A portable circular saw is an incredibly versatile and useful tool, but one drawback is that making long straight cuts on plywood and other sheets can be a little tricky, since it's often hard to keep the blade from wandering from the cutting line. But here's a way to make your circular saw function almost as well as an expensive stationary table saw or radial arm saw—with a simple cutting guide that guides the foot of your circular saw.
How a Cutting Guide Works
This DIY cutting guide works in exactly the same way as many expensive commercial cutting guides. It consists of three parts: a base, over which the foot of your saw will slide as it cuts; a fence, which will guide the side of the saw foot as you cut; and a stop, mounted perpendicular to the bottom side of the base, which allows you to quickly butt the cutting guide against the end of the workpieces before clamping it in place.
To use the cutting guide, place it over the workpiece with the stop butted against the end of the workpiece, and the edge of the base flush with a cutting line you marked on the workpiece. Clamp the base of the cutting guide in place onto the workpiece. Making a straight cut is now simply a matter of keeping the foot of the saw flush against the fence as you push the moving saw blade through the workpiece.
You may be able to make your cutting guide from scrap pieces of plywood or MDF you already have. But with a single sheet of new plywood or MDF, you can make a variety of cutting guides of various lengths, and probably have some leftovers for other projects.
It's best to make your cutting guides from high-quality MDF or solid-core plywood so the saw foot has a perfectly smooth surface to slide against. In most cases, the factory edges of a sheet of plywood or MDF provide these straight edges, but in some cases, you may want to have the sheet recut on a table saw or panel saw to assure edges that are perfectly straight.
Equipment / Tools
- Framing square
- 4 C-clamps or spring clamps
- Plywood or MDF
- Wood glue
- 3/4-inch wood screws
- Wood putty
- Primer and paint
Cut the Pieces
This project as shown builds a saw guide that is 24 inches long and 13 inches wide—a size that works well for cutting workpieces up to about 20 inches long. It consists of three pieces. The base, against which the bottom of the saw will slide, is 24 inches long and 13 inches wide. The fence is 24 inches long and 3 inches wide. The bottom stop, which allows for quick placement of the saw guide against the end of the workpiece, is 13 inches long and 2 inches wide; it will be attached perpendicular to the bottom surface of the base.
Adapt these dimensions, if needed, to make a cutting guide for cutting workpieces of other sizes. For cutting full 8-foot-long sheets, for example, you may want to build a cutting guide that is a full 8 feet long.
Mark cutting lines for these three pieces on a sheet of plywood or MDF. The factory edges of the sheet can usually serve as one of the edges of each piece; use a framing square to mark the other cutting lines. Make a note of which side of each piece has the factory edge; these will be the key edges during assembly.
Use your circular saw or a table saw to cut out the three pieces for the cutting guide.
Position the Pieces
Test-fit the three pieces for the cutting guide. Place the base piece flat on a working surface, with the long factory edge facing right and another factory edge at the bottom. Center the fence piece onto the base with the factory edge facing right; the base should extend by 3 inches on each side. Use a framing square to make sure the fence is positioned exactly perpendicular to the bottom factory edge. Draw a reference line along the edge of the fence to indicate its position.
Test-fit the 13-inch-long stop on the bottom of the cutting guide, using the framing square to ensure proper alignment. Draw a reference line to mark its position on the base.
Attach the Pieces
Remove the fence and stop and spread wood glue on the mating surfaces. Reposition the fence onto the base so it is aligned with the reference line, then clamp it in place. Also glue the stop against the bottom of the base, clamping it in place.
Allow the glue to dry for about 30 minutes, then anchor the fence and the stop to the base with 3/4-inch wood screws. Make sure the heads of the screws are slightly countersunk but make sure the tips of the screws don't poke through.
Clean up any excess glue with a damp cloth before it has a chance to dry.
Trim the Cutting Guide
Once the glue has dried, you still have an additional job to perform before your saw guide is ready to use. "Custom-fit" the guide to your saw by making one careful cut, gliding the shoe of the circular saw along the fence while you cut the base. A portion of the base will cut be cut away, making the cutting guide exactly fit your saw.
Once this is done, the guide will be able to make perfect cuts as long as you use the same saw. If you buy a new circular saw, chances are that you will need to make a new saw guide.
If you replace the blade on your circular saw—fitting it with a very thin panel blade, for example—you may find that the saw guide is no longer perfectly accurate, since it was originally tailored for a thicker saw blade. The discrepancy won't be huge, but it may be enough to cause problems if you are building fine woodworking projects. In this case, you may need to build another saw guide to match the new blade, or make a small adjustment relative to the cutting line each time you clamp the cutting guide in place.
Apply a Finish
Fill the screw holes with wood filler, then sand the surface of the saw guide. Apply primer and a coat or two of paint. This step will protect your saw guide from moisture damage and keep it operating like new for many years.
The same process shown here can be used to create cutting guides for other portable power tools, such as a portable jig saw or router.