You're a brave man CB!
Here's a ha'pence worth of norralot, which you've probably already thought through . . .
As an opening observation, I haven't personally ever had trouble truing up original sliprings of any make, be they singles with 360° brass - easy - or segmented for twins, a bit more care needed. I doubt you have either. So it should be possible to finish-turn a new making from Chateau Priory.
But I have had trouble with certain replicas, where the plastic (or whatever) has been softer, cutting tools can grab, and as you say rip the brass segment out. This isn't something I've experienced with the modern sliprings of decent quality like the ones you sell, but, without naming names, I fitted 4 very smart looking rings from another source when there was rupture in supply of the UK bits, and the HT brushes very quickly wore horrible grooves on the moulded track and then they fell to bits (brass segment ripped out) when I tried to rectify them. That didn't amuse me much, as they were expensive too.
Assuming the materials are broadly up for the task, maybe it does come down to tooling and cutting speeds?
The pic here is pinched from L H Sparey's book, which I've always relied on for these sorts of things. The diagram shows the angles he recommends for brass, on a knife tool. (Not that most of us would ever be able to grind one up that accurately.) A slightly blunted cutting edge maybe? He also reckons a tool height just below centre height will help. I don't know if this could be one of those jobs where a back toolpost could also help, but it sometimes does for some things? Not sure about suds either - wouldn't normally use on brass . . . but there are exceptions I believe.
When I've been truing these things myself (I've never made one completely from scratch) I've tended to use a very narrow-nosed tool, almost a parting tool, fraction below centre height, the tiniest depth of cut (half a thou maybe), relatively high speed and very fine feed, multiple passes, no suds.
What the heck - Good Luck!
