Make a Dodecahedron

[dog2bert]

I am trying figure out the angles to build a dodecahedron (12 sided) speaker cabinet.

How would I go about creating this in Sketchup as a whole and then just one side?

Also, how might I cut the sides at the table saw?

[AMarshall]

There are a lot of ways to do this of course, but here is one example using the circle tool.

Take advantage of the way that SketchUp draws circles - they are actually made up of segments. The default is a 24 sided polygon, but you can manually enter any other number, such as 12 in your case. Here is the view after you click on the circle tool, showing 24 sides in the bottom right corner.

Before you click anywhere to start drawing, type "12" and "Enter". The number in the bottom corner should update.

Now just draw a circle like you typically would: Select the center, and select another point on the edge or type in a dimension for the radius. Voila, 12 sided circle.

To model a single side, I would start with the offset tool. Make another 12 sided circle inside your first one with the offset tool, say 3/4" inside to represent the thickness of plywood.

Use the line tool to connect a few of the corners, to isolate one side of the cabinet. Then use the push/pull tool to create a 3D side.

Now you can get all fancy using the protractor to measure the angles if you like, but here is a simple geometry lesson. To get the degrees for each corner, just divide 360 by the number of sides. A square? 360 / 4 = 90 degrees per corner. A dodecahedron? 360 /12 = 30 degrees per corner.

I will leave detailed table saw instructions for someone else, but remember that the 30 degrees needed at each corner is split across 2 sides, so a 15 degree cut angle on each side gets you a dodecahedron when they are all joined up.

I hope that this helps and good luck with that glue up!

[dog2bert]

Thanks for your help. I was looking for the 3D figure.

[AMarshall]

Thanks for your help. I was looking for the 3D figure.

Oops! My mistake! Lol, that is harder.

The best I can give you on that one is to check the Google 3D warehouse - there are some premade models and even a few tutorials.

http://sketchup.google.com/3dwarehouse/details?mid=462c06af866552431a68ecf799d539a

To cut these on the table saw you could rig up a jig that has a pin/screw at the center of the side, then rotate it for each cut. Tilt the blade so that you end up with a beveled cut.

[dog2bert]

Your jig suggestion is exactly what I was thinking of.

I wanted the sketch up model to make a template for the holes for the jig.

I also wanted to figure out what the angle for the blade should be.

[PurpLev]

Are you still trying to make your dodecahedron?

[dog2bert]

Are you still trying to make your dodecahedron?

I know how to get the model into sketchup.

What I am looking to do is make the side 3/4" thick and then calculate the angle of which the sides meet.

[PurpLev]

I know how to get the model into sketchup.

What I am looking to do is make the side 3/4" thick and then calculate the angle of which the sides meet.

hmm. ok

So, if you have the model in SketchUp. use the push pull on each of the faces to 3/4", Then select 2 of the faces, and edit->intersect selected which will give you the geometry and lines where they cut each other. select all those lines, edit component for 1 face, and edit->paste in place. this will add the angled cut lines to the face, delete the other lines (that represented the 90 original straight cut) and you should have the shape with the beveled cut - measure and you're done.

of course - this all assumes each of the faces of the shape is a component.

IF you need more assistant - post up what you have, and maybe that would be easier to go from.

[rmac]

I know how to get the model into sketchup.

What I am looking to do is make the side 3/4" thick and then calculate the angle of which the sides meet.

You guys are making this too hard. Instead of messing with Sketchup, you can just look up the angle between the faces of a dodecahedron. This site, for example, shows it to be approximately 116.56 degrees. Fortunately, many other sites agree.

Then draw a little picture of the pieces you're trying to cut, like this:

If you want to cut the same bevel angle on each of your 3/4" pieces, you'd split the dihedral angle between them as shown in the picture. So 116.56 divided in half is 58.28 degrees, and to cut that on your table saw you'd tilt the blade 31.72 degrees from the vertical.

Make sense?

-- Russ

[PurpLev]

You guys are making this too hard. Instead of messing with Sketchup, you can just look up the angle between the faces of a dodecahedron.

True!

I just carried away with the 'use of SketchUp'. I find that sometimes tackling situations like this, and going the extra mile to make things work in SketchUp although can be harder than googling the answer- builds experience that comes in handy later on.

[dog2bert]

Here is my sketchup file for the template I plan on making:

dodecahedron template.skp

[GNiessen]

I don't know how big you want it. But I whipped up a 12" form. I found that sketchup was not too accurate on some of the angles, but was pleased when the last side matched up correctly. You should be able to scale this as you need. It would be easy enough to redraw it at another size as well, so if you have any problem I can post some drawing steps.

dodecahedron-12in.skp

[don johanson]

What I did was set the chopsaw at 30 degrees, and set the vertical bevel at 15 degrees. Now take you piece of wood put it against the fence and make your cut., then put that cut against the fence and continue until you have five cuts and you will be holding a pentagram, I use a razor knife to mark the board where I WANT to cut and to put a fine scratch on the chopsaw fence so that all the facets that I cut are exact. By doing this you will make a nice tight jointed dodecahedron.