://cad.onshape.com/documents/16c67c94012aca12218672df/v/fab1b635695aa14dec6e479c/e/3a676ea5ccfad6bde193fb44?renderMode=0&uiState=669f66ced58a2d525cb997fa

I cannot extrude if the rectangle I am extruding coincides exactly with the corner on the STL. If I move the rectangle by 0.0001mm it works, and I can later "move face" to get the faces to align exactly. However, the "move face" does not work always, and also this is a bit of a pain in the neck. 

Is there a way to make this mixed modeling easier / more robust? Or some other workflow I should try?

matti_suuronen

And one possibility would be to convert the stl in Fusion360 to a STEP, but that explodes the 75kB STL to a 1.4MB STEP if I use the Faceted algorithm. Works in this simple case nicely, but a more complex one might be rather a large file. And for prismatic I would need quite a bit of manual work, and for organic I would need a better license. So, ideally it would be be great if mixed modelling would allow these kind of modifications natively.

wayne_sauder

@matti_suuronen

 It seems like extruding as a new part and then using a boolean to combine everything is more robust than using extrude add. 

 I have not tested this idea extensively; do some further testing for your applications. 

S1mon

You are most likely running into issues of non-manifold behavior. The Parasolid kernel doesn't allow things to join at a point or edge. See: https://forum.onshape.com/discussion/20585/better-error-handling-and-training-around-zero-thickness-parasolid-limits

This part is so simple, it makes a lot more sense to remodel some or all of it natively, if you're making a lot of modifications.

Here I left the pyramid shape as mesh, but redid the flat bits. It only took a few minutes. The "R" doesn't match the font, but with some more work you could solve that other ways. It also wouldn't be that much harder to replace the mesh that's left with native features.

Right now, I just have most stuff aligned to the mesh, but it also wouldn't be that hard to make it fully parametric.

https://cad.onshape.com/documents/29967d6d1ee1c82ec4dcb57c/w/a3b26892e667752351729383/e/461ae2cf91bb9d63b77e0078