How to choose a precision square: side length, material and grade

To check a right angle on a mechanical part, choose a steel square to DIN 875/0 with a measuring side at least as long as the part to be checked. In the collection the sides range from 100 mm to 1000 mm. For quality control, hardened steel (or hardened stainless steel) is best, as it resists wear better; for bench marking out, plain non-hardened steel may be enough.

Types

• Stock square: it has a wide base that rests on the reference surface and a measuring side that is brought up to the workpiece. It is the most common type in the collection, for example the stock square Fervi S051/150. Sizes are given as side × base, from 100 × 70 to 1000 × 500 mm.
• Flat square: without the wide base of the stock square. It is brought directly up to the workpiece and is fine for checking and marking out on small parts.
• Bevelled-edge square: it has a chamfered edge, so the contact line is thin and the light gap between square and workpiece is easy to see when checking perpendicularity.

How to choose by job

The features to look at

• Side length: in the collection 100, 150, 200, 250, 300, 400, 500 and 1000 mm (with a 600 mm among the values given). Choose a side that covers the part: with a side that is too short, reading the error is less reliable.
• Material: steel, plain non-hardened steel or hardened stainless steel. Hardened is harder and wears less, but costs more and is pointless if the square is only used for marking.
• Standard: DIN 875/0 indicates the square's accuracy grade. If you have to certify a check, ask for confirmation of the grade on the model's product page.
• Accuracy: the collection lists 1/100 mm and 1/50 mm. The smaller the value, the tighter the tolerance.
• Section and construction: the blade section is 20, 25 or 29.5 mm by 5.5 mm thick. A sturdy section flexes less and is less sensitive to knocks.
• Weight: the gross weights given range from about 0.3 to 1.3 kg. For the same length, a heavier square is usually more massive and more rigid.
• Surface treatment: some models have a protective galvanised coating against rust.

Common mistakes

• Choosing a square that is too short for the part and then checking the angle "in sections".
• Using the square as a stop, a lever or a straightedge to hammer against: a fall or a knock can make it lose its angle.
• Checking with the part and the square dirty with swarf or oil: a few hundredths of a millimetre are enough to falsify the reading.
• Choosing the highest accuracy for a rough marking-out job, or on the contrary a non-hardened square for repeated checks.
• Putting the square away unprotected: it must be kept clean, lightly oiled and in its case.

Frequently asked questions

How do I check that the square is still at 90°?

Rest the base on a flat surface and draw a line along the side. Flip the square over and draw along the same line again: if the two lines coincide the angle is right, if they diverge the square is no longer true.

What is the difference between a stock square and a flat square?

The stock has a base that rests on the reference surface and gives stability. The flat square is used by bringing it up to the workpiece and is better suited to quick checks on small parts.

Which size should I get if I only have one?

A stock square of 200-300 mm covers most workshop jobs. For large parts or metal fabrication, move up to 500 mm or more.

How do I keep it in good condition?

After use clean it, wipe on a film of oil and put it back in its case, avoiding knocks and drops.

You will find all the sizes in the Precision squares collection: if you don't know which grade or length you need, ask for a quote or for advice from the AMES technicians.