How to cut a round mirror: circles and quadrants from EP3

How to cut a round mirror, or any curved one, was the question that decided challenge 3 of Arena do Vidro. Not the cut, not the break-out: the drawing. Both competitors had 20 minutes to reproduce a curved shape without touching the template to the glass, and both spent half the time trying to draw the curve by eye. Judge Vinícius, watching from outside, said the line that gives this post its name: he would focus on drawing circles to approximate the shape, in quadrants, the way Rui did.

This post is the step by step of that trick, the way I teach it in my shop. It works for round, oval and capsule mirrors, for any shape with a curve in it. The episode video is above, and the scene with Vinícius explaining is halfway through the challenge.

Why a curve drawn by eye never closes

A hand-drawn curve is a sequence of tiny wrist decisions. Every centimeter you decide whether to tighten or open the line, and you never decide the same way on both sides. The result is a shape that looks right from a distance and looks wrong under the caliper, which is exactly what Nathan showed at judging: glass left over on one side, missing on the other. A circle marked with a compass has no decisions in it: it has a center and a radius, and the line comes out the same at every point.

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The circles and quadrants trick, step by step

• Measure the whole shape first: total width and total height. Mark a rectangle with those dimensions on the glass, using a square. Every curve will fit inside it.
• Draw the vertical and horizontal center lines. Now you have four quadrants, and a symmetrical shape is identical in all four. You draw one and repeat.
• Look at where the curve starts and ends in each quadrant. Mark those two points on the glass, measuring from the center lines.
• Find the radius: the distance from the point where the curve touches the rectangle's edge to the center of that curve. A round mirror has one radius; an oval or capsule has two, one for the end and one for the side.
• Mark it with a glazier's compass, or with a nail at the center and a string with the marker at the end. Quadrant by quadrant, always from the same center.
• Before scoring with the cutter, measure from the center to both ends of each axis. If they match, it is symmetrical. That is what Nathan said was missing: measure here, here and here, and build the curves from the measurements.

What happened in EP3 when nobody used the trick

Janael drew the shape flipped, mirrored relative to the template, and only noticed afterwards. He had to switch models halfway through. That does not happen with center lines marked, because the reference is on the glass and not in your memory. Rômulo got closer to the shape, but at judging it came out that he had been tapping the cutter around the drawing, and that tapping leaves marks and runs that turn into cracks at the break-out. When the line is a clean compass arc, the cutter passes once and the break follows.

The judges also expected the competitors to use the sheet's straight factory edge as one side of the shape, saving a score. Nobody did. That is another detail the reference rectangle gives you for free: you line one of its sides up with the edge of the sheet.

Quadrants are what the machine does on its own

On the Router Glass computer, every curved shape is already made of arcs with a defined center and radius. That is why I said on the episode that I would cut one of those in about a minute: the machine does not draw by eye, it follows the geometry. Vinícius's trick is exactly about bringing that geometry to the hand. Anyone who masters it cuts round and oval mirrors by hand with confidence, and anyone who later buys a machine understands what it is doing.

How do you cut a round mirror without a template?

Mark a rectangle with the shape's width and height, draw the center lines and draw each curve as an arc with a compass, quadrant by quadrant. Never try to draw the whole curve by eye.

Which compass should I use for a large mirror?

A nail or suction cup at the center, a string that does not stretch and the marker at the end. For a small mirror, a glazier's compass with a diamond tip scores directly.

Why did the judge talk about quadrants?

Because a symmetrical shape is identical in all four quarters. You solve the geometry of one quarter and repeat it with the same measurements, and symmetry is guaranteed by the measurement, not by the eye.

If you want to see the same shape coming off the machine, the post about the EP3 shape on the Router Glass is in the links. To talk to me, WhatsApp +55 (31) 9095-8383.

Full episode 3 · Transfer pattern to glass: why EP3 banned the template · How to break out a mirror without chipping · Mirror cutting machine: the EP3 shape

Episode 3