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Re: question for coin shrinkers

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Hi Anthony,
A chemical explosion and a "coin shrinking" electrical explosion appear to have major differences. While a considerable volume of gas is typically evolved in the former case, much of the electrical energy (that's not expended in forming the coin and stretching the coil) apparently goes into kinetic energy in the ejected fragments. Although there is an initial pulse of high pressure (or even a shockwave near the actual arc plasma), the actual volume of gas in the chamber shortly after the electrical explosion is not much more than the that beforehand. The arc plasma disappears quite quickly, and the copper vapor rapidly cools and condenses.
I currently use 1/4" steel lining inside 1/2" thick Lexan. With replacement of the inner liners, this arrangement has withstood 6,000 shots at up to 6300 joules, and can be operated safely only to ~8000 joules. Based on my observations, I believe that doubling or tripling this energy level (as Bill wants to do) will require using thicker material or much more frequent plate replacement. The increased thickness not only provides brute strength against hypervelocity fragments, it also provides significant inertia, helping to prevent plastic deformation of the entire plate. Repetitive impact by hypervelocity copper fragments can do a surprising amount of damage. As an example, here's a "before and after" view of a fragment "catcher". Made from 3/4" thick steel, it required replacement after only ~1500 shots at 4000 - 6300 joules: 
http://205.243.100.155/photos/HVStuff/SteelBlocks2.jpg

Best regards,

Bert

Tesla list wrote:


Original poster: "Anthony R. Mollner" <penny831@xxxxxxxxxxxxx>
Interesting. I have no experience at all with quarter shrinking but I
 have a lot of experince with uhhh, exploding things. I can say that
1/2" steel plate is complete over kill. The most important thing here
is to have a vent of some type for the expanding gasses to escape,
other wise all you are making is a larger "bomb" that had you
detonated it in the open! The cabinet should have an indirect vent
that will allow the gases to vent but still deflect any fragments.
-----Original Message----- From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Thursday, December 28, 2006 1:52 PM To: tesla@xxxxxxxxxx Subject: Re: question for coin shrinkers 

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>
Hi Bill,
A 1/2" thick steel housing should work quite nicely for the case. Regular mild steel (such as A36) is fine, and I'm currently experimenting with 304SS and "abrasion resistant" AR400. The latter material is used for Humvee armor, but you can't easily drill or tap the stuff), but it is weldable. Thus far, I really don't see much difference in how any of these materials survive being struck by hypervelocity copper fragments, and I'd use plain old A36 in the 
future.
I would also recommend designing a cover latching mechanism to keep the box completely "closed" in order to prevent the force of the explosion from lifting the lid. Weights alone are usually not sufficient, particularly at higher energy levels. Sound deadening (on the exterior or sandwiched within a double wall) is also a great idea, especially if you are shrinking in a residential neighborhood. Please let us know what you did in this area and results... For insulating material, I've had excellent results using plain old HDPE or UHMW. The material is tough, resilient, and it gracefully responds to high shock. Indirect hits by wire fragments mark it up a bit, but I've used the material for parts of by shrinker and it has lasted for over 6,000 shots. Make sure that any HV bushings are located out of the direct line of fire from the wire fragments. The fragments will initially spray out radially from the work coil in a relatively narrow angle. The highest velocity pieces come from the 1-2 turns that are right above the coin. If you locate the HV feedthrough at right angles to the coil (i.e., along the centerline of the work coil axis of rotation), it should last a very long time with no problems. Be sure to provide sufficient creepage distance, since copper vapor and dust will collect on everything inside the blast shield. BTW, you may find it of interest that one of the Texas coin shrinkers (Paul Marshall) has taken measurements of the fragment 
 velocity  - some approached a hypervelocity of ~5,000 fps.
Good luck and play safely! Give me a holler if you have any other questions (630-964-2699). 
Bert -- *************************************************** We
specialize in UNIQUE items! Coins shrunk by huge magnetic fields,
Lichtenberg Figures (our "Captured Lightning") and out of print
technical Books. Visit Stoneridge Engineering at
http://www.teslamania.com ***************************************************
Tesla list wrote:
Moderated and approved by: Gomez Addams <gomezaddams@xxxxxxxxx> This is high voltage, but technically off-topic. Feel free to reply 
to me off-list. I will eventually be building a containment vessel
(possibly even with some sound insulation) for coin shrinking work
coils, after I get my main pulser rebuilt.  The chamber walls will
probably be made from 1/2" steel plate with 1x1" angle iron
ribs/stiffeners.  The pulser stores a maximum of 18kJ @ 10kV (wish
the voltage was higher). As for insulating materials for the HV
feed-through area, what materials have you used?  I was thinking of
using two layers with an air gap, possibly with 1/2" silicone resin
based G-11 on the inside. I gather the larger the chamber, and more
distant the walls from the work coil, the better the survivability.
I also assume that high modulus, high hardness materials like
phenolic laminates will tend to shatter, even the linen based ones.
What have your experiences been and what materials have you used? G-10? G-11? Polycarbonate? Phenolic? Filled polycarbonates? 
Delrin? - Bill "Gomez" Lemieux