The circular saw is a tool that is supposed to help you cut wood in a straight line. However, without a guide, this is very difficult. That's why someone invented the table saw.
First, with any tool, make sure your circular saw is sharp. If the blade is dull or chipped on one tooth, it will pull to the left or right. If the set on the blade is not wide enough for the material you are cutting, it will heat up, warp and do unpredictable things. For wet lumber you need a blade with a wide set or the blade will bind.
I remember on one job, The Canadian Syncrude (Oil Sands) Project in Northern Alberta, the company was to supply all the power tools and the carpenters were to supply their own hand tools. My partner, Kelly Johnson, was cutting some form ply with a circular saw for the huge forms we were pre-fabricating and he looked at his saw cut in the middle of the board. The cut was all over the place, off one side of the line and then off the other. He felt the circular saw blade and it was hot. It was a thin, cheap, dull blade. He took it off the circular saw and chucked it as far as he could into the bush, uttering some rude comments to goad it on its way. He turned around and right behind him was the superintendent of the job, glaring at him. Kelly told him we don't have to worry about using that circular saw blade again and if he expects us to do a good job, get us some decent saw blades. The boss walked away. We had some better quality blades within hours.
I've noticed that treated lumber is a tough one to cut with a circular saw. There are actually special blades for this material. I only use carbide blades and router bits now. You can throw those high speed steel circular saw blades and bits into the bush.
So, the first thing to know about how to cut a straight line with a circular saw is to install a sharp blade on it.
Manufacturers of circular saws put a little notch on the sole plate of the saw at the front edge, which lines up with the cut. Do not use this as a guide for the cut. This is a reference point only when you first put the saw on the board to line it up. Place the circular saw on the board with the line lined up with the edge of the blade and the circular saw straight with the line and the notch. Look at the edge of the circular saw blade to see where it is cutting on the line. This way you can immediately see if the cut is starting to wander and correct it. Pull back the circular saw and get the edge of the blade back on the line. Make sure that the circular saw blade is cutting the material out of the discard piece, rather than the piece you want as it shows in the diagram.
There are two circular saw positions that you can cut a board off: the circular saw can be on the piece you want or it can be on the piece you don't want (the discard piece). If the circular saw is on the discard piece, you can see the edge of the circular saw blade easier. If the circular saw is on the piece you want, you have to almost judge the thickness of the circular saw blade because you can't see the line through the blade. If unsure about where the blade is in relation to the line, pull the circular saw back a bit to check the cut on the line; if it's good, carry on; if it's off, pull it back to where it started to go off and correct it.
You will notice when cutting a stringer out that the circular saw is on the discarded part of the stringer for either the riser or the tread, but not for both, depending on which way you have the stringer laid out. So to cut a stringer with a circular saw, you have to be familiar with cutting on both sides of the circular saw blade.
Practice on a piece of scrap to get the feel of where the notch is in relation to which side the circular saw blade is cutting the line. As with any tool, the operator has to get familiar with his circular saw before getting good results with it.
When cutting a stringer with a circular saw, you only cut up to the line of the next tread or riser; don't cut over that line. That means you'll have more to cut on the underside of the stringer that the circular saw didn't cut. You finish that cut with a hand saw.
When finishing off the cut in the corners of the riser and treads with a handsaw, keep the saw 90 degrees with the stringer (perpendicular). This again is where practice makes perfect. If you are cutting too much off the underside of the stringer, adjust the angle of the circular saw until you have it right. The operator has to control the tool, not the other way around.
See my video on How to Lay Out a Stair Stringer on how to use a circular saw and hand saw to cut out the stringer.
With a little practice you can keep that circular saw cut straight!
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