Earlier this year my electronics hobby obsession drew me into the world of benchtop machining and DIY CNC tools. If you enjoy electronics but are looking for something that does more than beep and flash some lights, building your own computer-controlled machine tools can be a lot of fun. eBay can be a great source for many of the odds and ends as well as used equipment as fellow hobbyists upgrade to larger or more sophisticated tools. Even though I've found a lot of good industrial suppliers near where I live, I still check eBay regularly as the prices are often better.

Mini Machine Tools

Mini machine tools are hobbyist-sized versions of lathes and milling machines used to manufacture metal parts. There are roughly three categories of tools out there you might see:

1. Micro machine tools: Sherline, Taig, Proxxon, and the Emco Unimat all make similarly-sized tools well suited for modelmaking and similar types of small work. These were too small for me but they are very compact, light, and seem to hold their value very well. Sherline, Taig, and Proxxon all offer factory support/parts for CNC.
2. Older used "American Iron": Lathes such as Craftsman, Atlas, and South Bend were available in smaller sizes (~8" swing) and can be excellent tools capable of better work than most of their operators. You will not see these so often on eBay because they are HEAVY, often hundreds of pounds. Sometimes they sell for close to scrap metal prices because of this. If you know what you are doing and can move a 400lb machine, they can be great deals. These could be CNC'd by someone who knows what he's doing.
3. New imported (usually Chinese) machine tools: The best known of these are the "black and red" series of lathes and mills made (mostly) by Shanghai Sieg Industries. If you look at 7x10 lathes for instance you'll find that a lot of them look the same. Well, guess what, whether they were originally sold by Homier, Harbor Freight, Grizzly, Arc Eurotrade, or whomever, they were probably all made by Sieg. In the past some distributors were suggested to have higher QC standards but the quality seems to have become fairly uniform and is going up over time. There is good aftermarket support for these as well as very large and active owner communities so a used lathe or mill can be a good deal. If you look at new machines be sure to compare prices across different sellers because I've seen variation of 25% or more. Be sure to consider shipping costs too since these start around 100 pounds, and a dollar per pound is a good guesstimate for domestic ground shipping. Don't be suprised by the prices! A tool that's taken care of by its owner can be just as good in ten years as when it came out of the box. I typically see the Sieg tools going for 50% or more of new prices before shipping. Many of these can also be converted to CNC with aftermarket/homemade parts.

Hobby CNC

Hobby CNC runs the gamut from converted lathes and mills to completely homebrewed wood routers and engraving machines. This is an extremely broad subject to put it mildly. One thing to keep in mind is that most of the things used in hobby CNC machines are also used for many other purposes. Don't just look at things that have CNC in the title. It's not that these are junk--often the product is fine--but you'll miss the deals. There's an old joke that you can double the price of any ordinary consumer product by adding the words "for your boat" to the label. CNC is kind of the same way. Industrial and lab surplus is often not really clearly labeled for hobbyist purposes so people won't touch it.

Raw Materials

A lot of the materials that you are going to want to use to build machines, or to make things with them, you're not going to find at Home Depot. The good thing as a hobbyist is that you don't need big pieces (often you don't want them) and you can often use the professionals' leftovers. Search for scraps, cutoffs, offcuts, and shorts. Make sure to include shipping though when figuring out costs. Two materials you may not be familiar with but should be are PVC and Delrin plastics.

Most people know about PVC plumbing pipe but you can also get it in rod and sheet/plate form. PVC is fairly soft so it's as easy to cut as wood, but because it's plastic it's much better for tapping and drilling. Great for brackets, motor mounts, etc. Delrin is a special engineering plastic that is softer than aluminum but harder than most plastics. It is a great practice material because it behaves like metal but it's soft enough to be forgiving of stupid mistakes. One special quality it has is that it's extremely slippery so it is a great material for any kind of moving contact components. UHMWPE is even slipperier but as a result it is very hard to clamp and machine accurately, so it's hard to use for precision components. Plastic has been getting a lot more expensive lately as oil prices go up.

You will also find scrap aluminum and brass here. Ironically this is often going to be better machining stock than what you might find locally unless you have a real metal supplier. The most common materials are 360 brass and 6061 aluminum. If you buy larger pieces make sure you have access to a bandsaw to cut them down. It takes a long time to cut 1.5" aluminum billet with a hacksaw!