Published on 10/19/2026 · by Leandro Caldas, glazier, founder of Routeck and creator of the Arena
Glass cutting accuracy: the caliper at the EP3 judges' table
In Arena do Vidro EP3, when the clock hit zero, I said the judges would now decide which mirror came out the least bad. Then came a scene every glazier should remember: our pickiest judge pulled a caliper out of his pocket. His idea was simple: find the biggest gap between the cut mirror and the template. That is how you check glass cutting accuracy: not by eye, by measuring.
This post is not about who won. It was a tie, and the challenge itself is already covered here on the blog. It is about the judges' method: what they looked at, in what order, and how you can bring that to your own glass shop before a piece goes out to a customer.
What the judges measured in EP3
The evaluation had three layers, and they are worth paying attention to, because it is the same math a customer does when the mirror shows up at their house.
- Closeness to the template: the caliper came out to find the biggest gap between the cut and the drawing. It does not matter where it came out good, it matters where it came out worst.
- Edge quality: one judge said that, in terms of accuracy, the cut looked pretty good, but he pointed out the marks left from tapping and the runs the cut created along the edge.
- Piece integrity: Rômulo's mirror came closest to the drawing, but it cracked. Janael's did not crack, but it ended up far from the template.
Nathan also showed, right on the mirror, where the difference was: a lot of extra space in some spots, and what was left over on one side was missing on the other. For him, the right move was to take measurements at the key points first and only then draw the curves. In the end, Vinícius summed up the dilemma: one got closer to the drawing and cracked, the other did not get close at all. A tie.
Why the measurement that counts is the worst one
The caliper moment teaches something a lot of shops forget: glass cutting accuracy is not the average, it is the point that is furthest off. A piece that lines up on three corners and runs over on the fourth will not fit the niche. A mirror that is dead on in width but off along one stretch of the curve will show a gap in the frame.
That is why you check at several points, never just one. On a straight piece: width at the top, middle and bottom, height on both sides, and both diagonals. If the diagonals match, the piece is square. On a curved piece: measure where the curve changes direction, which are exactly the points Nathan pointed out at the judges' table.
How to check cutting accuracy in your glass shop
| What to check | With what | What to look for |
|---|---|---|
| Overall size | Tape measure or steel rule | Width and height at more than one point |
| Square | Tape measure across both diagonals | Equal diagonals on a rectangular piece |
| Curved outline | Template laid on the finished cut | The biggest gap between piece and template |
| Fine detail | Caliper | Exact distance at critical points, notches and holes |
| Edge | Eyes and fingers, with gloves | Tap marks, runs and chips |
Look at the third row. In the challenge, the template could not go on top of the mirror. In your shop, once the piece is cut, laying the template over it is the most honest check there is: the gap shows itself, and the caliper tells you how big it is.
Tolerance: how far off a piece can be
There is no single number that fits every job, and I am not going to make one up. Tolerance depends on where the piece is going. A mirror glued loose on a wall accepts more difference than one that goes into a frame, a bathroom niche or an aluminum channel. The rule of thumb: before you cut, ask where the piece will fit and how much clearance that fit allows. That clearance is your tolerance.
And the edge is part of accuracy too. The sheet in EP3 already came with one side cut straight, and one judge expected them to keep that edge. When they went to nip off a small chip, the edge that was right went crooked and started a run. A lot of the time a piece loses accuracy in the touch-up, when the glazier is already nervous.
Why measurement matters at delivery
In the Arena, an inaccurate piece costs points. In a glass shop, it costs the sheet, the time to cut it again and, worst of all, another trip to the customer's house. No amount of edging saves a mirror that does not fit: if it was cut off size, the bill is yours.
That is why, in the middle of the challenge, I mentioned that with the Router Glass I can cut a mirror in any shape, and that I would cut that one in about a minute, by myself. The machine works with an accuracy of ±0.5 mm, according to its spec sheet, and repeats the same piece as many times as you need. But measuring before and checking after is still the glazier's job, machine or no machine.
- How did the judges measure cutting accuracy in EP3?
- One judge used a caliper to find the biggest gap between the cut mirror and the template. The judges also looked at the edge, including tap marks and runs, and at whether the piece had cracked.
- What is the best tool to check glass cutting accuracy?
- It depends on what you are checking: a tape measure for overall size and diagonals, the template laid on the cut piece for curved outlines, and a caliper for fine details and critical points.
- What tolerance is acceptable when cutting a mirror?
- It depends on the fit. A mirror glued loose on a wall accepts more difference; a mirror in a frame, niche or channel has to respect the clearance of that fit. Ask where the piece is going before you cut.
The full EP3 is at the top of this post, and the caliper scene starts around the 8-minute mark. If you cut with accuracy and want to prove it in front of the judges, applications for the next Arena season are open here on the site.
Full Episode 3 · Mirror cutting machine: the EP3 shape in about 1 minute · Router Glass: cutting accuracy of ±0.5 mm