Ramstrike Error?

So the Royal Australian Mint use polygonal blanks for the 50-cent piece?
I'm surprised, given the size of the piece, since I can't see a way to upset a polygonal blank. I'd use a circular upset planchet, myself â€" the deviation from circularity isn't all that great. But they must have their reasons.

Nice find.




Yes, they were round in 1966 but they were apparently confused with the 20c piece which has fairly similar diameter. Using a polygonal planchet seems to have solved that.

Very minimal the-purple penny. Probably 1-2mm but at least you can tell the edges are affected. I was stunned to see how expensive they are especially when the lugs are much clearer.

jpl - missed your post somehow but it could be a washer coin. Nevertheless, it's a 50 cents coin and I wouldn't lose much sleep over it.

If the lugs aren't that high then it just hasn't rotated as much. I have a Fiji 50c that's very similar, only slightly out of alignment.

Publius, you earlier referred to a round blank filling out in a polygonal collar; not a chance it would work. Even if you started with a rimmed planchet the spread necessary to turn a round planchet into a dodecohedral coin is greater than the force needed to fill in a clip. Also, unless you start with a polygonal planchet, there is no such thing as an upset planchet.
Clipped planchet errors are equally distinct on both thin and thick coins. I have a lovely double clipped florin and that is as thick as a 20 cent piece.

I have here a standard Australian 50 cent coin.

I measure the breadth across the flats (inscribed circle) as 31.72 millimeters, & across the points (circumscribed circle) as 32.51 millimeters. We can estimate the motion of metal needed to fill the polygonal collar as that needed to fill a circular collar of a diameter the mean of the two circles, or 32.12 mm. Since a blank would typically be about 0.1 mm smaller than the inscribed circle in order to drop easily into the collar, this requires a motion of metal of 0.49 mm on the diameter (0.25 mm on the radius).

The World Bi-metallic Collectors' Club error page provides ample evidence that modern presses can produce more than enough force to spread blanks to this extent. Look toward the bottom of the page for rings or plugs struck by themselves, & one piece with an undersized plug. Particularly relevant are plugs which have spread far enough to pick up the reeding which should be on the edge of the ring ; these are off-center as well, of course, but their strong deviations from circularity indicate the extent to which this is possible, & greatly exceed the case of a full-sized circular planchet struck in a dodecagonal collar.

Mind you, I have a suspicion that the result of this operation might be to produce a kind of scalloped effect, in which the edge was thickest (rim highest) at the centers of the flats, & thinnest at the points. That this would be undesirable is obvious, & the use of a polygonal blank could help prevent it. As far as die life is concerned, I expect it would be a wash, because of the extra striking force needed to raise the rim when striking from a flat blank.

Ah, sorry, I wasn't too sure what you were getting at.
Still, I think back when the 50c was round quite a lot of force was required to get a decent strike to start with, so increasing the force required to strike a coin would have probably been considered cost ineffective because it would just wear out the dies quicker.




Really? I thought the florin/half-crown difference was pretty close to the 20c/50c difference.