Since this circular saw cutting jig gets placed up against the cut line, it makes it much faster than other cutting guides that require you to offset the guide some distance from the line. The design also helps prevent splintering and tear out of the thin plywood veneer. Also see my new post for tips on cutting plywood square with a circular saw.
What You'll Need
- 4' x 8' sheet of 1/4" tempered hardboard, melamine or plywood (3/8" or 1/2" will also work)
- Appropriate screws or staples depending on material
- 1 can of Rust Oleum AS2102 Anti-slip Spray Paint or similar
- Circular Saw
- Tape Measure
- 4' x 8' sheet of 1" to 2" rigid foam insulation
Measure the distance between the edge of the circular saw base and the circular saw blade on the bottom of your circular saw as shown.
You'll be measuring the longer of the two distances. It should be the side opposite where the arbor nut is. (Under the motor.)
Write down this measurement as you'll need it later.
Lay the rigid foam insulation on the floor or on a large worktable. The insulation will serve as your cutting surface so you don't have to rig up a bunch of sacrificial boards.
Mark the factory edge of one of the 8' lengths of the hardboard with some arrows and mark a cut line 3" from that edge.
Set the depth of cut on the circular saw to just over 1/4" (the thickness of the panel) and cut along the line. If you don't have a way of making a perfectly straight cut don't worry, we'll only be using the factory edge.
Take the circular saw fence to blade measurement you obtained in Step 1 and add the width of the previous cut (3") plus another 4-5" and mark a straight line that distance from your last cut on the hardboard panel and cut out that section. Again it doesn't have to be perfectly straight.
Align the first piece you cut on top of the second piece. Make sure the factory edge (marked by arrows) is facing the right. On the right side of the board make sure there is at least the width of the fence measurement from Step 1 plus 1". You can use a large t-square like a drywall square to help keep the pieces aligned.
Screw or staple the top guide to the base. You can glue it in place but using screws or staples will allow you to reposition the guide if the edge starts getting torn up. Make sure the tips of the screws or staples don't poke through the bottom piece or it could scratch the wood you're trying to cut in the future.
Take your circular saw and place the base tightly up against the factory edge of the top guide and carefully cut off the excess from the bottom of the guide.
You now have a circular saw guide that will allow you to rip a full length sheet of plywood. Your cut line will be where you place the edge of the guide. This makes it easy to make accurate and straight rip cuts in plywood with a circular saw.
On the back side of the guide that will be in contact with the plywood you're cutting, spray an anti-slip spray such as Rust Oleum AS2102 Anti-slip Spray Paint to help prevent the cutting guide from sliding around.
You can now use your new guide to make a shorter guide for cross cuts. Make it longer than 48" so you can add a guide to the bottom that will help keep the guide perpendicular to the plywood as shown. Just make sure the length of the board past the bottom guide is at least 48"
Using Your Circular Saw Guide
Using your DIY Circular saw guide is easy. You no longer need to know how far the blade cuts from the face like you do when you're cutting with other straight edges.
Let's assume we want to rip a 12" wide board out of a sheet of plywood. Simply place the plywood on top of the rigid insulation and mark a line 12" from one of the edges of the plywood.
Now place your plywood cutting jig on top of the plywood. The jig should be on the keep side of the line and the edge of the guide should be right on the line as shown. Clamp down the guide at each end to prevent it from slipping. Make sure the clamps won't get in the way of the saw.
Set your circular saw to cut just over the thickness of the plywood you're cutting plus another 1/4" (or the thickness of your guide) and carefully cut using your circular saw with the base pressed up against the guide.
Preventing Tear Out
Keeping the good side of the plywood face down on the rigid insulation, and keeping the depth of cut just below the plywood will help minimize tear out on the bottom of the plywood.
Since the guide is right up against the cut line, it will help prevent tearout on the "keep" side of the cut. To minimize tearout on the waste side you can first use a utility knife to score along the cut line. Placing blue painters tape along the line also helps.
The most important component to reducing tearout though is a good circular saw blade. The best circular saw blade to use for plywood is the Freud LU79R007 Perma-Shield Coated Ultimate Plywood and Melamine Saw Blade, 5/8-Inch Arbor 7-1/4-Inch. I've also had good success with the Freud LU79R007 Perma-Shield Coated Ultimate Plywood and Melamine Saw Blade. Both leave a near perfect edge on the plywood even without a guide, scoring or taping.