Just a question - is it odd that there is a pinch mark on the obverse & reverse 180* out from the clipped part? Is it a possibility that this coin was in a vise of some kind and the clipped portion was removed post-minting?
Quote: ..that's the Blakesley Effect mentioned above.
TIL
I used these resources: error-ref.com/blanking-and-cutting-errors/ error-ref.com/_curved_clips_/
so... If I understand what this site is saying (and I wish they had some diagrams at each bullet point) the clip's absence of material causes the surface pressure of the strike to be uneven in tons/sq-in across the die face. That imbalance in striking pressure results in a softening of the strike opposite the hole.
The power of the strike is lost to the deficit area. Sort of like how if you have an open differential, power goes to spin the wheel with the least resistance? Am I understanding this correctly?
It would be super cool to see a finite element analysis / force map of a normal strike and a clipped planchet strike. If anyone has details hit me please. I've subbed to this thread.
Quote: Just a question - is it odd that there is a pinch mark on the obverse & reverse 180* out from the clipped part? Is it a possibility that this coin was in a vise of some kind and the clipped portion was removed post-minting?
The Blakesley effect is produced when the blank is turned in the upsetting mill. That's when the rim is put on the edge, Kind of hard to explain. There are 2 rolling pins 180degress apart rolling the rim on, when one of the pins hits the clip, that moves the blank towards that pin and away from the other one. With no pressure the rim doesn't get formed at the Blakesley's spot.
I find it easier to first imagine the upset mill as two grooved rails. They move in opposite directions and are slightly closer to the exit than the entrance. The blank drops between them and the proto-rim is squeezed by the rolling motion as the blank moves towards the exit.
In reality, the upset mill is a fixed outer rim. The inner rim is motorized and turning. The effect is the same—the blank rolls between the grooved rails, and as the space between the rails slightly decreases, the proto-rim is pushed up on the edge of the blank: Viola, a planchet.
As John said, the Blakesley effect happens because every time the gap rolls around, the blank is narrower than the rail distance. So instead of a proto-rim getting shoved up on the opposite side, the blank simply moves sideways between the rails until it rotates past the missing metal again.
This is all before striking. Now the coin gets struck. The metal flow towards the gap and the Smith effect on the rim corners happen during striking. But regarding the rim, the striking process simply transforms the proto-rim into the final rim by smashing it between the two dies and the collar. Since the malformed planchet is missing metal on one side, and lacking a proper proto-rim 180 degrees opposite, nothing during the strike is going to fix that.
If the clip is small enough (and depending on the upset mill and striking machine), sometimes a little clip doesn't produce a visible Blakesley effect on the planchet, and the final coin rim seems normal opposite the clip. HOWEVER, if you are shopping for clipped coins on ebay, a coin show, or anywhere else, and you see a sizable clip but a missing Blakesley effect, that is a big red flag for a fake mint error coin.
ErrorRef.com is a very complete resource, but I found the above article the most helpful when I started on error coins. Also check out posts by Tropicalbats, he has posted many clipped planchet coins that are very well photographed and explained on CCF.
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Posted 05/20/2024 4:10 pm
