laying out a square / triangle

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I'd like to know what's the procedure to makea large eg. 600mm long side) square/triangle ? I'm concerned about making it as square as possible.I'd like to use the square in woodworking, so amconsidering using stainless or aluminium.
Reply:Use the 3 4 5 method to make it square, also for rt angle triangle. Not knowing what you know, do you know the 3 4 5 formula?
Reply:vjekoHere are some pictographs of the Pythagorean Theorem: aka 3-4-5 . . . For longer runs - any multiple of 3-4-5 works.http://www.google.com/search?q=pytha...HZ42Dq0Q_AUIBQOpus
Reply:OK, thanks, pythagoras theorem is clear, maybe my question was not clear and my concerns are unfounded (I have concerns because my smaller boughtsquares are all out). I was mainly concerned about accurate measurement andchecking before welding - do you just measure as accurately as possible / cutpieces and use a line perpendicular to an edge as check before welding ?For the line perpendicular to the edge, I was thinking of using a compass (make one)and bisecting two tangents.
Reply:Get it "square" with a square and tack it up. Clamp it tight.Then measure across, corner to corner. Most problems come from not having the iron the EXACT same length. 8 feet and 8  feet+ 1/64 are miles apart when trying to make a window frame or something
Reply:Pull an inch.... or a foot. Never trust the hook on a tape.
Reply:

Originally Posted by vjeko

OK, thanks, pythagoras theorem is clear, maybe my question was not clear and my concerns are unfounded (I have concerns because my smaller boughtsquares are all out). I was mainly concerned about accurate measurement andchecking before welding - do you just measure as accurately as possible / cutpieces and use a line perpendicular to an edge as check before welding ?For the line perpendicular to the edge, I was thinking of using a compass (make one)and bisecting two tangents.
Reply:Without a clamping table/fixture, it will be out of square when you are done welding it anyway.  Sometimes you are better off preparing for this by adding a little compensation, or devising methods for re-squaring.I've used the 345 on vary large layouts, like laying tile in multiple rooms with no square walls where the layout needs to transition into multiple rooms seamlessly.  Pick the layout line that has best match of the asymmetry, and then lay a perpendicular that is square to that layout.  Any multiple of 3-4-5 works.I second the advice to pull an inch or foot (not trusting the tape end), it's easier to "see", though harder to hold.
Reply:A method I have used is:start with a known straight edgePut the square against it and scribe a perpendicular lineFlip the square along the straight edge (Mirror the inital step)scribe another perpendicular line  from the same point of origininspect the scribe marks if they are on top of each other you have a known good squareif not a decision is made whether to bring into square or get another.This also a good method to determine a known straight edge. Scribe line flip over the scribed line and see if there is a belly.I have used this method to make fast squares for specific jobs, the largest of which is for building gates and follows the 345 method then checked with the above procedure.  It measures 6' x 8' , suitable for 20' gates without using more labor intensive methods as I have in the past.Hope this helps, and if not clear I can do some photos demonstrating.