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RE: Disc Construction
Hi Reinhard, my comments interspersed:
>Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>=20
>
>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:
>
>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 don=B4t 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 can=B4t 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.
The way I see it, the reason for not having a disk arbitrarily thin are:
1) Provide enough material so that the disk material does not fail and
allow the electrodes to fly outward. G10 is quite sturdy and I would
guess that your 1.5mm stock should not fail in this regard.
2) If the electrodes and mounting hardware are not identical on both
sides of the disk, i.e. they generate different moments as they rotate,
they will bend the disk if it is too thin. I don't know your geometry
and if this is an issue for you, but it probably is.
If the electrodes are screwed down such that they pull all layers of
disks together, there should be no need to glue them. Glueing is
probably a bad idea in that if the glue thickness is not uniform, the
disk will be unbalanced, and you don't want to have to turn the disk
faces down.
>c.) As these G-10 sheets are copper coated (35=B5m), 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.
Covering the copper with solder could unbalance the disk if not done
uniformly, not an easy thing to control. You will have to etch away an
inner circle to keep the hub that connects to the motor shaft insulated
from the electrodes, but I would think that you could keep enough copper
to make a good connection between electrodes, especially if you have two
layers. Any IR drop would be small compared to gap losses.
>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'm not convinced that electrode dwell time has too much bearing on quench
performance. In your case, the rim of an 8" disk rotating at 1500 RPM
will travel at 628 inches per second. Let's shoot for a quench time of
100 usec. The disk rim would travel 0.0628" in this time. Unless your
electrodes are each less than 0.03" wide, the dwell time will exceed the
target quench time.
>Coiler greets from germany,
>Reinhard
Regards, Gary Lau
Waltham, MA USA