Ok... I confess I have not built one of these. But stumbled across these sites a few weeks ago, and thought someone may be interested... never know...


http://www.philharrington.net/scotch.htm
How does the Scotch mount work? Let's begin by considering two facts. First, the Earth turns 360 degrees in 24 hours. That translates to 15 degrees per hour, or 5 degrees every 20 minutes. Next, one of the most common pieces of hardware is the 1/4-20 screw (1/4- inch in diameter, with 20 threads per inch). If this screw is turned at a rate of one revolution per minute (one RPM), then it will travel one inch in 20 minutes. Putting both of these facts together creates what I call the "20-minute Triangle. Using a little trigonometry, it turns out that if a 1/4-20 screw is placed exactly 11.42 inches from a pivot (e.g., a hinge) located at "A" and turned at one rpm, the triangle's hypotenuse will move at approximately the same rate as the sky. Anything mounted on the hypotenuse (such as a camera) will move right along with the stars.


http://www.astropix.com/BGDA/SAMPLE2/SAMPLE2.HTM


Now... LOL... I've saved the 2 links/pages above as PDF for future reference/use...