If you’ve been around woodworking hand tools for a while, you’ve likely heard of the hand saw hack that lets you saw perfectly square across a board without the aid of a pencil line to guide your cut. Wait! You haven’t seen this before? Here, hold my beer.
The concept is pretty simple as it is based upon simple geometry. If you can understand the principle of complementary angles, then you will get this easy peasy. The only requirement is a hand saw with a blade that is polished enough that you can see the reflection of the board you are sawing.
The gist of the trick is that you look at the reflection of the board in the saw and if the reflection of the board makes a straight line through the saw, then your saw is at exactly 90 degrees, square and plumb, to the edge of the board. However, if the reflection of the board angles at all, then your saw is not square to the edge of the board.
I’ve tried the trick, and I’ll admit it works OK, in a pinch, when you’re at the lumber yard and you need to crosscut a board to fit it into your hatchback and you forgot to bring a square and pencil with you. But it’s not something practical I would regularly rely on, for several reasons.
First, in order for this trick to work, the blade of your saw needs to be polished enough that you can actually see a reflection in it. While this may be no big deal if all you own are newly made saws, if you use vintage saws at all, chances are the patina on the blade will prevent it from showing much of a reflection, if any at all. That kind of kills the hack altogether, and, as far as I’m concerned, if your saw doesn’t come polished enough for it to reflect, it’s not worth polishing it just for this trick.
Second, this trick (I’m hesitant to even refer to it as a technique) only works for boards that are about as wide as your saw blade. In wide boards or panels, you lose the reflection of the edge of the board soon after the top of the blade gets beyond the edge of the board. So you can judge square for about the first 5 or 6 inches of the cut, but after that, you’re sawing by blind faith. This might be OK for a really rough crosscut, but for any cut that requires even a small amount of precision, I wouldn’t rely on it.
Third, it really only works for crosscuts. When you rip a board, the reflection is of the short end of the board, not the long edge. This means that you have a very short reference. Also, rips are usually long, so you quickly lose the reflection of the end of the board as the saw blade progresses through the cut. I hope you left lots of extra width.
Finally, there is a little catch to this trick that could screw you up even though you think you are sawing square. From the sawyer’s point of view, you don’t see a very long reflection – nothing like looking from the side of the blade as in these photos. Because of where you stand when you are sawing, if you happen to hold the saw at just the right combination of out-of-square and out-of-plumb, the reflection will appear to be a straight line from your point of view, even though the cut is neither square nor plumb. This doesn’t just happen at one combination of angles either. It can happen at an infinite combination of angles, it’s just that the out-of-square and out-of-plumb have to complement each other just right.
So really, I don’t see this parlor trick as practical in any way. In reality, it takes about 2.36 seconds to use a square and pencil to give yourself a reliable cut line to follow. Using the reflection in the saw blade is not going to save you any time. Further, if you need to make a really precise cut, like the shoulder of a tenon, are you really going to trust a hack like this to provide the accuracy that you need? I think not.
So while this might be a great little trick to pull out at parties, I think I’ll keep my square and pencil handy in my shop.