Before we even start to examine Mechanical Doubling Damage (MDD), let me start by explaining that MDD is a collectable error coin. I have several in my personal collection, but it is not Hub Doubling (Die Doubling). Coins that exhibit MDD can have a very dramatic appearance, but they should not sell for much of a premium. If the coin has a value of $3 and the dealer is asking $3.50 or $4, you might want to add the coin to your collection.

If a dealer or collector is asking several hundred dollars for a coin with MDD, run... don't walk away. It is a damaged coin. This is not saying they are not collectable, just that one should not pay a high premium for a coin with MDD.

Now let’s take a minute to examine Mechanical Doubling Damage.

We know what MDD is, but there are two schools of thought on how it happens. If the die is loose in the coinage press, some movement of the die can occur after the striking of the coin. At the instant the hammer die (obverse die) starts to move back up to the starting position, mechanical doubling occurs. At this point the pressure is being reduced on the die. Since the striking of the coin is complete, the minting process is complete. Anything that happens when the pressure on the dies is reduced is damage to the coins surface.

Adjustment screws that hold the die firmly in the coinage press can become loose. This is a mechanical problem, so it is called Mechanical Doubling Damage.

One group feels the die bounces and creates a second image. The second group feels that the die twist and forms a second image. I personally feel the die does twist and that it does not bounce.

Examine the top of the letters below. Notice the secondary image to the top of the letters are flat and shelf like in appearance. It looks like someone took a bulldozer and pushed the letters down. There is no separation of the any letter at the corners or on the serifs. The doubling has a very sharp and angular appearance.

On this second example shown below, there is a corner on the second image to the left of the letter “E”. BUT, there is no separation on the main image at the serifs. ("Serif" is the corners or ends of the letters or numbers on a coin.) This is mechanical doubling damage.

On the third photo, we have separation at the lower right of the “N” & “T”. Or do we ? Look at the primary image; is there any separation of the serifs?    NO (If you need to see a bigger image of the serifs, just click on this image.)

You have looked at three different appearances that Mechanical Doubling Damage can display. It is also important to know that mechanical doubling can appear on both sides of the same letter. This usually will not happen on a doubled die.

It is also important to know that a coin can have mechanical doubling damage and also be a true doubled die. Yes, both can be found on the same coin. Also, more than one class of doubled die can be found on the same coin.

Let’s examine the key points a doubled die should display.

Characteristics of a Doubled Die

Look at the photomicrograph on the right. Notice the white arrow points to the separation of the designer’s initials. The top of the “J” is almost completely separated from the primary image.

The black arrow points to the separation of the serifs on the lower right side of the number “1”. Both show rounded secondary images.

On the second photomicrograph, notice that the secondary image is rounded and not flat. The black arrows point to the separation in the serifs. The serifs show separation at the top of the letter and at the bottom of the letters. (If you need to see a bigger image of the serifs, just click on this image.)

The serifs only need to show a separation at one serif, but this is a good example. It is also important to know that many 19th century coins have letters with flat surfaces. But the serifs will still show separation on a true doubled die.

Are you ready for a little test? Study the images below and determine which ones are Mechanical Doubling Damage and which ones are Doubled Dies. After examining the image, you will find the answers at the bottom of the page.

Coin 1	Coin 2
Coin 3	Coin 4
Coin 5	Coin 6
Coin 7	Coin 8

Coin 1 - Mechanical Doubling	Coin 2 - Mechanical Doubling
Coin 3 - Doubled Die	Coin 4 - Mechanical Doubling
Coin 5 - Doubled Die	Coin 6 - Mechanical Doubling
Coin 7 - Doubled Die	Coin 8 - Doubled Die

We hope you have learned a little more about Doubled Dies and Mechanical Doubling Damage.

What is a Doubled Die?	What is Mechanical Doubling?
What is an RPM, OMM?	What is an RPD, MPD, OD?