Lips and ledges

Just wondering how people calculate dimensions and compensate for die swell etc so that when sliced the filament is laid in the right place.

Let's take a canonical example of a hollow box with a lid, where the lid sits with flush sides to the base by incorporating a lip that is half the thickness of the walls of the base. With the right dimensions I should be able to get a good friction fit, especially with the layer ridges. Here is an example I knocked up in OpenSCAD to demonstrate:


If the narrowest feature we can print is one filament width wide (let's call this 1W), then the minimum wall width to support this must be twice the filament width (2W).

Here is a simulated (and magnified!) cutaway view of filament being laid to create a wall 2W wide, with a lip of 1W wide):

	OOOOOOOOOO...
	OOOOOOOOOO...
	O
	O


	 O
	 O
	OO
	OO
	OO
	OOOOOOOOOO...
	OOOOOOOOOO...

Thing is, with swell and some general inaccuracy of my setup this will result in a lip that is too tight, and in this particular case where the lip is only 1W, the plastic will delaminate when I try to push the lid and base together.

Now if I increase the wall width to give a bit of clearance then the filament that makes up the wall perimeters ends up having a cavity in the middle, which tends to make it weak.

Now I could increase the wall width to 3W, but now I have another problem: the lip now wants to be slightly under 1.5W, and when sliced this will get rounded down to 1W, which means my friction fit is now way too loose despite having increased the overall widths!

Is there even a way to ensure at these kind of scales (1W, 2W) that there is enough material to make a tight fitting friction fit without weakening the supporting perimeter structures?

As far as I know there is no accurate way right now. I normally add 1-3mm inside (depending on how large the item is. And if it's an important piece I create a 2-3 layer test sample of both parts then print and see how well they fit and adjust from there. This way I don't waste time and material printing up the whole thing just to find it does not fit.

The way I do it with something like that is I model the object with a wall twice the extrusion width, slice it and if it doesn't slice properly I fiddle with the perimeter extrussion width until it does, sometimes it's a matter of setting the extrussion width to 0.38 instead of 0.4mm, a very small difference.
As for clearance, I think it depends a bit on the printer, with mine I usually make a gap of 0.15 to 0.25mm for a good fit without forcing the pieces together too much.