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If may offer a little advice it looks to me that you took bigger cuts towards the finish than you need and maybe you cut it dry, (assuming the tool was on centre and sharp also the speed was a max of 60rpm for steel). Thread finish cuts are around 0.001" - 0.002" just like shaving, you need a good cream, (lubricant) and you don't want any stubble.
His theory is that this will give you better formed threads since for the last few cuts you get both leading and trailing edge cutting. I did this. Wonder if that was part of my problem?
But is there such a thing as a 'half-engaged' half-nut?