Why did I do it

For two main reasons:

To learn a little VRML programming
Because I lost my slide rule and can't affort to buy a new one

Not willing to spend money on the real thing I decided to build my own.

Because I do not like to get tired or dirty I soon settled for a virtual one.

I already did a few things in VRML, so it was a logical choice to use it for the slide rule.
VRML 2.0 allows for basic animations, and for more complex things you can use JavaScript Nodes.

All you need is a PC, and a little time :-)