> > I looked into this a couple of years ago with the intention of making > > a 24 sector pack for an HP7900 (actually part of an HP9880). Starting > > from a 12 sector pack of course. > > > > This project got interupted by a house move and I've not gone back > > to it yet, but I did discover there is no alignment ridge or anything > > between the hub and platter. The platter fits on the flat top of the > > hub, there is a clamping ring that is then screwed down to anchor > > it. > > Did you construct an engineering drawing of the hub based on your > observations?
No. There is no need for what I want to do. I just need to put an extra notch between each of the sectoring notches, I think (I will have to investigate what happens with the index notch, the one at the odd spacing). It was my intention to put the hub on a dividing head, carefully get one of the existing notches over a slitting saw, then rotate and cut the next notch, etc. > > My intention was to put the hub on a spare spindle (I happen to > > have a load of RK05 drive spares), put the platter on, turn it round > > by hand and use a lever-type dial gauge to get minimum run-out. > > That's like the procedure for centering a work piece in a 4-jaw chuck. Sure, done that often enough. > With care and patience you can get it centered to .001 inches (25 um), And as you said the only real requirment is to get it balanced to avoid vibration. I think a 1 thou runout would be good enough for that. Of course the disk would have to be formatted after this modification but that's not a problem. In fact for my application (the HP9880) it may not be necessary. After battling through manuals and microcode, I have realised that the thing actually treats it as a _12_ sector pack, starting a read or write on alternate notches only. That means an electronic modification (easier for me) would let you use normal 12 sector packs that I have over a hundred of.... -tony