Barry,

Thanks for the kind words.  I'll try to keep more coming.

Regarding spring thickness:  There is a direct relationship between the 
stress level in the spring, its thickness and how sharply it is bent as 
defined by its radius of bending.

Approximately:  max stress = Elastic modulus * thickness / (2 * min bend 
radius)

So for a given max stress we get   thickness = 2 * max stress * min bend 
radius / Elastic modulus

This obviously works for any units, so long the units of thickness and 
radius are the same, and stress and Elastic modulus are also.

Putting some numbers to that:
desired max stress 72,000 psi
min bend radius = 2.5"
Elastic Modulus from a table of metal properties = 3E7 psi (or a little less)

So   thickness = 2 * 72,000 * 2.5 / 3E7 = 0.012"

Bending the spring more sharply requires thinner material to stay below the 
same max stress.

For a spring bent to this radius, hacksaw blades and taping blades are 
quite a bit too thick.

One issue is that you want to keep the stress level much lower than the 
yield stress of the spring material to stay far away from any issues like 
creep or hysteresis losses.  Clock makers seem to be designing their 
springs to stay under 40% of yield.

When hardened , the 17-7 stainless appears to achieve a yield strength 
above 280,000 psi, so we're in great shape, only 25% of yield.  Even 
without hardening, the 17-7 is not so bad (198,000 psi), but from Dave's 
reports the hardening process for this material is not all that 
difficult.....bake it at 900 +/- 10 deg F for 1 hour, then let it cool.

If you want all the gory details about the material, and more, 
see.    http://bnordgren.org/seismo/17-7_Stainless.zip

You can watch the seismo make wiggles on Larry's 
page  http://psn.quake.net/currentseismicity.html    Thanks, Larry
Big quake this morning.

Regards,
Brett

At 04:18 AM 8/16/2009 -0700, you wrote:
>Dave
>I'm interested in finding out more and impressed also. I reduced the size 
>of the STM style vertical 12" but like your design. Didn't you try a 
>hacksaw blade(s)  before? I don't remember the thickness but would the 
>taping blade material STM mentioned work? I'm not sure about my heat 
>treating abilities (proper annealing etc). I have read Bretts excellent 
>documentation in the past.
>Regards
>Barry


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