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I used "use model silhouette" on page 2 (not "fit to material" as earlier post). Both machine and material were dead firm, and both roughing and finishing were with 8mm spiral. Feed speeds were in the 50mm/sec range, and stepovers were 3mm and 0.2mm. The tall vertical edges were cut afterwards - there was no registration or toolpathing problem as the item was not moved and the final shape was spot on. Given that I had 1mm machining allowance, the cutting position got to be at least 2mm out of where it should have been. On the roughing page, I had it set to "raster Y" / "Z level" / "none" (profile).
It is possible that it was confused by only using a top surface for a model.
Anyway, thanks for replies - I will be more careful to look at the full combined preview in future - currently I just check the finished preview for surface quality.
R.
