Square your squares
This may be well known to "old timers" but might prove useful to newbies. I have an aluminum speed square that wasn't square, causing me some consternation (and bad language). I finally figured out that I could verify its accuracy...or lack thereof...with a set of plastic drafting squares that have been in my desk drawer for years. I did the old two-line test on the 45-45 drafting square and determined that it was, in fact, accurate. Then I laid it on the speed square...and verified that it was not.
How to solve this problem? After some thought it dawned on me that the cast iron table of my table saw is perfectly square so I lined up and clamped the speed square on the corner and (gently) filed away aluminum until I was satisfied (it didn't even begin to cut the cast iron). Now the two-line test is perfect...or as nearly as I can make it.
Another out-of-square square was my cheap combination square. A Starrett it ain't. The angle was determined to be a degree or two less than a perfect right angle. As you all know the blade of a combination square rides in a slot in the handle and if that slot is not perfectly square to the handle it ain't accurate. Since mine was less than a right angle, I wrapped a small piece of 120 grit sandpaper around the blade of my 6" steel ruler and gently sanded away at the rear part of the slot until it was a perfect right angle. It took many tries but I got it so the two-line test is as perfect as I can make it.
I can't emphasize enough that if you're going to try this, work a short time and then re-test. I don't know if a square would be repairable if you went past your objective.
Last edited by 163481; 04-15-2017 at 04:54 PM.