Dick Johnson submitted this entry from his Encyclopedia of Coin and Medal Terminology. Thanks. I've added an image from a related 2017 NGC article. -Editor
Mechanical Doubling. Tiny lines or offset around the edge of lettering because of loose dies in the coining press. Also called double line
by collectors, it shows a shift to one side forming a double outline of letters or numbers near the rim. The phenomenon is not in the die – it is not a doubled die – but is a
chattering of a loose die during striking, particularly after a long press run. It has been described as a twisting of the die as it withdraws pulling small metal relief from its
fixed position leaving large heavy relief, as a portrait or device, unaffected. It is cleared up by tightening the set bolts in the chucks holding the die in place. Such errors
are usually not collected by numismatists because of their minor interest in the field.
A coin with mechanical doubling can be discerned from double die coins by the clarity of the letters or figures. True double die strikes (with a die that has two identical
images) have distinctly formed letters, two distinct over-lapping complete letters. Mechanical doubling is a distortion of letters which look misshapen; the top of their letters
or figures are smaller than their base, with a movement of the upper part of the letters or figures to one side.
Reference:
NM19 {1991} Margolis, p 321-324.
To read the complete NGC article, see:
Doubled Dies vs. Machine Doubling
(https://www.ngccoin.com/news/article/5688/Double-Dies-vs-Machine-Doubling/)
Book lovers should be word lovers as well.
Looking for the meaning of a numismatic word, or the description of a term? Try the Newman Numismatic Portal's Numismatic Dictionary at: https://nnp.wustl.edu/library/dictionary
Or if you would like a printed copy of the complete Encyclopedia, it is available. There are 1,854 terms, on 678 pages, in The Encyclopedia of Coin and Medal
Technology. Even running two a week would require more than 19 years to publish them all. If you would like an advance draft of this vital reference work it may be obtained
from the author for your check of $50 sent postpaid. Dick Johnson, 139 Thompson Drive, Torrington, CT 06790.
Wayne Homren, Editor
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