Fixing negative volumes with netfabb

I’ve blogged before about using Netfabb to fix problems in STL files, especially when I’ve downloaded them from Last night I had a problem that the desktop version of Netfabb wouldn’t fix. After spending some time searching for understanding, I found that the cloud version of netfabb fixed the STL automatically.

How to justify one’s 3D obsession

The STL files I was trying to print were the excellently modelled Doctor Who inspired Tardis and Dalek cookie cutters by David Wilson. My wife and several of her friends are committed Whoovians, so there were some serious brownie points to be gained here.

I know I can, I know I can…

From the thingiverse page it was clear that several people had successfully printed these objects. Ed Nisley’s prints were especially impressive. David’s photo of the cooked and decorated gingerbread daleks was motivation enough. But when I loaded the Tardis stamp or outline into Netfabb Studio Basic (version 4.8.0, the most current), the objects displayed in dark red, instead of green, and a big red exclamation mark in a warning triangle showed in the bottom corner of the screen.

The problem is imaginary

Using my google-fu I eventually realised that the dark red colour in Netfabb indicates “negative volume”. So printing the STL correctly should have sucked PLA up from the print bed and returned it back through the extruder onto the coil 😉 As if the designer had somehow extruded out the back of the object and turned it inside out in some sort of Heinlein-ian distortion of reality (See Heinlein’s short story “and he built a crooked house” for where my mind was going with this).

Slic3r and Pronterface had done their best to make sense of this strangeness, but without success. This sort of strangeness (and others…) happens now and then as a result of the wide range of CAD and design programs people use to make cool things. The trick is in working out how to fix the problems.

Fixing it with Netfabb

Netfabb understood what was wrong — which is more than I could say of myself — but I couldn’t get it to un-invert the part.

More searching revealed that the free on-line cloud-based version of netfabb fixes more stuff than the free desktop basic version. So I uploaded the STL to the cloud netfabb, and it crunched it and twisted it and returned me a link to a fixed STL file. I downloaded that fixed file, and checked it in my desktop version of netfabb, and all was green (instead of red).


The rest of the production chain

Then I sliced the new STL file in the latest version of Slic3r, and printed it through Pronterface, into Marlin which lives inside my my MakerGear Mendel Prusa.

All is well. Brownie points on their way

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