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Thread: 3-4-5 Square Clarification when squaring boards, items, plywood, etc...

  1. #16
    Join Date
    Mar 2003
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    SE PA - Central Bucks County
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    David, assuming your sled's fence is perfectly perpendicular to the blade, just be sure to use the SAME reference edge for both 90º cuts relative to that edge. It does require you to flip the material to do this but then you will have three sides with the correct relationship and can then complete the last cut using either of the two previous cuts to get a "perfect" rectangle. I do this a lot on my slider to square up various random off-cuts so that I can use them. Obviously, this gets more challenging as the size of the workpiece increases, however...
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  2. #17
    Join Date
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    The first step toward getting square cuts is to align your table saw. Contrary to popular belief, saws do not always come properly adjusted from the factory. My son purchased a Rigid 4512 last year and the first thing we did after getting it assembled was to check the alignments. We used a dial indicator attached to the miter gauge to true up the blade to the miter slots. We started at about .007" out of alignment and ended up at .001", not perfect but better. Then we adjusted his fence alignment using the same set up. On his saw the fence was surprisingly close, but we got it even closer. The miter gauge on that saw is not a precision piece of metal. We used a high quality drafting triangle to get it square, but any time he changes it it has to be reset. I prefer the Kreg miter gauge, which I use on my saw.

    Since you have built a cross cut sled, have you taken the steps needed to make it square to the blade using the five cut method? If not, you should.

    One more point. Many squares you buy are not "square" Lower cost combination squares are notorious for being out of whack as are most carpenters squares. Thee are methods for checking and adjusting both.
    Last edited by Lee Schierer; 12-23-2018 at 3:24 PM.
    Lee Schierer
    USNA '71
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  3. #18
    Join Date
    Apr 2017
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    Michigan
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    A panel that is too big to fit on your power saw is a good place for the 3,4,5 method.

    Make one long side straight. Lay out a 3,4,5 triangle on it and you will have one 90 degree angle. Layout another 3,4,5 triangle on the opposite end of the straight side. You now have two parallel sides and a pair of 90 degree angles between them. Measure the desired distance along the two parallel sides and connect these endpoints. Trim to the lines and you have a rectangle.

  4. #19
    Join Date
    Apr 2008
    Location
    East Virginia
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    830
    Or just measure diagonally, corner-to-corner, one way and then the other. Diagonal lengths should match.

  5. #20
    Quote Originally Posted by Jacob Reverb View Post
    Or just measure diagonally, corner-to-corner, one way and then the other. Diagonal lengths should match.
    This doesn't necessarily prove square, it only proves out of square.
    Attached Images Attached Images

  6. #21
    Quote Originally Posted by johnny means View Post
    This doesn't necessarily prove square, it only proves out of square.
    I think that if you make sure the lengths of the parallel sides are exactly equal, the diagonal measurement will prove that something is square. If the parallel sides are not of equal length, you can have equal diagonal measurements and the panel will not be square.

    So in your picture, if the top line and the bottom line are exactly equal length and the two lines on the sides are exactly equal length, equal diagonal measurements will prove square.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

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