The following is Mike Diamond's Collectors' Clearinghouse column from the July 20 issue of Coin World:
When a planchet is fed into the striking chamber, it’s supposed to be perfectly centered over the anvil die. This positioning is ordinarily enforced by the surrounding collar, but it often holds even when the collar is not deployed.
An unconfined planchet that is perfectly centered when struck becomes an oversized coin known as a broadstrike, sometimes called a “centered broadstrike”(most hobbyists omit the modifier).
A misfed planchet either falls short of the striking chamber or shoots past it. In either case, a serious misfeed results in an off-center strike, unless the planchet completely misses the striking chamber.
A modest misfeed of 30 percent or less may result in an off-center strike or an uncentered broadstrike. In an off-center strike, some of the peripheral design is cut off at the edge of the coin on at least one face. In an uncentered broadstrike, the design is complete on both faces. Although it’s technically part of the design, the design rim itself is not taken into account when determining whether a poorly centered strike is an off-center strike or an uncentered broadstrike.
Three factors determine whether a poorly-centered planchet becomes an uncentered broadstrike or an off-center strike. These are (1) the extent of the misfeed, (2) striking pressure, and (3) design.
The extent of misfeed is calculated as the maximum radial distance from the edge of the planchet to the internal margin of the coin’s unstruck crescent. The measurement is then converted to a percentage of the coin’s standard diameter.
We can tell that the illustrated 1998 Lincoln cent is a minor off-center strike because the ST of TRUST is cut off. None of the reverse lettering is cut off because the lettering is a bit farther from the design rim (the design factor).
When striking pressure generates very little expansion, even a slight misfeed can result in an off-center strike. When striking pressure is substantially elevated, even a significant misfeed can be converted into an uncentered broadstrike.
Striking pressure is a function of ram pressure (the tonnage delivered to a planchet of normal thickness) and minimum die clearance. The smaller the clearance the greater the expansion.