Reply To: How square should a square be?
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Return that combination square to the store if you can. If you can’t, then do something to dispose of it.
You can find used Starrett, Brown & Sharpe, Muitoyio (spelling?) and several other machinist grade combination squares on eBay in the USA and UK. A missing scriber or damaged bubble level is of little consequence to their usefulness in woodworking but will generally lower the cost. Likewise, a little corrosion on the blade isn’t a big deal if the marks are incised. A quick scrub and you are back in business.
Baring finding a quality used square, one brand to look for is PEC but I’m not sure if it is distributed in the UK. I can get factory second PEC combination squares, 12″ for $20US and they are of good quality. Just have the occasional ding or dent that will not affect their use in woodworking. Here is the mail-order side of the store where I’ve purchased PEC squares: http://store.harryepstein.com/cp/Squares/7131-4R.html
Good quality squares have a rib running through the slot in the head that can be filed to fine-tune the angle of the beam and thus the squareness. A quality 12″ combination square should be less than 0.01″ out over its full extension length (11″) for woodworking, and less than 0.005″ is the “B” grade machinist square.
One additional note on vintage combination squares, there are cast iron heads that can be rather brittle, cracking or breaking if dropped. Starrett still offers one model with a cast head that (I believe) is not ductile cast-iron. So as always with used tools, caveat emptor.