I posted part of this in reply to another thread, and Mick suggested I post it here so I have modified it a little and added some more reading material to it.
For those of us who don't have a plasma cutter or oxy-acetylene or propane torch set, we resort to power tools to help with demolition and destruction projects. Here's some pointers to make your work safer, easier and more economical:
Here's a tip to save blades when cutting thick metal with the Sawzall-- Don't use full speed, it will overheat the blade and burn it, basically softens the teeth until the melt down into a smooth blade which is useless, needs to have sharp teeth. Go slower on the tool speed. Also what i find to help is to cut with one portion of the blade, then pull the tool back and let a different part of the blade do the cutting for a moment, then switch back, this keeps the heat spread out a little to cut back on the first problem I mentioned.
I've been using Sawzall's for years to cut all kinds of metal for fabrication, couple years ago I added a Chop-Saw to my fleet, that was a great addition but nearly useless for scrapping.
A grinder with a cut off wheel is a great tool to have, but the cut off wheels don't last long on thicker/heavier metal, but they definitely earn their place in my toolbox. I use 4.5", I find to be a good all around size, and they are offered in 4" and the larger 6 or 7" sizes which I often find too large, however they are sometimes nice to have. You might pickup a cheapie grinder for $20 somewhere (often less at Harbor Freight), name brand ones run $50+ (I have two DeWalt $50 ones that have lasted me for many years now), but the cheap ones are well worth the money and generally are not too disappointing given the cost. Given the rotary action, they are handy for cutting things the sawzall just wants to push and pull if it grabs too hard.
Also, we have successfully cut sheet metal siding with a blade for the circular saw (AKA SKIL SAW) that is a metal blade with slots cut from the outside of the circle toward the center, odd looking blade but works great for metal siding, if you needed to cut through the thin sheets of siding or thin sheet metal, that works great, blade seems to last a long time.
A jigsaw equipped with a metal blade will work in a pinch but very slow going and the blades are too weak for anything very much. If you already own one of these and do not own any of the above listed tools, it might get you by in a pinch but I recommend to find a grinder or sawzall ASAP. Jigsaws were made for woodworking and the metal blade should never have been invented for them. If you're fabricating with thin sheet metal and need to cut pieces with curves and such, you might find a metal blade useful but buy a full pack of them, they break easily. Not at all useful for demolition of any sort.
For Sawzall blades- stay away from DeWalt blades they are somewhat disappointing. I find Lenox brand to be my absolute favorite, and Milwaulkee blades work real well too. Lenox blades aren't cheap but they will last a little longer. In general, you get what you pay for with Sawzall blades. Cheaper blades are fine for wood but the metal takes a toll on them. There are two kinds of blades available in many lengths- low and high profile. The low profile ones are great in tight places and the high profile ones will hold up a lot better if you have room to work with the larger size. They last longer for a couple reasons- one, the higher blade gives the heat more room to spread out and lets the blade run cooler (makes teeth last longer) and two- the larger blade is stronger and doesn't bend NEAR as easy when it gets pinched. However, all of these blades are reduced to the same size where they connect into the blade holder on the saw, so this will always be a weak point under extreme conditions.
If you're cutting wood, use a razor sharp large tooth blade, these will cut through anything wooden. If you're cutting metal there are a few different "tooth count" blades, this is measured in "TPI" - Teeth Per Inch. For all around use, I find 14 TPI to be great. I rarely use anything else and I use the Sawzall to cut anything from 3/8" to sheet metal. There is a third kind of blade, called a Demolition blade. These are meant to quickly cut through general construction material, wood, pipes, nails, etc. They are normally more expensive than metal blades but they are meant to take a lot of abuse. They are offered with carbide tipped blades and such. I rarely use these.
Sawzall blades are not real cheap but can be a valuable asset, follow the first paragraph and you'll find they last a lot longer than just ripping wide open full blast at something, especially in anything thicker than 1/8" or so. Same thing goes for cut off wheels on grinders- slow down and take your time not forcing it and the disc will last longer. If you force it these things have a tendecy to grab and can take you for a ride. Use the safety guard and T-handle on the grinder when possible and always wear safety glasses please, especially with the grinder. They fling stuff everywhere and everytime I try to save a few moments finding the glasses I regret it and get a smoldering hot chunk of metal in my eye!!!
Good luck and happy scrapping!