Re: Disc Construction

To: tesla-at-pupman-dot-com
Subject: Re: Disc Construction
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Sat, 01 May 1999 19:21:31 -0600
Approved: twf-at-verinet-dot-com
Hi Nick,

>Original Poster: NickandSim-at-aol-dot-com
> why use G-10? At 1500rpm you could make the rotor out of
>plywood and it would stay together (remember the square function in the
>centrifugal force equation).
>If you're running faster than that a good way the keep discs together is to
>build a disc holder like the one below
>(use courier to view)
>
>     X
>     X
>   /|X|\
>   ||X||___
> =====| ___> to motor
>   ||X||
>   \|X|/
>     X
>     X
>XXXX=disc material ==== threaded rod of diameter of your choice
>all other lines indicate boundary of material you machined this from
>This spreads the load of centrifugal force around a much wider conatct area
>than if you just epoxied or keyed the disc to the shaft.


I hope the plywood wasn´t meant too seriously. I wouldn´t trust (ply)wood
having HV across it. But seriously, this is very similar to how I machined
the stainless hub, except that I don´t need the threaded rod. You can´t
go out too far with your load spreader disks (represented by the slants),
because you don´t want the HV to arc from the electrode to the hub, etc.
The SS-306 hub was final trued (with a lathe) while it was actually mounted
on the motor armature. This way I can compensate for the 8/100mm runout
that the armature shaft has (making it very smooth running, now) As to using
G-10: you never know if you might want to upgrade the system. Plus I don´t
like taking chances with rotating electrodes ("tungsten bullets"). Its not just
the centrifugal (centripedal) force, that you have to consider. Using G-10
has a few advantages to it:

a.) It is pretty heat resistant and doesn´t carbon track immediately.
b.) Due to the fibers, it is shatter resistant (i.e: if an electrode comes lose
and hits the spinning disk, this won´t disintegrate). Or if you (due to some
quirk) hit the resonance frequency of the material, the fibers will hold up
longer. Should the disk shatter, then the fibers will slow down the
"speeding" parts considerably. Using 3 disks (together) instead of one will
reduce the possibility of hitting self resonace of the material, as they will
dampen each other out.
c.) Good insulating properties w/o being hygroscopic.
d.) light weight vs. strength.
e.) pressure stable (if you put electrode holders into a wooden disk and
tighten
     them, they will loosen in no time, because the wood fibers crush, when you
     tighten the holders. This crushing takes time, so even if they are tight
     one day, there is no way you can guarantee that they will still be 24 hrs.
     later.
f.) G-10 has a uniform thickness. This means you don´t need to machine the
     front and rear surface, just the outer edge.
g.) As my electrodes will be on one side only (not all the way through the
     holder), the forces which act on the front and rear disc surfaces are very
     different. This means you don´t want a flimsy material that will bend and
     twist (even if it is only minor) as it runs.

G-10 DOES have one disadvantage. tho: It is hell on cutting tools. But as I use
tungsten-carbide cutters, this is of no real problem.


Coiler greets from germany,
Reinhard