Building and civil
Sheet-metal flat patterns — adding the dimensions does not work
A flat pattern is not the sum of the outside dimensions: the outside stretches when you bend it, and the amount subtracted is each shop measured value, not a standard
This drawing comes from one YAML file. No shape was moved by hand.
Notes in the source (in Japanese)
展開長は、外側寸法を足しただけでは合わない。曲げると外側が伸びるので、その分（伸び値）を曲げの数だけ引く。50 ＋ 30 − 2.9 ＝ 77.1。
外は伸び、内は縮み、あいだに伸びも縮みもしない面（中立面）がある。その面の長さが展開長。中立面は板厚の真ん中ではなく、内側へ寄る。
伸び値は規格ではない。 材質・板厚・内R・金型で変わり、各工場が実測データとして持っている ——だから図面にはどの伸び値で展開したかを書く。この図の 2.9 も例。
Source (YAML)
examples/gallery/238-板金の展開図.zumen.yaml
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