Thread: So, how do I really work out if my try square is square?

I wouldnt worry if a square was out by 0.002mm. That margin is pretty irrelevant in woodworking. If your squares matches up to the lines by eye when doing the flip trick (assuming the line is "clean" and straight), then its as square as it needs to be in my opinion. A lot of woodworkers get too anal about something being out by thousands of an inch. I'd rather spend my time making something rather than worrying if my gear is out by the most neglible margin that probably makes zero difference.

I saw squares being made at the Bridge City factory in Portland, Oregon, USA when I was there in 2000. The squares were measured against a standard 90 degrees block. It was all done by machines in an area that was as clean and sterile as any hospital operating theatre.
I imagine that some engineering works would have equipment capable of fine measurements of that nature.
A square is a woodwork tool. I would have thought that measurement against an engineering square (traditionally made from a single piece of metal, but now often made from fancy "plastic" material) would provide adequate accuracy.
The make a line, flip it over, make another line gives a very good indication of accuracy, provided that the start is a point on a dead straight edge. You can measure how far out of square by measuring the gap between the two lines. The error is 1/2 the gap at the distance from the start point on the straight edge. With a good ruler that could be measured to .5 mm, with a vernier measure it could be measured to 1/10th, if not 1/100th mm.

Do the line with a utility knife or similar, much finer and easier to see any deviation.

I agree with Dean in principle, that is before I spend an hour farting about with some inconsequential detail on something


Cheers................Sean, pedant

Reality check...imagine that 0.2mm is the height of a door - about 2m. That makes 0.02mm look like one tenth the height, or 200mm - a bessa block. Now 0.002mm is one tenth of this - 20mm.

now divide this by ten thousand. That is how small 0.002mm is. In other words, if you do the line scribe flip scribe thing, and the lines look parellel, go make something, it will be square.

Check it against both sides of your table saw blade.

Or fix something vertical aligned to your square and check the other side.

0.002mm that is 2 microns or a 50th of a human hair. One wave of visible light is approximately 1/6th of a micron (633 nanometres) so you are trying to physically measure 3 light waves, good luck.

Sorry about that, I bet you meant 0.002" or 2 thousandths of an inch in old money or half a human hair.

More than half a century ago when I was an apprentice toolmaker we would check our squares against a steel cylinder that had been ground on a cylindrical grinder. The way this worked was that the sides of the cylinder were made parallel with great accuracy ie the measurement is the same each end of the cylinder, the measurement itself is not important. One end of the cylinder would have a small annular surface at the outside of the circular end. This small area would be ground while the cylinder was turning thus making it square to the side of the cylinder. The rest of the end was just cupped in

When the cylinder stands on a flat surface the vertical side is square to the surface it is standing on.

Now it occurred to me if you turned a piece of wood so that it had two rings on the diameter – the same distance apart as the length of the blade of your square – and you made these rings very smooth and the same diameter as each other they could act as the side of the cylinder and you don’t have to produce an accurate cylinder along the whole length. Cut a small annular surface while turning and this will give you a square end. The accuracy of the cylinder will depend on how accurately you can make the diameter of two rings, but as size doesn’t matter you can keep working on the larger one until you have two exactly the same. I suggest you use a mic or a digital caliper.

I agree with the other posts in that there is no point doing this for woodwork but I thought that if you just wanted to know how accurate your square is this could be a way for you to do it.

Good luck
The best from the West
Ray

Ray, welcome aboard mate!

Going by yer avatar, the cancer sticks are keeping you young


Cheers..................Sean

.002mm sure thats not .002"(thats a big difference with measurements in the low decimals)
One thing that amazes me is when companys claim that their 4"/102mm has a tolerance of .002"/.05mm and then their 8"/203mm has a tolerance of .002"/.05mm... if they are measured on the same machinist's block wouldnt the 8" have the tolerance of .004"/.1mm?

But like they say whats the point, if all your woodwork meausres accurately to .2mm you should earn the term FIGJAM! A piece of wood is not like a ground straight edge...

All,
Thank you for your responses. I think some of you may have taken me a little too literally when I said 0.002mm, that was just a figure of speach / exapmle of some of the crap you read from some marketers when you are reading their ads.
If my wood working was going so well that I needed to worry about tolerances this tight, I would be a genious.
What I was really asking was is there a way of very accurately measuring the accuracy of a square. That has been answered, so thanks.
And also, welcome Ray, and thanks for your input, it's appreciated.
Anthony