Well, there are a few answers to that question. The first is, because I can. The second, and more important answer is, I can use the transformer for all manner of OTHER projects. For example, you could wire up a couple of these bad boys together and make a welding machine. Or, you could do what I intend to do, which is to make an electrochemical etching machine for putting my maker's mark on the knives I make and sell.
So... without further ado, let's jump right into this. But first, a couple of comments. For this Instructable, I will assume you know how to take apart a microwave oven. Ideally for what we want, you want an older oven, one that uses a big transformer - the bigger the better, really. Why? Because bigger (and older, to a point) typically means both more power and higher quality. Ideally, you want to find one of those old Japanese or American microwaves from the late 70s, 80s, or even early 90s.
Also, keep in mind that you are working with high energy electronic parts. Keep that in mind. Make sure everything is unplugged from a power source, and ensure that you are working in a well-lit, safe, clean workspace. I tend to do a lot of my work like this at my local Techshop, which is where I actually completed this task.
Step 1: Tools/Parts List
Microwave Transformer - if you're unsure what the transformer looks like, check out the first picture. It's the box in the middle of the coiled wiring with the red wires coming out on one side, and the white and black wires coming out of the other side.
Also, a note on safety. When you're tearing apart the microwave, please, please please be careful that you beware the capacitor inside the microwave. It looks like a small metal can with two metal tabs on top, and it should be near the transformer. Make sure you short it out to make sure it doesn't have any lingering charge in it. The best approach for that is to just put a screwdriver or something metal you aren't connected to across the two metal terminals. Some capacitors have and automatic discharge feature, but you can't be too careful here.
Drill Press (with bits)
Step 2: Removing the Stuff We Don't Need
Now, it's important to note that you should do absolutely everything in your power to prevent any damage to the bigger windings, as these are necessary to produce the type of power we will need. If you do damage the heavier gauge windings, you could create shorts in the winding which will allow current to bypass certain parts of the coil, thereby resulting in a smaller coil. No joy.
By removing the smaller windings, we free up space to provide even larger gauge wire windings, thereby giving us higher amperages with the concomitant lower voltages.
The best option is to CAREFULLY use a hacksaw to cut through the smaller windings. Why a hacksaw? Well, simple. You can easily control where the blade goes, and the cuts are smaller, thereby reducing the chance of nicking the larger windings.
Also, when sawing through the smaller windings, make sure you stick close to the laminate block, since the windings are tight and don't give you a whole lot of room for cut through them. Take your time doing this. From time to time, if you think you're getting close to cutting through the windings, use a screwdriver to peel away some of the thoroughly cut windings to get a better view of your progress.
Step 3: Pounding Out the Windings... or Not.
Well, that was a non-starter. Right idea, wrong execution (at that point). So learn from my mistake instead. Use a drill press to give yourself some room and reduce the stuffing effect of pounding on the small windings. If your transformer is like mine and has aluminum or copper-coated aluminum as the winding material, using a punch will simply push the windings into the hole and mushroom, i.e., peen out, inside of the hole, making it more difficult to clear the hole.
A better approach at this point, as I mentioned above, is to use a drill press to drill clearing holes through the windings. This will allow more motion in the transformer, allowing you to go back later and punch out the windings that are left.
It's important to not breathe in the metal shavings produced when you're drilling into the windings. That stuff is nasty.
Step 4: Pounding Out the Windings, Redux.
First, chuck up the transformer in your vice, then proceed to use your punch and hammer to get what's left of the windings out. That third picture is what your goal is. You want all the windings out. The windings will look something like the last two pictures, and unless that is copper (possible for older microwaves) toss it in the recycling bin.
And there you have it. It's simple enough, but this way you can save yourself some money and scavenge parts for which you would normally have to pay someone.
Hope it helps!