Gererating bad STL.

[rob_a_johnson]

We are generating bad STL. (it seems)

Other bodies in the file are generated diferently and they print fine.  They were extruded from sketch.

The wing skin was a t-spline extrusion then shaped to final form. It does not want to print now. The internal structure of the wing was extruded from sketch as well and it appears to be fine. (I combined the two, skin and structure then cut the wing into 6 peices to fit the printer) 

The wing skin looks fine in preview but when you switch to layer/toolpath mode the skin has jagged holes in it! 

I have tried every permutation of the STL export box, ASCII/binary and low/med/high. I have also tried taking it to Meshmixer first, repairing the STL, the loading that repaired STL into Cura. I can discern no difference in the output using all those methods with the exception that “Repairing” it in Meshmixer actually made it worse.

Oddly enough, the only thing that has helped is feeding invalid parameters into Cura! I did it by accident at first but it is repeatable!

It Looks fine in normal view. Only when I started printing did I realize I had a problem.

So then I threw it into layer/toolpath mode and the problem becomes obvious! Big jagged holes! 

Now this is weird. I set shell thickness down and it cleared up the problem I was actually trying to take it UP to 2 mm to see if that would help but I left the . in place and made it .2 by accident.  The nozzle is .5 so a .2 shell width is impossible (as denoted by the box turning red) yet it cleared up the render! Weird right? 

[GrAndAG · Accepted solution]

STL is fine, actually.

I think all problems because the wing shell has thickness exactly 0.5. As STL triangulates surface, actual thickness of walls will vary and in some places it can be less than 0.5. So, when you ask Cura to print with wall thickness 0.5, Cura just throw away peaces which are thinner. But, when you set wall thickness as 0.2, Cura includes all walls. Actually, you can try to print with settings of wall thickness 0.4 (or 0.45) in Cura even with 0.5 nozzle. As physically the nozzle is 0.5, you will get 0.5 walls, but maybe underextruded a bit.

Below is what Cura 15.04 suggests to print with nozzle 0.4 and 0.5.

Also, I loaded STL back into Fusion and did some measurements. That's the distance between 2 random chosen wall edges - 0.493073 mm. They are even not parallel.

[rob_a_johnson]

Well first off I want to say thank you very much for digging into this and getting to the root cause.   I am very impressed! You are sharp! 

You gave me a lot to think about! I will try the following.  

1)  First I will try to print as is with the shell thickness set down and see how that goes!  

2) I will try setting the wing skin to .6 (?) That was wayyyyy back in the time line. I hope that change will flow all the way back through!  (I have been massively impressed so far with how changes will ripple back through the timeline! Of course, it does have limits)

I have not “shelled” the main body yet.

Wait, now that I think about it, instead I will create a complex curving test piece today and “shell” it at different thicknesses to see which one works best in the STL.

Again thank you for your fine research!

[doughaas]

why when I export stl it scales the item down.  I drew the item as a 4 dia but when I open in slicer it is very small.  I am guessing that the stl file is in mm.  any idea how to change or what I need to do to be able to open in slicer full size as drawn, thanks, Doug