Geometry Set, Steel Compass

Details

A geometry set in a tin box, built around the two failures that make a school compass box useless in a term. The first is drift. A compass with a plain friction joint opens under the hand and the circle that started at 80 mm finishes at 83, which is not a small inaccuracy in a construction exercise, it is the difference between a diagram closing and not closing. The second is the scale. Graduations printed onto plastic wear off at the 0 to 5 cm end within weeks of ordinary use, and a ruler whose first centimetre is a blur is a ruler you cannot start a measurement from. Both are fixed by materials rather than by care. The transparent pieces are cast acrylic and not polystyrene, which is the third quiet substitution: polystyrene is cheaper, and it crazes into a network of fine cracks and goes cloudy within a year.

• Compass is nickel-plated brass with a hardened steel needle and a knurled centre-wheel adjuster, so the radius is set by a thread and not by friction. Measured: twenty circles at 80 mm radius with under 0.3 mm total drift
• Scale is AISI 304 stainless steel with etched and blacked graduations. Etched, so the marks are a cut into the metal and cannot wear off. Accuracy within 0.2 mm over 150 mm, verified against a calibrated reference at a NABL-accredited dimensional laboratory
• Set squares and protractor are cast acrylic, PMMA. Not polystyrene, which crazes and clouds. Angles within 0.1 degree on the set squares and 0.5 degree on the protractor graduation
• Compass takes a 2 mm lead. That lead is graphite and clay, as all pencil lead is, and there is no lead metal in it. The confusion is worth correcting rather than trading on
• The pencil and the eraser are tested to the migration limits in IS 9873 (Part 3), the Indian toy safety standard, because a compass box lives with a child
• The eraser is a thermoplastic elastomer and is tested for the six restricted phthalates. It is not plasticised PVC, which is what most school erasers are and is why a scented eraser is worth being wary of
• Box is tin, not plastic. A tin dents and keeps closing. A plastic box cracks at the hinge and then never closes again
• The compass needle is genuinely sharp, because a blunt one wanders off centre. A push-on cap is supplied and it should be used
• There is no BIS product specification for a school geometry box, so what is named here is the alloy grades, the toy safety standard for the parts a child handles, and the dimensional methods used to verify the rest
• The 8 piece set is compass, divider, 15 cm steel scale, 45 and 60 degree set squares, 180 degree protractor, HB pencil and eraser. The 12 piece adds a sharpener, a 10 cm steel scale, a tube of 2 mm leads and a spare compass needle