Using offset to create thin features

[mortisj]

First of all, thanks gumyr for creating an awesome tool.

I am puzzling over the offset() function. I'm trying to create a hollow box with no openings using the example "Offset Part To Create Thin features", but if I leave off the "openings" argument, or set it to None or [], the resulting box is solid. No big deal as I can make the hollow box by subtracting the offset box from the original box, but perhaps the documentation for the offset() function could be updated to reflect this behaviour? Or perhaps the offset+openings functionality could be moved to its own function, hollow() maybe?

Thanks.

#!/usr/bin/env python
import build123d as bd
box = bd.Box(width = 100, length = 100, height = 100)
box = bd.offset(box, amount = -10, openings = [])
box = bd.split(box, bd.Plane.XY, keep = bd.Keep.BOTTOM) # to reveal the inside
bd.export_step(box, "box.step")

[jdegenstein]

I agree this can be a bit confusing, the key is that the default mode for offset is Mode.REPLACE. What this means in practice is that when openings are specified the return value of the offset operation is the entire box, not just the interior. Conversely, when openings are not specified the return value is the interior of the box, and by reassigning box = bd.offset ... you are replacing the original shape with the offset shape. Here are two examples that might help illustrate the above further:

import build123d as bd
box1 = bd.Box(width = 100, length = 100, height = 100)
box1 -= bd.offset(box1, amount = -10) #note the subtraction here
# box1 = bd.split(box1, bd.Plane.XY, keep = bd.Keep.BOTTOM) # to reveal the inside
box2 = bd.Box(width = 100, length = 100, height = 100)
box2 = bd.offset(box2, amount=-10, openings=box2.faces().sort_by(Axis.Z)[-1])

[gumyr]

Note that offset isn't "shell" - as Jern described it's creating a new part that is an offset of the original part. What you do with that part is up to you. If in a BuildPart scope, offset(amount = -10, mode=Mode.SUBTRACT) will remove the newly offset part thus creating a shell.

[mortisj]

Thanks, I get it now. The openings parameter acts as though it sets mode=mode.SUBTRACT.

I think the underlying mechanism goes a bit deeper than that but I'll stick with that mental model for now. :-)

Cheers.