A try-square is not always at hand when it is desired to saw a stick, and when it is handy some mechanics prefer to work by “guess” than otherwise. When a bright, straight saw is placed upon a stick or on the edge of a board, the reflection of the stick or board in the saw
is sufficiently well defined to permit of placing the saw so that the reflected image coincides with the object reflected, forming a continuous straight line. If the sawing is done while the image and the stick are in line, the stick will be cut at right angles.
It is obvious that a line may be drawn at right angles to the stick by arranging the saw as shown in Fig 2. If, after forming this line, the saw be placed across the stick so that the line and its reflected image and the stick and its reflected image form a square, with the
reflected image and the stick lying in the same plane, as shown in Fig. 3, the stick may be sawed at an angle of forty-five degrees, provided the saw is held in the same position relative to the stick.
Scientific American – July 26, 1890
– Jeff Burks
Another very good reason to keep the plate bright and shiny.
Looks like there are too many fingers in the grip! Love the 45 degree trick
My current saw has got a coated matt black blade.. That makes this trick kind of difficult.
But the saw does have a 90 and a 45 degree angle in the handle relative to the back of the saw. So there is no guessing.
Does anyone have tips on polishing up a saw plate that has been stained from rust? I have several electrolytically de-rusted Disstons but they are not shiny enough to pull this off. I’d like to clean them but not remove the Disston engraving.
There was a thread recently on sawmillcreek about using Autosol metal polish and a ball of aluminum foil. I haven’t tried it yet, but the poster’s pics looked good.
Never tried the foil with it but I can vouch for the Autosol polish, does a pretty good job with just a cotton cloth and a little goes a long way.
I will try to find Autosol. I assume that it’s found in an Auto parts store?
A Wrinkle in Sawing…
Maybe if I hold four shiny saws at the correct angles it will create a tesseract wormhole?
I wonder when a woodworking tip last appeared in Scientific American?
I’ve seen this tip several times. It sounds great, but the problem is that it doesn’t actually work! It assumes that the saw is held perfectly vertical. If the saw is off by a small amount, then lining up the reflection will lead you to make a cut that is not square (and out-of-vertical as well).
So it assumes you can somehow eyeball perfectly vertical, but can’t do the same thing to judge perfectly square. Not a very useful tip, IMHO…
I use the reflection all the time. I can see if I am square and plumb. If I’m out of plumb then the reflection shows up bent in the Z axis. If I’m out of square, it is apparent in the XY axis.
When cutting joinery (tenon shoulders, removing the half-pin on the tailboard), this is a solid gold confirmation that I’m plumb.
malletstorwardsnone is partially correct: There exist pairs of non-square/non-plumb angles that cause the reflection to line up when viewed from a particular viewing angle, but only from that viewing angle. So you do have to move your head around to verify that you aren’t seeing one of these “false positives.”
Ah, yes, thanks steveschafer. There is an added detail. For an arbitrary out-of-square angle, it is possible to tilt the saw until the reflection appears to give a straight line (at least a 2D straight line– I don’t think the reflection image contains enough information to say that it’s definitely not straight in 3D, unless maybe you can see some reflected geometry of other things in the room?). But this false positive isn’t invariant to head movement. So as you move your head around, the line will go out of true. But when you have true square and plumb, the image is invariant to head movement.
I’m glad that I was at least partially correct! Perhaps I can work out the mathematics & geometry; it might be about time to re-visit the topic in Scientific American.