Framed mirror measurements: cutting 2 cm to save the deadline

Framed mirror measurements are a calculation that looks simple and trips up good people: the mirror has to fit the frame, and the frame changes size after welding. In the Arena do Vidro finale, Rômulo left the square tube too tight for the mirror, the weld pulled it in and the glass would not go in. With the clock running, he did the only thing possible: he cut about 2 cm off the mirror, resized the project and delivered.

This post is about that decision. First the physics of the problem, why welds pull and how much clearance metal needs. Then the part I find most valuable in the episode: how to resize mid-project without losing the deadline or your head. The finale video is above.

Why the weld pulls the frame in

When you weld square tube, the weld bead heats up, melts and then cools. As it cools it shrinks, and that shrinkage pulls the two bars toward each other. In a four-corner frame, each corner pulls inward a little, and the inside opening ends up a few millimeters smaller than what you measured before welding. Rômulo knew that. He said it himself at judging: "I slipped; I know the weld pulls; I could have left a few extra millimeters". Knowing it and applying it under pressure are different things, and that is what the finale showed.

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The framed mirror measurements I use

• Start with the mirror, not the metal. Cut the mirror to its final size and measure with a tape what actually came out, not what was on paper.
• Add the clearance: a few millimeters per side so it goes in without forcing, plus a reserve for what the weld will pull. On thin tube the pull at each corner is small, but there are four corners.
• Cut the metal to that inside measurement plus clearance. Weld two opposite corners first, check square and the inside measurement, then close the other two.
• Once welded and cool, measure the opening again. If the mirror goes in with a small gap, it is right. If it goes in tight, do not force it: glass forced into a metal frame cracks at the first temperature change.
• If the weld pulled too much and the mirror does not fit, the fastest way out is Rômulo's: cut the mirror, do not rebuild the frame.

Resizing mid-project: what Rômulo got right

He had 60 minutes, a frame already welded and a mirror that did not fit. Redoing the metal meant cutting the tube again, welding again and waiting for it to cool, with Janael already assembling his piece on the other side. Cutting the mirror was one score and one break. He chose the mirror, took off about 2 cm, and recalculated the rest of the project for the new size. The piece stood on its own, as the rule required, and was judged whole. He lost points on finish, not on the decision.

In a glass shop it is the same calculation every day. When one part of the project does not line up, you weigh the cost of redoing each side: time, material and what the customer will notice. It is almost always cheaper to take a little off the mirror than to touch the frame, as long as the new size still meets what the customer asked for. Two centimeters on a full-length mirror, nobody sees. A blown deadline, everybody sees.

The other side: flat bar and Janael's frame

Janael had never MIG welded and used the help the judges allowed: Rui welded his frame in under 10 minutes. Peterson had said he would start with the flat bar, the thin one. Rômulo himself remarked that thin flat bar is deceiving: easy to bend, but flimsy. Each metal asks for its own clearance and care, and I explain that better in the flat bar vs square tube post, linked below.

What are the right framed mirror measurements for a square tube frame?

The inside opening of the frame after it is welded and cool, minus a few millimeters of clearance per side so it goes in without forcing. Measure the finished frame, not the drawing, because the weld pulls.

How much does the weld shrink a square tube frame?

A few millimeters in total, adding up the four corners. On thin tube the pull per corner is small, but on a tight mirror those millimeters are the difference between fitting and not fitting.

Is it better to cut the mirror or rebuild the frame when it does not fit?

Almost always cut the mirror, if the new size still works for the customer. That is what Rômulo did in the Arena do Vidro finale: took off about 2 cm, resized and delivered on time.

Want to talk about framed mirrors or about Arena do Vidro? Message me on WhatsApp at +55 (31) 9095-8383.

Full episode 4: the finale · Flat bar vs square tube mirror frame · Floor mirror: what the final rule demands · Rômulo Sullivan, from Monte Glass

Episode 4