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Circular saw blades can be expensive. If you’d like to extend their life and get your money’s worth out of them, a few minutes spent sharpening them is well worth your time. To sharpen them by hand you’ll need a table clamp and a file. For most blades, you’ll need a diamond file to get the best results. It will also be much faster with a diamond file than any other kind.
Remove the blade from the saw. Never try to sharpen the blade of a circular saw while it’s still in the saw. There’s no safe way to steady the blade and eventually, you’ll wind up cutting yourself when the blade gets away from you. Some newer circular saws have a blade release switch, which makes it remarkably easy. Older saws will require you to loosen the arbor nut with a wrench.
Clamp the blade in a vertical position with the teeth facing toward you. Be careful not to over-tighten the clamp too much or you’ll bend the blade.
Mark the tooth you’re starting on with a piece of tape or a felt tip pen or marker. This way you know when you’ve come full circle. If all the teeth are angled in the same direction with respect to the blade you’ll be able to sharpen it in one pass. If half of them are angled to one side of the blade and the others are angled to the other side, you’ll have to make two passes, one for each set of teeth.
On each tooth, hold the file flush against the tooth and make four smooth strokes back and forth along the plane of the tooth. Stop and inspect the tooth. If you now have a sharp edge, proceed to the next tooth. If not, give it another four strokes. Continue until you’re satisfied you’ve completely sharpened that tooth. Remember the total number of strokes you used.
Proceed to the next tooth, or if the teeth are beveled, half to one side and half to the other, you’ll skip the next tooth and proceed to the third one. Sharpen that tooth using the same number of strokes as you did with the first one. Continue in this manner all the way around the blade, skipping every other tooth until you get back to your starting point.
You’ll need to release the blade after two or three teeth have been sharpened. Rotate the blade in the clamp until dull teeth are facing up, tighten the clamp, and continue sharpening. You’ll probably have to do this three or four times until you’ve sharpened all the teeth that angled, or beveled, in the same direction.
Once you’ve finished sharpening one set of teeth, remove the blade from the clamp, and turn it around. Re-clamp the blade and mark your beginning spot again. Repeat the process you did above until all the teeth on the second set have been sharpened. Remove the blade from the clamp and put it back in the saw.