This is my finished, working Digi-Comp, the mechanical computer from the 60s. You can read about it here http://www.retrothing.com/2006/02/build_your_own_.html
I was originally going to build my own version out of cardboard. I actually built 2, but I first used the incorrect plans, so the pieces wouldn't fit together. My second version had the correct size, but it was too floppy. At this point my arms are sore and my wife told me that I should just break down and buy the kit, http://mindsontoys.com/dc1_main.htm for my birthday. I hadn't had such a great birthday gift since I was a kid!
I had some trouble putting it together because I had a dyslexic moment and installed the crank backwards, but Tim from Minds on Toys very helpfully pointed the error out after looking at some photos. He was very, very friendly and helpful. Thanks Tim.
I guess I will write more about what I do with it as I work through the workbook. This is going to be an important learning tool at Crunchy Granola Academy, an alternative school at which I am the headmaster.