'Calibrating' a square

[PeterC]

Trying to find out which of my assortment of squares and adjustable angle measurerers is, well, square. Not sure what to use, so far thought of a die-cast box or the machine-made end of a kitchen work top. Do these seem reasonable? I'm trying to cut a shelf to go into a corner; the piece of material/corner is out-of-square and I'd like to check which one. I could just use an adjustable one to get a match but that wouldn't help with future projects. There seems to be quite a variations between the various instruments.

[John Bryan]

You need material with a straight edge, then place the square up to the edge and draw a line onto the material. Rotate the square 180 degrees and measure the drawn line against the square blade. If the square is out the blade and line will not coincide.

[williamwright]

Put a try square on a perfectly straight edge and mark the line. Turn it over and mark it again. Split the difference.

Use Pythagoras's theorem.

Bill

[Andy Bennet]

Treat yourself to a digital angle rule. You can copy angles and set any angle you like. example:-

I expect cheaper ones are available elsewhere.

[PeterC]

Looking like the worktop, given the size of the squares (30cm to 60cm). The 2 squares look as if they might be adustable but the one screw is hex socket and I haven't yet found a key among the scores in the box! The variable ones can have an allowance or might be tweakable - haven't yet looked closely. The combination square is 'as is' - no adjustment. I was using the 2 lines (not marking, just comparing with one line) but only on the work piece and the best edge is a bit dubious. I know how critical small angles can be as I once wanted to mod. some front forks to kill the trail to use with a trike conversion kit. Very small angles, tapering tubes with varying profile and no useful datum. Did manage to get to zero trail and the handling was good.

[RJH]

Yep - any straight edge, draw a line, then reverse. Found each of the 3 Lidl/Aldi squares I had were a bit out - the hard way ;-)

I've just been using a roofing square to cut some shelving and work out the angles to fit a sloping chimney breast alcove. It's similar to this:

So won't go out of square. And measures angles - but not as accurately as the digital type (assuming that's calibrated properly)

[PeterC]

Tempting, but I have a few squares and, at my age, not sure about buying more. There's a digital one on SF - at twice the price - that looks OK but, weirdly, goes to 280mm:

The squares: couldn't find an allen key to fit. Eighth 'bounced' off the corners; 3mm went in but not far enough to use, then there are 2 pins through so probably not much adjustment. I got them as close as I could with a screwdriver that has a metal handle and a hard rubber 'tip' of about 250g, so there's a bit of wobble room there. The 2 Powerfix adjustable squares are bit out on calibration but still useful to give the angle as-is for marking the workpiece. No adjustment even with the screwdriver and one has a lot of hysteresis. The newer combination square (from SF a few years ago) is way out - enough that I'll be taking some 'adjusting' measures with a hacksaw. No name, no country. The older one (made in England, no name) is almost spot on, so I swapped the bits (the old rule is a bit rusty and hard to read) and now have a good one.