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remember to allow for the bending away from your cutting tool the work piece ... a general rule is that stock should not be unsupported more than 2 to 3 diameters from the chuck . the result of the vector of the cutting force is that the outboard end is larger ...it bends away from the cutter more.
either support the outboard end, or cut at a remedial angle ... according to the tolerance you need . even threading with many passes can show tapered threads ....
wider tolerances can be our friend .... but still unplanned tapered cuts leave us a little depressed .....we know they are there .
so include in your testing kit some known straight ground shafts to sweep when " chucked " .