While there are many kinds and sizes of circular saws on the market, I would like to talk about what I believe to be the three most important divisions. They are: plunge saws with guide rails, worm drive or hypoid saws for construction use and standard-drive circular saws suitable for both home and construction use. Before I dig into all of that, however, I'd like to take a quick look at the fundamentals concerning these saws, in general..
A circular saw allows you to take a relatively small tool to a large work piece and cut it without too much back-breaking work. In the past, the price for this convenience was inaccuracy because there was no really effortless way to force a circular saw to cut straight along a pencil line. For rough construction work like roofing and decking, this was not problematic. For fine woodworking, however, the circular saw was not the tool of choice. Most woodworkers rely on the table saw to get the long, straight cuts they need and for good reason. The fence on a table saw gives the constant reference point needed for linear cuts.
Sometimes, however, using a table saw to trim the top of a huge conference table, for instance, turns out to be impracticable, especially when trying to trim off the ends at 90 degrees to the sides. That's when a very focused approach using a circular saw seems to deliver the best final result. I would draw a pencil line using a long straightedge exactly where I wanted the trim cut to go. I would then carefully measure the distance between the inside (or outside) of the saw blade and the adjacent edge of the foot plate of the saw. The next step would be a second pencil line, parallel to the first one and separated from it by the distance I measured between the inside (or outside) of the saw blade and the adjacent edge of the foot plate. I would select a perfectly straight board (ripped straight on the table saw, if necessary) and clamp this across the table top as a guide along the second pencil line. Then, I could make a pretty straight cut along the first pencil line. I would then do this for the other end of the table top.
In the past few years, this process has become a whole lot easier. There are now several brands and designs of plunge saws that run along metal guide rails, cutting right next to the edge of the rail without cutting into the rail itself. These guide rails don't even need to be fastened down to the surface being cut because they have material underneath that keeps them from sliding around. If you feel more comfortable clamping down the guide rail, this can be done, as well. It's a simple matter to lay the guide with its edge along the cut line and then to take the saw and run it down the rail, cutting right next to the lip of the rail.
Because these saws are plunge-type saws, you can commence and/or stop a cut in the middle of a sheet of plywood. You could cut out a window or door opening, for instance and have it come out clean and square every time. The best thing about using these track saws is the confidence they give you: You KNOW you can do a perfect job, quickly, accurately, over and over again.
Another kind of circular saw I've used a lot over the years, mostly for construction, is the worm drive saw sold by Skil. A framing carpenter needs to be able to cut a lot of lumber all day long. While accuracy is always desirable, it is not as critical to the framer as it would be to a finish carpenter or cabinet maker. Speed is the thing that the framer wants on his side and he (or she) does not want to be saddled with a saw that's not up to the job. He doesn't want his circular saw to stop in the middle of cutting a 2 x 10 joist or have the sole plate hang up every time it goes across the edge of another board. He does not want to have the saw blade sliding around the saw arbor. What he wants is clean, fast accurate cuts: In short, he wants power.
A worm drive saw delivers the constant drive he needs because there is no slack or play anywhere between the powerful motor and the saw teeth that are doing the cutting. The worm gear cuts down a bit on saw blade RPM but trades this off for torque. It is torque, more than speed, that powers a saw blade through thick, wet boards. A diamond-shaped arbor makes it virtually impossible for the blade with a matching diamond-shaped hole to rotate around the arbor. Keeping the number of saw teeth down to 18-24 teeth on a 7 1/4' saw blade also helps. The only problem with early Skil worm drive saws was the weight of the tool with a large motor and all that gearing. The solution was to use magnesium instead steel wherever possible in the construction of the saw. Modern magnesium worm drive saws weigh only about 14 to 15 pounds which is more than a standard circular saw but controllable in the strong hands of a muscular framer.
The final category includes the kind of saw that most people think of as a 'circular saw.' It is lighter than the worm drive saws and, for most uses, it's plenty powerful enough. Some of these saws are now also be made out of magnesium parts and weigh just over 10 Lbs. This makes them easy to use by the average do-it-yourselfer. Did I mention that they are a lot cheaper than the other types?