Explaining the insert system

Ear piece showing some of the inserts exploded

I've modeled the parts to have insert details whenever they're raised from a surface in such a way that nooks and cranies that defy smooth sanding appear.

This mostly has no function mechanically for the 3D models, except in rare cases like for the nut fastening to the right in the picture above. The system is mostly there to enhance the 3D print post sanding quality later.

Insert as used on my Dalek gun (see my other blog), in this case it also served a mechanical purpose to easier attach led lights and wiring on the back of the tab

Finished Dalek gun, the handle parts use the insert system, the barrel doesn't and had to be sanded twice as long and with more attention to detail because of it. 

Why do this

If you print parts on a flilament printer you get lines from the layer by layer material addition process, think of it as cutting profiles out of sheets of paper and stackning them, the sides will get tiny stairway-like steps.
This needs to either be filled with primer or sanded down smooth to give realitstic smooth cast- or milled-and-polished-looking prop surfaces after painting. You can also smooth with acetone, but it melts the surface details and will often not remove all print lines without sanding beforehand anyway. Either way having insert parts like this help with the end quality from all of these methods and add just a little bit of time in after-assembly. Time and effort you save up on easier sanding.

Why? -So I can sand smooth here...

Why not just use an SLA printer?

It's true, layer lines are far less apparent with an SLA vat printer than a Filament printer simply because the layer height can be set much finer, but that process is messier with stinky chemicals and far more expensive per print. It's also far more uncommon to be able to print large parts.
As it is the parts on this project is about as large as I can fit in my Zortrax M200 which has a fairly large print volume for a filament printer.
It's also the printer I own and have at home so of course I'm adapting the models for optimal performance for my equipment. ;)

However, it's easy to fuse assemblies to print as a solid piece later, not so easy to separate like this so I get both choices when adding inserts according to my system.

Don't get me wrong, SLA printers are a dream for some projects.
Some day when the good ones are inexpensive enough and I have a well ventilated workroom for it, I'll get an SLA printer. at the very least for printing tabletop miniature models and perhaps if it's large enough even for larger parts. This means no more sanding required, or far less of it at least. ;)