The FrankenKern has two spindles. One of them is a $50k ISO20 grinding spindle from Fischer SFJ. It runs at 20k RPM and takes whatever tools fit in an ISO20 tool holder. The other is a $4k 1/8" PCB cutter from NSK. It runs at 50k RPM and takes anything on a 1/8" shank. It'll have three spindles but that's another story.
The crazy-expensive grinding spindle clamp is 60mm, to fit a 60mm Fischer SFJ. The NSK is 50mm. Not only that the digging end of the Fischer protrudes about 50mm more than the NSK would like to. So I basically need a 75mm sleeve with set screws in it, that's 60mm on the outside and 50mm on the inside, preferably as straight as possible (both spindles have a runout of 1 micron or less).
The unpleasant discovery, though, is that your magic $50 McMaster drill bushing is Rockwell 61. As in, "will burnish your cheap shitty Chinese Amazon drill bits to a shiny bluntness" even before you acknowledge that you're trying to drill a curved surface with a 40-year-old drill press.
So you wander into Western Tool, as one does, and say "I need to drill and tap some 3mm holes in this" and they look at it and they look at you and they hem and haw and you say "and it's 1144 so my understanding is you can't anneal it, drill it, tap it and re-temper it" and they go "hmmm" and they get on the phone with drill manufacturers and they come back and say "so we've got $10 bits that are rated to Rockwell 45, I can get you this one that's rated to Rockwell 80, good luck with the tapping, you're better off thread milling it" and you sit there going "where the hell am I going to find a machine with enough precision and rigidity to thread mill a 3mm hole" and then you mentally stare at the camera like the monkey kid in Jumanji.
You also decide to look up the thermal expansion coefficient of 1144 and recognize that at the circumference you're working with you get about a micron per degree c and the difference between 0.0015" shim stock and 0.0020" shim stock is 75 degrees and it's already got between 0.003" and 0.0035" clearance so you get yourself about $10 worth of stainless steel shim stock of Amazon and put your drill bushing in the oven and decide to write a pubski post about it while you wait for it to heat up for a second time because yes indeedy, a 0.004" feeler gage will totally fit between a bushing at 350 and a spindle at 40 and when that thing tightens down? It's gonna take a blowtorch to get it off again prolly.
Wish me luck.
EDIT: well that was just this side of catastrophe. Thermal expansion of 1144 is 11.5 microns per minute. Thermal expansion of 303 is 17.2 which means I had time to get it partway on before it locked up like the 405 on the day before Thanksgiving a good inch away from anywhere useful. Some judicious blowtorching some gentle persuasion via claw hammer and we again have four parts but I'm thinkin' (A) 1 shim is enough (B) we're gonna let that spindle hang out in the freezer overnight.