Fitting a panel that has drifted

Prevention first, because it is most of the answer. A panel assembled freehand on a bench will drift; a panel assembled into a fixed corner mostly cannot.

1. 1Two battens, one right angleScrew or clamp two straight battens to your board at a true 90°. Check the angle with a square you trust, not by eye. Everything builds from that corner.
2. 2Cut line taped down underneathTape the intact cut line to the board inside the corner, so every piece has something to be checked against as it goes down.
3. 3Work outwards from the cornerBottom left to top right for most people. Horseshoe nails or push pins hold the growing edge steady so nothing creeps as you add to it.

Two sides can both be the right length while the panel is a parallelogram. The diagonals are what tell you whether it is square, and they are the measurement to take early and often.

Measure corner to corner both ways. If they differ, the panel is racked, and every further piece will make it worse. Take the pins out, ease the whole assembly back against the battens, and re-pin.

When a panel has already drifted, the instinct is to take a little off everything remaining. That is the one approach guaranteed not to work: it spreads a local error across the whole panel and leaves you with a smaller panel and the same misalignment.

1. 1Lay the whole thing back on the cut lineDo not take it apart. Slide the assembly over the drawing and find the first place where the glass stops following the lines. The error started there, not where you noticed it.
2. 2Lift only back to that pointEverything before it is fine and does not need touching. Most drifts trace back to one or two pieces.
3. 3Check those pieces against their patternsUsually one is over-ground, or one was cut outside the line and never checked. That is the piece to recut.
4. 4Rebuild forwards, checking every second pieceAgainst the cut line, not against its neighbor.

Not every drift is worth chasing. The question is whether it will show.

• Under about 1/32 inch (0.8 mm) across a whole panel: absorb it into the border came and carry on. Nobody will ever find it.
• A gap in one seam in the middle of a busy pattern: a slightly wider solder line there is invisible. A slightly wider line along a long straight border is not.
• Anything that makes the outside edge out of square: fix it. The frame will not negotiate.
• Anything over about 1/8 inch (3 mm): recut. It will not hide, and it will fight you at every stage from here on.