Strike doubling plagued 2006 coinage from the one cent coin through the silver and gold collector and bullion issues. The most recent examples we?ve seen were on the 2006 San Francisco Old Mint commemorative gold coins and the silver and gold American Eagles. It is the most common form of doubling found on coins.
Strike doubling, the most common form of doubling, is shown at the right on a 2006-D nickel obverse.
It is sometimes referred to by others as ?Machine Doubling Damage,? ?Mechanical Doubling,? ?Shelf Doubling,? etc. While the terms may vary among specialists, what most agree upon is that it is a very minor variation that occurs during the manufacturing process, that it is not a die variety and that it is too common to quote any extra value on. Collectors often misdiagnose it as the more valuable hub doubling (doubled dies) or repunching. What makes these so valuable is that the doubling is not only present on a coin but on the die (doubled die) itself.
Strike doubling is generally accepted to be caused by die bounce or slide due to looseness in the press or tooling-to-die assembly, which in turn causes excessive vibration during press operation, much in the same way as excessive vibration may set up in a running automobile with a broken or loose motor mount.
In effect, the vibration causes a die to bounce or slide against the struck coin within the split second after it is struck. Mint technicians state that this form of doubling is usually eliminated when loose bolts, etc., are tightened back down.
The most common form of strike doubling is characterized by a flat, shelf-like area of doubling bordering a design that represents metal from the original raised image that has been smashed down into the field of the coin by the die. This varies from hub doubling, which is usually raised and rounded.
Ken Potter is the official attributer of world doubled dies for the Combined Organizations of Numismatic Error Collectors of America and for the National Collectors Association of Die Doubling. He also privately lists other collectible variety types on both U.S. and world coins in the Variety Coin Register. More information on either of the clubs, or how to get a coin listed in the Variety Coin Register may be obtained by sending a long self-addressed envelope with 63 cents postage to P.O. Box 760232, Lathrup Village, MI 48076, or by contacting him via e-mail at KPotter256@aol.com. An educational image gallery may be viewed on his Web site at www.koinpro.com.