Re: Rotary gaps -- machine work

Tesla List wrote:
> >From chip-at-poodle.pupman-dot-comTue Aug  6 20:28:50 1996
> Date: Tue, 6 Aug 1996 17:02:00 -0600 (MDT)
> From: Chip Atkinson <chip-at-poodle.pupman-dot-com>
> To: Tesla List <tesla-at-poodle.pupman-dot-com>
> Subject: Rotary gaps -- machine work
> Greetings,
> I've been working on a rotary gap for a while now (5 minutes at a time).
> The problem I'm having is making the disc run true.  The disc consists of
> two aluminum hubs with a 1/2" thick x 12" dia. disc of G-10 epoxy
> fiberglass.
> After reworking the hubs, they are sitting in the 3 jaw chuck.  A dial
> gauge shows that they have been faced so that there is much less than
> 0.001" runout(?).  I then put the disc on the faced hub and put the dial
> gauge on it.  It shows about 0.002" at the center, and about 0.020" or
> more out of true at the edge.  Looking at the motion of the dial, I can
> see that the disc isn't just out of true, but is actually slightly warped
> or something. The needle indicates one "major" high point, and one
> "minor" high point.  These high points are not 180 degrees apart either.
> After a little shimming, I can get things a little better, but not good
> enough.
> This leads me to the reason for the posting.  How to true up the darn
> thing?  The three ideas that I have are
> 1) Face the G-10.  I don't like this too much because it may weaken, or
>    at least make it all fuzzy.
> 2) Build up a rim of epoxy on both sides and face that.
> 3) Ignore the out of true on the disc, put on my brass acorn nut
>    electrodes, and face the points of them.
> Does anyone have any thoughts or suggestions?
> TIA!
> Chip

Hi Chip

I faced this same problem when I made my first rotary. I was using some 
filled plastic material of some kind. My disk was absolutely flat but it 
would not run true with the large aluminum washers which were supplied 
with the work arbor I wanted to use to mount it to the motor shaft. 
Eventually I determined that the hole which I had drilled in the center 
was 1. too big in diameter and 2. not perpendicular to the plane of the 
rotor. I filled the hole with a mixture of epoxy and glass fiber and let 
it set up well. I then made very sure that the drill press table was 
absolutely perpendicular to the drill chuck. I then clamped the rotor to 
the drill press table and cut a new hole of the exact required size using 
a Forstner bit which cuts extremely smooth walled holes. I found that the 
rotor could then be mounted to the arbor without the aluminum washers and 
it runs true. By the way, if you are using work arbors to mount the rotor 
you might consider running the threaded end of the arbor shaft in a ball 
bearing mounted in a pillow block. This really helps if you have any 
imbalance in the rotor itself. Actually you will probably have to 
dynamically balance the rotor but I like the added safety of the extra 

I have built 4 more rotors since #1 and they all work fine.