RK05 packs

Paul Koning paulkoning at comcast.net
Wed Sep 7 11:17:09 CDT 2016

> On Sep 7, 2016, at 11:56 AM, tony duell <ard at 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.


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