Checking Circular Saw Accuracy: Time for a Tune Up
Have you spent time checking circular saw accuracy? Did you even know that you should?
Whether guiding against a rafter square or straightedge or just following a line freehand, your saw needs to be in tune for efficient cutting. That means having accurate cutline markers and the shoe set parallel with the blade.
Checking Circular Saw Accuracy at 90 Degrees
Circular saws have two cutline marking notches, one for square cuts and another for 45° bevel cuts. To check their accuracy, start with the square-cut marker. Make sure the blade is set at exactly 90° to the saw shoe (every saw in our shootout has an adjustable set screw for this calibration) and cut into a piece of wood a few inches guiding the saw against a square or straightedge. Stop the blade and back the saw out of the cut while keeping it against the edge guide.
See if the kerf made by the blade lines up with the marker. If it’s off, carefully transfer the actual kerf position and width on the saw’s shoe with a permanent marker or, scratch awl. Draw on white tape if need be to see the lines better. It helps if the device you’re guiding the saw against is clamped to the wood for this marking operation.
If the kerf is in line with one side of the marker but thinner than the notch, mark the missing side below the notch. For truly accurate cutting, you want to be able to cut precisely to either side of your mark consistently.
Checking Circular Saw Accuracy at 45 Degrees
Repeat this process for the bevel cutline marker once you make sure the saw blade is set at 45°. Cutting a flat piece of wood and measuring the resulting angle is often more accurate than measuring the angle between the saw’s shoe and blade. Any waiver in the flatness across the shoe—which unfortunately is not uncommon—can throw off your measurement.
Blade to shoe parallelism is important whenever you rely on guiding the saw along a square or straightedge for accuracy. Like the rudder of a ship, the saw will follow the direction the blade is pointing. If the blade skews away from the edge, the saw will track out into the material, and if the blade skews toward the edge, the blade will flex and bind, stopping all forward progress. Raising the blade so the least amount of blade needed is in the cut can help by essentially increasing the blade’s flexibility. But the more blade that’s in the cut, the more it helps keep the cut straight and square.
When in Doubt, Read the Manual
To fix a saw that is out of alignment, check the manual or your brand’s technical service department for advice. You can verify this misalignment by measuring the distance from the edge of the shoe to the blade at the farthest forward and rearward points.
My trusty old corded saw has adjustments for blade parallelism but I’ve never seen the equivalent on cordless saws. If there are bolts connecting the body of the saw to the shoe, you may have to loosen all of them and re-tightening them with the saw twisted in the desired direction. Failing that, you may have to carefully bend or pry some of the linkage parts that hold the body and shoe together or possibly twist the body and shoe apart. If it’s not an easy fix and the saw is new, take it back. If it’s not returnable, do what it takes to fix it. For the greatest degree of cutting accuracy, you need a saw that will guide straight and true.