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Re: Disc Construction

Hi Reinhard,

> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
> Hello All,
>
> As you know, I am presently constructing my SRSG. I have run
> into a major problem: the disc material. The thickest G-10
> (which I would like to use) I can find is only 1.5mm (0.059")
> thick (being circuit board material). What I was thinking of doing
> is taking 3 of these sheets for a total of 4.5mm (0.177"). As I
> have a lathe, trueing the disc (sheets are square) is not a big
> problem. Will this work and should I:

I would be concerned that 4.5mm is too thin to hold up to the stresses
presented to the disk (even for G10) unless rpm's were kept relatively
low and/or the electrodes were close to center. If your thinking of
stacking 3, why not stack 6? I would go with a high strength epoxy
between laminates. The bolt pattern for the hub is further added
strength.

> a.) just bolt them together? I will be using 4 or 8 electrodes
>      (depending on the answer I get to my other post) and the
>      disc will be bolted to a hub made from solid 306 stainless
>      (which is a real pain in the butt to machine) via 8 more
>      screws. This means I have sixteen screws (in two circles)
>      total holding the 3 G-10 sheets together.
>
> Or should I:
>
> b.) Slightly rough up the inner surfaces and epoxy the three
>      sheets together? The problem is I donīt have a laminating
>      press, so all I can do is pressure clamp them (while drying)
>      between two pieces of thick sheet metal with normal C-clamps
>      and two pressure bars. I can get the epoxy on pretty evenly,
>      but I canīt guarantee a 100% parallel surface (due to slight,
>      but entirely possible, variations in the glue thickness. I would
>      hate to have to true the front and rear of the disc.

As I mentioned, I think you could do both and feel good about the disk. I
don't think slight variations would be too critical. You should
eventually spin balance it considering the method.

> c.) As these G-10 sheets are copper coated (35ĩm), could I
>      etch a circle on the innermost and outermost surface
>      (first and third sheet) to connect my tip holders? I will be
>      using 5mm (3/16") thoriated tungsten tips and be mounting
>      them in brass screws. I could cover the copper with solder
>      to increase the effective thickness.

I'm not too sure what you've got planned here with the copper. I would
etch the copper off completely.

> d.) Are the 3/16" TTRs (thoriated tungsten rods) large enough
>      for 6kVA input power? I think so. If I use 4 rods, each rod
>      would have to handle 1.5kVA (which is a lot less than they
>      are rated for in TIG welding, esp. the thoriated ones) and
>      with 8 electrodes, each would only have to handle 750VA,
>      which has got to be underrated. The 5mm is a sort of
>      compromise between VA handling and dwell time (motor is
>      a 1500 rpm modified unit), although I think even the 4
>      electrodes should be able to handle 8-10kVA easy enough.
>      Oh, before I forget the disc will be 20 cm (~8") in diameter.

I think the 3/16" should be fine for 6KVA up until they erode beyond use.
The smaller the electrode/higher the power, the faster this is going to
happen of course.

Take care,
Bart