Published 25 August 2026 at 02:22
A try square is the reference for ninety degrees. But that raises an uncomfortable question: how do you know this particular try square is actually square? Comparing it to another try square doesn’t help — that only tells you they differ, not which one is right. And buying an even better try square just pushes the same question one step further.
The answer is older than most measuring instruments and requires no better reference at all. You flip the square.
What it is
A steel try square with base, in accuracy class DIN 875/0.
- Size 500 x 330 mm
- DIN 875/0 — the finest of the common accuracy classes for try squares
- Hardened and ground reference surfaces
- Supplied with an inspection certificate
The flip method
The principle is simple and the result is exact. Place the try square on a surface plate against a reference edge, then set a dial indicator or scribe a line along the blade edge. Then rotate the square 180 degrees around its vertical axis and set it up against the same line again.
If the square is exactly true, the two positions coincide. If the blade deviates from square by some angle, it will lean one way in the first position and the other way in the second — and the difference between the two measurements is twice the actual error. Half the measured difference is therefore the try square’s deviation.
The elegant part is that the method needs no square reference whatsoever. It only needs a flat surface and a way to compare the same line twice. Any error in the setup — the reference edge itself not being perfectly straight — affects both measurements equally and is therefore automatically cancelled out.
Why the base exists
The foot isn’t there to make the square convenient to set down. A try square without a base has to be held against the workpiece, and then the operator’s pressure and tilt become part of the measurement. With a base, the square stands stably on the surface plate under its own weight, both hands are free for the dial indicator, and a significant source of error disappears from the picture.
The 500 x 330 mm size is also a deliberate choice. A long blade means a small angular deviation produces a large deflection at the far end — an error invisible on a ten-centimeter square becomes measurable on a half-meter one. It’s the same leverage principle behind the flip method, just applied to geometry instead of procedure.
And the inspection certificate is what makes the accuracy class useful. DIN 875/0 states what the deviation may be at most. The certificate states what it is on this particular unit — and once you know your square’s actual deviation, you can factor it in instead of treating it as zero.
Three typical use cases
- Machine alignment: checking columns, tables, and spindle angle against the bed.
- Inspection: squareness of machined and welded parts against a surface plate.
- Toolmaking: setups and checks where squareness is the precondition for everything measured afterward.
Why it pays off
Squareness is the reference that almost never gets checked and that everything else rests on. If it’s wrong, the error propagates through the entire chain: a skewed setup produces skewed machining, which is measured against that same skewed reference and therefore looks correct — right up until the part has to fit together with something made elsewhere.
Knowing you can verify your own reference, with a method that takes ten minutes and requires no expensive instrument, is therefore worth more than one more decimal on the datasheet. It’s the difference between trusting your equipment and being able to prove it holds up.
