I've pulled a few partial collars from circulation and scored this one yesterday. Well worn but what would you expect from 11 years circulation:
(http://www.australian-coins.net/gallery/albums/userpics/10124/Tilted_partial_collar2.jpg)
I've always thought these were due to the coin not sitting properly within the collar but there seems to be a couple of reasons some more plausible than others.
The "tilted coin" explanation is the one linked to the Purple Penny site:
http://www.australian-threepence.com/blog/2013/07/an-explanation-for-partial-collar-coin-errors.html and comes from the Royal Mint
Prior to a coin blank being struck it is often fed through a rimming machine to be pre-rimmed before being struck into a coin. This process raises the edge of the coin and reduces the diameter of the planchet to allow for a reduced striking pressure (thus increasing the die life) and helps the coin wear better over the circulating life of the coin. Now if a coin blank missed this process of being rimmed it would therefore be more likely to not feed properly into the press resulting in an error just like a partial collar strike as it was just too large to fit correctly into the collar die.
So he is saying that the coin looks like this:
(http://www.australian-coins.net/gallery/albums/userpics/10124/Tilted_coin.JPG)
That's always been my understanding but I'm not totally comfortable with this as I can't see how you could get two fully struck up sides when the coin remains tilted. The faces of the die always remain parallel so for the coin faces to be fully struck then the blank also has to be flat and a tilted partial error isn't possible.
The other explanation comes from http://hermes.csd.net/~coneca/content/glossary.html as
Tilted partial collar
When the collar jams in a tilted position, the result is a coin with an unlevel split edge. See also Partial collar, Reverse partial collar.
which looks like this:
(http://www.australian-coins.net/gallery/albums/userpics/10124/Tilted_collar.JPG)
I suppose a third explanation is a combination of the two where the blank not fitting within the collar sits on top and when it is struck this causes the collar to warp. I can see that for the larger coins but for a smaller frail planchet like the 5 cent I wouldn't have thought it likely.
Anyone have any strong views one way or the other? Does the term "tilted" refer to the coin or the collar????
TIA
Quote from: 'Steveqld' pid='29553' dateline='1380426541'Does the term "tilted" refer to the coin or the collar????
The collar. Even if the coin isn't properly rimmed, and therefore doesn't sit flat, a hundred tons of pressure will
make it sit flat. But in the process it will push the collar out of the way, possibly jamming it for further strikes.
So yeah, a combination of the two.
By the way, while it doesn't specifically address this issue, there is a very informative thesis written in 2000 available from the University of Pretoria.
http://upetd.up.ac.za/thesis/available/etd-07032006-151623/
I would recommend reading the abstract in the front section. Then definitely grab section 3:
http://upetd.up.ac.za/thesis/submitted/etd-07032006-151623/unrestricted/03sectionB-C.pdf
While things get rather technical pretty quickly, even a light skim should prove valuable. The presses in Canberra were similar if not identical to the ones used by the SA Mint at the time, if memory serves. But even if they're long gone, the principles remain the same.