If The Blade Sees Its Shadow, It’s Another 64th Of Accuracy

If you’ve bought a miter saw in the past few years, you might have noticed the LED “laser” that came with it. The goal was to show where on the piece the saw was going to cut. But over a year or two, you might have found the laser to have drifted or skewed into a crooked line. [Fisher] decided that his after-market laser wasn’t entirely accurate enough and added a shadow line instead. (Video, embedded below the break.)

The blade has a thickness (known as kerf), and with a laser to one side, you can only accurately cut on one side of the line. A shadow line works differently. By shining a line at the top of the blade, you get a mark where the blade will cut precisely. You can also see your marks as the laser doesn’t shine over them. Previously, [Fisher] had tried to use LED strips, but after a comment suggested it, he found a sewing light on a gooseneck. It worked great as a small compact light fitting the blade housing. After some quick modifications, hot glue, and duct tape, the light was installed, and the wires were routed while still allowing the saw its full range of motion.

The result is impressive, with a clear shadow on even darker hardwoods. Just the few test cuts he made seemed entirely accurate. Of course, you can always go deeper down the hole of accuracy and measurement. But overall, [Fisher] has a great little mod that speeds up his workflow more accurately.

43 thoughts on “If The Blade Sees Its Shadow, It’s Another 64th Of Accuracy

  1. I was hoping this involved Fresnel lenses or something. Correct me if I’m wrong, a Fresnel lens on the light would make the shadow appear sharp much earlier, and give better contrast too, right?

    1. It’s certainly possible to stimulate an infinitely far away light source with a point source and a lens, but it requires specific spacing and the right kind of lens, and in general would be more complicated and more expensive and more bulky. So I think it made sense to try this way first, and the results are probably “good enough.”

    2. It’s a really quick modification if you watch the video…
      On another note:

      I highly doubt that lamp will stay secured to its place because hot glue sets much too quickly when used on aluminium because of the quick heat transfer.

      Guess vibration will make it fall into the blade sooner or later…

      1. I’m sure the “alabama chrome” will be right behind the hot glue. But as a proof of concept, it’s perfect. I think two small screw holes through the metal housing into the light’s plasic shell would work ok. Similar for clips to hold the wires.

      2. I’ve had good success with glueing aluminimium with hot snot, but you have to pre-heat the aluminimiminium or you have to use enough hot snot to heat up the aluminiminiimiuum with that.
        And of course you also have to clean it. If the hot snot only adheres t the sawdust inside the guard it won’t work very well.

        I’ve glued a spoon / knife rack to ceramic tiles with this (in a rental apartment, so I could not drill holes in the tiles) and it’s holding up for years now.

  2. Nice hack! But FYI those stick on cable guides are trash. If they have ANY shear load on them the will fall off within a month or 2. Serious, 1 ethernet cable is enough to pull them off.

      1. Maybe there’s some brand that’s good, but I’ve never seen it. I was an industrial engineer when I was younger, and I saw these used many times. All different brands and types, every installation fell apart at some point.

  3. reminded again of the usa being pretty much the only country in the world with inches and those weird fractions..
    And accuracy with a drop saw is just practice, and occasionally end stops on the bench.. :-)

      1. “numbers to the right of the binary point” – Touché – Nice one, I never thought of it like that!

        In all honesty, I always though the USA rejected the imperial system on 16 December, 1773, in Boston, when they threw some tea into the harbour. Seems not everyone did though.

        1. There were multiple attempts to bring the metric system to the colonies, unfortunately they were defeated by pirates, storms and short-sighted politicians. One of the first attempts at bringing the kilogram to the USA was struck by a storm. The ship ended up off Montserrat where it was attacked by pirates who threw the kilogram over the side as junk.

  4. I like this idea a lot. Wondering – if you look at the light (time 6:21), would that tend to get somewhat coated in fine sawdust? Especially for sappy or moist wood? I’d think the crisp shadow line would get diffuse over time. Maybe frequent swiping with a soft brush would keep sawdust at bay…

  5. Y’all are aware that most newer (in the last 10+ years afaik) miter saws are sold with an led shadow line guide, and dewalt sells one as an add-on to any of their saws made in the last 18+ years.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.