[Mavigogun]

Second-hand machine, the previous owner did not attend to keeping tool holder tapers clean, and the ISO30 spindle taper suffered aluminium chip intrusion, gouging, as did the tool holders. The spindle has been reground, balanced, and draw bar adjusted by the manufacturer, with a reported result of 1-2 microns of runout- however, a deep, sub-surface gouge all the way around the tapper could not be removed. They reckon it should not impact performance- what say you? Are there implications for the tool holder tapers?

[metalmayhem]

it would have to be an extremely bad gouge to cause any problems . If the grinder says it's good then I'd take their word for it .
I've worked on a couple mills over the years that had a lot of damage to them . One was a matsuura that had the pull stud come loose and the loose holder beat the snot out of the taper . It was reground with a lot of gouging remaining and it was fine for the hard steels we cut with it . That was over 20 yrs ago , I recently saw the machine posted for sale by the same company and I'm willing to bet it's still running the same spindle .
another was a hass that was damaged and had bad run out . I machined an aluminum taper on the lathe then mounted it into the vise . I added lapping compound and brought the spindle up and down on the taper numerous times until the bulk of the damage was gone . That took a while . There was some gouging left but the the humps and run out was gone and the machine was great after that .

as far as your tool holders go - they will be fine and there won't be any noticeable marks on them if it's been properly ground

[Mavigogun]

...as far as your tool holders go - they will be fine and there won't be any noticeable marks on them if it's been properly ground

Excellent reflection- much thanks!