I started the adapter plate today.
I first made it in cardboard, then transferred it to the alloy with about 5mm added all the way round just in case.
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I used the circular saw for the first few big cuts but I think I killed the blade so had to resort to the grinder and jigsaw.
It's a pity as the Circular saw was doing a great job while it lasted.
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The garage was a mess when I finished and I'll not be cutting on the bench again.
That dust gets everywhere ...
Did some cement board in a bathroom a few years back, never again ...