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COIN SHRINKING: in a B-field of improved uniformity

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Bert

Mon Mar 23 2015, 02:37AM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Interesting work, Sig! I mostly agree with your conclusions.

It looks like we have similar Maxwell capacitors - mine are Maxwell model 33252, 70 uF at 12 kV, arranged in a bank of two or three caps in parallel. I've found them to be very reliable. Maxwell spec's them for a lifetime of 100,000 shots at 100 kA/shot, and after shrinking well over 6,000 coins, the only failure I encountered was during a cold snap in the winter of 2014. The Quarter Shrinker is housed in an unheated enclosed porch. Unbeknownst to me, the castor oil dielectric fluid "froze" during an extended cold wave. I now know (with 20-20 hindsight and a discussion with GA's capacitor engineers) that when castor oil freezes at around 14F (-10C), small amounts of dissolved water come out of solution and are absorbed by the kraft paper dielectric. This significantly reduces its dielectric strength. During charging, one cap in the pair abruptly faulted at about 8500 volts, dumping about 5 kJ of energy into the short-circuited cap. Fortunately, the welded metal case held, preventing any fluid leakage, but the capacitor was destroyed, The remaining caps now have strip heaters to keep them toasty all winter long.

I'll be very interested in your progress towards more permanent work coils, When I first started shrinking coins, I also looked at various techniques to keep the work coil together including experimenting with fiberglass and resin. However, the heaviest radial forces seem to be confined to the 1 or 2 turns that are directly over the coin. I was unable to find any approach that would keep this portion of the coil from self-destructing at anywhere near normal shrinking energies. I think it may be an energy density thing associated with coin shrinking, since epoxy-reinforced work coils DO work quite well for commercial electromagnetic metal forming. These EM forming systems seem to spread out repulsion forces more evenly across the work coil or they use field shaping techniques. For example, I looked at using a heavy slotted cylindrical conductor (a "flux concentrator") between the work coil and coin to more evenly distribute forces across the entire length of the work coil. While this might prolong the work coil's life, I concluded that the presence of the gap in the concentrator would cause an unacceptable "kink" in the coin at the gap. And, that double-build 200C magnet wire was easy to use for multiple coin diameters and, even if sacrificial, was still very cost-effective.

I agree that coin wrinkling may be mechanical buckling. Once it begins, further shrinking may intensify the effect. And, some coins, such as US small (golden) dollars, seldom show any wrinkling even though magnetic forces are similar. I found that wrinkling can be prevented, or at least minimized, by using the appropriate dowel material to hold the coin firmly in position perpendicular to the coil H-field while also constraining the perimeter of the coin during the initial stages of shrinking. Choosing the right material is important. I initially used wooden dowels, but these quickly shattered and often left an imprint of the wood grain on the coin's faces. I also tried a variety of polymer rods. Denser or harder polymers caused the surface features of the coins to become unattractively flattened. After much experimentation, I got the best results with natural UHMW rod for US clad coins, and nylon for some silver alloy coins. Curiously, clad coins sometimes contain hidden cladding defects between the outer Ci-Ni layers and the inner Cu substrate. During shrinking, the improperly bonded regions cause interesting force imbalances and "Mutant coins"

Raising the natural frequency of the LC circuit does have some interesting effects on the final shape of coins even when using identical energy levels. A lower bank capacitance and higher voltage results in the outer edge of the coin becoming significantly thicker along the perimeter than in the center - a property likened to "toroiding". Using a higher bank capacitance reduces, but does not eliminate this effect, particularly for large coins. Using even higher bank capacitance seems to reduce shrinking efficiency - probably because of reduced di/dt. The "sweet spot" for single-layer coils seems to be with "square" shaped ~10-turn coils with diameter approximately equal to length. We use #10 through #14 AWG round 200C double-build magnet wire to cover various coin sizes from dimes through Silver Eagles (0.7 - 1.6"). Work coil current measurements with a Pearson CT shows that work coils seem to stay intact during most/all of the first current half-cycle, even though the shape of the coil may change dramatically prior to self-destructing. There is some evidence that the inertial of the work coil helps "keep things together" during the critical 1/4 cycle to the first primary current peak, and it's not especially critical that the coil stay intact after this point.

Signification

Wed Apr 01 2015, 07:16PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Bert,
6000 coins...WOW!!! I'm at about 30 (..don't have a convenient setup yet) ...don't relax during the procedure, no matter how many--it just takes one tiny slip... Also you seem to be on track to advancement in the art. What is your trigger method?
It does seem like our setups will be similar in time and space! Yes, I looked at the Maxwell capacitors that I have (2) and they are the same model--are yours orange? I an a bit concerned...It has been several years since I used them and they were outside (unenclosed back porch) in some rather hard freezes!! Do you have a source for these? I have just received the following from McMaster Carr: (all 1-1/4" dia x 2' long for ~1" quarters). I ordered then after reading your message:
1) Easy-to-Machine ABS Rod
2) Self-Lubricating MDS-filled Nylon Rod (Black)
3) Self-Lubricating Oil-Filled UHMW Polyethylene rod
all to be cut for end pieces.

I have not found a great formula yet for reusable windings (haven't really tried so far either) but hope to have the time soon. As I mentioned, I will try keeping the coin in a constant B-strength region, which require a first try of centering the the coin in the ends of the 1-1/4" rods. I plan to drill a 'coin sink' pit with a depth of 1/2 the coin thickness on each rod end. I tested this by using a 1" wood bit (with those nasty teeth at the outer edged filed away). I found this difficult to center. Then I discovered that on the inside of plastic caps (for spray paint etc.) the center has a small cup that is -exactly- 1-1/4" ID and usually a dimple dead center in which to punch a hole, after it is slid over the rod--it's a perfect fit! I also have 8, 9, 10, 12 gauge wire and 6 and 8 gauge insulated wire. I previously used 9 turns of 10AWG magnet.

I want to try insulated wire buried in fiberglass-cloth resin. Since I am not placing the coin edge at the outer ID of the coil, there will be a 1/8" gap all around the 1-1/4" ID which I plan to have filled with the potting. I realize this may require more cap energy (maybe close to the one klugesmith mentioned for coin-shrinking use-I don't recall the specs, but they caught my attention), but if it works--so what, right? Is efficiency that important here if you get a reusable coil that puts out excellent coins? One sort of wild thought was to form a Brooks config with a 3x3 coil. Here, I am worried about divergent B-Fields, but have learned, especially with shrinking, the unexpected seems to work more often than not. I want to work on tuning, decaying oscillations, critical damping...
I have found that PAM non-stick (for cooking) to be invaluable in removing the cured cloth resin.

I do have some samples of potted coils that I want to post photos of. I can't believe I let people take my last coin!!!!!!!!!!!

...more when I get started again...

Bert

Tue Apr 07 2015, 08:34PM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Signification wrote ...

Bert,
6000 coins...WOW!!! I'm at about 30 (..don't have a convenient setup yet) ...don't relax during the procedure, no matter how many--it just takes one tiny slip... Also you seem to be on track to advancement in the art. What is your trigger method?

We used to use a trigatron for many years but it sometimes unexpectedly self-discharged (surprise - BANG!!). I HATE 6 kJ surprises. Sometimes it failed to fire (particularly at lower voltage levels). And, it required excessive maintenance. After many years we converted to a a solenoid-driven switch (see below). This has been relatively trouble-free ever since and reliably works over a wide range of voltages. The only problem is that some of the zinc in (from the brass electrodes) is vaporized, and deposited as zinc oxide on the inner surfaces of the Lexan housing. I would strongly recommend using some type of housing around the gap to reduce the noise level when the switch fires.

Signification wrote ...

It does seem like our setups will be similar in time and space! Yes, I looked at the Maxwell capacitors that I have (2) and they are the same model--are yours orange? I an a bit concerned...It has been several years since I used them and they were outside (unenclosed back porch) in some rather hard freezes!! Do you have a source for these?

No such luck. I got mine off eBay over 15 years ago, and mine are also orange. Had to drive to central Ohio to pick them up. Simply storing them in a cold place should be OK as long as they have thoroughly warmed up before applying voltage to them. Maxwell specs their storage temperature range as -20C - 60C (-4F - 140F). However, the Maxwell engineer I talked with said that these caps could be stored at even colder temperatures as long as they had fully stabilized to a temperature above -10C (14F) before voltage was applied.

Signification wrote ...

I want to try insulated wire buried in fiberglass-cloth resin. Since I am not placing the coin edge at the outer ID of the coil, there will be a 1/8" gap all around the 1-1/4" ID which I plan to have filled with the potting. I realize this may require more cap energy (maybe close to the one klugesmith mentioned for coin-shrinking use-I don't recall the specs, but they caught my attention), but if it works--so what, right? Is efficiency that important here if you get a reusable coil that puts out excellent coins?

Increasing the space between inside of the coil and coin reduces coupling and seems to decrease shrinkage "efficiency". I've always try to minimize this space but not so much that flashovers occur between the coin and coil.

Signification wrote ...

One sort of wild thought was to form a Brooks config with a 3x3 coil. Here, I am worried about divergent B-Fields, but have learned, especially with shrinking, the unexpected seems to work more often than not. I want to work on tuning, decaying oscillations, critical damping...
I have found that PAM non-stick (for cooking) to be invaluable in removing the cured cloth resin.

Many years ago, an experimenter in the Pacific Northwest told me that he was efficiently shrinking coins using a multilayer coil wound with something like 16 - 18 turns of vinyl-insulated hookup wire, so your proposed technique might work as long as you can prevent the coin from twisting relative to the now shorter coil. He was using relatively small Sprague 14uF 20kV energy discharge caps off the surplus market.

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