Why can't I slice something I made in Sketchup?

I'm still learning the whole 3d development side of things and wow is it hard. Yesterday I made an open cube in Sketchup and for whatever reason Slic3r wouldn't slice it. My son wanted me to make a marble trapped in a box, so I measured the marble and using Sketchup I made a box that was the correct size. The sides were 4mm and overall length/width/height was 22mm. It took me about an hour to make sure all walls were there and filled in, but it looked perfect. I then exported it as an STL file and opened it in Netfabb, which told me I had errors. It told me the errors were the inside walls of the cube. It tried to repair them, but ended up adding extra walls for some reason.

I thought maybe Slic3r would just slice it, so I loaded my config file and did a quick slice on my file and got the following error:
Modification of non-creatable array value attempted, subscript -1 at print.pm line 891

This is just a simple open cube, what could possibly be wrong?

I've attached it in hopes someone can help.

THANKS!

I just imported my file into Sketchup again and I noticed that the lines I used to make my cube were changed. I guess it did this when it exported it as an STL file. Originally I had nice even concentric lines that I used to make the cube and for some reason now they are randomly placed, even though it still looks like a cube, the lines are in different places. I also tried adding more lines to make more smaller triangles and that didn't help.

I tried fixing your file but it was very messed up. Not sure how or what happened but I just whipped up the same object for you that works (took 10 minutes).

I appreciate that, but I would like to know why mine didn't work. I started with a square that I used the push/pull tool to bring it to a cube. I then used the ruler to make marks 4mm in on all sides, connected the dots, deleted the middle piece and then made the insides complete.

Quote
tmorris9
I tried fixing your file but it was very messed up. Not sure how or what happened but I just whipped up the same object for you that works (took 10 minutes).

What program did you use? Did you go about it the same way that I just described?

An easier way to do this:

1) Just make a cube the right outer size. Nothing fancy, just a cube.
2) Tell Silc3r to do zero top layers - now it's an open cube
3) Tell Slic3r you want zero fill - now it's a box
4) Tell Siic3r you want 10 perimeter layers. With a .4 mm head you now have a very solid 4 mm wall.

Takes very little time that way. You can even print it up as a Spiral Vase if you want to .

Quote
uncle_bob
An easier way to do this:

1) Just make a cube the right outer size. Nothing fancy, just a cube.
2) Tell Silc3r to do zero top layers - now it's an open cube
3) Tell Slic3r you want zero fill - now it's a box
4) Tell Siic3r you want 10 perimeter layers. With a .4 mm head you now have a very solid 4 mm wall.

Takes very little time that way. You can even print it up as a Spiral Vase if you want to .

I wish I understood all of this. I created my cube and opened it in Slic3r, but couldn't find all of these settings. Even more than that though is I don't understand how you would even begin to figure that out. Hopefully with time I'll learn Slic3r and how to use these options, but for now I guess I'm stuck with Sketchup. I think you recommended a Slic3r tutorial which I haven't had a chance to study yet, but I'm sure it would be time well spent.

Have you still got the .skp file?

---

_______________________________________
Waitaki 3D Printer

Quote
uncle_bob
An easier way to do this:

1) Just make a cube the right outer size. Nothing fancy, just a cube.
2) Tell Silc3r to do zero top layers - now it's an open cube
3) Tell Slic3r you want zero fill - now it's a box
4) Tell Siic3r you want 10 perimeter layers. With a .4 mm head you now have a very solid 4 mm wall.

Takes very little time that way. You can even print it up as a Spiral Vase if you want to .

This won't make what he was trying to. He was making a jail box with holes on every side.

Quote
vetteguy112233

Quote
tmorris9
I tried fixing your file but it was very messed up. Not sure how or what happened but I just whipped up the same object for you that works (took 10 minutes).

What program did you use? Did you go about it the same way that I just described?

I use Lightwave 3D a very complex 3D program normally used to make movies and animation. I have had it for many years and make images for advertising in it. It's just a nice coincidence that it works for 3D printing as well.

I did play a little with Sketchup at first but figured it was easier using a program I already know inside and out.

Here is the SKP file, hope it helps figure out what/where I went wrong???

I'm scared to put time into any other project. This was suppose to be a very easy hollow cube and yet for some reason it has errors and won't slice. Any and all help/advice is much appreciated since I'm at the mercy of those who have already been in my shoes and know what is wrong.

Well, I decided to try again and this time I was successful! I did a 50mm hollow cube and used a different method. Last time I made my measurement marks in the center of each side and then connected each line with the line tool, then deleted that square in the middle that I just made. I did that for all 6 sides, which left me with a non 3d interior, which I had to add lines to make it 3d again. This time I just used the push pull tool and it was much much much faster and it didn't have errors or any problems slicing.

I still would like to know why the other method didn't work.

Edited 1 time(s). Last edit at 11/18/2013 10:51PM by vetteguy112233.

Quote
vetteguy112233

I still would like to know why the other method didn't work.

You had some flipped trianges at the back of the cube (as you saw).
Instead of doing an auto repair in Netfabb, do this instead..


Click "Standard analysis".
Click the red cross.
Select the "actions" tab.
Click "fix flipped triangles"
Click "Apply repair"
Done!

---

_______________________________________
Waitaki 3D Printer

Quote
vetteguy112233


I still would like to know why the other method didn't work.

because you have some surfaces inside the volume which make a faulty stl when exported

in sketchup choose the wireframe view and inspect your cube you wil see a lot of surfaces inside the volume
use the eraser tool to erase the useless lines and you will get a clean part to export

there is some free pugin to verify your parts are correct solids but it is a bit complicated for a beginner

go ahead try antry again, inspect the inside of yours parts the answer if often here with faulty stl

you will succed, i did, why not you

A bit of background into why some models slice properly and others don't.

As you know, slicing and processing a 3D model generates a set of g-code instructions that tell the printer where to put plastic in order to give us a physical object. Pretty simple really. We can clearly see the 3D model on the computer screen, and it's obvious which parts are supposed to be solid. There's a clear separation between the space inside the object and the space around it. It seems logical that the computer, or design software, also knows exactly which parts are of the model are solid. And to a certain extent it does. Then we export the model to to an STL file.

But what information actually ends up in the STL file? Surely it's some kind of definition of a "solid" model.

Nope. It's triangles.

Just a big list of triangles. And they're not necessarily in any kind of order! No solids. No surfaces. No lines or curves. No model hierarchy. It's just triangles. Each triangle is defined with three vertices and a normal (ie. direction the triangle is facing). In many cases the normal is just ignored and the direction is calculated based on the clockwise/anticlockwise order of the 3 vertices (I won't bore anyone with the maths). Basically the whole file is a massive list of vertex [x,y,z] locations, grouped into threes.

So, let's say we were manually slicing a layer at a certain height, how would we approach it? Firstly, we find all the triangles that intersect with the layer. We then draw a line across each triangle precisely where it crosses the layer. If the STL file is good then all these lines should form one or more nice 2D rings (one for each part or "island" in that layer). These are our external perimeters for the layer. We know which direction each triangle is facing, so a little bit of maths tells us exactly what areas of the layer are the insides and what are the outsides. The insides belong to solid shapes and infilled appropriately. [Note: this is just the general starting point to define the shape of the layer. Calculating the appropriate paths and g-code is much more complex.]

But what if something is wrong with the triangles on that layer? Maybe there are gaps between a couple of them and the external perimeters we drew aren't continuous lines. We no longer have a definite shape for the island. And maybe a bunch of the triangles are facing the wrong way. How do we now know which parts are insides and which parts are outsides? What if we find a degenerate triangle, where one or more of its vertices are in the same place? Some of the trigonometry starts to break and give undefined or infinite results.

There are various solutions to these problems, but in many cases they involve making assumptions which may no longer represent the model as originally intended. Consider an example of slicing a layer where we find a square with all the faces pointing inwards. It's pretty safe to assume that the model is accidentally inside-out, and that we shouldn't be extruding plastic all over the print bed with a hollow square in the middle. But what if that's what the user really did want? They might have been intending to make a faceplate with a cut-out for installing a switch. An assumption has to be made one way or another.

STL is popular for its simplicity. A moderately experienced programmer can throw together a basic exporter within minutes. It's also simple to load and slice into layers, as the whole thing boils down to some vector mathematics, which computers are very very good at. You don't need a perfect mesh to display a rendering on a computer screen, but it's a different story trying to get enough information from the mesh to turn it into something physical. On a screen a triangle is always displayable as a triangle, regardless of whether or not it belongs to a manifold surface. The human brain is also very good at automatically filling in missing or gabled information to imagine a perfect solid, with clearly defined insides and outsides, whereas computers are not.

When you're designing a 3D model it's good to have an understanding of the mesh topology and the way it's affected by various operations in your modelling program of choice. Programs like Blender, 3ds Max and Sketchup give you direct control of the mesh. With a bit of experience you can produce perfect meshes. CAD software or parametric modellers such as OpenSCAD, Autodesk Inventor and SolidWorks do not give you direct control over the mesh. These programs use mathematical descriptions to define surfaces and volumes, which are then tessellated (turned into a triangle mesh) for display on the screen and for exporting to STL. They will often need to be cleaned in another program prior to printing.

As confusing as this is....it actually makes sense! As mentioned I tried to have Netfabb fix my model and it added all kinds of lines. I now understand why it did this. I will try the other methods mentioned about looking inside and erasing non needed sections, but I'm learning!

A big thanks to everyone who took the time to answer.

To the viewers of the tread. jf pion has the only real answer to the Original Poster's question based on the proper review of this model.

ANSWER:
"because you have some surfaces inside the volume which make a faulty stl when exported

in sketchup choose the wireframe view and inspect your cube you wil see a lot of surfaces inside the volume
use the eraser tool to erase the useless lines and you will get a clean part to export

there is some free pugin to verify your parts are correct solids but it is a bit complicated for a beginner

go ahead try antry again, inspect the inside of yours parts the answer if often here with faulty stl

you will succed, i did, why not you"

Every other post in this thread has nothing to do with this OP's issue and makes this thread completely too long. On top of jf pion's correct answer, the OP's model has reverse orientated faces (blue instead of white in Sketchup). Sketchup does work well with reverse faces on polygons (in the OP's model there are 8) and also does not work well with them exported into STL.

TheZionPhil:
Of all the comments in this thread it's you who is making an unnecessary comment. Please think twice before 'necroing' a thread.

From my point of view, a lot of help was offered and the original poster even learned more than anticipated. People invested time in this and they did so with grace. Kudos for that!

Ps. Waitaki already summed up the part about reversed faces so I really do not understand your comment.

Edited 1 time(s). Last edit at 01/23/2015 08:51PM by Ohmarinus.

---

http://www.marinusdebeer.nl/

In Sketchup
draw a square
select it --- pull it up 4mm
draw another square atop the one you just did, 4mm smaller on all sides
select new inside square --- delete it
select perimeter square ---- pull it up size of cube -4mm (for top)
Draw another square atop perimeter
select square -- pull it up 4mm
select it ---- group it

cubic model with 4mm walls

select group --- export as .stl

you can check it in netfabb and repair it

This will test the "bridging" on your printer

Hope this helps

We can Skype --- share screens to work it out?