Fun with CAD

[GregHolmberg]

So here's a fun experiment. I modeled a Gore Medium steel-string in CAD. Well, just the body so far.

Here's how I did it. First, I used R.M. Mottola's very useful "G" Thang tool to draw the outline. I used the printed Drawing No. 4 from the book, and visually tried to find the radii and centers of the curves in the drawing. I think I came pretty close.

Here's the gthang file if you want to load it into G Thang and edit it.

I then used the OnShape.com CAD tool. This is free to use, but your documents will be public. Much like Google docs, this runs in your web browser and documents get automatically saved on their machines.

I exported a DXF file from G Thang and imported it into OnShape. This is just a flat 2D outline of the guitar. After some fix-up of the outline, I was able to build some 3D objects: an arched top, and arched back, and sides connecting them. I made the top and back parallel, since I'm new to CAD and don't know how to do anything else! I spaced them to give 110 mm outside dimension. I also created a 3D object that fills the cavity inside the box. I did not include the linings, head block, or tail block.

Screenshot from 2024-04-25 20-33-13.png (101.08 KiB) Viewed 10749 times

Here's a link to the CAD project. You can copy the project and edit it for yourself.

You might ask, why go to this trouble? I got a few interesting numbers from this exercise:

• The area of the top: 144,405 mm²
• The area of the free part of the top: 140,935 mm²
• The volume of the cavity: 15,223,307 mm³

That first one is quite close to the number given in the book (144,215 mm², page 4-62), only 0.6% difference.

For the volume, after subtracting the volume of the braces, linings, and blocks, I get 13,879,553 mm³.

These numbers are useful when using the four-degree-of-freedom model spreadsheet (docs).

Yes, I'm still hoping to accurately predict the frequency response of a guitar. The real test will come when I actually build the guitar!

Greg