> On Sep 7, 2016, at 11:56 AM, tony duell <a...@p850ug1.demon.co.uk> wrote:
>
>
>> The other question is the disassembly of the pack and the installation
>> of the new hub. How is that done? What are the concentricity
>> requirements for the platter? Is there a mating surface (exterior cylinder
>> surface on the hub) or are platter and hub aligned in some fixture and
>> then clamped to hold the platter in position? Clearly the pack would be
>> reformatted, so a small amount of runout would be ok, but it would have
>> to be small enough that the vibration is controlled.
>
> 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?
> 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. With
care and patience you can get it centered to .001 inches (25 um), give or take.
Another option would be to make a centering jig, one that holds the hub and
platter. If the outer diameter of the platter is held to tight tolerances that
would work well; if that dimension is not tightly controlled then centering
with an indicator gauge will work better.
With a spare spindle like that, you'd also be able to verify the balance
afterwards, by spinning up the finished platter and checking the vibration
level.
paul
