Plywood can be tricky to cut accurately due to its size. This technique can be used to cut panels to size, within 1/16th of an inch without specialty equipment.

Equipment

• Circular saw
• A long straight edge - I use a 4’ aluminum level
• 2 small clamps, such as 12” trigger clamps
• Speed square or combination square
• Pencil
• Sacrificial board or similar way to elevate (2-3” should be enough) the plywood panel directly below the cut line

Technique Overview

This technique executes an accurate straight cut by guiding a circular saw along a precisely positioned straight edge. It accounts for the dimensions of the saw & and its cutting blade.

Tutorial

1. Adjust the circular saw’s blade depth to slightly deeper than the plywood’s thickness.
2. Plan the cut: As a righty, I like to control the saw with my right hand and travel right-to-left with the saw’s left edge against the straight edge. The workpiece for the cut is on the near side of the straight edge (closer to me).
3. Position the plywood over the sacrificial board, so that the cut line is directly over the board for as much of the cut line as possible.
   1. The sacrificial board should support both the workpiece side and remainder of the panel, so that neither falls to the ground once cut. This step also serves to elevate the plywood panel slightly, so that it’s possible to get a clamp tight under it.
   2. Depending on the size of the panel, this step can be executed at waist-height (e.g. with several saw horses or a big enough work table) or on the ground. For making wide cuts on full-size panels, I prefer the ground, so that I can walk alongside the cut without having to stretch over the workpiece. This allows me to have firmer control of the saw throughout the cut & is therefore safer. I also find working close to the ground tends to reduce the risk of large panels sagging or bending slightly, which can create space between the panel & straight edge, making it more difficult to guide the saw.
4. Plan the measurements: This step requires measuring from an accurately rectangular edge (i.e. a “factory edge”) and marking the position to clamp the straight edge, allowing for the distance between the edge of the saw’s baseplate and the blade and the width of the blade itself (known as a “kerf”). In this setup, the straight edge glides along the shorter side of the base plate (one and one-half inches).
   1. If starting from a previously cut panel that is not accurately rectangular, use the straight edge and speed square to align perpendicular to one side, close to the imperfect edge, but with enough space to guide the saw. Clamp that side down, then repeat on the other side to create a perpendicular guide on both sides. The exact measurements on this cut don’t matter, since the imperfect edge will become scrap.
   2. Assuming a workpiece on the inside of the straight edge, the measurements in these steps should be taken from the inside edge to, spanning the width of the workpiece, blade kerf, and distance between the blade and edge of the saw’s baseplate that glides along the straight edge.
5. Mark the measurements: this example walks through cutting off a 24” workpiece panel from a factory edge.
   1. Starting from the inside factory edge on the right side of the cut, measure the workpiece width: 24”
   2. Add the blade kerf width: In my case, 1/8”. I usually make a mark here and extend it slightly using a speed square for the next step.
   3. Add the width between the edge of the baseplate & blade: In my case, 1+1/2”. This happens to be the same as the 1+1/2” standard carpentry mark line on my speed square, so I use the square to line up the mark line with the previous mark, then mark a slightly longer line.
   4. Repeat these steps for the far (left) side of the cut.
   5. In this example, the final marked lines should end up at 25+5/8” inches from the inside edge on both sides. Double check the measurements.
6. Align the straight edge with the longer mark line on one side and clamp it down. Do the same on the other side. Double check squareness with the speed square on both sides. Consider if the clamp position will interfere with the saw’s path during the cut. If so, use a piece of scrap of the same width as the workpiece to extend the clampable space.
7. The straight edge is positioned to allow the saw’s baseplate to glide alongside the length of the cut while the blade lands just outside the far edge of the workpiece.
   1. Plan how you will control the saw for the full width of the cut. I like to walk alongside or over the cut holding the saw just above it to make sure I can reach and comfortably control the saw the whole way through before starting the cut.
   2. Make the cut!

I’ve found this technique to be accurate enough for finish quality work. When cutting small panels (2’ or smaller rectangles), I typically start with this, cut the panels slightly larger than I need them, then finish the cuts to exact dimensions on a table saw.