For making V6 or Mk8 nozzle what tolerance specification we've to follow? Both nozzles use M6 threads, which ISO thread specification do these nozzles use?
And what should be the tolerance and surface finish (internal/external) of the nozzle bore?
I found the drawings but nothing about the tolerances.
I've found the V6 nozzle drawing here and the Mk8 nozzle dimensions as depicted below:
The short answer is it really doesn't matter. None of these surfaces are interfacing with other rigid parts. They're conducting filament that's being melted. The specifications are likely optimized for favorable flow properties, and following them as closely as your tooling allows is probably a good idea, but there's no hard tolerance cutoff.
CNC Kitchen has a video producing a working nozzle with a mini lathe and had good luck without any rigorous process for tolerances.
It should be noted that the exactness of the nozzle orifice size and other dimensions are for the most part not relevant to the accuracy of extrusion; that's pretty much all on the filament diameter and extruder motor motion, which determine the volume of material extruded. In practice you can produce 0.4 mm extrusions pretty much equally well with an 0.3 mm orifice or an 0.5 mm orifice; at most you'll have some minor differences in behavior on overhangs. And presumably your error is not going to be anywhere near 25%.
As Trish noted, the threads do have to interface with another rigid part. However they're governed by ISO standards for metric threads not this particular design. I use this tolerance calculator site: https://amesweb.info/Screws/metric-thread-dimensions-calculator.aspx