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What are you using to check the edges straightness, you sure it's straight?
Even eyeballing it looks iffy. I also offered up the edge of a 1m metal level, and a frikken laser level.
It's not the most egregious of curves, but over 2.4 meters it's noticeable.
NotThamesWater-
Maybe the mdf has been exposed to moisture and warped ever so slightly?
If you can't find anything good enough at home, I'd wonder over to the sheet cutting section in your local builders merchant and see what's there, take your laser level with you and take the straightest offcut you can find?
I have eyeballed the 2.4m sheets I've been getting from Selco and they've been good.
Aluminium box section could be good, but I'd want to look at it before purchasing as there will always be duds. Builders depot perhaps?
Hovis
I made mine with the factory edge of an mdf sheet. I didn't consider if it would be straight or not. Maybe I should have... but all my cuts using it have been good.
What are you using to check the edges straightness, you sure it's straight?