Re: [TCML] G-10 blistering

[FIFTYGUY@xxxxxxx]

In a message dated 6/14/08 2:00:43 P.M. Eastern Daylight Time,  
bartb@xxxxxxxxxxxxxxxx writes:

> I have 8 flying electrodes on my disc. Each of these  flying
> electrodes is a 3/8" x 1.5" long solid tungsten rod.  

Some notes with bare tungsten press-fit to  G-10:
 
    When I assembled the 3/8" tungsten rods into my  12" G10 disk (sound 
familiar?) I did some testing in a scrap piece of G10 (the  corners of the squares 
I cut off to make a circle from the square piece of  3/4" thick G10). I 
drilled a hole with the same distance-to-edge as the actual  rotor holes and reamed 
it out to .373" to begin with. The rods were .375"  dead-nuts, to the best of 
my measuring ability. I had a .374" reamer in  case they ended up too tight. 
It took about two tons of force to get  the rods into the holes, or to adjust 
them once they were in. So I reamed  all 8 rotor holes to .373".
    Another thing I wanted to check was the retaining  ability of G10 at 
elevated temperatures. So with a torch and a non-contact  thermometer I carefully 
warmed a rod up to 300 deg F (past the 284 deg F max  service temp of G10!). 
Guess what? That press-fit rod now freely slid back and  forth in its hole! 
After it had cooled, it was nowhere near a tight fit as it  had been.
    There has been discussion here that adding a  retaining setscrew thru the 
rotor edge actually weakens the G10. Maybe the  best bet is clamping shaft 
collars:
 
    From McMaster-Carr (_www.mcmaster.com_ (http://www.mcmaster.com) ):
 
6157K13 
One-Piece Aluminum Clamp-on Shaft Collar 3/8" Bore, 7/8"  Outside Diameter, 
3/8" Width
In stock at $2.25 Each
 
    or if you prefer thermal mass over  conductivity:
 
6435K13    
One-Piece Clamp-on Shaft Collar Black-Oxide  Steel, 3/8" Bore, 7/8" OD, 3/8" 
Width 
In stock at $1.99 Each
 
    The plain steel would probably suffer in  that environment. They also 
offer stainless steel for substantially more  cost and lower thermal 
conductivity.    
 
    I've seen some folks use shaft collars as the  sole means of retention on 
their RSG's. Of course you'd have to re-balance  after adding the collars to 
both sides of the rods. Would add about $40 for my  8-electrode RSG, but 
certainly seems worth it if it keeps the tungsten slugs  from escaping! Might have 
to watch out that the collars don't arc to the  stationary electrodes.
    Some folks use rings on the faces of the disk,  but I dunno how they 
secure the rods. I also dunno how  you would  get heatsinking from it unless you 
tack-welded the collars to the disk after  everything was assembled. 
    Maybe the best would be to use a solid aluminum  rotor with an insulated 
hub or belt drive. I've got all the parts for a timing  belt drive, but I 
figured I'd start simple.
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities






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