Drafting with printing in mind part 2: Holes and Gaps

Posted by LGM | Monday, September 23rd

Holes and Gaps  3D printers require "water-tight" models, no holes in the exterior surface.

Once tolerances have been correctly accounted for, the model must be checked for openings. To 3D print a model, the input file must be one continuous, solid object, usually in the STL file format. The printer must be able to clearly distinguish between the inside and outside of the object it is printing. There cannot be holes or gaps in the model, because the 3D printer will not be able to tell what is inside the object and what is outside. This applies quite literally to a house - the exterior of the building must be sealed. There are tools that can help identify an close holes and gaps, like CADspan, but the majority of these areas should be found and closed within the original CAD model for best results.

To do this we should focus on 4 of the usual suspects.

1) Walls meeting the roof.

There are two places I often find problems here. See images below for illustration.  Images are of real models in the SketchUp Warehouse.

The first is simply having walls stop vertically before the roof joins. This happens frequently when people model all of the truss work and they draw the walls up to the rafter tails which leaves a gap in between.  This issue is also very prevalent in REVIT and AUTOCAD because they draw a vertical surface until they hit something else and then stop.  If you have a solution for this please leave a comment below.

The second place is where a wall should jog back between two roof pieces. The most frequent location for this is on Dormers, where the wall of the dormer does not go quite far enough back.  This issue seems to pop up in models from a great number of 3D CAD packages.  

Through the multitude of change requests we are all likely to miss some small changes so it is important to check connection points. The roof wall as described above creates the largest holes, but the following can create a large number of low volume holes.

2) The connection between windows/doors and walls.

I like to punch windows and doors in to solid walls rather that through the walls, but I don't frequently need to produce interior visualizations. If you need to punch all the way through the wall to show interior relief, I would suggest making the hole the size of the glazing rather than the casing. This way you are sure there is a thin overlap of solids. Make sure the door touches the jam and floor. If the door is floating it can be a real head scratcher when you can't figure out why your resolution is not as tight as it should be if using a shrink wrap tool, or why your model is hollow if you are not.

In REVIT, creating a library of 3D printing Windows, Doors, Etc can come in really handy here.  You can alter the 3D printing set to have different offsets so that your resolution looks its best when printed, and you can offset the casing to make sure it intersects the walls rather than theoretically touching them.  In SketchUp you should be making your windows and doors components, so you could simply offset them just before printing if you are having a hard time finding a hole that is creating a problem fo you.

3) Entry-way ceilings.

Not sure why these are always missing other than they are never seen, but we need them. If you have recessed elements in your model (covered patios, covered entrances, car ports) just have a look to see if they have a ceiling.

4) Chimneys.

Go ahead and seal them in. No one will be using them so there is no reason to leave an opening in to your model.

5) Staircases.

Spiral staircases are the most frequent offenders, but many traditional staircases leave an opening in your model. Check to see that your staircases exiting your model have some sort of "barrier" between the inside and outside.

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