Transfer pattern to glass: why EP3 banned the template

Learning to transfer a pattern to glass is the part of the job nobody films. Everyone wants to see the score and the break-out, but the mirror goes wrong before that, when the shape moves from paper to glass. That is why, in challenge 3 of Arena do Vidro, the most important rule was not about cutting: touching the template to the glass was forbidden. You looked at the shape, measured it, and had to reproduce it exactly, in 20 minutes, cutting only.

In this post I explain what that rule was measuring, why at the 10-minute mark neither competitor had the right shape on the glass, and the path judge Vinícius pointed to. The episode video is above, and the first 10 minutes are worth watching with that in mind.

What the no-template rule really measured

If I let you lay the template on the glass and trace around it, I am measuring your hand on the cut. That matters, but challenges 1 and 2 had already measured it. Take the template off the glass and what is left is a different skill: reading a shape, understanding which measurements it is made of, and rebuilding those measurements on a blank sheet. Vinícius said it out loud during the challenge: the hard part would not be the cut, it would be transferring the drawing.

In a glass shop this happens every day with no template at all: the customer sends a photo of a mirror they saw online, or a sketch on a small piece of paper, and you have to pull a full-scale shape out of it. Anyone who only knows how to trace a template freezes right there.

▶

Why nobody had the shape at 10 minutes

Half the time was gone and both were still marking, erasing and measuring. It was not a lack of cutting skill. Both tried to draw the curve by eye, like copying a picture, and a curve drawn by eye never closes the same way twice. Janael actually drew the shape mirrored, flipped relative to the template, and had to switch models halfway through. Rômulo, who cuts freehand with no trouble, said afterwards that marking and drawing is the terrible part for him. Each one got stuck at the same stage, for different reasons.

The path the judge pointed to

Vinícius said what he would do: focus on drawing circles to approximate the shape, in quadrants, the way Rui did in the demonstration. Instead of trying to draw the whole curve at once, you split the shape into arcs, find the center and radius of each arc, and mark it with a compass or a fixed point and a string. The curve closes properly because it does not depend on your hand, it depends on the measurement.

• Look at the shape and break it into segments: straight, curve, straight, curve. Each curve is an arc of a circle.
• Measure the overall dimensions on the template: width, height, and where each curve starts and ends.
• Mark those points on the glass first, with a ruler and square. Only then think about the curve.
• For each curve, find the center of the circle that passes through the points and mark it with a compass. Quadrant by quadrant.
• Check symmetry by measuring from the center to both sides before scoring with the cutter. A flipped drawing only shows up after the cut.

What the judging showed

Nathan judged with a caliper and the criticism was the same for both: the shape did not match the template because the drawing did not match. On Janael's mirror he said there was too much space left over and that he should have measured at three points and built the curves from them. Rômulo's shape was closer, but the glass cracked on the break-out because he scored both lines before breaking and the run went inward. The result: a tie. Both lost at the same stage, the transfer, not the cut.

What I do in my own shop

I said it on camera and I will repeat it here: with the Router Glass I cut that shape at any size, and one of those would take me about a minute. That is not to put anyone down. The machine removes exactly the stage that brought both of them down, because the drawing goes into the computer ready and the cutting wheel follows the line. But anyone cutting by hand needs to master the circle method, because it is the only way the curve comes out the same every time.

How do you transfer a pattern to glass without a template?

By breaking the shape into straight lines and arcs. Mark the start and end points of each segment with a ruler and square, then draw each curve with a compass from its center. That is what judge Vinícius recommended in EP3.

Why did challenge 3 ban touching the template to the glass?

To measure shape reading, not just the hand on the cut. Anyone can trace a template; rebuilding the drawing from measurements is what sets apart people who make custom mirrors.

What was the most common mistake in EP3?

Trying to draw the curve by eye. Both competitors spent 10 minutes on it, one drew it flipped and the other ended up far from the shape. The cut itself was the fast part.

If you want to see how the Router Glass makes that shape in about a minute, the post about it is in the links. And if you want to talk to me about the machine, message me on WhatsApp at +55 (31) 9095-8383.

Full episode 3 · How to cut a mirror by hand with a template · Glass cutting accuracy: the caliper at the EP3 judges' table · Mirror cutting machine: the EP3 shape · Router Glass

Episode 3