The Art of Scrounging
Of the three major skills needed to build robots (electronics, programming, and mechanics) the last was clearly my weakest. I began by putting things together using wood, and tape and glue, but soon realized that, besides looking sloppy, my projects never lasted. Furthermore, in Minnesota the relative humidity goes from near 100 in the summer to near 0 in the winter. As I quickly learned, wood is a sponge and will change shape dramatically under such humidity fluctuations. I had a reasonable assortment of wood working tools (drill press, table saw, etc.), and found out that you can use them pretty well on aluminum.
The only problem with aluminum is it's expensive! Go to your local home improvement center, and you will be shocked by what they want for a small piece of right-angle channel. And unfortunately, in the few years since I first wrote this, the price has risen as much as four times (thanks to China's development!). Here's where I learned my first of many lessons about the value of surplus. I found several sources of cheap aluminum stock in the bins at local recycling houses, with prices ranging ~$.75-1.50 per pound (now $3 per pound). The stuff is used, and you have to hunt through various shapes, but we're talking 60-90% off the new price.
And sometimes, that aluminum has all sorts of nicely drilled and tapped holes in it. So I learned to adapt my machines to pieces I found. Unless you have access to a well stocked machinist's shop (and their skills), it can be very worthwhile to keep hunting for motion control machinery that you can adapt to your desires, as opposed to building it from scratch. Nearly all of the machines I've built are composed partly (or entirely) of machinery that was ultimately destined to be melted in the recycling process.
A few caveats:
START SMALL: you will learn a great deal from a small prototype, and it may save you untold headaches, time, and money that can be wasted by launching into something big before you know what you're doing. Notice that my first CNC device was an egg-plotter.
SAFETY FIRST: you should always be aware of three key pieces of your body to protect: EYES, LUNGS, EARS. Don't skimp on protective gear (goggles, masks, earplugs).
In my opinion, the key tool that is crucial for you to do reasonably decent work, is a drill press. And it doesn't have to be big and expensive--the $100 cheap import is fine for starters. Although others will undoubtedly have differing opinions on this, I find that you can build just about anything if you can put holes in the right place. (OK, some sort of saw is useful too).
[Dad, you need to write more here. You only covered aluminum, not mechanics].