FoldRule: a geometry set that folds up for storage

FoldRule: a geometry set that folds into a pocket

FoldRule puts a 160 mm ruler, a 180 degree protractor, a compass and a set of circle stencils on one card. It prints flat in a single piece and folds down to 72 x 60 x 7.6 mm, small enough to live in a pencil case or a blazer pocket.

School geometry sets rarely survive the year. The case cracks early, and the compass spends the rest of term as a loose steel spike in a backpack. This replaces the set with one print that uses about 12 g of filament.

It comes off the bed flat. Twist it once to free the hinge, and it folds.

What is on the card

• 150 mm scale running the whole long edge, continuous across the fold

• Inch scale to 5.9 in, graduated in 1/8 in

• 180 degree protractor, dual reading, 2 degree graduations

• A compass with no spike. A pivot hole plus pencil holes every 5 mm from R10 to R60. Put a pencil tip in the pivot, another in the radius hole you want, and turn. It draws circles up to 120 mm across, and nothing in the bag can stab anyone.

• Circle stencils at 6, 10, 14 and 18 mm

• A ring-binder hole, so it clips into a binder or onto a backpack zip pull

Folded shut, every marking faces inward, so the scales stay readable all year.

Why it prints well

The hinge is a print-in-place pin hinge with a lap joint rather than a living hinge, so it will not fatigue and snap after a few weeks in a schoolbag. The lap joint also stops the card bending backwards, which is what lets it work as a straight edge instead of a floppy toy.

• No supports and no hardware

• 0.4 mm nozzle, 0.15 to 0.20 mm layers, 3 perimeters

• About 12 g, roughly 1 h 15 m

• Fits any bed of 155 x 60 mm or larger

The markings are raised 0.5 mm. Drop a single colour change into your slicer at 3.0 mm and the scales come out in a contrasting colour on a single-extruder printer, with no MMU or AMS involved. That is what makes them readable across a classroom desk.

Print the hinge test coupon first if your tolerances are unproven. It is a 32 mm slice of the identical hinge and takes about six minutes.

Make it yours

The .scad file is fully parametric and works in the Printables Customizer.

• Put a name on the fold panel. It is the most effective way to stop the card being lost, and it is the reason a class set is worth printing.

• Metric only, or metric with inches

• Change the card size or the protractor graduation

• One tolerance parameter, hinge_clr, matches the hinge to your printer

For teachers: a class set of thirty, each with a name on it, is about six hours of printing and roughly a third of a spool.

Limits

On a calibrated printer this holds about 0.3 mm over the full 150 mm, which is right for schoolwork. It is not a machinist's rule, and I would not set up a lathe with it.

Licence: CC BY-SA. Remixes are welcome. I would like to see a left-handed layout and a version with a set-square edge.