Spoilboard surfacing - strange problem/result

[Voicecoil]

Because if you hadn't had a good skimming before I was going to suggest a very similar prognosis to the one you've just come up with! Though it might be more than a matter of the rails being bowed - obviously they're less stiff in the middle than at the edges, so the spindle can move about more.

[Lee Roberts]

Personally I thought at first that the spindle wasn't quite perpendicular to the bed and needs minute adjustment. It looks to be spot on when going in what ever the directions are to the left and right of the pic (top to bottom of pic), whereas you can see tiny lines/ridges on the cuts that go from left to right of the pic (the problem areas).