Precision squares
A precision square checks the squareness between two surfaces. Start from the form, meaning the shape that suits your job: flat for a quick check, with a seat when you want it to stand on its own, knife edge when you are looking for the thin line of light. Then decide on angle, DIN 875 accuracy grade and material.
A single L-shaped blade with no base. The simplest and most economical form, for a quick squareness check and for marking out.
One leg carries a wide base, so the square stands on its own on the plate or the workpiece. Choose it when you want both hands free.
A knife edge that touches the workpiece along a single line. You check against the light: the more even the line of light, the squarer the surface. The most sensitive form of check.
One leg thin and the other thick. The thick one sits firmly on the base, the thin one reaches into tight spots where a normal square will not go.
Granite squares, cubes and reference squares. They do not rust and do not burr, which is why they hold their geometry over time. For a quality control station.
The angle adjusts and locks, for checking and marking out at angles outside the standard ones.
The grade is the maximum permitted deviation from 90°. DIN 875 defines four of them: 00, 0, 1 and 2. The higher the number, the more open the tolerance: on a 100 mm leg it is 3, 7, 15 and 30 μm respectively, roughly doubling at each step. The tolerance also grows with the length of the leg, so always compare grades at the same size. Below we start from GG 2, the usual choice, and work up.
30 μm on 100 mm, the most open tolerance in the standard and the most common grade. For normal work in the machine shop, at the bench and marking out, it is considered quite sufficient. Most of our squares are GG 2.
15 μm on 100 mm, one grade tighter than GG 2. It does not exist as a product of its own: you pick it as an option on the same square, which comes in GG 0, GG 1 and GG 2.
7 μm on 100 mm. The grade of quality control and toolmaking, when GG 1 is not enough but you do not need a master.
The tightest grade in DIN 875, at 3 μm on 100 mm. This is the square you check your other squares with, not the workpiece. In our collection you will find it mostly on knife edge squares.
Above the four DIN 875 grades and, in our collection, only in granite. The reference square of the calibration laboratory, where you check your masters themselves.
Hardened stainless steel. The most common choice for precision squares, because it does not rust and holds its edge.
Hardened steel. Economical and solid, but it needs protection from moisture because it rusts.
It does not rust and does not burr: a knock will not raise a burr that would corrupt the measurement. The choice for the highest accuracy grades.
Galvanised steel, on fabricator squares that reach up to 2000 mm in height. For marking out and general use on large fabrications, not for metrology checks.
Tolerance depends on length
The permitted deviation is given in μm and grows with the length of the leg: a GG 1 square allows 15 μm at 100 mm and 25 μm at 300 mm. Full tables by grade and length in the Precision Squares DIN 875 guide.
Check a square by reversal
Rest the square against a straight edge and scribe a line. Flip it 180° and scribe again from the same point. If the two lines diverge, the gap you see is double the actual error.
Checking with a line of light
Rest the square on the workpiece with a light source behind it. An even line of light means a square surface, while a wedge of light shows the deviation.
Calibration certificate
Precision squares can be supplied with a calibration certificate, for traceability in an ISO certified environment.
Which accuracy grade should I choose?+
For normal work in the machine shop, at the bench and marking out, GG 2 is the standard choice and is considered quite sufficient. Step up to GG 1 when you want one grade more on the same square, and to GG 0 for quality control and toolmaking. GG 00 is not for the workpiece: it is the master you check your other squares against. A granite reference square is only needed in a calibration laboratory.
What is the difference between a flat square and one with a seat?+
A flat square is a single L-shaped blade. A square with a seat has a wide base on one leg, so it stands on its own on the surface plate and leaves both your hands free.
Why choose a knife edge square?+
The knife edge touches along a single line, so even a small deviation shows clearly as a wedge of light. It is the most sensitive way to check, which is why knife edge squares are grade GG 00.
Granite or steel?+
Granite does not rust and does not burr, so it holds its geometry and suits a quality control station. Steel and stainless are more economical, lighter and more practical for use on the machine.