<o =""></o>The answer of course is “When it is not square”! <o =""></o>
With great care and patience, I tuned my new Jet cabinet saw last weekend. Everything was tuned to .002” or less. I ripped a test piece and then crosscut it on my Windhaven miter gauge. I then checked the angle with my square (nothing fancy, wooden handle bought at Home Depot years ago). My cut was off quite a bit – I don’t know to what degree, but certainly more than I was willing to tolerate. Knowing how diligent I was in the tuning of my saw, I decided to check for square against one of those measuring devices sold by Incra – the ones with the very small holes for marking and the littler red fence. I have never considered using this to check for a 90 degree angle, but I did anyway. My cut was perfectly square according to it. I then picked up a speed square and checked with that. Once again, my test cut looked great. I decided I needed a machinist’s square to confirm the mixed results. Neither HD or Lowes have such a thing, so I drove to the local Woodcraft store and checked my test cut with an Incra (guaranteed square) square. Now I felt better, as my saw was obviously well tuned and the problem was the original square I was using was not square at all (I should have just checked it by scribing a line with it and then flipping it over, but I did not think of that – DUH!!) In any event the 2 tools I had in my shop in which I did not have much confidence proved to be more accurate than the square I assumed was dead-on. Lesson learned.
<o =""></o>BTW, I bought the Incra square!!