How to cut exact and identical rectangles with a circular saw?

Question

Following this question What tools do I need to make a straight cut in laminated chipboard? , I bought a circular saw.

So far, I used it to make long straight cuts which didn't require high precision for the absolute positioning and it's working great.

The lengths involved in this question are not greater than one meter for the rectangle I would like to cut.

I would like to use the circular saw to make two identical rectangles from a larger piece of wood. The dimensions of the rectangles do not need to be precise, they should just be equal. My first attempts have not been successful, and I had these problems (formulated as questions for a better fit here)

• How to improve absolute positioning of the blade ?
• How to duplicate a rectangle ?
• How to ensure that a piece of wood is rectangular ?

Is there a way to do this without a table saw ?

I have a few ideas of jigs in mind, but looking at a table saw, everything I have thought of would be easily done with a table saw, and everything I could build would probably not be precise enough to be useful.

Even by cutting the pieces that need to have the same dimensions together, in the end there is always a little difference between them, because the cut is not following a perfect line, the edge is not perfectly linear, my guide is not perfectly aligned, and so on..

I also think having them cut at the shop would solve all these problems.

Answer 1

Try this little trick.

1. Cut two rectangles from your plywood an inch or two larger than what you want your finished pieces to be. Be sure one face of these rectangles are a straight factory edge.

2. Now stack the two pieces together and clamp them with the factory edges one on top of the other, nice and flush.

3. Scribe a perfectly square line using a builders square perpendicular to the factory edge on both sides separated by the dimension you want the piece to be.

4. Now take a perfectly straight piece of wood and clamp it onto the surface exactly at the dimension of the distance of the edge of your blade to the edge of the saw frame. You now have created a guide to run your saw against while making a cut through both pieces of plywood at the same time.

5. Repeat this procedure on the other side.

6. For the last cut parallel to the factory edge, measure precisely the distance and mark on either side, draw a line between the marks. This line should be square to the end cuts and parallel to the factory edge.

7. Now reattach the cutting guide at the blade to frame dimension as you did for the edges and cut both pieces at once again.

You have now made three cuts, two at a 90 degree to the factory edge and one parallel to the factory edge. Since both pieces were cut at the same time, they should be identical.

Answer 2

The usual answer for repeatability is "make a jig". Build a frame out of 2x4s, attach it to the sheet that you want to cut, and then set the base of the circular saw inside the frame. That will give you a very repeatable hole.

Answer 3

You circular saw almost surely has a flat base extending to the side (usually to the left) which can be moved along a guide bar for straight cuts.

Here's how you do a straight cut: you position a guide bar (big thick ruler, a straight board, or a piece of rolled metal used in place of a ruler) appropriately and fix it to the material being cut with clamps so that it doesn't change its place while the cut is being done. Then you just move your saw along the guide. You will have to practice on some scrap material, but this is all relatively easy to do.