Levelling and Fixing the Strongback

2026-03-29

Continued working on the strongback for mounting the fuselage. I discovered that the main 40×80 mm beams have suffered some warping despite all my precautions. I took great care with heat input; welding at 110 A, very fast, and only in short sections of ~1 cm.

To counteract the warping, I made a small relief cut with the angle grinder on the opposite side of the weld bead, and later I will run a TIG pass to close the gap. That should pull the beam straight.

It is not critical though — the wooden beams will need to be planed anyway, but I want to start from an even better baseline.

Using panels for lateral stiffness.
Using panels for lateral stiffness.
Finally: levelled and bolted to the floor.
Finally: levelled and bolted to the floor.

TIG Pass to Straighten the Beam

Later that day I ran the TIG passes and managed to level the entire jig perfectly, or at least within 1 mm over the full length of 4.5 m. That is more than good enough; there is probably more twist, warp, and thickness variation in the lateral wooden beams that sit on top of the table. The pictures show the effect of the first pass. After cooling down, I made two more passes, but did not take any more photos as I had to sand down the shims under two of the feet.

Deviation before the first pass.
Deviation before the first pass.
Relief cut.
Relief cut.
Weld to build up counter-tension.
Weld to build up counter-tension.
After the first pass.
After the first pass.

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